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The role of the vasti in patellar kinematics and patellofemoral pain

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The role of the vasti in patellar kinematics and patellofemoral pain

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Content THE ROLE OF THE VASTI IN PATELLAR KINEMATICS AND
PATELLOFEMORAL PAIN
by
Christopher M. Powers
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Biokinesiology)
May 1996
Copyright 1996 Christopher M. Powers
UNIVERSITY OF SOUTHERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES. CALIFORNIA 90007
This dissertation, written by
C hristopher Michael Powers
under the direction of .................. Dissertation
Committee, and approved by all its members,
has been presented to and accepted by The
Graduate School, in partial fulfillment of re
quirements for the degree of
DOCTOR OF PHILOSOPHY
Dean of Graduate Studies
Date • ............................U U J M
DISSERTATION COMMITTEE
Chairperson
Christopher M. Powers Helen J. Hislop Ph.D. P.T.
The Role of the Vast) In Patellar Kinematics and Patellofemoraf Pain
Medial vasti insufficiency is commonly accepted as being responsible for
lateral patellar tracking, however, electromyographic (EMG) studies supporting
this hypothesis are inconclusive. With the advent of kinematic magnetic
resonance imaging (KMRI), it was the purpose of this study to establish the role
of the vasti in contributing to patellar kinematics. Additionally, gait analysis was
performed to assess vasti activation patterns in this population and to identify
compensatory gait mechanisms characteristic of this disorder. Twenty-six
female patients with patellofemoral pain (PFP), and 19 controls participated in
this study. Subjects underwent KMRI of the patellofemoral joint using a device
that allowed resisted knee extension from 45 to 0 degrees. Following KMRI,
EMG analysis (indwelling electrodes) of the vastus lateralis (VL), vastus medialis
longus (VML), vastus medialis oblique (VMO) and the vastus intermedius was
conducted using the sam e apparatus. Gait analysis then was performed with
EMG, joint motion (Vicon), stride characteristics (Footswitch Stride Analyzer),
and force plate data (Kistler) being collected simultaneously during level walking,
stairs and ramps. KMRI data were assessed for medial/lateral patellar
displacement, tilt, and sulcus angle at six positions of knee flexion. Normalized
VL:VMO and VL:VML EMG ratios were calculated at the sam e angles. EMG
timing during gait was determined by footswitches. Stepwise regression was
employed to determine if the sulcus angle or EMG ratios were predictive of
patella tracking patterns, while two-way Anova’s (repeated measures) were
utilized to compare gait data between groups and conditions. The sulcus angle
was predictive of lateral patellar tilt and displacement at 0 degrees, supporting
the premise that bony structure is the primary determinate of patellar stability.
The VL:VML EMG ratio was inversely related to patella malalignment suggesting
recruitment of the VML was in response to, and not a cause of abnormal
tracking. During gait, there were no EMG differences indicative of compromised
patellar stability, however, PFP subjects demonstrated reduced vasti EMG
compared to normal. This was indicative of a quadriceps avoidance pattern
which was accomplished through a reduced velocity rather than a kinematic
adjustment at the knee. Such a compensation reduced the forces acting on the
patella while preserving loading response mechanics.
IN MEMORY OF
JO H N B. POW ERS
ACKNOWLEDGMENTS
A project of this magnitude could not have been undertaken, let alone
completed, without the guidance, support, and participation of a number of
individuals. To begin with, I would like to acknowledge the financial support of
the Bauerfeind Corporation, the Foundation for Physical Therapy, as well as the
California Physical Therapy Fund Inc. whose contributions m ade this
investigation possible. In addition, my deepest appreciation goes to the
subjects who donated their time and energy to participate in this study.
Primarily, I would like to thank my dissertation committee, whose
mentoring has made my doctoral education first rate. Namely, Dr. Arthur Hsu
for sharing his extensive knowledge of anatomy and biomechanics, and for
whom it has been a pleasure working under as a teaching assistant. To Dr. Stan
Azen for making the field of biostatistics understandable and even enjoyable,
and to Dr Frank Shellock for introducing me to the area of kinematic imaging
and providing me with the facilities andf expertise to complete such a study.
Special thanks go to Dr. Helen Hislop who had the confidence in me from the
very beginning, and who has been a tremendous source of unwavering support,
and to Dr. Jacquelin Perry who has been both a role model and an inspiration. It
has been an honor working in her laboratory over the past five years.
Additional thanks go to a number of individuals who contributed to this
work in a variety of ways. Dr. Marty Pfaff, who wrote the image quantification
software for this study, which literally saved me a year of drudgery, Dr. Todd
Molnar and the Southern California Orthopaedic Institute who provided me with
the patients, and Pete Sullenberger (AKA Captain Plastic) who was instrumental
in the construction of the custom MRI positioning device. Special appreciation is
extended to Ernie Bontrager and Sreesha Rao who have been a constant
source of engineering support throughout the duration of my studies. In
addition, I would like to acknowledge the contribution of the many M.P.T.
students that I have have the pleasure of working with over the past 5 years, for
their assistance in data collection and processing during portions of this study.
This study could not have been completed without the technical support
from both the Pathokinesiology Laboratory, as well as Tower Imaging Center
and St. Johns Hospital. The assistance of Carlos Williams, Sandy Hardy, Leo
Casados, Claire Alonzo, Lisa Edwards, Angel Sorino, and Arturo Aguilar is
greatly appreciated.
I would also like to acknowledge my colleagues (both past and present)
from the Pathokinesiology Laboratory: Leslie Torburn, Grace McNelis, Sara
Mulroy, Craig Newsam, Kristine Lassen, Lara Boyd, and Joanne Gronley, as well
as my fellow advanced studies students, particularly Beth Fisher and Kathy
Sullivan who have not only been a tremendous source of encouragement, but
have provided me with great friendship during the past few years. In addition, I
would like to thank the faculty members of the Department of Biokinesiology and
Physical Therapy for their mentoring, especially Dr. Lucinda Baker who has
always been there to lend friendly advice and reassurance.
Finally, I must thank all my friends and family who have been both
understanding and supportive of my endeavors in pursuing this degree. And of
course to my parents, Michael and Angela Powers, who have been encouraging
of my academic pursuits and have always provided me with the love and
support to accomplish my goals. I could not have made it this far without them.
■ * *
i n
TABLE OF CONTENTS
Acknowledgments...................................................................................................iii
List of Tables.............................................................................................................vii
List of Figures.......................................................................................................... viii
CHAPTER I: OVERVIEW...................................................................................... 1
Specific Aims................................................................................................. 5
CHAPTER II: LITERATURE REVIEW.................................................................7
Functional Anatomy of the Patellofemoral Joint....................................... 7
O sseous structure.............................................................................. 7
Synovium............................................................................................. 10
Vascular supply................................................................................... 10
Soft tissue stabilizers.......................................................................... 11
Biomechanics of the Patellofemoral Joint..................................................14
Function of the patella........................................................................14
Patellofemoral contact a re a s................................... 16
Patellofemoral joint reaction forces...................................................17
Normal patellar kinematics................................................................21
Normal Function of the Quadriceps Femoris............................................22
Anatomy............................................................................................... 22
Role of the quadriceps femoris in knee extension........................ 24
Role of the quadriceps femoris in patellar stabilization.................25
Role of the quadriceps femoris in normal and
pathological gait..................................................................................26
Patellofemoral Pain Syndrome.................................................................... 29
Pathophysiology of patellofemoral pain.......................................... 31
Excessive lateral pressure syndrom e..............................................35
Patellar subluxation............................................................................38
Etiology of patellar subluxation...............................................40
Bony abnormalities........................................................40
Abnormal bony relationships....................................... 43
Abnormal subtalar joint pronation............................... 45
Soft tissue abnormalities............................................... 47
Passive structures...............................................47
Dynamic structures.............................................48
Quadriceps Femoris Function in Patellofemoral Pain.............................. 50
The Role of Radiology in the Evaluation of Patellofemoral Joint
Disorders........................................................................................................ 54
Standard roentgenogram s................................................................54
Computerized tom ography...............................................................59
Magnetic resonance imaging........................................................... 61
CHAPTER III: PATELLAR KINEMATICS AND VASTI ACTIVATION IN THE
NORMAL AND PATELLOFEMORAL PAIN POPULATIONS........................ 66
Introduction................................................................................................... 67
Quantification of patellar tracking using kinematic magnetic
resonance imaging............................................................................. 67
Factors contributing to abnormal patellar tracking in subjects
with patellofemoral pain....................................................................69
Materials a > id M ethods............................................................................... 71
Subjects..............................................................................................71
Instrumentation................................................................................. 73
Kinematic magnetic resonance imaging............................... 73
Dynamic EMG........................................................................... 74
Procedures......................................................................................... 76
Kinematic magnetic resonance imaging..............................76
Dynamic EMG..........................................................................78
Reliability of KMRI and EMG m easurem ents......................79
Data Management.............................................................................. 81
Kinematic magnetic resonance imaging............................... 81
Dynamic EMG...........................................................................84
Data Analysis....................................................................................... 85
Results and Discussion.............................................................................. 87
Quantification of patellar tracking using kinematic magnetic
resonance imaging............................................................................87
Results....................................................................................... 87
Case reports...................................................................91
Discussion............................................................................... 96
Factors contributing to abnormal patellar tracking in subjects
with patellofemoral pain......................................................................103
Results........................................................................................103
Reliability.......................................................................... 103
Kinematic magnetic resonance imaging.................... 104
Dynamic EMG.................................................................106
Relationship between EMG ratios, sulcus angle
and patellofemoral joint congruency........................... 108
Discussion.................................................................................112
Summary....................................................................................................... 122
CHAPTER IV: PATELLOFEMORAL JOINT DYSFUNCTION AND ITS
INFLUENCE ON GAIT..........................................................................................123
Introduction...................................................................................................124
Timing and intensity of vasti contraction during gait activities 124
Loading characteristics in subjects with patellofemoral pain 125
Influence of quadriceps strength and patellofemoral pain
on gait characteristics and joint motion..........................................127
Materials and M ethods................................................................................129
Subjects.............................................................................................. 129
Instrumentation..................................................................................130
Dynamic EMG........................................................................... 130
Motion analysis.........................................................................131
Force plate analysis................................................................. 133
Stride analysis........................................................................... 133
Torque testing........................................................................... 134
Visual analog pain scale.......................................................... 134
Functional Assessment Questionnaire..................................135
Procedures..........................................................................................137
Data management...............................................................................140
Dynamic EMG........................................................................... 140
v
Motion.......................................................................................141
Force plate d ata......................................................................141
Torque......................................................................................143
Data analysis......................................................................................144
Results and Discussion..............................................................................146
Timing and intensity of vasti contraction during gait activities 146
Results......................................................................................146
Subject characteristics.................................................146
Dynamic EMG............................................................... 147
Onset....................................................................148
Cessation............................................................ 151
Mean Intensity.................................................... 152
Knee motion..................................................................154
Discussion............................................................................... 155
Loading characteristics in subjects with patellofemoral pain 160
Results......................................................................................160
Subject characteristics.................................................160
Vertical ground reaction force.....................................163
Peak loading rate..........................................................163
Knee flexion...................................................................164
Discussion............................................................................... 165
Influence of quadriceps strength and patellofemoral pain
on gait characteristics and joint motion......................................... 170
Results...................................................................................... 170
Subject characteristics................................................. 170
Relationship between knee extension torque,
pain and functional assessm ent score...................... 170
Stride characteristics................................................... 172
Joint motion................................................................... 175
Discussion............................................................................... 179
Summary.......................................................................................................185
CHAPTER V: SUMMARY AND CONCLUSIONS............................................187
References............................................................................................................... 191
vi
LIST OF TABLES
Table 3-1. Subject Characteristics......................................................................72
Table 3-2. MRI Measurement Reliability: Intraclass
correlation Coefficients.......................................................................87
Table 3-3. EMG Ratio Reliability: Intraclass Correlation Coefficients............ 103
Table 3-4. Partial Correlations for Bisect Offset................................................109
Table 3-5. Partial Correlations for Patellar Tilt...................................................109
Table 4-1. Subject Characteristics......................................................................146
Table 4-2. EMG Onset: Vastus Medialis Oblique and Vastus Lateralis.........147
Table 4-3. EMG Cessation: Vastus Medialis Oblique and Vastus Lateralis.. 152
Table 4-4. EMG Intensity: Vastus Medialis Oblique and Vastus Lateralis......153
Table 4-5. Subject Characteristics..................................................................... 160
Table 4-6. Stride Characteristics....................................................................... 161
Table 4-7. Force Plate Characteristics...............................................................164
Table 4-8. Knee Flexion ............. 165
Table 4-9. Subject Characteristics..................................................................... 170
Table 4-10. Maximum Knee Extensor Torque...................................................171
Table 4-11. Scores for the Visual Analog Pain Scale and Functional
Assessment Questionnaire..............................................................171
Table 4-12. Stepwise Regression Results for Predicting Velocity,
Stride Length, and C adence............................................................175
vii
LIST OF FIGURES
Figure 2-1. Representation of the articular facets of the patella.....................8
Figure 2-2. Inferior aspect of the distal end of the fem ur................................ 9
Figure 2-3. Anatomy of the arterial supply of the patellofemoral joint........... 11
Figure 2-4. Fiber orientation of the quadriceps musculature in relation
to the fem ur....................................................................................... 13
Figure 2-5. Contact areas on the patella as a function of knee flexion
at 20, 45, 90 and 135 degrees.........................................................17
Figure 2-6. Compressive forces on the patellofemoral joint............................ 18
Figure 2-7. Increased compression between the lateral patellar facet
and femoral condyle as a result of excessive lateral tilting
of the patella....................................................................................... 35
Figure 2-8. The angle between the quadriceps tendon and the
patellar ten d o n ..................................................................................39
Figure 2-9. Schematic representation of the Wiberg classification
system for patella morphology............................. 41
Figure 2-10. Patient positioning used to obtain a Merchant axial view of
the patellofemoral joint................................................................... 55
Figure 2-11. Congruence angle..........................................................................56
Figure 2-12. Patellofemoral joint relationships as proposed by Laurin 58
Figure 3-1. Patient set-up on the nonferromagnetic positioning device
used for imaging.................................................................................75
Figure 3-2. Placement of the patient and positioning device within
the MR system ...................................................................................77
Figure 3-3. Experimental set-up used to assess vasti activity using
the MRI positioning device...............................................................80
Figure 3-4. Methods used to measure bisect offset........................................ 82
Figure 3-5. Methods used to measure patellar tilt............................................83
Figure 3-6. Methods used to measure the sulcus angle................................. 84
Figure 3-7. Reliability results for the bisect offset, patellar tilt,
and sulcus angle measurements in seven normal subjects....... 88
viii
Figure 3-8. Bisect offset in normal subjects from 45 to 0 degrees
of knee flexion.....................................................................................89
Figure 3-9. Patellar tilt in normal subjects from 45 to 0 degrees
of knee flexion.....................................................................................90
Figure 3-10. Sulcus angle in normal subjects from 45 to 0 degrees
of knee flexion..................................................................................91
Figure 3-11. Example of a series of axial plane images obtained from
a normal subject from 45 to 0 degrees of knee flexion.............92
Figure 3-12. Bisect offset data for normal and patellofemoral pain
patients from 45 to 0 degrees of knee flexion.............................93
Figure 3-13. Patellar tilt data for normal and patellofemoral pain
patients from 45 to 0 degrees of knee flexion.............................94
Figure 3-14. Sulcus angle data for normal and patellofemoral pain
patients from 45 to 0 degrees of knee flexion.............................94
Figure 3-15. Example of a series of axial plane images from patient # 2
from 45 to 0 degrees of knee flexion............................................95
Figure 3-16. Comparison of patellar tilt between the patellofemoral pain
and normal groups from 45 to 0 degrees of knee flexion..........104
Figure 3-17. Comparison of bisect offset between the patellofemoral pain
and normal groups from 45 to 0 degrees of knee flexion..........105
Figure 3-18. Comparison of sulcus angle between the patellofemoral pain
and normal groups from 45 to 0 degrees of knee flexion..........106
Figure 3-19. Comparison of the vastus lateralis:vastus medialis oblique
electromyographic ratio between the patellofemoral pain
and normal groups from 45 to 0 degrees of knee flexion..........107
Figure 3-20. Comparison of the vastus lateralis:vastus medialis longus
electromyographic ratio between the patellofemoral pain
and normal groups from 45 to 0 degrees of knee flexion..........108
Figure 3-21. Relationship between the vastus lateralis:vastus medialis
longus electromyographic ratio and the bisect offset for
both patellofemoraTand normal subjects at 27 degrees
of knee flexion.................................................................................110
Figure 3-22. Relationship between the sulcus angle and bisect offset
for both patellofemoral pain and normal subjects at zero
degrees of knee flexion.................................................................. 111
Figure 3-23. Relationship between the sulcus angle and patellar tilt
for both patellofemoral pain and normal subjects at zero
degrees of knee flexion.................................................................. 112
Figure 3-24. Axial plane Images obtained from one normal subject and
three patients with patellofemoral pain.........................................121
Figure 4-1. Camera placement with respect to the data acquisition field
and the VICON system measurement volume............................. 132
Figure 4-2. Example of Visual Analog Scale scoring........................................135
Figure 4-3. Functional Assessment Questionnaire...........................................136
Figure 4-4. Anatomical marker placement for motion analysis.......................139
Figure 4-5. Parameters used for force plate analysis...................................... 142
Figure 4-6. Interval selection for the determination of peak loading rate...... 143
Figure 4-7. Onset and cessation of vasti activity during free and fast
walking for patellofemoral and control subjects............................ 148
Figure 4-8. Onset and cessation of vasti activity during ascending and
descending stairs for patellofemoral and control subjects..........149
Figure 4-9. Onset and cessation of vasti activity during ascending and
descending ramp for patellofemoral and control subjects..........150
Figure 4-10. Mean intensity of vasti contraction between patellofemoral
pain and control subjects for all conditions te ste d ..................... 154
Figure 4-11. Comparison of maximum knee flexion during the loading
response phase of the gait cycle between patellofemoraT
pain and control subjects for all conditions teste d ..................... 155
Figure 4-12. Relationship between peak vertical ground reaction force
expressed as a percentage of body weight and walking
velocity.............................................................................................. 162
Figure 4-13. Relationship between time to peak vertical ground reaction
force and walking velocity...............................................................162
Figure 4-14. Relationship between peak loading rate expressed as body
weight per second and walking velocity....................................... 163
Figure 4-15. Correlation between Functional Assessm ent Questionnaire
score and the Visual Analog Pain sc o re ...................................... 172
Figure 4-16. mean velocity between patellofemoral pain and normal
subjects for all conditions tested....................................................173
Figure 4-17. Mean stride length between patellofemoral pain and normal
subjects for level and ramp conditions..........................................173
Figure 4-18. Mean cadence between patellofemoral pain and normal
subjects for level and ramp conditions..........................................174
Figure 4-19. Ankle motion for patellofemoral pain and normal subjects
for all conditions tested................................................................... 176
Figure 4-20. Knee motion for patellofemoral pain and normal subjects
for all conditions tested................................................................... 177
Figure 4-21. Hip motion for patellofemoral pain and normal subjects
for all conditions tested................................................................... 178
xi
CHAPTER I
OVERVIEW
Patellofemoral pain (PFP) has emerged as one of the most common knee
d i s o r d e r s .105 Despite its high prevalence in the general population, the etiology
and treatment of PFP remains highly controversial. PFP is widely believed to be
caused by malalignment of the patella.50'123* 143* 169 Clinically, lateral patellar
displacement is the most common abnormality, and is postulated to create
abnormal areas of pressure between the patella and the fem ur41 This in turn,
leads to basal degeneration of the deep layers of the patellar articular
cartilage 46 Pain is believed to be caused by nerve endings in the subjacent
bone, which are subjected to pressures that would normally be absorbed by
healthy cartilage 46
The cause of abnormal lateral patellar tracking appears to be multifaceted
with components being defined by both static and dynamic elements. Static
considerations include factors that increase the lateral component of the
quadriceps pull as a result of bony configuration (ie. increased Q angle, femoral
anteversion) or tightness of non-contractile elements (lateral retinaculum,
iliotibial band fascial expansion), while dynamic factors involve the contribution
of the quadriceps femoris. Most of the research literature has focused on the
dynamic factors associated with patellar instability and probable vastus medialis
oblique (VMO) insufficiency. This structure has been described as the primary
mediai stabilizer of the patella opposing the vastus lateralis (VL) in preventing
patellar subluxation.99 VMO insufficiency has been associated with muscle
atrophy,38* 180 impaired motor control,10 as well as inhibition owing to pain,111
and effusion.170
l
Documenting imbalances between the VMO and VL in patients with PFP
has been of primary interest to the practicing clinician as conservative treatment
of this disorder has focused primarily on restoring normal function of the medial
stabilizers. Several investigators have studied the electromyographic (EMG)
activity of the dynamic patellar stabilizers in subjects with PFP, however, the
results of these studies are equivocal. While som e authors have found
significant differences in VMO and VL activity in patients with PFP,109> 169> 189
others have n o t . 1 5 ,1 0 3 , 1 1 8 ,1 8 5 Direct comparisons of these studies are difficult
because of differences in experimental technique and methods of assessing
EMG data.
One likely reason for the inconsistent results in these investigations is the
inherent variability exhibited by subjects diagnosed with PFP. Since etiology of
PFP has been considered a dynamic entity, it is logical that a deficiency of the
medial stabilizers should result in lateral displacement of the patella. It has been
documented through radiological examination however, that less than 5 0
percent of this population demonstrate isolated lateral s u b l u x a t i o n .1 5 2 ,1 5 9
Subjects in these studies were shown to exhibit lateral tilting, medial subluxation
and rotational abnormalities without patellar lateralization, indicating that the lack
of medial stabilization may play only a partial role in PFP.
If medial quadriceps deficiency is responsible for patellar malalignment,
then it stands to reason that subjects with documented lateral subluxation may
exhibit imbalances between the dynamic patellar stabilizers while subjects that
demonstrate different malalignment abnormalities (ie. isolated lateral tilt) may
not. To date, only one study by Moller et al.118 has attempted to control for this
potential variation. Patients in that study were classified as having lateral
subluxation or idiopathic chondromalacia based on radiological examination.
2
Their imaging techniques however, involved passive joint positioning and,
therefore, could not be used to determine the influence of the active musculature
in contributing to patellar malalignment. This study failed to show a significant
difference in muscle activity between groups.
Recently, a new diagnostic technique has been developed that uses
kinematic magnetic resonance imaging (KMRI) to asse ss patellar tracking during
dynamic knee extension.159 The advantage of KMRI over standard procedures
(ie. x-rays and computerized tomography) is that the patellar mechanism is
assessed during functional activity instead of passive positioning, giving
diagnostic information related to the kinematics of the joint throughout a range
of motion. Studies comparing the active and passive techniques suggest that
patellar kinematics are influenced by the degree of quadriceps contraction with
resisted knee extension providing more of a physiologic assessm ent of
patellofemoral joint mechanics.15® * 159
While classification systems of patellar malalignment have been
developed for images obtained through standard radiographs95* 113 and
computerized tomography,152 quantified assessm ent of patellar motion through
KMRI has only recently been described.19 This radiological information
combined with dynamic EMG would provide valuable information regarding the
influence of muscle activation on patellar kinematics.
An underlying assumption in correlating EMG to patellar kinematics is the
relationship between electrical activity and muscular force. Muscle activation
however, also involves patterns of initiation and cessation, which are considered
by motor control scientists to be important factors in normal movement.25 For
example, asynchronous timing of vasti contraction has been theorized as being
contributory to patellar instability. Evidence in support of this hypothesis was
3
presented by Voight and W ie d e r" l8 2 who found a delay in VMO activation
compared to the VL in patients with PFP during a reflex contraction (patellar
tendon tap). These results however, were recently disputed by Karst and
Willett,87 who reported no vasti timing differences in their patient group during
both voluntary and reflex conditions. Although the conflicting data presented in
these studies can be explained by methodological differences, continued
research is necessary to establish whether timing differences do exist in this
population.
Despite the fact certain treatment programs incorporate biofeedback
training to restore neuromuscular timing of the dynamic patellar stabilizers,m
abnormal timing patterns have not been documented in this population during
activities that typically reproduce PFP. A piiot study of 10 PFP subjects
conducted by this author did not find any significant timing differences between
the vasti during gait activities. The VMO however, did demonstrate a greater
mean intensity than the other vasti, indicating a difference in function and
neuromuscular control. This preliminary investigation was limited by both
sample size and a wide patient symptom variability. Subjects in this study
exhibited a variety of EMG and gait patterns that appeared to be related to the
degree of pain and dysfunction. A larger experimental group would provide
additional information regarding neuromuscular and functional deficits as well as
compensatory gait mechanisms in this population.
Since conservative treatment is concerned primarily with restoring
dynamic stabilization of the patella, it is of importance to fully understand the
function of the extensor musculature and its role as a possible mechanism of
patellofemoral joint pathology. The primary purpose of this dissertation is two
fold. Initially, the study w ill focus on describing normal and abnormal patellar
4
tracking patterns based on KMRI, and to establish the role of the extensor
mechanism (via EMG) in contributing to patellar kinematics (Chapter 3).
Secondly, the neuromuscular control patterns of the vasti will be evaluated over
a wide variety of gait activities (Chapter 4). In addition, this chapter will explore
the kinematic and kinetic factors associated with the ambulation patterns of this
population, as well as the influence of pain and strength on gait.
Information regarding the function of the vasti in patients with PFP will aid
clinicians in the continued development of scientifically sound treatment
programs and provide a basis from which PFP can be m anaged. More
importantly, however, w ill be the significant contribution towards the
understanding of the etiology and long term prognosis of this complicated
disorder.
SPECIFIC AIMS
The principal aims of this study were to:
1) assess the reliability of obtaining patella motion data with KMRI during
resisted knee extension.
2) establish normal ranges of patella motion (tilt and horizontal displacement)
based on KMRI during resisted knee extension.
3) compare patella motion obtained with KMRI between normal subjects and
patients with PFP during resisted knee extension.
5
4) asse ss the reliability of obtaining EMG activation patterns of the vasti during
resisted knee extension.
5) establish the relative EMG activation patterns of the vasti during resisted knee
extension in normal subjects.
6) determine if vasti EMG activation patterns vary between normal subjects and
patients with PFP during resisted knee extension.
7) assess the influence of vasti EMG activity in predicting specific patellar
alignment patterns in subjects with PFP.
8) compare EMG activity patterns of the vasti (onset, cessation, mean intensity)
between normal subjects and patients with PFP during functional gait activities.
9) compare loading characteristics (vertical ground reaction forces, peak rate of
loading, and loading response knee flexion) between normal subjects and
patients with PFP.
10) asse ss differences in measured gait characteristics and joint motion
between normal subjects and patients with PFP during functional gait activities.
11) define the influence of quadriceps weakness and pain on stride
characteristics and joint motion in subjects with PFP.
6
CHAPTER II
LITERATURE REVIEW
FUNCTIONAL ANATOMY OF THE PATELLOFEMORAL JOINT
The patellofemoral mechanism play a key role in normal knee mechanics,
with its function being reflected in its anatomical design. The ability of this joint
to accept and redirect forces, is dependent on a host of factors including
osseous structure, as well as contributions from various soft tissues (quadriceps
musculature, quadriceps tendon, patellar ligament and the retinaculum). An
understanding of the anatomical structure of this joint is necessary in
appreciating both normal and pathological function.
OSSEOUS STRUCTURE
The patellofemoral joint is located anteriorly to the knee and consists of
the articulation of the patella and the trochlear surface of the femur. This joint is
an integral part of the extensor mechanism and has all of the characteristics of a
"true joint". 126
Contained within the quadriceps tendon, the patella has the distinction of
being the largest sesmoid bone in the body.122 its axial length is approximately
4 to 4.5 cm, while its width is estimated at 5 to 5.5 cm for the general population.
Thickness of the patella varies considerably, attaining a maximum height of 2 to
2.5 cm at its central portion (Fig. 2-1 ).14°
The patella consists primarily of cancellous bone covered by thin
compact lamina. While the anterior surface is slightly convex in all directions, the
7
posterior surface is largely occupied by an oval articulating surface. The inferior
portion of the posterior surface is non-articulating and accounts for 25 percent of
this area.41
The articular surface of the patella is divided into medial and lateral facets
by a vertical ridge (median ridge) that roughly bisects the patella (Fig. 2-1).21
Although similar in size, the lateral facet is slightly larger than the medial.184 The
medial facet is subdivided by a less prominent vertical ridge that separates the
medial facet proper and the smaller "odd facet" (Fig. 2-1). Orientation of this
portion of the articular surface varies considerably among individuals. The odd
facet has been found to be in the sam e plane as the medial facet or to deviate
from it by as much as 60 degrees.41
Superior
Lateral
Medial
Odd
Inferior
Figure 2-1. Representation of the articular facets of the patella. From Fox JM &
Del Pizzo W. eds. The Patellofemoral Joint New York, McGraw-Hill, 1993.
(Reproduced with permission)
8
The femoral condyles form the trochlear groove which provides the
articulating surface of the femur. Similar to the patella, the trochlear surface also
is divided into medial and lateral facets, with the lateral facet being larger and
extending more proximally and anteriorly than its medial counterpart (Fig. 2-2).
This orientation of the lateral femoral condyle provides a bony buttress which
assists in providing lateral patellar stability.58 The median ridge conforms to the
medial border of the lateral condyle while the secondary ridge of the medial facet
conforms to the medial condyle.41
Patellar surface
Lateral
condyle
Medial
condyle
Intercondylar
notch
Figure 2-2. Inferior aspect of the distal end of the femur. From Hertling D &
Kessler RM. Management of Common Musculoskeletal Disorders: Physical
Therapy Principles and Methods (2nd edition). Philadelphia, J.B. Lippincott Co.,
1990. (Reproduced with permission)
The articular surfaces of the femur and patella are covered with aneural
hyaline cartilage. Articular cartilage of the patella is the thickest found in the
body, reaching a maximum of 4 to 5 mm in its central portion, while cartilage
thickness reduces from the median ridge to the medial and lateral borders. The
9
cartilage covering the trochlear surface of the femur is much thinner than that of
the patella being 2 to 3 mm in thickness.41
SYNOVIUM
The synovial lining of the patellofemoral joint is essentially the synovium of
the anterior portion of the knee and consists of three portions: suprapatellar,
peripatellar, and infrapatellar. These three portions blend imperceptibly with
each other allowing free communication with the knee joint proper.147 The
peripatellar synovium creates a small synovial fold or fringe less than 1 cm broad
which surrounds the patella and is generally regarded as the true synovium of
the patellofemoral joint.41 Inflammation or scarring of this synovial fold can
produce symptoms similar to that of patients with articular cartilage
degeneration and is commonly associated with chondromalacia 54
VASCULAR SUPPLY
The vascular anatomy of the patellofemoral joint structure dem onstrates
relative independence from the knee as it receives its circulation from its own
vascular tree.41 The patellofemoral joint receives its blood supply from a
vascular anastomosis with arterial input from the superior and inferior medial
genicular arteries, the superior and inferior lateral genicular arteries and the
anterior tibial recurrent artery (Fig 2-3). These vessels are derived from the
popliteal artery and form a peripatellar circle which richly supply the anterior or
patellar portion of this joint. Two other deep anastomotic system s arising from
the sam e arterial branches form the blood supply to the knee.151
10
Lateral superior
genicular a . ...
Lateral inferior
genicular a.
Anterior tibial
recurrent a.
1
Suprem e
genicular a.
Medial superior
genicular a.
S aphenous branch
Medial inferior
genicular a.
Medial tibial
recurrent a.
Figure 2-3. Anatomy of the arterial supply of the patellofemoral joint. From
Fulkerson JP & Hungerford DS. Disorders of the Patellofemorai Joint (2nd
edition). Baltimore, Williams & Wilkins, 1990. (Reproduced with permission)
SOFT TISSUE STABILIZERS
The lack of a tightly closed capsular cuff necessitates external
assistance in achieving patellar stability within the trochlear groove. Fulkerson
and Hungerford41 have described both passive and active soft tissue structures
that provide this stability and anchor the patella to the knee.
Passive stabilizing structures include the patellar ligament inferiorly and
the medial and lateral retinaculum. The patellar ligament is typically in line with
li
the long axis of the tibia but is often orientated slightly laterally. This creates a
slight lateral pull on the patella.41
The lateral retinaculum is comprised of two distinct portions, a thinner
superficial layer and a thicker deep layer. The deep layer is further divided into
three fibrous components which connects the patella to the iliotibial band and
assists in preventing medial patellar excursion 46 As most of the lateral
retinaculum originates from the iliotibial band, this structure is drawn posteriorly
with knee flexion placing a lateral force on the patella.136
The medial retinaculum forms a tough fibrous layer which assists in
limiting lateral patellar excursion. The lateral retinaculum is usually thicker than
the medial retinaculum and is generally accepted as providing stronger lateral
support.57
Active stabilizers of the patella consist of the four heads of the quadriceps
femoris (vastus lateralis, vastus medialis, vastus intermedius and the rectus
femoris) which fuse distally to form the quadriceps tendon. These muscles can
be identified at their insertion into the patella and provide dynamic control of the
patellofemoral joint.
The rectus femoris (RF) inserts into the anterior portion of the superior
aspect of the patella with the superficial fibers continuing over the superior
aspect of the patella ending in the patellar tendon. The vastus intermedius (VI)
inserts posteriorly into the base of the patella but anterior to the joint capsule.
Both the vastus lateralis (VL) and vastus medialis (VM) insert into their
respective sides of the patella and reinforce the medial and lateral
retinaculum.35.176
The lower fibers of the VM insert more distally on the patella and at a
greater angle from the vertical compared to the VL. In a detailed anatomical
12
analysis, Lieb and P e rry ® 3 determined the angle of insertion of the various
heads of the quadriceps with respect to the vertical axis. The fiber alignment in
the frontal plane were as follows: VL, 12 to 15 degrees laterally; RF, 7 to 10
degrees medially; VM (upper fibers), 15 to 18 degrees medially and the VM
(lower fibers), 50 to 55 degrees medially (Fig. 2-4). The fibers of the VI were
found to lie parallel to the shaft of the femur.
RF 7-10
VML 15-18'
VMO
50-55
Figure 2-4. Fiber orientation of the quadriceps musculature in relation to the
femur (VL= vastus lateralis, VI = vastus intermedius, RF= rectus femoris, VML=
vastus medialis longus, VMO= vastus medialis oblique)
13
The distinct and abrupt change in the fiber orientation between the
superior and inferior portions of the vastus medialis lead these authors to
consider each as separate entities in their mechanical study. The lower fibers
were designated as the vastus medialis oblique (VMO) and the upper fibers as
the vastus medialis longus (VML). The fiber orientation of the VMO m akes this
structure particularly effective in providing medial patellar stability."
In summary, the bony confines of the trochlea combined with the passive
and active soft tissue stabilizers define the limit of patella excursion and
contribute significantly to stability of the patella. The balance between medial
and lateral stability is essential for maintaining appropriate alignment of the
extensor mechanism and normal biomechanics of the patellofemoral joint.
BIOMECHANICS OF THE PATELLOFEMORAL JOINT
FUNCTION OF THE PATELLA
The primary function of the patella is to facilitate knee extension.'*" This
mechanical attribute has been described in detail and has been shown to
increase the functional lever arm of the extensor mechanism.88 Documentation
of strength losses in subjects that have undergone patellectomy supports this
concept. Peeples and Margo1 *27 found that approximately 20 percent of post
patellectomy patients failed to achieve satisfactory muscle function while
Fletcher et a l87 reported a 49 percent reduction in the torque output of the
extensor mechanism after patellectomy. Similarly, a study conducted by H ill et
al.60 found that subjects with good to excellent surgical results had a 45 percent
14
loss of quadriceps strength compared to a 60 percent loss of strength in
subjects with a fair surgical result.
In a mechanical study using human cadavers, K a u f e r 8 8 calculated full
post-patellectomy extension may require as much as a 30 percent increase in
quadriceps force. In addition, these authors demonstrated that the patella
contributed to the knee extension moment arm throughout the entire range of
motion, providing significant mechanical leverage for the extensor mechanism.
The quadriceps lever arm varies throughout the knee range of motion
with reported maximum values ranging from 4.9 cm at 30 degrees of flexion, 165
to 7.8 cm at 15 degrees of flexion. 130 The effectiveness of the patella
diminishes with full flexion as the patella sinks into the trochlear groove reducing
the anterior displacement of the quadriceps tendon. The extensor lever arm is
only slightly reduced with full extension (4.4 cm). 105 Quantification of the
contribution of the patella to the total quadriceps lever arm was m ade by
K a u fe rO S who found that at 120 degrees of flexion, the patella accounted for 0.4
cm of the total lever arm compared to 1.1 cm at 30 degrees, and 1.8 cm at full
extension. The 1.8 cm contribution of the patella at full extension accounted for
approximately 30 percent of the total lever arm in his study.
Apart from improving the moment arm of the quadriceps, the patella
provides protection for the articular cartilage of the trochlea and prevents
excessive friction between the quadriceps tendon and the femoral condyles,
permitting the patellofemoral joint to tolerate high compressive loads. The
patella also acts as a guide for the converging heads of the quadriceps,
facilitating transmission of the muscular forces to the patellar tendon.38
15
PATELLOFEMORAL CONTACT AREAS
The articulating surface of the patella on the femur has been shown to
vary throughout the range of knee motion with first contact occurring at 10 to 20
degrees of flexion 47>® 3>69 using cadaver knee joints mounted in such a way
as to reproduce weight bearing forces, Goodfellow and Hungerford47 assessed
patellofemoral contact areas at 20,45,90 and 135 degrees of flexion using a dye
technique. Movement from extension to 90 degrees of flexion resulted in a band
of contact that moved from the inferior to the superior pole (Fig. 2-5). The odd
facet m ade no contact during this range. Between 90 and 135 degrees, the
patella rotated laterally with the ridge between the medial and odd facets making
contact with the medial condyle. At 135 degrees, the odd facet and the lateral
portion of the lateral facet made contact as did the quadriceps tendon (Fig 2-5).
The point of articulation of the odd facet during the last part of flexion differed
among investigators with som e reporting distinct contact as early as 90 degrees
of flexion.154
The overall pattern of patellar contact area increased with increasing knee
flexion which served to distribute joint forces over a greater surface a r e a .69 The
contact area as m easured by Mathews et al.110 increased steadily from 2 cm2
at 30 degrees to a maximum of 4.5 cm2 at 90 degrees. At 120 degrees of
flexion, the contact area was reduced to less than 2 cm2 reflecting a small
weight bearing surface.110 Compensation for this reduced surface area at
higher degrees of knee flexion was made by the contact of the quadriceps
tendon against the femoral groove which has been shown to reduce the total
patellofemoral contact stress by one third.®2
16
s
20° 45 * 90 °
S
L
Figure 2-5. Contact areas on the patella as a function of knee flexion at 20, 45,
90 and 135 degrees. (A-B= path of medial margin of contact zone, l= inferior,
S= superior, L= lateral, M= medial). From Goodfellow J, Hungerford DS, Zindel
M. Patellofemoral joint mechanics and pathology. J Bone Joint Surg (BR), 1976;
58:287-290. (Reproduced with permission)
PATELLOFEMORAL JOINT REACTION FORCES
Patellofemoral joint reaction force (PFJRF) is the m easurem ent of
compression of the patella against the femur and is dependent upon the angle of
knee flexion as well as muscle tension.69 The resultant of the quadriceps force
and patellar tendon force is equal and opposite to the PFJRF, which evokes
compressive stresses on the patellofemoral articular cartilage (Fig. 2-6J.20
17
J S ! L £ O R c r f \
Figure 2-6. Compressive forces on the patellofemoral joint are determined by
the resultant of Mh and Mo as schematized. From Fulkerson JP & Hungerford
DS. Disorders of the Patellofemoral Joint (2nd edition). Baltimore, Williams &
Wilkins, 1990. (Reproduced with permission)
Using the law of cosine, Maquet108 presented the following calculation of
PFJRF:
PFJRF= Fq2 + Fp2 + 2Fq FpcosO
where Fq = quadriceps force, Fp= patellar ligament force and 6 = patellar
mechanism angle. In calculating the PFJRF many authors have assum ed that
Fq =Fp because of the low coefficient of friction of the articular cartilage, thus
18
assuming a frictionless patellar pulley system.5> 142> 185 This assumption has
been refuted by Buff, Jones, and Hungerford,20 who m easured the ratio
between Fq and Fp experimentally using eight cadaver knees. The direct
measurement of differences between quadriceps force and patellar ligament
force for most angles of flexion confirmed Maquet’s mathematical analysis of the
forces transmitted through the patella. The ratio of the tension in the quadriceps
tendon and patellar ligament (Fq/Fp) varied from 1.55 at 70 degrees to 0.86 at
10 degrees of flexion. Furthermore, the difference in Fq and Fp influenced both
the magnitude and direction of PFJRF. These results were in agreem ent with
Huberti et al.,02 who found similar ratios in six cadaver knees. These authors
also ascertained that a shift in location of the patellofemoral contact area caused
the patella to act as a lever arm to change the tendon/m uscle force ratio.
Indirect measurement of Fq and Fp has been the standard of most
biomechanical models as forces can be estimated using kinetic, kinematic and
anthropometric data. Reilly and Martens'!42 reported PFJRF’ s during various
activities such as level walking and ascending and descending stairs. Using a
force plate and stroboscopic photography, these authors calculated the moment
at the knee and used photographs to determine the quadriceps tendon lever
arm. Results indicated that the PFJRF for level walking was 0.5 times body
weight compared to 3.3 times body weight during stair climbing. Using similar
techniques to quantify the moment at the knee while running, Scott and
Winter153 calculated the PFJRF with a function created by van Eijden et al.180
to predict the quadriceps/patellar ligament force ratio. Patellofemoral joint
compressive forces were found to increase to 7.6 times body weight during the
initial phase of stance.
19
When considering the forces transmitted through the patellofemoral joint,
an assumption is made that the forces are transmitted equally across the area of
contact between the patella and femur. As mentioned earlier, this surface area
changes dramatically throughout the range of motion and has significant
implications for patellofemoral contact stresses. Using calculated PFJRF’s,
Mathews and colleagues110 determined the patellofemoral contact stresses
through direct measurement of patellofemoral contact areas in 15 cadaver
specim ens throughout the range of knee flexion. By dividing the patellofemoral
joint reaction force by the patellofemoral contact area, a determination of
patellofemoral contact stress was made. The greatest patellofemoral contact
stresses in this study occurred at 30 degrees of knee flexion while walking down
a ramp (7.4 N/mm2).
The patellofemoral joint can be loaded under two circumstances, with
body weight applied from above as in squatting or by applying a weight at the
ankle and having the subject extend the knee. Both these methods are
commonly used as rehabilitation exercises and have distinct implications
regarding patellofemoral biomechanics and contact stresses. In the case of
applying weight from above, the flexor lever arm, quadriceps tension, patellar
tendon tension, PFJRF, and the patellar contact area increases with increasing
knee flexion.69 The increased joint forces are dispersed over a greater surface
contact area as the knee flexes to 90 degrees. Conversely, when a weight is
applied at the ankle as with knee extension exercises, PFJRF’s increase as the
knee extends to zero as a result of the weight moving further from the axis of
rotation of the knee. This requires increased quadriceps force which translates
to increased patellofemoral joint forces. The increased force combined with a
20
diminishing patellar surface contact area results in greater patellofemoral contact
stresses.69
Hungerford and Barry69 experimentally evaluated these loading
conditions and determined that knee extension with a 9 kg boot equals contact
stresses of flexion under body weight at 55 degrees of flexion. These results
indicated the importance in understanding the biomechanics, anatomy and
contact pattern of the patellofemoral joint when prescribing exercises for
rehabilitation.
NORMAL PATELLAR KINEMATICS
Normal patellar movement is characterized by smooth tracking that has
been described as a "C" curve in the frontal plane with the concavity facing
lateral as the knee moves from full extension toward full flexion.69 During initial
flexion, the patella is drawn into the trochlear groove from a slight lateral position
making contact with the femur at approximately 10 degrees.69 From 10 to 30
degrees the patella moves anteriorly from the axis of rotation of the knee as a
result of prominence of the femoral trochlea.5 Beyond 30 degrees, the patella
settles in the deepening trochlear groove which provides increasing stability 64
Three dimensional analysis of patellar tracking was undertaken by Koh et
a l90 using intracortical pins and a motion analysis system. Results for one
normal male subject indicated that from 0 to 50 degrees of knee flexion, the
patella flexed 30 degrees, tilted laterally 10 degrees and shifted laterally 8.6 mm.
Minimal patellar rotation was evident in this study. The values for patellar motion
were similar between seated and squatting positions.
Comparable results were described by Reider and colleagues141 in the
assessm ent 20 cadaver specimens. Average patellar movements during 0 to 90
21
degrees of flexion were found to consist of 14 mm of lateral shift, six degrees of
lateral rotation and 12 degrees of lateral tilt. This pattern of patellar motion was
designated as Type I. A small minority of these specim ens had a patellar
tracking pattern that was distinctly different in two of these factors. This sub
group (Type II) demonstrated only 7 mm of lateral shift and no tilt component
(neutral). Cutting of the lateral retinaculum had no effect on the tracking
patterns of either the Type I or II groups. No hypothesis was offered for cause
of the motion differences between groups.
Using a highly accurate roentgen stereophotogrammetric analysis
method, van Kampen and Huiskes181 studied three dimensional patellar
tracking in four cadaver specimens and reported results similar to those
mentioned previously. The authors determined that patellar tracking patterns
however, were highly influenced by induced tibial rotations, with the tilt and shift
component more affected during the first part of knee flexion. In general, the
patella followed the obligatory rotations of the tibia. This finding indicated that
motion of the patella is determined by the anatomical characteristics of the distal
femur in relation to the tibia in addition to the forces exerted by the soft tissues.
This would explain individual differences in patellar motion patterns and possibly
account for potential pre-disposing factors to pathology.
NORMAL FUNCTION OF THE QUADRICEPS FEMORIS
ANATOMY
The quadriceps femoris muscle consists of four distinct heads (RF, VL,
VM, VI), with the VM being divided into two distinct portions based on fiber
22
orientation (VMO and VML). The upper and lower fibers of the VM are
occasionally separated by a fascial p la n e d
The RF takes its origin from the anterior inferior iliac spine and the portion
of the ilium superior to the acentabulum. This is the only component of the
quadriceps femoris that crosses the hip. The vasti have broad origins from the
anterior and lateral shaft of the femur, extending as high as the greater
trochanter and as far posterior as the linea aspera.147 In an anatomic study of
the VMO, Bose et al.14 found that this structure originated chiefly from the
tendon of the adductor magnus as well as the medial intramuscular septum.
They reported the significance of this attachment in terms of adding resilience to
the action of the VMO and stated possible implications for the etiology and
treatment of patellofemoral instability. Hanten and Schulties55 discussed the
attachment of the VMO to the adductor magnus and presented evidence for
increased electromyographic (EMG) activity compared to the VL during a hip
adduction exercise with the resistance placed at the joint line.
The quadriceps fuse to form a common tendon that envelopes the patella
and inserts on the tibial tuberosity. As a result, the vasti are powerful extensors
of the knee joint and provide anterior stability. By virtue of its proximal
attachment, the RF acts to flex the hip and together with the vasti assists in knee
extension.
All muscular components of the quadriceps femoris share a common
innervation and are supplied by branches of the femoral nerve, with the branch
to the vastus medialis being relatively larger than the rest.147 In som e cases,
the VMO has been shown to be supplied by a separate nerve tru n k ." This has
led som e authors to suggest that this muscle can be recruited separately from
the rest of the vasti97> 1" or may be more susceptible to reflex inhibition
23
resulting from effusion. 170 Effusion has been hypothesized to prevent activation
of the alpha motor neurons in the anterior horn of the spinal cord thus leading to
muscle atrophy.174 Contraction of the VMO independent of the VL has been
demonstrated through EMG biofeedback training and can have an influence on
the VMO:VL EMG ratio in subjects with patellofemoral pain 97» 189
Considered as whole, the cross-sectional area of the quadriceps femoris
muscle group in adult males is approximately 148 cm2, which is three times the
area of the hamstring musculature.86 The VL is the largest of the vasti with a
cross sectional area of 43 to 64 cm2 which is twice the size of the VM.16* 183
This size differential between the medial and lateral stabilizers of the patella has
implications for the potential loss of dynamic equilibrium, especially in the
presence of VMO insufficiency.38
ROLE OF THE QUADRICEPS FEMORIS IN KNEE EXTENSION
Visual medial quadriceps atrophy and the loss of terminal knee extension
led early investigators to assign the responsibility for this motion to the
VM.36-166 This misconception of selective function of the various heads of the
quadriceps femoris has erroneously been the basis for many therapeutic
exercise programs for the knee.66
In a mechanical study using amputated limbs, Lieb and Perry88
determined that the loss of terminal knee extension was the result of mechanical
disadvantage rather than specific muscle action. They demonstrated that a 60
percent increase in quadriceps force was necessary to complete full extension
indicating that extensor lag was a function of general quadriceps weakness. In
addition, these authors noted that the oblique fibers of the VMO could not
complete full extension without the assistance of the other vasti. Thus, the
24
authors concluded that the only selective function of the VMO would appear to
be patellar alignment.
A follow-up EMG analysis by the sam e authors supported the results of
their mechanical study. 100 Quadriceps function under isometric conditions was
assessed by fine wire EMG throughout the arc of knee motion. The consistent
EMG values for all positions indicated muscular activity of all portions of the
quadriceps during the entire range of knee extension. The VMO showed no
difference in activation with respect to the other quadriceps muscles except that
its action potential count was consistently two times greater than that of the
other components. The decreasing torque values in approaching full knee
extension confirmed the previous observation that this terminal range is an
inefficient posture. Such findings were shared by Pocock131 who cited no
differences in the action potential patterns or the timing of activity during
extension. These EMG results subsequently have been reproduced by
numerous authors confirming the integrated function of the quadriceps femoris
during extension, regardless of knee position8.54* 78* 149
ROLE OF THE QUADRICEPS FEMORIS IN PATELLAR STABILIZATION
As the VMO was widely accepted as preventing lateral subluxation of the
patella,8-93 its relationship with respect to the VL has been thoroughly studied.
The EMG activity of the VMO and VL during knee extension while weight bearing
was undertaken by Reynolds et al.,143 to determine the role of these muscles in
providing patellar alignment. Both the VMO and VL demonstrated similar low
levels of activity. Reynolds and colleagues'!43 stated that this is understandable
since the a balance of dynamic factors acting at the patellofemoral joint should
be exhibited. Similar results were also reported by Mariani and C aruso- !99 who
25
found the EMG activity of the VM and VL in normal individuals to be similar in the
last thirty degrees of extension with the tibia free.
Displacement of the patella during maximal electricai stimulation of the
VMO and maximum voluntary isometric quadriceps contraction was quantified
by Koh et al 90 Isolated VMO contraction resulted in medial rotation, tilt and shift
while isometric quadriceps contraction reversed this orientation with lateral
rotation, tilt and shift. These results suggested that the knee extensors tend to
pull the patella laterally. Smaller displacements occurred at 30 degrees of knee
flexion compared to full extension implying that quadriceps contraction has less
influence on patellar tracking at 30 degrees. This reflected the increased stability
of the patella as it moves into the trochlear groove with increasing knee flexion.
Results of this study must be interpreted with caution as data were collected
from only one normal subject.
The above studies supported the conclusions of Lieb and P e rry " who
stated that VMO and VL oppose each other in a critical balance, preventing
excessive lateral subluxation of the patella. This controlled balance is essential
in preventing pathology of the extensor mechanism and disorders of the
patellofemoral joint.1"
ROLE OF THE QUADRICEPS FEMORIS IN NORMAL AND PATHOLOGICAL
GAIT
During normal gait, initial contact with the floor is made with the heel. The
knee and ankle are in a neutral position and the body weight vector falls
posterior to the ankle and posterior to the knee.1" The posterior position of
the body weight vector with respect to the ankle generates a plantar flexion
torque that brings the foot to the ground. This plantar flexion torque is
26
restrained by the pre-tibial musculature (tibialis anterior, extensor hallicus longus
and extensor digitorum longus) which contract eccentrically to lower the foot to
the floor. This muscular action also draws the tibia forward. The combined
effects of restrained foot drop and active tibial advancement roll the body weight
forward on the heel and has been described by Perry129 as the heel rocker.
The heel rocker also has an interactive effect on the knee. The forward
pull on the tibia places the body weight vector posterior to the knee creating a
flexor moment.129 The quadriceps contract eccentrically to decelerate this
flexor moment and provide stance stability. This activity also provides a shock
absorbing mechanism to reduce floor impact and limits the vertical displacement
of the body center of mass. In general, 15 degrees of knee flexion is evident
during loading response.129
Activity of the vasti begins in terminal swing extending the knee in
preparation for initial contact. This activity provides knee stability in preparation
for stance. Muscle intensity increases rapidly during loading response, reaching
a peak of 25 percent of maximum.129 The RF demonstrates a much different
pattern of activity with a short period of action between late pre-swing and early
initial swing. This activity functionally serves to assist in hip flexion which in turn
provides a passive mechanism against knee flexion in pre-swing. The peak
intensity of RF rarely exceeds 20 percent of maximum.129
For normal level walking, the maximum knee flexor torque has been
determined to be approximately 2.8 kgm which represents the functional
demand that the quadriceps must overcome to ensure s ta b ility .163 Further
analysis of the dem ands of flexed knee posture was undertaken by Perry and
colleagues130 using instrumented cadavers. These authors reported that 15
degrees of knee flexion represented an extensor demand of 500 newtons which
27
corresponded to a muscular demand of 22 percent of maximum. This level of
effort was considered by Monod129 to be slightly above the level that allowed
for continuous contraction without fatigue (16 percent of maximum). As knee
flexion progressed to 30 degrees, the extensor demand doubled (51 percent of
maximum), demonstrating a rapid rise in the quadriceps force requirement
It is the increasing demand with knee flexion that subjects with weak
quadriceps avoid. This is accomplished by reducing the heel rocker effect or by
keeping the body weight vector anterior to the knee thus reducing the knee
flexion torque.129 This compensatory gait deviation has been described by
Berchuck et al.12 as a "quadriceps avoidance" pattern.
A study by Dillon et al.33 found that subjects with chondromalacia
significantly reduced the amount of stance phase knee flexion compared to
normal controls. While it was not clear whether subjects in this study reduced
knee flexion as a result of pain or weakness, this finding illustrated the primary
compensatory mechanism employed by such subjects to reduce quadriceps
dem and and therefore minimize patellofemoral joint forces. Pilot work done by
this author has found similar knee flexion deficits in this population combined
with decreased EMG activity of the vasti.
The primary penalty for reduced knee flexion during weight acceptance is
a reduction in the shock absorbing capacity of the quadriceps,129 which may
contribute to an increased rate of lower extremity loading.139 This is of
particular concern as it has been shown by Radin and Paul133 that articular
cartilage wear is more sensitive to loading rate, rather than the magnitude of
loading. Radin et al.139 suggests that this impulsive loading may be a precursor
to degenerative osteoarthritis.
28
EMG activity of the various heads of the vasti during gait was recorded in
normal subjects by Adler et al. J to assess differentiation in intensity or timing of
contraction. As expected there was no difference in the timing or intensity of the
VMO or VML and the remaining vasti during normal and fast walking. These
results confirmed previous investigations that the vasti work in concert to
achieve knee extension and stance stability. Voight and Wieder182 presented
evidence showing that the activation of the VMO was delayed compared to the
VL during a patellar tendon tap reflex in subjects with PFP, however, vasti timing
during gait in subjects with PFP has not been reported in the literature.
PATELLOFEMORAL PAIN SYNDROME
Patellofemoral pain (PFP) is the most commonly encountered disorder
involving the knee 38» 105 Over a five year period, Devereaux and Lachmann32
demonstrated that 25 percent of all knees evaluated in a sports injury clinic were
diagnosed with PFP, while McConnell111 reported that PFP affects one in four of
the general population. In addition, a study of 100 painful knees conducted by
Outerbridge125 reported a finding of chondromalacia patella in 40 of 50 women
and 15 of 50 men. While patellofemoral related problems occur with an
incidence of two to one in females vs. males, men outnumber women when
athletes are studied.38
The symptoms of PFP are multiple.15* 38.145 Pain is generally
characterized as being diffuse and arising from the anterior aspect of the
knee.145 Pain along the medial border of the patella is the most common
complaint, however, retropatellar pain and pain along the lateral border is often
29
reported.41 Generally, onset is insidious and progression is slow. PFP is often
activity-induced and aggravated with functions that increase patellofemoral
compressive forces such as ascending and descending stairs, inclined walking,
squatting and prolonged sitting.34* 98 Orthopedic assessm ent is usually positive
for a patellar grind test and discomfort with palpation of the medial and lateral
facets of the patella. Swelling, loss of motion, and a sensation of giving way or
instability may also be present.15
A great deal of confusion exists regarding the terminology of this
disorder. Patellofemoral pain has been referred to by a number of diagnoses
including patellofemoral chondritis, anterior knee pain, quadriceps or VMO
insufficiency, patellar subluxation, patellofemoral dysfunction, patellar
compression syndrome and chondromalacia.15 In general, PFP refers to pain
originating from the patellofemoral joint. Fulkerson and Hungerford41 have
stated that the term "anterior knee pain" is a broader diagnosis taking into
consideration the other possible pain producing structures that support this
joint. Such structures include the patellar tendon,41 prepatellar and retropatellar
bursae,34 synovial plica,18 and small nerve injury in the lateral retinaculum.44
Chondromalacia refers to morphologic softening of the patellar articular cartilage
and its use as a diagnosis is inappropriate unless confirmed arthroscopically or
by an arthrogram which exhibits softening or fissuring of the underside of the
patella.106 Insall106 has reported that the relationship between chondromalacia
and pain is poor and rejected its use as diagnosis for any purpose other than to
describe the pathological process of articular cartilage degeneration.
30
PATHOPHYSIOLOGY OF PATELLOFEMORAL PAIN
The cause of patellofemoral pain is not clearly understood and may
consist of multiple origins. As the retinaculum, synovium and the capsule of the
knee are richly innervated and subject to abnormal stresses, these structures
are potential sources of pain.39 In contrast, articular cartilage is generally
accepted as being aneural and has been dismissed as a cause of symptoms.
The subsequent effects of pathological degeneration of the articular cartilage,
however, are considered by many to play a substantial role as a source of
patellofemoral pain.46
The stages of patellofemoral articular cartilage breakdown have been
discussed in detail.41,-46,7’ 4,124 Based on macroscopic observation,
Outerbridge^24 described the stages of patellofemoral cartilage pathology as
follows:
Grade I: softening and swelling of the cartilage
Grade II: fragmentation and fissuring in an area half an inch in diameter
Grade III: fragmentation and fissuring in an areagreater than one half in diameter
Grade IV: erosion of cartilage down to bone
Insall74 stated that grade IV of the Outerbridge classification system
should be more properly characterized as osteoarthritis, as significant erosive
changes and exposure of subchondral bone is present. Insall74 also expanded
on the Outerbridge system by reporting articular cartilage dam age of the femoral
articulating surface in grade IV degeneration.
Goodfellow et al.46 proposed a different classification dividing cartilage
pathology into two categories, surface and basal degeneration. Surface
31
degeneration was described as age dependent as it was encountered more
frequently and in a more advanced form when comparing young and middle
aged adults. This process involves changes in the articular surface of the
cartilage that can progressively deepen to the subchondral bone plate thus
affecting all layers. The appearance of surface degeneration has been
described as "surface fissuring",124 while Goodfellow and colleagues46 report
surface flaking and more advanced fibrillation.
Age dependent degeneration has been shown to appear more commonly
on the medial facet.46* 124> 186 Outerbridge124 described the shearing of the
medial patellar facet on the rim at the upper border of the medial femoral
condyle as the likely cause for degeneration at this site. This occurred from
about 10 to 30 degrees of flexion as the medial facet of the patella m ade contact
with the medial articulating surface of the femur.
It is generally accepted that surface degeneration of the articular cartilage
is not a cause of symptoms due to the lack of pain receptors.21* 46 This was
further supported by observations that the presence of articular cartilage
dam age was typically age related and usually asymptomatic.28* 124 Devas and
Golski81 found that 50 percent of the adult population over 30 years of age has
som e form of cartilage damage, however, only a small percentage reported
pain. Conversely, significant patellofemoral pain has been shown to occur when
the articular cartilage appears normal.46* 96 Leslie and Bentley96 examined 78
patients with a diagnosis of PFP and found through arthroscopy that 49 percent
of the patellofemoral joints demonstrated healthy articular cartilage. The results
of these studies indicated the lack of correlation between clinical symptoms and
surface degeneration of the articular cartilage.
32
Basal degeneration as described by Goodfellow et al.,4® is characterized
by degeneration of the deep layers of the cartilage. Involvement of the surface
layers usually occurs late in its development if at all. Stage I of basal
degeneration is seen initially as a smooth and intact articular surface with
discrete softening of the layers underneath. The determination of this
"fasiculation" can only be determined by palpation.4® The presence of a blister
indicates Stage II, which may lead to rupturing of the surface fibers (fasiculation).
These authors stated that fissuring of the deep surface of fasiculated cartilage
was evidence of a lack of cohesion between the thick collagen bundles with the
cartilage being held together by intact tangential fibers at the articular surface.
Basal degeneration is usually pathological and symptomatic.4® As this
type of lesion leads to disorganization of the deeper collagen fibers, the
subadjacent endplate is subjected to pressure variations that would normally be
absorbed by healthy cartilage. This in turn stimulates pain receptors in the
subchondral plate.
As with surface degeneration, basal degeneration typically occurs on the
medial facet, more specifically the ridge that separates the odd facet from the
medial facet. The cartilage on this ridge is subject to high compression loading
and shearing as the patella glides off and on the facets of the femoral condyles
making this area susceptible to injury.47 This mechanism explains the
frequency of this lesion among young and athletic populations and suggests
that variations in patella alignment and biomechanics could contribute to
pathology.
Abnormal patellar tracking or malalignment is commonly believed to be
responsible for articular cartilage degeneration or chondromalacia®9.75 In a
study of 25 patients seen for recurrent patellar dislocation, Heywood59 reported
33
that 19 demonstrated significant degeneration and erosion, two were classified
a s arthritic, and the presence of osteochondritis dissecans was observed in two
additional patients. Only two of these subjects were found to have normal
articular cartilage. These distinct degenerative changes were attributed to
repetitive subluxation where the patella would slide laterally across the lateral
femoral condyle and back again in regaining its central position. The effects of
this repetitive trauma resulted primarily in medial facet damage. In a prospective
study of 105 arthrotomies for chondromalacia patella, Insall et al.,75 reported
that the most frequently encountered initiating factor was patellar malalignment.
Surgical re-alignment of the patellar mechanism in this study resulted in
satisfactory results in 79 percent of the subjects. Unilateral patellar
malalignment was induced in 40 young and mature rabbits in a study conducted
by Moller and colleagues,1'19 who described cartilage degeneration in all knees
after only six weeks. The degenerative changes were more pronounced in the
more mature animals. This study further delineates the role of patellar
malalignment in patellofemoral joint pathology.
In summary, pain is the major symptom in patellofemoral dysfunction.
Despite the fact that articular cartilage is aneural, the subchondral bony layer is
richly innervated and is likely the source of patellar pain. From a mechanical
standpoint, abnormal stresses placed upon the subchondral bone as a result of
articular cartilage degeneration is the most likely stimulus for this pain, it has
been documented that abnormal patellar tracking and alignment is associated
with chondromalacia and degenerative changes of the articular cartilage and
may be the primary etiological factor in patellofemoral pain. Two of the most
common patellar malalignment syndromes, excessive lateral pressure syndrome
and patellar subluxation will be discussed in more detail.
34
EXCESSIVE LATERAL PRESSURE SYNDROME (ELPS)
The concept of excessive lateral pressure as a causative factor in
patellofemoral articular cartilage pathology was first described in detail by Ficat
et al.36 These authors characterized ELPS as a tilt/compression syndrome
where the patella was tilted laterally which increased the compression between
the lateral facet and the lateral femoral condyle (Fig. 2-7). This patellar posture
also served to unload the medial facet.
Tilting of the patella can be isolated or associated with lateral patellar
subluxation.152 Chronic lateral tilt is determined radiologically and has been
shown to have a deleterious effect on the articular cartilage. Increased density
of the subchondral bone underlying the lateral facet, combined with a decrease
in the medial facet subchondral bone density are signs of the pressure
differences exhibited in this syndrome.41
Figure 2-7. Increased compression between the lateral patellar facet and
femoral condyle as a result of excessive lateral tilting of the patella (axiat view).
From Fulkerson JP, Hungerford DS. Disorders of the Patellofemoral Joint (2nd
edition). Baltimore, Williams & Wilkins, 1990. (Reproduced with permission)
Lateral Medial
35
Lateral facet overload and deficient medial facet contact can lead to
articular cartilage degeneration at both sites. Abnormal articular cartilage
loading of the lateral facet may cross the threshold of cartilage resistance
leading to the failure.41 The primary area of lateral facet degeneration
corresponds to the areas of contact in the 40 to 80 degree knee flexion
range.154
The mechanism of medial facet articular cartilage dam age in ELPS
appears to be different from lateral facet degeneration, as this area is
susceptible to deficient contact. This degeneration is attributed to impaired
nutrition resulting from the decreased flow of synovial fluid which is normally
pumped in and out of the pores of articular cartilage surface during activities that
result in joint compression.101 Seedholm et al.154 has stated that areas of
relative contact deficiency only develop mild degenerative changes and are most
probably asymptomatic. When combined with the shearing of the medial facet,
however, as with lateral patellar subluxation, more extensive medial facet
degeneration can o ccu r41
The natural history of ELPS has been described as congenital tilting of the
patella followed by adaptive shortening of the lateral retinaculum. Congenital
anomalies cited as possible causes of ELPS include genu varum, femoral
anteversion and dysplasia of the hip 41 The significance of a tight lateral
retinaculum is the increased postero-lateraf pull on this structure with knee
flexion. This in turn accentuates lateral facet compression. Insall74 stated that
adaptive shortening of the lateral retinaculum was more likely the result of
habitual lateral patellar tracking where the VM becam e stretched and the VL
contracted. Regardless of the cause of this shortening, both authors agreed
36
that excessive tightness of the lateral retinaculum was a secondary adaptive
change.
Disruption of the medial stabilizers (VMO and the medial retinaculum) has
also been implicated as a possible cause of ELPS.34 Measurement of static
pressure distribution on the retropatellar surface was undertaken in a
mechanical study conducted by Ahmed and colleagues 3 Results from 24
cadaver specim ens demonstrated that a release of VMO tension created a
pressure shift that was transferred almost entirely to the lateral facet of the
patella. In addition, the change in the orientation of the pressure zone
suggested a considerable rotation of the patella relative to the femur.
Evidence exists which supports both the retinaculum and the dynamic
medial stabilizers as contributing to ELPS. Fulkerson and Schutzer42
demonstrated the effectiveness of surgical release of the lateral retinaculum in
reducing lateral patellar tilt. Based on pre and post surgical computerized
tomography evaluation, these authors reported a mean tilt improvement of six
degrees at 10 degrees of knee flexion and 15 degrees at 20 degrees of knee
flexion. These improvements brought the tilt angles of these subjects well within
the normal range as demonstrated in the control group. Douchette and Goble34
illustrated the importance of VMO weakness and tightness of the lateral
structures in contributing to ELPS by demonstrating a decrease in patellar tilt
with an eight week quadriceps strengthening and iliotibial band stretching
program. In addition, 84 percent of the subjects were pain free following the
conservative exercise program. Results of these studies suggest the
contribution of both dynamic and passive factors in the etiology of ELPS.
37
PATELLAR SUBLUXATION
Abnormal patellar tracking such that transient medial or lateral
displacement occurs during flexion and extension has been documented as a
cause of articular cartilage damage and pain.59* 75 In general, subluxation
typically involves increased lateral displacement of the patella,41 however,
medial displacement can also occur.156* 161 This excessive motion results in a
feeling of instability and discomfort.43 Fulkerson and Hungerford41 described
three types of subluxation: minor recurrent, major recurrent and permanent
lateral subluxation. Minor recurrent subluxation deviated little from the normal
patellar course and was not associated with clinically apparent relocation. With
major recurrent subluxation, the patella cam e across the lateral trochlear facet
and returned to the trochlear groove with an audible snap. Permanent lateral
subluxation was a stable lateral displacement where there was no centering of
the patella.
The natural tendency of the patella to track laterally has been described
by Fulkerson and Hungerford41 as the “ law of valgus". This is a result of the
valgus orientation of the lower extremity, where the relationship of the anterior
superior iliac spine of the pelvis to the midline at ground contact forms a 10
degree angle between the femur and the tibia. As the quadriceps follow the
longitudinal axis of the femur, the quadriceps angle (Q angle) is formed which
creates a lateral force vector acting on the patella (Fig. 2-8). This predisposes
the patella to lateral tracking forces with quadriceps tension.73
Clinically, the Q angle is measured as the angle formed by the
intersection of the line drawn from the anterior superior iliac spine to the
midpoint of the patella and the line drawn from the tibial tubercle to the midpoint
of the patella.167 The Q angle for women is greater than that of men as a result
38
of a wider pelvis, averaging 15 to 18 degrees. The average for men is
approximately 12 degrees.58 This variation between the sexes may partially
explain the greater incidence of PFP in females, as a larger Q angle would create
a larger valgus vector and therefore a potentially larger predisposition to lateral
tracking.80
Quadriceps
Q angle
Valgus
vector
Patellar
tendon
Figure 2-8. The angle between the quadriceps tendon and the patellar tendon
forms the Q angle. The Q angle produces a valgus force on the patella. From
Hungerford DS, Barry BS. Biomechanics of the patellofemoral joint. Clin Orthop
1979; 144:9-15. (Reproduced with permission)
39
Etiology of patellar subluxation
As mentioned previously, resistance to the inherent lateral tracking
forces is provided by both static and dynamic structures. Disruption of the
normal equilibrium of forces may lead to patellar malalignment and associated
pathology of the patellofemoral joint. The possible mechanisms of abnormal
patellar tracking have been discussed thoroughly in the literature. Awareness of
these mechanisms and how they influence patellofemoral joint mechanics is
essential for formulating effective treatment programs and the understanding of
the etiology of PFP.
Bonv abnormalities
Anatomical variations of the patella and/or distal femur have been shown
to contribute to potential recurrent s u b l u x a t i o n .1 4 6 * 184 Patella or trochlear
dysplasia compromises the inherent stability afforded by the bony structure thus
making the patella more susceptible to malaligning forces.
Wiberg164 proposed a three part classification system of patella shapes
based on axial view x-rays with the following descriptions:
Type I- Both facets were slightly concave and symmetrical with the medial and
lateral facets being equal in size (Fig. 2-9).
Tvoe II- The medial facet was distinctly smaller than the lateral facet. The lateral
facet remained concave while the medial facet was more flat (Fig. 2-9). This was
the most common patella form found.
40
Type III- The medial facet was considerably smaller with marked lateral facet
predominance (Fig. 2-9). This was considered by Wiberg as a frank dysplastic
form. Other morphologic dysplasias of the patella include patella magna, patella
parva, the pebble and hunters cap deformities, as well as the half-moon
patella.41
W IB E R G I
W IB E R G II
W IB E R G III
Figure 2-9. Schematic representation of the Wiberg classification system for
patella morphology. From Fox JM & Del Pizzo W. eds. The Patellofemoral Joint.
New York, McGraw-Hill, 1993. (Modified with permission)
Patella instability has been associated with a shape that dem onstrates a
small medial facet and a much larger lateral facet.9.184 In a study of 59 patients
with demonstrated subluxation, Baum and Bensahal9 reported that 42 (71
percent) of these subjects were found to have a Wiberg Type III patella. Similar
findings have been reported by others for this population.102,146 Fulkerson
and Hungerford41 stated that the association between instability and patella
type was probably more of an "effect1 1 rather than the "cause" of maltracking.
41
These authors hypothesized that if the patella was chronically malaligned during
skeletal development, then its ultimate shape would be the result of secondary
morphologic changes.
Apart from its shape, the position of the patella with respect to the
trochlear groove creates a potential causative factor in lateral subluxation.
Patella alta as described by Insall73 is evident when the resting position of the
patella is above the femoral groove. The high riding patella does not sink
adequately into the trochlear groove with knee flexion and is thus prone to lateral
displacement. Patella alta is determined with a lateral roentgenogram and is
considered positive when the patellar tendon is 20 percent longer than the
patella. The excessive length of the patellar tendon is thought to be the primary
cause of this condition.77
Insall and colleagues75 reported that of 84 knees evaluated for PFP 24
(29 percent) had documented patella alta. This value was relatively small
compared to the 40 subjects or 48 percent of this population that demonstrated
excessive Q angles. Despite this difference, these authors cited patella alta as a
common cause of patellofemoral joint dysfunction. Fulkerson and Hungerford4
stated that patella alta should only be considered a major contributor to
patellofemoral pathology when it is found in association with other malalignment
factors.
Probably a more influential bony etiologic factor in patellar subluxation is
femoral trochlea dysplasia. The trochlear groove of the femur, especially the
larger anterior protrusion of the lateral femoral condyle, provides significant bony
stability for the patella.38 The normal trochlear facet (sulcus) angle was
established by Brattstrom17 using a sophisticated radiological technique.
Evaluation of 100 normal knees revealed that the values for both sexes were
42
similar, with a mean angle of 143 degrees for males and 142 degrees for
females. Higher sulcus angles represented a more shallow trochlear groove
and were found to been associated with recurrent patellar subluxation.17.179
According to Hvid et al.,79 a sulcus angle of greater than 150 degrees was
representative of trochlear dysplasia. Brattstrom17 determined through
extensive measurements that trochlear dysplasia was the result of a rising of the
depth of the sulcus rather than a decrease in the height of the facets.
Regardless as to the cause, Brattstrom stated that trochlear dysplasia was the
most important etiological factor in recurrent patellar subluxation.
Abnormal bony relationships
Abnormal skeletal alignments have been shown to have a profound effect
on the magnitude of the Q angle and the subsequent laterally directed
component of the quadriceps force.63.75 Huberti and Hayes63 documented
the deleterious effects of an increase Q angle by measuring patellofemoral
contact pressures in 12 fresh cadaver specimens. These authors found that a
10 degree increase in the Q angle resulted in a 45 percent increase in peak
contact pressure at 20 degrees of knee flexion. In half of these specimens, the
area of patella contact shifted laterally, with the peak pressures being evident on
the medial portion of the lateral facet.
An increased Q angle is often present when rotational malalignments of
the femur and tibia exist. Such abnormalities include femoral anteversion, genu
valgum, tibial torsion and lateral displacement of the tibial tubercle 75> 98.126
In the transverse plane, the neck of the femur forms an angle of about 15
degrees with the transverse axis of the femoral condyles. This places the
femoral head anterior to the femoral condyles. An increase in this angle is
43
termed femoral anteversion and is associated with internal femoral rotation.®8
Femoral anteversion and subsequent internal femoral rotation causes the
trochlear surface of the femur to be placed more medially in relationship to the
tibial tubercle and serves to functionally increase the Q angle.12® Clinically, this
entity is manifest with a toed in gait and the appearance of "squinting patella".98
External tibial torsion can com pensate for this deformity by straightening out the
long axis of the leg, however this also displaces the tibial tubercle more laterally
resulting in an even larger Q angle.8® Fulkerson and Hungerford41 have stated
that isolated femoral anteversion rarely results in patellar problems, however,
femoral anteversion combined with compensatory tibial torsion increases the
risk of lateral patellar displacement.
A laterally displaced tibial tuberosity with respect to the midline of the tibia
and the anterior superior iliac spine will act to increase the Q angle.12® This
anatomic variation is typically the result of increased external tibial torsion.41
Documentation of external tibial torsion in subjects with PFP has not been clearly
delineated giving doubt as to its clinical importance. Heywood59 reported that
only two patients out of 54 demonstrated this abnormality, while Brattstrom17
failed to find any significant difference between patients and normal subjects.
Trillat and colleagues,178 as well as Hauser,®® discussed the importance
the laterally displaced tibial tuberosity in contributing to patellar malalignment
and described surgical procedures to correct this deformity. This involved
transfer of the tibial tuberosity medially and distally to reduce the effects of the
valgus forces created by an excessive Q angle. The effects of this surgery were
studied by Hehne,57 who reported that medial transfer of the tibial tuberosity
resulted in a significant decrease in the total patella contact area, especially the
contact area of the lateral facet. This reduction in contact area caused a 25
44
percent increase in the average patellar contact pressure both medially and
laterally leading the authors to dispute the effectiveness of this procedure.
Increased valgus angulation of the femur and tibia in the frontal plane is
described as genu valgum which is often the result of a tight iliotibial band or
femoral anteversion.66 Likewise, genu valgum is postulated to increase the
valgus force vector of the quadriceps,'104.'126 however, the presence of this
bony orientation is not consistent with PFP patients. Heywood60 reported only
seven cases of genu valgum in his population of 106 patients, while
Hughston 65 found only one case in 111.
Abnormal subtalar joint pronation
The relationship between motion at the subtalar joint and the lower
extremity has been well documented.61.128,148,177 The motions of pronation
and supination occur at the subtalar joint and assist in normal locomotion.
Pronation occurs during the initial phase of stance and is in response to the
lateral point of contact of the heel on the floor with respect to the line of body
weight. This motion acts as a shock absorbing mechanism and serves to
reduce rotatory stresses at the ankle joint. Supination begins slowly after the
forefoot contacts the floor which functionally serves to lock the midtarsal joints
providing a rigid lever during terminal stance.126
Normal subtalar joint pronation occurs during the first 25 percent of the
gait cycle during which the tibia internally rotates 20 degrees.?2 This is in
response to the inward rotation of the talus as it falls into the space created by
the inferior and lateral movement of the anterior portion of the calcaneus.126
Subsequent supination results in external rotation of the tibia.
45
Excessive subtalar joint pronation has been described as a
compensatory mechanism for several anatomic conditions which include tibial
varum, equinus resulting from a tight triceps surae, plantarflexed 5th ray,
forefoot varus and rearfoot varus.81 > 148 Of particular concern are the bony foot
deformities that functionally invert the foot relative to the ground. In order to
achieve medial rearfoot and forefoot contact during gait, excessive subtalar joint
pronation is required. Subtalar joint pronation required to com pensate for a
bony deformity is considered abnormal if the amount of pronation is excess of
the normal amount needed for locomotion or occurs at the wrong time (ie. when
the foot should be supinating).148
If excessive pronation is evident in midstance, then excessive internal
rotation of the tibia will be evident.177 To achieve knee extension during
midstance, Tiberio177 postulated that the tibia must externally rotate relative to
the femur to ensure adequate motion for the screw-home mechanism. In order
to com pensate for this lack of tibial external rotation, the femur internally rotates
on the tibia providing the necessary rotation for extension.177 The internal
rotation of the lower extremity resulting from subtalar joint pronation serves to
increase the Q angle and the lateral component of the quadriceps vector.81
While this theoretical model of the influence of excessive subtalar joint
pronation on the patellofemoral joint has been widely accepted as an etiological
factor in PFP, there is little objective data to support this theory. In the only
study to date comparing subjects with PFP to a control group, Messier et al.114
found no significant differences in maximum pronation, maximum pronation
velocity and total rearfoot movement in the 36 runners evaluated. This lead
these authors to state that rearfoot movement variables were not significant
etiologic factors in the development of PFP. Further research in this area is
46
warranted to substantiate evaluation of the subtalar joint in the clinical evaluation
of subjects with PFP.
Soft tissue abnormalities
Both contractile and non-contractile soft tissue structures can contribute
to the lateral forces acting on the patella. While these factors may be present in
conjunction with the abnormalities already described, assessm ent of the
potential influence of these structures in contributing to lateral patellar tracking is
necessary in formulating an effective treatment.
Passive structures
As mentioned previously, the lateral retinaculum has been implicated as
a cause Excessive Lateral Pressure Syndrome and is capable of exerting a
lateral force on the patella, potentially contributing to subluxation. Since the
lateral retinaculum has an extensive attachment to the iliotibial band, contraction
of the tensor fascia latae may exert a dynamic lateral force through this
connection.38 In som e cases, this attachment has been found to be excessive
causing recurrent dislocation of the patella.83 Further evidence was given by
PunielloJ36 who demonstrated a strong relationship between iliotibial band
tightness and decreased passive medial patellar glide in a group of 17 subjects
with patellofemoral dysfunction. Twelve of these subjects dem onstrated a
positive Ober’s test and hypomobility of medial patellar glide compared to only
two positive findings in a control group with no iliotibial band tightness. In
addition, Hughston and Deese67 found a high incidence (50 percent) of medial
patellar subluxation following lateral retinacular release, indicating that this
structure also plays a role in pulling the patella laterally. These studies support
47
the concept that a functional anatomic relationship exists between the patella
and the passive lateral structures of the knee.
Dynamic structures
A lack of equilibrium between the VMO and VL is widely accepted as a
principal cause of patellar subluxation 55,66,123,143 a s such, most of the
research literature has focused on the dynamic factors associated with patellar
instability and probable VMO insufficiency. It is this structure that has been
identified as the primary structure capable of counteracting the VL in maintaining
patellar alignment. VMO insufficiency has been associated with muscle
a t r o p h y 8.38 h y p o p l a s i a ,3 8 inhibition due to pain and e f f u s i o n ^ O . 175 ancj
impaired motor control.10
According to Fox,38 hypoplasia of the extensor mechanism is found to
som e varying degree in 40 percent of the population. This hypoplasia manifests
itself as incomplete development of the VM as it is the last of the quadriceps to
develop phylogenetically.38 The effect of an underdeveloped VM is a patellar
alignment that is influenced by an overpowering VL, more specifically, the patella
is situated more laterally and proximally. The more hypoplastic the VM, the
more lateral the position of the patella. In addition, the superior and lateral pull
of the patella can cause the development of patella alta. This hypoplasia also
has been theorized to have an influence on the development of the tibia where
the unchecked pull of the VL can result in external tibial rotation, lateral
placement of the tibial tubercle and genu recurvatum.38
F o x 3 8 theorized that the VM is the weakest muscle phylogenetically and
therefore the first component of the quadriceps to atrophy after injury or disuse.
The potential muscular imbalance between the medial and lateral dynamic
48
stabilizers as a result of this atrophy or weakness was considered by this author
to be the major predisposing factor responsible for "hypermobile patella
syndromes". Atrophy of the VMO was observed by Smillie,166 who stated that
the apparent wasting of the vastus mediatis was associated with the inability to
complete terminal knee extension. To the contrary, Lieb and Perry99 stated that
apparent atrophy of the VM was the result of a thinner fascial covering (half the
thickness) compared to the VL which made atrophy of the VL less perceptible.
Atrophy of the quadriceps muscle group is largely thought to be caused
by reflex inhibition,179 with the stimulus being pain, 179 or effusion.27 inhibition
is the result of afferent stimuli from receptors in or around the injured knee which
prevent activation of the alpha motor neurons in the anterior horn of the spinal
cord.174 Spencer and colleagues179 reported that infusion of only 20 to 30 ml
of saline into the knee joint exceeded the threshold for quadriceps inhibition.
This is in contrast to much larger volumes proposed by other authors (100
ml).27 In an attempt to ascertain if selective inhibition of the different heads of
the quadriceps was possible, these authors examined the Hoffman reflex of the
individual muscles after intra-articular infusion of saline into the knee. Although it
appeared that the VM was more affected by small amounts of induced effusion,
there were no statistically significant differences. This supported previous work
that reflex inhibition resulting from effusion affects the entire extensor
mechanism and does not predispose the patellofemoral joint to an imbalance of
dynamic forces.
A neural component of VMO insufficiency was proposed by Bennett and
Stauber,10 who hypothesized that underdevelopment of this muscle may result
from a deficiency in motor control. These authors observed an eccentric muscle
contraction deficit with isokinetic testing that demonstrated a rapid reversal with
49
training. The short return to normal eccentric strength combined with a rapid
decrease in symptoms lead to a conclusion that an error in the appropriate use
of the VM exists in this population. Exactly how the training of the quadriceps
relieved pain was not presented in this study.
It is apparent from the literature that a host of factors have been
postulated as being contributory to lateral patellar tracking. Although many
theories have been presented in the literature, further research is necessary to
substantiate these claims.
QUADRICEPS FEMORIS FUNCTION IN PATELLOFEMORAL PAIN
Documenting imbalances between the VMO and VL in patients with PFP
has been of primary interest to the practicing clinician, as conservative treatment
of this disorder typically focuses on restoring normal function of the dynamic
stabilizers.55-97 It is this functional imbalance that is widely accepted as a
cause of PFP.38 Despite the interest in the function of the quadriceps femoris in
patients with PFP, adequately controlled studies are few.
Mariani and Caruso109 first investigated the electromyographic (EMG)
activity of the dynamic patellar stabilizers in eight subjects with lateral
subluxation of the patella, and found a marked decrease in activity of the VM
compared to the VL in seven of these cases. Despite incorporation of a control
group, no statistical analysis or quantification of EMG was performed in their
study. Qualitative analysis of surface EMG also was made on the sam e eight
subjects after a surgical realignment procedure of the extensor mechanism. VM
activity was found to return to normal in all patients except one, leading the
50
authors to postulate that dysplasia of this structure was a secondary event,
dependent on the static alterations in the extensor mechanism. There were no
quantified data however, to support this hypothesis.
In a similar study conducted by Souza and G ross,169 electromyographic
activity of the VMO and VL was assessed via surface electrodes in a group of
nine PFP subjects. These authors calculated the VMO:VL EMG ratio during
isotonic and isometric contractions and reported no significant differences
between groups or conditions. Perceiving a potential flaw in the normalized
EMG data if the normalizing contractions were themselves imbalanced, the
authors also analyzed the non-normalized EMG data. The non-normalized
results showed a significant difference between the two groups with the patient
population demonstrating a lower VMO:VL ratio. These findings supported the
work of Mariani and Caruso,109 in that abnormal muscle activation patterns
were present in subjects with PFP. However, the authors did not take into
consideration that the amplitude of the non-normalized surface EMG signal
could have been affected by electrode placement, differences in muscle bulk
within the pickup range of the electrode, or the amount of subcutaneous tissue
between the electrode and the muscle. This methodological limitation was
enough to seriously question the validity of these data.
To differentiate between two mechanisms underlying PFP, neuromuscular
and mechanical, Boucher et al.15 compared two groups based on a diagnosis
of PFP and Q angle. The control group had a normal Q angle and had no
history of PFP, while the patient group had an excessive Q angle and were
diagnosed as having PFP. There were no group differences in the integrated
EMG between the vasti during isometric knee extension, suggesting the neural
drive was not affected in these patients. However, when the five patients
51
demonstrating the largest Q angles were analyzed separately, there was a
significantly lower VMO:VL ratio compared to the control group. This finding
lead these authors to speculate that VMO insufficiency may have more of a
mechanical origin, with the mechanical disturbance exhibited first, and then
followed by atrophy.
Further support for abnormal activation patterns in subjects with patellar
subluxation was presented by Wise and colleagues, 189 who noted VMO:VL
ratios ranging from 1:1.3 to 1:2.6 in six subjects. This finding w as strictly
observational as they had no control group nor statistical comparisons.
While the above investigations have indicated muscular imbalance in PFP,
som e authors have reported no such abnormalities. MacIntyre and
Robertson103 found no significant differences in EMG patterns of the
quadriceps in eight women runners with PFP compared to a normal control
group. A study of 18 subjects conducted by Wild et a!.,185 also reported no
differences in magnitude of contraction between the VMO and VL. Moller et
al.113 described similar findings in 28 patients. Based on history and clinical
examination, 17 of these patients were diagnosed as idiopathic chondromalacia
while 11 were found to have lateral subluxation. These authors cited no
differences in muscular activity patterns between the two groups, however, the
EMG of the involved knees were decreased compared to the contralateral non-
symptomatic knees. Neither of the groups demonstrated differences in VMO
and VL activity that suggested a muscular imbalance that would be contributory
to patellar subluxation.
Further evidence refuting the contributory effects of imbalanced
quadriceps activity in patellar subluxation was presented by Moller et al.116 in a
follow-up study. Increases in lateral patellar displacement in subjects with
52
patellar instability were documented by comparing standard x-rays (axial view)
with the quadriceps relaxed and fully contracted. Despite the significant
increase in patella lateralization compared to normal controls, there were no
significant differences between activity of the VL and the VMO.
Grabiner and colleagues48 evaluated the hypothesis of a feedforward
activation pattern (larger relative initial VMO activation compared to that of the
VL) in eight subjects with PFP. No differences between muscles were noted
during various isometric conditions, however, the authors did report decreased
excitation of both muscles compared to normal controls.
Results of the previous studies regarding potential muscular imbalances
of the extensor mechanism appear equivocal, indicating that diminished medial
stabilization may play only a partial role in PFP. Direct comparisons between
these studies are difficult as a result of differences in experimental technique and
methods of assessing EMG activity. The lack of control groups and statistical
analysis in som e of these investigations further contribute to the inconsistent
findings.
Another confounding variable in these studies could be the lack of a
hom ogeneous patient population. As discussed earlier, a diagnosis of PFP can
encom pass a large area of clinical disorders that are not necessarily related to
patellar subluxation. A majority of these investigations did not adequately
describe the patient selection criteria necessary for a controlled experimental
group. Of the above studies, only one has attempted to control for the wide
variation of biomechanical abnormalities exhibited in this disorder, however, the
criteria utilized for patient classification was based on clinical examination.1'18 It
appears that more objective methods of documenting patellar instability and
53
classifying PFP patients is necessary to determine the role of quadriceps
function in patellofemoral disorders.
THE ROLE OF RADIOLOGY IN THE EVALUATION OF PATELLOFEMORAL
JOINT DISORDERS
The use of radiography to demonstrate the presence of malalignment,
subluxation, dislocation or other disorders of the patellofemoral joint is an
important tool in the diagnosis and treatment of PFP.115 To quantify the
relationships between the patella and the femoral trochlea, many indices have
been developed using standard radiography, computerized tomography (CT)
and magnetic resonance imaging (MRI). These indices provide the clinician with
valuable information regarding treatment and understanding of the etiology of
p p p 1 1 5
STANDARD ROENTGENOGRAMS
Conventional radiography of the patellofemoral joint has evolved over the
past century with information being obtained with three primary views:
anterior/posterior (AP), lateral and tangential (axial). The AP view is commonly
obtained supine and is adequate for determining patellar fractures or dysplasia;
however, this view distorts patellar alignment as there is a natural tendency for
the legs to roll into external rotation.41
The lateral view is traditionally taken in the lateral decubitus position and
is useful in demonstrating the functional relationship between the patella and the
tibia and the patellar facets to the fem ur41 Patellofemoral chondrosis may also
54
be diagnosed with this v ie w . 115 patella alta is best determined with the lateral
view using the measurements described by Jacobsen and Berthensen.79 The
measurement consists of determining the ratio of patellar tendon length
{distance from tibial tubercle to the inferior pole of the patella) to the greatest
diagonal length of the patella. This technique was found to provide the most
accurate determination as it was independent of the amount of knee flexion.
The upper limit of this ratio has been determined to be 1.2 at the 90 percent
confidence level79
The axial view provides information regarding patellofemoral congruence,
contact, and displacement.115 There are several techniques that are commonly
used; however, the Laurin and Merchant methods provide the best results and
have been demonstrated to be simple and reproducible.41
The Merchant view is taken with the patient supine and the knee flexed to
45 degrees over the edge of the table (Fig. 2-10). 113
A
Figure 2-10. Patient positioning used to obtain a Merchant axial view of the
patellofemoral joint. From Fulkerson JP, Hungerford DS. Disorders of the
Patellofemoral Joint (2nd edition). Baltimore, Williams & Wilkins, 1990.
(Reproduced with permission)
55
In studying 200 asymptomatic knees, Merchant et al.113 defined a
normal "congruence angle" which was developed to demonstrate subtle patellar
subluxation. The congruence angle was determined by the angle formed by the
line bisecting the sulcus angle and the projection of a line from the sulcus to the
lowest point on the articular ridge of the patella (Fig. 2-11). This angle was
reported as being positive or negative based on the position of the articular ridge
of the patella with respect to the bisection of the femoral sulcus. A negative
value was indicative of a medial patellar shift while a positive value indicated a
lateral shift (Fig. 2-11).
MEDIAL (-> \ | LATERAL (+ )
\ I
SULCUSN& iM 'S'
'ANGLE
Figure 2-11. Congruence angle. The sulcus angle ETI is bisected by a neutral
reference line TO. The apex of the medial patellar ridge is connected to the
lowest point of the trochlear sulcus. When this iine (RT) is medial to the neutral
reference line, the angle is given a negative value; when lateral a positive value is
given. From Fox JM & Del Pizzo W. eds. The Patellofemoral Joint New York,
McGraw-Hill, 1993. (Reproduced with permission)
Normative data established by these authors for asymptomatic knees
revealed a congruence angle of -6 degrees, while the average for symptomatic
knees was +23 degrees. An angle of +16 degrees was determined to be
abnormal at the 95th percentile.
56
The Laurin technique95 is similar to that of Merchant and is performed
with the patient supine and the knee flexed to 20 degrees. The attempt to image
the patellofemoral joint at angles less than 20 degrees is based on the
assumption that subluxation may be more evident owing to the inherent
instability in this range. Indices developed by Laurin et al.95 included the lateral
patellofemoral angle, the patellofemoral index, and lateral patellar displacement.
The lateral patellofemoral angle evaluated the relationship between the
femoral sulcus and the patella giving information regarding the tilt of the patella.
This angle was formed by lines connecting the apices of the femoral condyles
and the limits of the lateral facet (Fig. 2-12a). This angle was found to open
laterally (indicating medial tilt) in 97 percent of normal patients while 40 percent
of patients with patellar subluxation demonstrated an angle that was open
medially (lateral tilt)95 Exact degrees were not given for this particular
measurement.
The patellofemoral index, as described by Laurin and colleagues,95 was
more sensitive to subtle changes in patellar tilting that was not evident with the
lateral patellofemoral angle. This index was computed as the ratio between the
thickness of the medial patellofemoral interspace and the lateral patellofemoral
interspace (Fig. 2-12b). The interspace was defined as the shortest distance
between the patellar facet and the femoral condyle. In comparing 100 control to
100 patients with chondromalacia, these authors established that the
patellofemoral index was considered norm&l at 1.6 or less. Ninety-three percent
of the chondromalacia group demonstrated a patellofemoral index that was
greater than 1.6. This measurement has subsequently been demonstrated to be
valid only at 20 degrees of knee flexion, therefore necessitating accurate
positioning for assessm ent.115
57
The final index described by Laurin et al.95 w as for lateral patellar
displacement which was m easured by a line joining the summits of the femoral
condyles (horizontal) and a perpendicular line marking the highest point of the
medial condyle (Fig. 2-12c). Analysis of 230 radiographs found that 97 percent
of normal controls demonstrated some lateral displacement with the medial
edge of the patella being lateral to the perpendicular line of the medial condyle.
Thirty percent of the patients with chondromalacia demonstrated excessive
lateral displacement, while 50 percent of the subluxing patella group showed the
sam e orientation95 These authors stated that the incidence of lateral
displacement in the chondromalacia group appeared low (30 percent) and that
the development of more sensitive criteria to identify this abnormality may be
necessary.
Figure 2-12. Patellofemoral joint relationships as proposed by Laurin. a) Lateral
patellofemoral angle. Two lines, one connecting tne apices of the femoral
condyles (AA) and the other along the lateral facet of the patella (BB) are
defined, b) Lateral patellofemoral index. This represents the ratio between the
narrowest medial patellofemoral distance measured at the lateral edge of the
medial facet (A) and the narrowest lateral patellofemoral distance (B). c) Lateral
patellar displacement. A line is drawn connecting the highest points of the medial
and lateral femoral condyles (AA). A perpendicular to that line at the medial
edge of the medial femoral condyle (B) is then drawn. From Fox JM & Del Pizzo
W. eds. The Patellofemoral Joint. New York, McGraw-Hill, 1993. (Reproduced
with permission)
58
COMPUTERIZED TOMOGRAPHY
CT may be used to obtain sequential images of the patellofemoral joint at
varying degrees of knee flexion thus enabling patella alignment to be assessed
more accurately. An axial view with a mid-transverse tomographic slice is the
view of choice and gives adequate assessm ent of patellar alignment. More
exact relationships between the patella and the femur can be m ade as CT
provides specific sections through the femoral trochlea thus omitting overlap
and distortion 4 -1
The accurate imaging offered by CT allows for more reliable reference
planes which makes it possible to note a variety of different tracking patterns.
Particularly useful is the plane of the posterior femoral condyles which are more
defined compared to traditional radiographs.4 -1 In addition, as the anatomy of
the anterior femoral trochlea changes considerably throughout knee flexion, the
posterior condyles remain consistent providing a more stable reference in
determining patellofemoral joint relationships. Fulkerson et al.42 demonstrated
that the posterior intercondylar line was a more reliable reference point for
determination of the patellar tilt angle compared to the more variable anterior
femoral trochlear margin. The mean variability demonstrated with the anterior
reference plane was almost three times as great as the mean variability of the
posterior trochlear reference plane (12.1 degrees vs. 4.8 degrees). Fulkerson
and Hungerford4 -1 have reported that the patellar tilt angle should always be
greater than seven degrees and can commonly have a range to 20 degrees in
asymptomatic knees.
Utilizing more sensitive CT criteria, subtle patellofemoral joint
abnormalities have been documented. Delgado2^ examined 24 normal knees to
59
study the position of the patella at varying degrees of knee flexion. At 90
degrees, 96 percent of the patella were centered in the trochlear groove. This
value dropped to 63 percent at 60 degrees and 29 percent at 30 degrees. In full
extension 13 percent of the patella were centered with the quadriceps relaxed
and only four percent with quadriceps contracted. Despite the lack of specific
indices to quantify patellar centering, this study refuted previous reports that the
patella rests comfortably in the femoral sulcus in full extension and that a
deviated posture of the patella is evident in asymptomatic subjects.
A comparison of a conventional radiographic axial view to CT was
undertaken by Sasaki and Vagi’ !50 to evaluate the usefulness of CT for
investigating abnormalities of the patellofemoral joint. Mean lateral tilt and lateral
shift values were obtained on 20 subjects with patellofemoral pain. The mean
values for the CT scan were significantly higher than those obtained with
conventional radiography (lateral shift: 12.1 vs. 31.4 degrees; lateral tilt: 17.2 vs.
31.8 degrees). The authors hypothesized that the increased values seen with
CT were probably the result of imaging at angles close to full extension where
patellar stability was compromised. Since conventional radiography is limited to
20 to 30 degrees of flexion as a result of the overlapping of images and poor
definition, it appears that patellofemoral joint indices may be angle
dependent.150 A comparison of normal controls and symptomatic subjects in
the sam e study demonstrated significantly greater lateral tilt (15.0 vs. 31.8
degrees) and shift values (14.0 vs. 31.4 degrees) for the patellofemoral
population. In addition, when the quadriceps were contracted at full extension,
there was an increase in both lateral tilt and shift for both groups. The mean
lateral shift values increased on the average of 19 degrees for the symptomatic
group and 11 degrees for the control group. Tilt values increased only five
60
degrees for the patients and three degrees for the controls. The CT method of
evaluating the patellofemoral joint was deem ed to be more valuable than
conventional radiography as a more accurate understanding of the pathological
state of the subluxing patella could be observed at full extension.150
Using modified parameters and criteria established by Merchant et al.,116
Schutzer and colleagues152 identified three distinct patterns of malalignment
based on CT evaluation at 0 to 30 degrees of knee flexion. Twenty-four patients
were compared to ten normal controls in this study. Six patients were found to
have patellae that were laterally displaced and non-tilted, seven were laterally
tilted and laterally displaced and five patients were both laterally displaced and
laterally tilted. Four patients had no evidence of patellar tilting or lateralization.
The authors discussed the awareness of different patterns of malalignment as a
significant advantage of CT, especially when surgical intervention was being
considered.
MAGNETIC RESONANCE IMAGING
MRI has been reported to be a useful modality in evaluating the
patellofemoral joint, and has been found to be reliable for the imaging of soft
tissue structures,41 and the assessm ent of patellar alignment.161 The ability to
give a direct picture of hydrated structures gives this modality the advantage of
viewing patellar articular cartilage.41 Yulish et a!.101 undertook a study to
determine if MRI could accurately demonstrate the patellar articular cartilage and
show the detail necessary to stage chondromalacia. Arthroscopy was used as
the standard of reference with findings compared to MRI. In this study, MRI was
able to depict focal areas of swelling, surface irregularities, areas of thinning and
cartilage loss with exposure to subchondral bone. The surgical findings agreed
61
with those from MR images in 17 of 19 patients indicating that MRI could be
used as an accurate means of assessing patellar cartilage and may be an
alternative to diagnostic arthroscopy when chondromalacia patella is suspected.
Analysis of patellar alignment using MRI was conducted on 20 normal
subjects (10 males, 10 females) in a study by Kujala et al 93 The patellofemoral
joint was assessed with sagittal and axial views with the knee flexed to 0,10, 20,
30 degrees. There were increased values of lateral patellar displacement, lateral
tilt, and the congruence angle was directed more laterally at zero degrees
compared to 30 degrees indicating a tendency towards subluxation at the
beginning of knee flexion. This finding was in agreement with the results of
Delgado29 for CT of normal subjects. When males were compared to females,
there was a significant increase in the lateral displacement at 20 degrees in the
female group, suggesting the femoral condyles gave less support to the patella.
The authors stated that this finding may partially explain the greater incidence of
patellofemoral problems in this population.
In a follow-up study, Kujaia et a l94 utilized MRI to analyze the
patellofemoral relationships during the first 30 degrees of knee flexion in a group
of women with recurrent patellar dislocation. As expected, the dislocating group
demonstrated greater mean lateral displacement, lateral tilt, and congruence
angles compared to control subjects for all positions evaluated. The femoral
sulcus angle was found to be greater in the dislocation group (indicative of a
more shallow groove) which was indicative of an anatomical predisposition to
recurrent patellar dislocation.
Using similar passive positioning techniques in 130 patients with
suspected tracking abnormalities, Shellock and colleagues161 reported that
only 26 percent of subjects demonstrated lateral displacement of the patella as
62
opposed to 41 percent with medial subluxation. Although these results were
based on qualitative assessm ent, such findings indicate that individuals with
patellofemoral disorders may exhibit varying types of patella malalignment. This
would suggest that one form of patellar malalignment cannot be generalized to
this population.
Recently, a new technique of MRI has been developed that has the ability
to assess the patellofemoral joint during active movement as opposed to the
passive positioning techniques described previously. This technique utilizes a
fast, spoiled gradient-recalled acquisition in the steady state (GRASS) pulse
sequence that allows for MR imaging during active movement.159 This form of
imaging is believed to offer important diagnostic advantages which includes the
ability to asse ss the contribution of the activated muscles and other soft tissue in
providing patellar alignment and tracking information.15? A nonferromagnetic
device with a cut-out area permits uninhibited movement of the patella within the
magnetic field as the knee extends from a flexed position. This active MR
technique was compared to traditional passive positioning by Shellock et al.,159
in order to delineate differences in assessing abnormal patellar tracking and
alignment. These authors found no appreciable differences between the two
techniques, however, it must be emphasized that no m easurem ents of
traditional patellofemoral joint indices were taken. The authors did report that
the image quality of the active MR technique was adequate and that this
technique was considerably shorter in duration (eight seconds vs. four minutes).
To simulate physiologic loading of the patellofemoral joint, a
nonferromagnetic device was developed by Shellock et al.,159 which produced
a resistance of 30 ft-lbs/sec throughout a 45 degree arc of knee extension. The
intent of using this device during active MR imaging was to determine if an
63
externally applied force would have an effect on patellar kinematics com pared to
active non-resisted knee extension. A striking difference was observed between
the two conditions with the resistive MR images, demonstrating a greater
amount of patellar deviations. Of the 23 patellofemoral subjects tested, seven
were considered normal with the active MRI’ s. When, however, the sam e
subjects underwent MRI evaluation with the externally applied force only one
demonstrated normal tracking. Even though quantified values were not
reported in this study, these results suggest the importance of the dynamic
stabilizers in determining patellofemoral joint kinematics. These authors
hypothesized that performing active MRI examination under loaded conditions
can enhance the ability to identify joint pathology as this resisted movement
would more likely provoke an abnormality, if present. The authors stressed the
importance of additional research in this area.
Quantitative assessm ent of kinematic MRI has only recently been
described A 9 The limitations of such analysis is the absence of clearly defined
and constant reference points as the view of the bony anatomy changes
throughout an arc of motion. This requires defining indices based on constant
bony landmarks, regardless of the knee angle. Brossmann et al.19 attempted
such quantification, however, the methods utilized for the determination of
patellar tilt and lateral patellar displacement were dependent on the anterior
femoral condyles (as shown by the axial view) for landmarks. As mentioned
earlier, the variability of measurements using these landmarks is much greater
than the variability of measurements obtained using the more stable posterior
femoral condyles.42 Although this study documented significant differences in
all patellofemoral indices between normal and PFP groups, more accurate and
reliable spatial measurements are necessary. A review of the literature has
64
revealed no reports of the accuracy or reliability in obtaining image
measurements.
In summary, sufficient evidence exists to substantiate the role of
radiography in identifying patellofemoral abnormalities. Statistically significant
correlations between certain radiographic indices and patellar instability have
been made for conventional x-rays, CT and MRI and provide useful information
regarding patellar position and tracking. The transverse mid-patella section
gives the best view to assess tilting and subluxation and gives information
regarding the depth of the trochlear groove.
Radiography has demonstrated that the patella is most unstable as the
knee approaches full extension and that lateral displacement and tilt are
common findings in asymptomatic knees. Conventional radiography is limited in
ranges less than 30 degrees of flexion as views are subject to distortion due to
overlapping of structures. CT can provide imaging at angles less than 30
degrees, and therefore, can give more accurate information regarding patellar
stability. MRI has the advantage of viewing soft tissue structures such as the
articular cartilage and with the advent of kinematic imaging, views of the
patellofemoral joint through a range of motion are possible. By observing the
effects of the contracting quadriceps, additional information can be gained
regarding the etiology of PFP.
Interpretation of numeric values from different studies is difficult because
of differences in radiological techniques and methods of determining specific
indices. More standardized techniques and determinations of patellar
malalignment would aid in consistent classifications. This would contribute to
the understanding of the role of the quadriceps mechanism in patellar
malalignment.
65
CHAPTER III
PATELLAR KINEMATICS AND VASTI ACTIVATION IN THE
NORMAL AND PATELLOFEMORAL PAIN POPULATIONS
The vasti are the prime movers of the patella,35,41 and are responsible
for the forces experienced by the patellofemoral joint.108 Although abnormal
patellofemoral joint mechanics associated with patellar instability has largely
been attributed to dynamic factors such as medial vasti insufficiency,5!41'63
there is little evidence to support this premise. One reason for the lack of
objective data in this area has been the inability to quantify patellar kinematics in-
vivo. With the advent of technological advances in Magnetic Resonance
Imaging (MRI), in depth analysis of patellofemoral joint kinematics is now
possible. This information is essential in understanding the role of the vasti in
contributing to patellofemoral joint dynamics and patellofemoral pain (PFP).
This chapter will provide a detailed description of the techniques
employed to quantify patellofemoral joint motion, and explore the determinants
of patellar instability in subjects with PFP. The introduction, results and
discussion sections will be divided into two different topics: 1) Quantification of
patellar tracking using kinematic magnetic resonance imaging (KMRI), and 2)
The factors contributing to abnormal patellar tracking patterns in subjects with
PFP. These two topics will share a common methods section.
66
INTRODUCTION
QUANTIFICATION OF PATELLAR TRACKING USING KINEMATIC
MAGNETIC RESONANCE IMAGING
Abnormal patellar tracking is considered to be the primary contributing
factor associated with the pathomechanics of PFP,43.59.75!119 yet patella
orientation and joint congruency are typically evaluated through static radiologic
imaging such as planar x-rays and computed tomography.95* 113 For objective
information regarding patellar kinematics, cadaver studies121 > 141 > 181 as well in-
vivo techniques using cortical bone pins98 have been used. Although these
types of analyses relate patellar motion to a coordinate system established
within the distal femur, they are limited in their ability to accurately asse ss the
position of the patella within the femoral trochlear groove.
Recently, a new diagnostic technique has been developed that uses
KMRI to asse ss patellar tracking.159 This imaging technique employs a fast,
spoiled gradient-recalled acquisition in the steady state (GRASS) pulse
sequence, that has sufficient temporal resolution to permit imaging during active
movement (ie. one image per second). The advantage of this type of imaging is
the opportunity to assess the extensor mechanism dynamically throughout a
range of motion, thus providing a physiologic assessm ent of patellofemoral joint
mechanics. This is a substantial improvement over static imaging, as studies
comparing active and passive procedures have indicated that patellar motion is
significantly influenced by the degree of quadriceps contraction 85,150,158,159
Another advantage of KMRI over conventional radiography, is the ability
to obtain axial views of the patellofemoral joint during terminal knee extension.
Images obtained through standard radiographs are subject to distortion at this
67
point in the range and, therefore, are generally limited to knee flexion angles
greater than 30 degrees.50 This is a substantial limitation as the patella is
considered to be the most unstable as the knee extends from 20 to 0
degrees.152
Up to this point, interpretation of KMRI data has been mainly
qualitative.158-160 The primary difficulty in quantifying patellofemoral joint
relationships during dynamic motion has been the inability to rely on consistent
bony landmarks from which measurements can be taken.1 ? 1 For example, as
the patella has a tendency to move superiorly during knee extension, it is often
above the level at which the femoral sulcus can be defined.85 This is of
particular concern as many patellofemoral joint indices rely on the ability to
determine the limits of the anterior femoral condyles and the trochlear
groove.85* 118
Using motion-triggered MRI, Brossmann et al.18 provided quantitative
documentation of patellar tracking patterns in 13 subjects with extensor
mechanism disorders, and found significant differences in lateral patellar
displacement and lateral patellar tilt compared to asymptomatic volunteers. In
an attempt to overcome the difficulty in obtaining m easurem ents when the
femoral sulcus could not be delineated, these authors transferred reference lines
from other image sections from the sam e time frame. While this method
permitted image quantification, the failure to use reference lines obtained at the
sam e section location as the midsection of the patella did not allow for a true
representation of the relationship between the patella and its articulation with the
distal femur.
The images obtained in the study conducted by Brossmann and
colleagues18 were during active knee extension however, no external resistance
68
w as applied. Consequently, it is unlikely that the extensor mechanism was
functionally stressed and, therefore, the true contribution of quadriceps
contraction to patellar tracking was not assessed. Since PFP is typically
reproduced under activities that require substantial quadriceps forces,34.98 it
appears logical that assessm ent of patellar kinematics should be m ade under
similar conditions.
To date, there has been no quantification of patellar motion under a
functional load as assessed by KMRI. This information would offer a more
physiologic assessm ent of patellar motion, and provide information regarding
the mechanics of this joint when placed under stress. The purpose of the first
phase of this study, w as to describe a method to quantify patellar tracking
patterns (patellar glide and tilt) during resisted KMRI, and to a sse ss the day to
day reliability in obtaining these measurements. In addition, normative data as
well as patient examples will be presented.
FACTORS CONTRIBUTING TO ABNORMAL PATELLAR TRACKING
PATTERNS IN SUBJECTS WITH PATELLOFEMORAL PAIN
Patella malalignment is considered to be an important etiological factor
contributing to the development of PFP. The cause of patellar instability appears
to be multifaceted with components being defined by two distinct categories:
static and dynamic. Static considerations include abnormal bony
configuration"* 7,41,70,98,126,179,184 0r tightness of non-contractile
elements,38-83.138 while dynamic components involve unequal activation of the
quadriceps femoris .33,41,55,66,123
From a dynamic standpoint, lateral patellar subluxation has been
attributed to insufficiency of the medial vasti, namely the vastus medialis longus
69
(VML) and the vastus medialis oblique (VMO).109* '169.'189 As a result of its
more horizontal fiber orientation, the VMO has been given the distinction of
being the primary medial stabilizer of the patella.99 This premise has formed the
basis of conservative care for PFP as improving VMO strength is thought to be
essential in overcoming the lateral pull of the vastus lateralis (Vl_).55> 97>i11
Despite the large emphasis on VMO rehabilitation in this population,
assessm ent of VMO strength in-vivo is not possible. In lieu of this limitation,
electromyography (EMG) has been employed to establish the activation patterns
of this muscle with the rationale that decreased activity of the VMO relative to the
VL is indicative of compromised medial patellar stability. Although numerous
studies have compared the activity of the VMO to the
VL,15>^03,1 ISjISQ.ISS.IS9 there is no general consensus as to whether VMO
insufficiency (as determined by EMG) exists, or more importantly, is predictive of
abnormal patellofemoral joint function.
Apart from dynamic factors, Brattstrom'17 has reported that dysplasia of
the femoral trochlea is the most important etiological factor in recurrent patellar
subluxation. Because the lateral femoral condyle is larger and projects further
anteriorly than the medial condyle,38 the trochlear groove is thought to provide
bony stability against laterally directed forces. Although som e authors have
reported that the decreased depth of the intercondylar sulcus is a primary cause
of patellar instability^7^ 07 others have hypothesized that patellar instability is
the result of the patella resting above the trochlear groove (ie. patella
alta).45< 75> 76>117
Although bony abnormalities as well as muscle imbalance have been
implicated as being contributory to abnormal patellar alignment, the relationship
of these factors to actual patellar tracking patterns has not been established.
70
With the advent of KMRI, quantification of patellar movement throughout an arc
of resisted knee extension is now possible.135-157 This diagnostic technique
has a distinct advantage over static imaging procedures in that the contribution
of the extensor mechanism to patellofemoral joint kinematics can be
a ssessed .159
Given the ability to quantify patellar kinematics in-vivo, the purpose of the
second phase of this investigation was to assess the influence of vasti activation
(as determined through EMG) and the depth of the intercondylar groove on
patellar tracking patterns in normal subjects and patients with PFP. It was
hypothesized that patellar malalignment would be associated with a shallow
trochlear groove and decreased vastus medialis activity. This information will
have broad implications for the treatment of PFP, and provide useful information
regarding the etiology of patellofemoral joint instability.
MATERIALS AND METHODS
SUBJECTS
Two groups of female subjects were recruited for the MRI portion of this
study. Only female subjects were studied owing to the higher incidence in the
general population (2:1; females vs. males), and the inherent biomechanicai
differences between the sexes.39-93
Twenty-three female subjects with a diagnosis of PFP served as the
experimental group, while 12 females with no history of knee pain or dysfunction
served as the control group. The two groups were similar in age, height, and
weight (Table 3-1).
71
Table 3-1
Subject Characteristics
PFP (n=23)__________Normals (n=12)_______ p-value
Aae fvrs)
m ean 26.8 28.1 ;3s :
. sd . 8,5 5.0
Height (cm)
m ean 165.6 168.4 .• • ::-.%29
| sd ; 7.2 .
8,0
iWeight £kg)
i l i i l s p l l i l l l l l l l l p l l
mean 62.2 61.2 :: ,76 : i
sd 8.1 8.0
PFP= patellofemoral pain
Subjects for the PFP group were recruited from the Southern California
Orthopaedic Institute as well as local physical therapy clinics, and were
screened to rule out ligamentous instability, internal derangement, patellar
tendinitis, or large knee joint effusion. Only those subjects meeting the following
inclusion criteria were admitted to the experimental group of this study:
1) pain (vague or localized) originating from the patellofemoral joint
articulation (only patient histories relating to overuse or indsidious onset were
accepted)
2) readily reproducible pain with at least two of the following functional
activities commonly associated with PFP:
a. stair ascent or descent
b. squatting
72
c. kneeling
d. prolonged sitting
e. isometric quadriceps contraction
PFP subjects were excluded from the study if they reported:
1) previous knee surgery
2) acute traumatic patellar dislocation
3) neurological impairment that would influence gait
The control group was recruited from the University of Southern
California, and Rancho Los Amigos Medical Center. These subjects were
selected based on the sam e criteria as the experimental group except that
subjects had no:
1) previous history or diagnosis of knee pathology or trauma
2) current knee pain or effusion
3) pain or discomfort with any of the activities previously listed
4) limitations that would influence gait
INSTRUMENTATION
Kinematic Magnetic Resonance Imaging
KMRI of the patellofemoral joint was conducted with the transmit and
receive quadrature body coil of a 1.5T/64 MHz magnetic resonance imager3
using a fast spoiled GRASS (FSPGR) pulse sequence. T1 weighted, spin echo,
axial plane imaging was performed using the following parameters: time to
repeat: 6.5 ms; time to echo: 2.1 ms; number of excitations: 1.0; matrix size: 256
a. General Electric Medical Systems, 3200 N. Grandview Ave. Waukesha, Wl
73
X 128; field of view: 38 cm; flip angle: 30 degrees; and a 7 mm section thickness
with an interslice spacing of 0.5 mm. Acquisition time was six seconds to obtain
six images (ie. one image per second).
Imaging was performed using a specially constructed, nonferromagnetic
positioning device that permitted bilateral knee extension against resistance (in
the prone position) from 45 degrees flexion to full extension (Figure 3-1 ).& This
device allowed uninhibited movement of the patellofemoral joint and normal
rotation of the lower extremities. This is of importance as it has been reported
that patellar tracking may be influenced by tibial rotation. 181
Resistance was accomplished through a pulley system that provided a
constant one foot lever arm. This allowed the application of a constant (isotonic)
torque throughout the entire range of motion. Weights constructed of
nonmagnetic stainless steel (316L)C supplied the resistive force for this
maneuver. These plates were placed upon a moveable carriage which was
attached to the pulley apparatus (Figure 3-1).
Dynamic EMG
Fine wire, indwelling wire electrodes were used to record the intensity of
vasti muscle activity. The electrodes were bipolar in configuration, and were
m ade of nylon-insulated 50 micron wire (nickel-chromium alloy). The wires were
passed through the cannula of a 25 gauge hypodermic needle with the distal
ends staggered and folded over the needle tip as described by Basmajian and
DeLuca.7
b. Captain Plastic, P.O. Box 27493, Seattle, WA
c. Esco Corp., 6415 E. Corvette St. Los Angeles, CA
74
After insertion into the muscle of choice, the wires were secured to a
reference (ground) electrode and the signals were fed directly into a differential
amplifier/FM radio transmitter unit.d The differential amplifier had a common
mode rejection ratio of 60 dB. The EMG signals were then telemetered from the
transmitter to the receiver unit where the signal was band pass filtered (150-1000
Hz) and amplified to a gain of 1000. The raw signal was sampled and digitized
by a DEC 11/23 data acquisition computer.© The sampling rate was 2500 Hz.
Figure 3-1. Patient set-up on the nonferromagnetic positioning device used for
imaging. A pulley system consisting of a one foot lever arm (background) and a
moveable carriage (foreground), allowed resisted knee extension from 45
degrees to full extension. This device permitted uninhibited movement of the
patella and natural rotation of the lower extremity. Velcro straps were used to
secure the thigh and tibia to the apparatus.
d. Biosentry Telemetry Inc., 20270 Earl St., Torrance, CA 90503
e. Digital Equipment Corp., 146 Main St., Maynard, MA 01754-2571
75
PROCEDURES
This study involved two different testing sessions: KMRI to determine
patellar kinematics, and EMG evaluation to assess the vasti activation pattern.
Prior to testing, all procedures were explained to each subject and informed
consent was obtained. A ll procedures for this portion of the study were
approved for human subjects by the Los Amigos Research and Education
Institute of Rancho Los Amigos Medical Center. After all procedures has been
adequately explained, subject age, height, and weight were recorded to
determine group homogeneity.
Kinematic Magnetic Resonance Imaging
All Imaging was performed at Tower Imaging Center in West Los Angeles.
Subjects were asked to remove all jewelry and change into a hospital gown
which was worn during testing. Subjects were placed prone on the positioning
device with care being taken to allow for normal lower extremity rotation. After
this position was achieved, velcro straps were placed around the thighs and
positioning device for stabilization purposes. Resistance was then set at 15
percent body weight. Once the subject was secured, the positioning device was
placed within the magnet bore (Figure 3-2).
After familiarization with the knee extension apparatus, subjects were
instructed to practice extending their knees at a rate of approximately nine
degrees/sec. This rate ensured six evenly spaced images throughout the 45
degree arc of motion (including the 45 degree position), and would therefore
permit imaging at 45, 36, 27, 18, 9, and 0 degrees of knee flexion.
Approximation of this rate was made by the Principal Investigator with the use of
a stopwatch.
76
Figure 3-2. Placement of the patient and positioning device within the MR
system.
Once the subject was able to reproduce the desired rate of motion in a
smooth and even manner, imaging commenced. Subjects were instructed to
initiate extension upon verbal command and continue until full extension had
been reached. This procedure was repeated at three different image planes in
order to asse ss the entire excursion of the patella in relation to the trochlear
groove (ie. three slices were obtained at each angle of knee flexion).
Dynamic assessm ent was repeated if the rate of knee extension was too
fast or too slow, or not performed in a smooth manner. In addition, assessm ent
was repeated if six adequate images were not obtained. An adequate image
was one in which the medial and lateral borders of the midsection of the patella,
the trochlear groove, and the posterior femoral condyles were well defined.
77
Dynamic EMG
Following KMRI, all subjects underwent EMG analysis at the
Pathokinesiology Laboratory, Rancho Los Amigos Medical Center. This typically
occurred within 24 hours of the KMRI evaluation. Subjects were appropriately
clothed allowing for exposure of the lower extremity under study to
accommodate the instrumentation.
Sterilized, fine wire electrodes were inserted into the VMO, vastus VML,
and VL using the technique described by Basmajian and Deluca.7 After
insertion of the electrode into the muscle of choice, the needle was immediately
removed. Electrodes were secured within the muscle by having the subject go
through full passive range of motion followed by a maximal contraction. Wires
were taped to the skin with a 2 cm loop to prevent dislodging during testing.
The insulation of the exposed wires was removed using tweezers coated with
sandpaper, and the wires were then wrapped around ground plates taped to the
subjects lateral thigh. Electrode placement was confirmed by local muscle
contraction and the pull of the patella induced by mild electrical stimulation.
After confirmation of electrode placement and signal integrity, five
seconds of EMG data was collected with the subject resting supine to determine
background noise at the time of recording. To allow for comparison of EMG
intensity between subjects and muscles, and to control for the variability of
electrode placement, EMG data were normalized to the EMG acquired during a
maximal isometric knee extension effort. This was done on a LIDO isokinetic
dynamometer* with the subject seated with the hips flexed to 90 degrees and the
knee flexed to 60 degrees. A velcro strap was placed around the pelvis to
ensure proper stabilization. Sixty degrees of knee flexion was used as this
f. Loredan Biomedical Corp., P.O. Box 1154, Davis, CA 95617
78
position has been found to result in the greatest torque output in normal
f e m a l e s , 100 anc| provides greater patellar stabilization within the trochlear
groove.41 This could potentially minimize quadriceps inhibition resulting from
the pain associated with patellar instability.
Vasti activity then was recorded during active knee extension using the
positioning device described previously for the KMRI. Procedures for subject
positioning, setting of the device resistance, and familiarization practice was
identical to that reported above. To ensure the sam e rate of knee extension
during the KMRI, signals from an electric goniometer positioned at the axis of
rotation of the knee were fed into an oscilloscope to provide visual feedback
(Figure 3-3). Movement of the knee then could be matched to a generated
signal with the desired six second time interval. Once the requested rate of knee
extension could be consistently achieved, six seconds of EMG were recorded
while performing this maneuver. Five trials of data were collected.
Following the knee extension trials, the maximal isometric muscle test on
the LIDO dynamometer was repeated, with the maximal EMG being recorded.
This was done to ensure that the intramuscular electrodes had not been
displaced during the testing procedure.
Reliability of KMRI and EMG measurements
Since MRI and EMG data were not collected simultaneously (owing to
magnetic interference), reliability of these m easures had to be established to
ensure agreeable results could be expected between testing sessions. In
addition, determination of the number of trials to be averaged for consistent data
was necessary.
79
In order to assess the reproducibility of the testing procedures and the
m easurem ents used to obtain the kinematic MRI and EMG data, seven of the
normal subjects underwent repeated testing. A ll repeat testing took place within
24 hours of the initial testing session using the identical procedures outlined
above.
Figure 3-3. Experimental set-up used to assess vasti activity using the MRI
positioning device. Subject positioning and resistance was identical to that
reported for KMRI. To ensure the sam e rate of knee extension during the KMRI
assessm ent, electrical signals from an electric goniometer positioned at the axis
of rotation of the knee were fed into an oscilloscope to provide visual feedback
(background).
80
DATA MANAGEMENT
Kinematic Magnetic Resonance Imaging
Although both patellofemoral joints were imaged simultaneously, only the
data from the involved side were analyzed. Prior to analysis, all images were
screened to ascertain which best visualized the mid-section of the patella
(maximum patellar width) at each angle of knee flexion. Once this was
determined, measurements for these images were obtained. Only images
containing a mid-patella slice were analyzed.
To more accurately assess patellofemoral joint relationships at the
various degrees of knee flexion, m easures that were independent of the shape
of the patella and the anterior femoral condyles were used. This was done to
avoid measurement variability resulting from the continually changing contour of
these structures when viewed at different angles of knee flexion, and to allow
assessm ent of patellar orientation when the intercondylar groove was not well
defined. A ll measurements were made with a computer-assisted program and
included assessm ent of medial/lateral patellar displacement, medial/lateral
patellar tilt, and the sulcus angle.
Medial and lateral patellar displacement was determined by the "bisect
offset" measurement as described by Stanford et al.171 and modified by
Brossmann et a!.19 The bisect offset was measured by drawing a line
connecting the posterior femoral condyles and then projecting a perpendicular
line anteriorly through the deepest point (apex) of the trochlear groove. This line
intersected the patellar width line, which connected the widest points of the
patella (Figure 3-4). The perpendicular line was projected anteriorly from the
bisection of the posterior condylar line to obtain data when the trochlear groove
was flattened (Figure 3-4). A ll bisect offset data represented the extent of the
81
patella lying lateral to the projected perpendicular line and was expressed as a
percentage of total patellar width.
Figure 3-4. Methods used to measure bisect offset. This was determined by
drawing a line connecting the posterior femoral condyles (AB), and then
projecting a perpendicular line anteriorly through the deepest portion of the
trochlear groove (CD) to a point where it bisected the patellar width line
(EF)(left). In order to obtain data when the trochlear groove was flattened, the
perpendicular line was projected anteriorly from the bisection of the posterior
condylar line (right). The bisect offset was reported as the percentage of patellar
width lateral to tne midline.
Medial and lateral patellar tilt was measured using a modification of the
technique described by Sasaki and Y agiJ50 The patellar tilt angle was reported
as the angle formed by the lines joining the maximum width of the patella and the
line joining the posterior femoral condyles (Figure 3-5). A ll tilt m easurements
were reported in degrees.
D
D
Lateral C Medial
B IS E C T O F F S E T
82
L ateral M ed ial
P A T E L L A R T I L T
Figure 3-5. Method used to assess patellar tilt. This angle was defined as the
angle formed by lines joining the maximum with of the patella (AB) and the
posterior femoral condyles (BC). A ll tilt measurements were reported in
degrees.
The final index measured was the sulcus angle. As reported by
Brattstrom,17 the sulcus angle was defined as the angle formed by the highest
points of the medial and lateral anterior femoral condyles and the lowest point of
the intercondylar sulcus (Figure 3-6). In order to obtain data when the trochlear
groove lacked discernible depth, the center of the sulcus angle was defined by a
perpendicular line that was projected anteriorly from the bisection of the
posterior condylar line (Figure 3-6). The estimation of the center of the sulcus
angle was based on the evaluation of normal images which showed that the
deepest portion of the intercondylar groove typically overlies the midpoint of the
posterior condyle interval. A ll sulcus angles were reported in degrees.
83
Lateral Medial
S U L C U S A N G L E
Figure 3-6. Methods used to measure the sulcus angle. This angle was defined
by lines joining the highest points of the medial and lateral condyles and the
lowest point of the intercondylar sulcus (AB and CB)fieft). In order to obtain
data when the trochlear groove lacked discernible depth, the center of the
sulcus angle was defined by a perpendicular line that was projected anteriorly
from the bisection of the posterior condylar line (right). A ll sulcus angle
m easurements were reported in degrees.
The values for patellar tilt and bisect offset were obtained by averaging
two measurements, while the assessm ent of the sulcus angle was m ade by the
averaging of four measurements. This determination was based on the results
of the reliability analysis (see below).
Dynamic EMG
Digitally acquired EMG data collected during the knee extension
maneuver were full wave rectified and integrated over 0.25 second intervals.
Intensities were reported as a percentage of maximum muscle test. In order to
assess whether electrode displacement occurred during testing, all EMG data
collected during the end isometric muscle test were screened for discernible
84
drops or rises in intensity. If a noticeable change was evident for a particular
insertion, then all acquired runs were examined to determine where the drop or
rise occurred. A ll trials subsequent to that point then were normalized by the
end muscle test EMG.
Intensity of VL, VMO, and VML activity was assessed at points in the
range of motion that corresponded to the angular position at which the MR
images were obtained. EMG intensity data were further analyzed to obtain
VL:VMO and VLVML ratios at the sam e points in the range of motion. Individual
ratios were calculated three times and then averaged. This was done to obtain
more reliable results as detained by reliability testing.
DATA ANALYSIS
BMDP statistical softwares was used for all data analysis. Prior to
analysis, descriptive statistics for all data were performed to determine the
means, standard deviations and normality of distribution for all variables. The
data were tested for normality of distribution using the Wilks-Shapiro W statistic.
A ll significance levels were set at p<.05.
Reliability of the KMRI measurements (sulcus angle, bisect offset, and
patellar tilt) was assessed using the intraclass correlation coefficient (ICC).
Multiple one way analyses of variance (ANOVA’ s) with repeated m easures were
used to compare each measurement between sessions at each designated
angle of knee flexion. The mean squares between subjects and the mean
squares within subjects were substituted into the ICC(1) equation described by
Bartko.6 This analysis was repeated for each measurement to obtain correlation
g. BMDP Statistical Software Inc., 1440 Sepulveda Blvd., Suite 316, Los Angeles,
CA 90025.
85
coefficients for each of the number of averaged values (ie. ICC’s were calculated
for the data obtained by averaging two m easures for both sessions, and was
compared to averaging three, four, and five m easures for both sessions).
A similar analysis was used to assess the day to day reliability of
obtaining the EMG ratios (VLVMO and VL:VML). As with the KMRI data,
multiple ICC’s were calculated for both ratios, for each designated knee flexion
angle. In addition, the effects of averaging multiple trials on the ICC values were
determined as described above.
To determine whether patellar indices or EMG ratios varied between
groups or angles of knee flexion, a 2 X 6 (group x angle) analysis of variance
(ANOVA) with repeated m easures on one variable (angle) was performed. This
analysis was performed for each of kinematic variable and EMG ratio.
Significant main effects were reported if there were no significant interactions. If
a significant interaction was found, the individual main effects were analyzed
separately.
Stepwise regression analysis using forward stepping was performed to
determine if any of the independent variables (VLVMO ratio, VLVML ratio, or
sulcus angle) were predictive of patellar tilt or horizontal displacement
(dependent variables). This analysis was repeated for both dependent variables
at each angle of knee flexion. In order to control for differences between the
subject populations, the grouping variable was forced into all regression
equations.
86
RESULTS AND DISCUSSION
QUANTIFICATION OF PATELLAR TRACKING USING KINEMATIC
MAGNETIC RESONANCE IMAGING
Results
Results of the reliability testing showed that the bisect offset index had the
highest ICC value (.85) when averaged across all angles of knee flexion (Figure
3-7). Patellar tilt demonstrated moderate reliability with an average ICC value of
.79 (Figure 3-7). The sulcus angle demonstrated the poorest reliability across all
positions (ICC=.67)(Figure 3-7). In addition, it was determined that taking the
average of two measurements produced adequate ICC values for patellar tilt
and bisect offset, while the average of four measurements was necessary to
obtain consistent results for the sulcus angle (Table 3-2).
Table 3-2
MRI Measurement Reliability: Intraclass Correlation Coefficients
(averaged across all angles of knee flexion)
# of averaged
m easurements Sulcus angle Tilt___________ Bisect offset
■■■IIIBlllBIIlllBiiilillllllSilllllilliillillBillfiBlBlliBWl
87
s
9
— I
2
5
I
a *
B IS E C T O F F S E T
ICC=.85
so
DAY 1
DAY 2
K N E E FL E X IO N (degrees)
2
§
T I L T
o - -
ICC=.79
-lO- ■
DO AO ID
K N E E FL E X IO N (degrees)
S U L C U S A N G LE
t r>
u i t no- -
at
&
a
ICC=.67
im V s ~ " Jo 5b 66 56 56 56 16 ife 4”
K N E E FL E X IO N (degrees)
DAY 1
DAY 2
DAY 1
DAY 2
Figure 3-7. Reliability results (between day one and day two) for the bisect
offset, patellar tilt and sulcus angle measurements in seven normal subjects.
The intraclass correlation coefficient (ICC) reported for each index is the average
ICC across all angles of knee flexion. Data presented for all indices represent
the average of five measurements at each angle of knee flexion.
88
Results of the bisect offset calculations showed that slightly more than
half of the patella was lateral to the center of the sulcus at all angles of knee
flexion (Figure 3-8). During knee extension, from 45 degrees to 18 degrees, the
patella demonstrated subtle medial motion with a reversal towards increasing
lateral movement from 18 degrees to full extension (Figure 3-8). Maximum
lateral displacement was evident at 45 degrees (57 percent of the patella lateral
to the midline), while minimum lateral displacement occurred at 18 degrees (51
percent of the patella lateral to the midline). On the average, 54 percent of the
patella was lateral to the center of the sulcus at full extension.
60T
U J
L L I
60--
48
60 46 40 36 30 26 20
K N EE FLEX IO N (degrees)
16 10
Figure 3-8. Bisect offset in normal subjects (reported a percentage of patellar
width lateral to the midline) from 45 to 0 degrees of knee flexion. Error bars
represent one standard error (of the mean).
89
Measurement of patellar tilt revealed lateral tilting at all angles of knee
flexion. During knee extension, the magnitude of lateral tilt decreased steadily
from a maximum value of nine degrees at 45 degrees of flexion to a minimum
value of four degrees at full extension (Figure 3-9).
Figure 3-9. Patella tilt in normal subjects from 45 to 0 degrees of knee flexion
(reported in degrees). Positive values indicate lateral tilt. Error bars represent
one standard error (of the mean).
In contrast, the sulcus angle demonstrated increasing values throughout
the range of motion. As the knee extended, the sulcus angle at the midsection
of the patella progressively increased from 139 degrees at 45 degrees of flexion
to 148 degrees at full extension (ie. became progressively more shallow) (Figure
3-10). An example of a normal series of axial images is presented in Figure 3-11.
3
Q :
t---------- &
60 48 40 36 30 36 30 16 10
K N E E FL E X IO N (degrees)
90
K N E E FL E X IO N (degrees)
Figure 3-10. Sulcus angle in normal subjects from 45 to 0 degrees of knee
flexion (reported in degrees). Error bars represent one standard error (of the
mean).
C ase Reports
The three patellofemoral pain subjects evaluated in this study displayed a
wide range of patellar tracking patterns. Patient #1 displayed a normal
kinematic pattern for both patellar tilt and displacement, as well as a normal
sulcus angle throughout the range of motion (Figures 3-12, 3-13, 3-14). Patient
# 2 presented with substantial tracking abnormalities, with values of 19 degrees
of lateral tilt and 85 percent of the patella lateral to the midline evident at full
extension (Figures 3-12, 3-13, 3-14). This patient also demonstrated an
increased sulcus angle reaching 155 degrees at 0 degrees of flexion (Figure 3-
14). The kinematic image series for Patient # 2 is presented in Figure 3-15.
91
45 degrees 3fi degrees
27 degrees
9 degrees
Figure 3-11. Example of a series of axial plane images obtained from a normal
subject from 45 to 0 degrees of knee flexion. The patella is evenly centered
within the femoral trochlea throughout the range of motion.
92
Patient # 3 demonstrated normal values for sulcus angle, patellar tilt, and
horizontal displacement at 45 degrees of flexion, however, as knee extension
progressed, severe deviations were evident. At zero degrees of flexion, the
maximum horizontal displacement was 125 percent of the pateila lateral to the
midline. In addition, there was 35 degrees of lateral tilt (Figures 3-12, 3-13, 3-
14). This patient also presented with convexity of the femoral sulcus as evident
by a value greater than 180 degrees at full extension (Figure 3-14).
Bisect Offset
0)
~ 120
*o
^P atien t #1
- a Patient #2
-•Patient #3
Normal
C 3
Q _
45
Knee Flexion (degrees)
Figure 3-12. Bisect offset data for normal and patellofemoral pain patients
(reported a percentage of patellar width lateral to the midline) from 45 to 0
degrees of knee flexion.
93
Patellar tilt
c o
a)
£
O)
a >
■a
2 0 -
< D
3
0 27 18 9 45 36
Knee Flexion (degrees)
Figure 3-13. Patella tilt data for normal and patellofemoral pain patients from 45
to 0 degrees of knee flexion (reported in degrees). Positive values indicate
lateral tilt.
Sulcus Angle
co
a >
180
9
a> 160-
O)
c
<
3 1401
O 1
3
CO
^-Patient #1
a Patient #2
-♦Patient #3
•■•Normal
120
18 45 36 27 9 0
Knee Flexion (degrees)
Figure 3-14. Sulcus angle data for normal and patellofemoral pain patients from
45 to 0 degrees of knee flexion (reported in degrees).
94
45 degrees
0 degrees
Figure 3-15. Example of a series of axial plane images obtained from Patient
# 2 from 45 to 0 degrees of knee flexion. The patella dem onstrates substantial
lateralization and lateral tilting as the knee extends. There is also progressive
shallowing of the femoral trochlea.
95
Discussion
The results of the reliability portion of this study indicate that consistent
data for horizontal patellar displacement, patellar tilt and sulcus angle can be
obtained between testing sessions using KMRI. According to the criteria
established by Richman and colleagues,144 the bisect offset index was found to
be very reliable throughout the arc of motion, while the patellar tilt and sulcus
angle measurements demonstrated moderate reliability. The fact that these
indices were repeatable is significant because consistency is necessary if such
data are to be interpreted with any degree of confidence.
Reliability for the sulcus angle improved substantially with the averaging
of multiple measurements, suggesting that the determination of this index based
on only one assessm ent is not adequate. As demonstrated in Table 3-2, the
ICC value doubled after four measurements were averaged, while the averaging
of five measurements did not further improve repeatability. Satisfactory ICC
values for both patellar tilt and bisect offset were obtained after one
measurement, with only a small improvement in the ICC being evident with the
averaging of two or more measurements. The fact that repeatability was
improved with multiple measures, may have implications for clinical or research
purposes, especially if tools with inherent measurement error are used (ie.
goniometers and rulers).
To date, only one other study has addressed the repeatability of
assessing patellofemoral joint relationships. Using static MRI techniques at 20
and 0 degrees of knee flexion, Koskinen et al.^1 reported that the m ost variable
data were the sulcus and the congruence angles. The inconsistency of these
two indices was attributed to the finding that the determination of the depth of
femoral sulcus at a fixed knee position varied significantly depending on the
96
location of the image slice. For example, significantly different results for both
the sulcus angle and the congruence angle were found with an image plane
inferior to the mid-section of the patella compared to a mid-patella image plane
at the sam e time frame.
For the purposes of dynamic imaging, the sensitivity of m easurements to
image location becom es problematic, as a fixed imaging plane can lead to
differences in the patella cross-section obtained at the varying angles of knee
flexion. To avoid this potential limitation, the dynamic sequence in the present
study was repeated at three different patellofemoral joint cross-sections,
therefore increasing the probability of a mid-patella view throughout the range of
motion. Despite obtaining multiple patellofemoral joint slices, the error
associated with obtaining identical imaging planes between testing sessions was
reflected by the lower ICC value for the sulcus angle compared to the other
indices. This finding supports the observations of Koskinen and colleagues,9"1
and suggests that care must be taken in using an index that is susceptible to
landmark variation based on the location of the image plane.
The consistent results obtained for both horizontal patellar displacement
and patellar tilt were most likely related to the m easurements utilized in this
study, as these indices were selected based on the fact that they are primarily
dependent on the posterior femoral condyles as the principle reference points
instead of the more commonly used anterior femoral condyles. This premise is
corroborated by the work of Fulkerson et al.,42 who determined that the use of
the posterior femoral condyles in measuring patellar tilt produced less variable
data compared to using the anterior femoral condyles as guides.
The use of patellofemoral indices that are independent of the anterior
femoral condyles was further supported by the sulcus angle results. Although
97
our average sulcus angle at 45 degrees corresponded nicely to that of Merchant
et al.,113 as well as Aglietti and colleagues,2 for the sam e measurement (139 vs.
138 and 137 degrees respectively), there was a progressive increase (flattening)
in the sulcus angle as the knee extended. This finding was also reported by
Kujala et al.93 using serial imaging from 30 to 0 degrees of flexion, and suggests
that the patella has a tendency to ride superiorly within the femoral trochlea
during extension. Given the continually changing limits of the trochlear groove
at the mid-section of the patella (ie. the depth of the trochlea and/or the height
of the medial and lateral femoral condyles), comparison of m easurem ents
obtained with these landmarks, at different knee flexion angles, would be
difficult. For example, data reported for indices such as the congruence angle
as described by Merchant113 have been strictly limited to a specific point in the
range (ie. 45 degrees of knee flexion). Whether this measurement would be a
valid indicator of patellar instability at different angles of knee flexion has not
been determined. In addition, Stanford et al.171 noted that small differences in
defining bony landmarks resulted in large measurement variation, which would
further limit the ability to use such measurements during dynamic imaging.
In a study of normal subjects evaluated by ultrafast computed
tomography, Stanford and colleagues171 reported another limitation in the use
of indices that incorporate the anterior femoral condyles as reference points.
These authors reported that at full extension, the patella would often ride
superiorly above the level at which the femoral sulcus could be defined. For
such subjects, measurements such as the sulcus angle, congruence angle, as
well as the lateral patellofemoral angle (as described by Laurin et al.95) were not
possible. Although our normal data showed a tendency for the patella to move
98
superiorly during knee extension, at no point was the femoral sulcus indefinable,
it was problematic however, in one of the patient examples (Patient #3).
The normal kinematic pattern of patellar horizontal displacement was
characterized by a slight medial shift, followed by a slight lateral shift at the end
range of extension. This pattern of movement is similar to the data of Nagamine
et al.121 as well as van Kampen and Huiskes,181 who studied patellar
kinematics in cadaver specimens. Although these authors analyzed a knee
flexion movement with the tibia fixed, their motion plots compared favorably to
the results of the present study (initial medial patellar shift followed by
progressive lateral translation with increasing knee flexion). In addition,
kinematic studies assessing patella movement during knee extension also have
found that the patella translates medially with decreasing knee flexion.90.-141
Since the amount of patella excursion in these studies was reported in
millimeters, comparison to the results of the present study can only be made
with respect to the overall pattern of movement, and not magnitude. If one
assum es however, a typical patellar width of 5.5 cm as reported by Fulkerson
and Hungerford,41 and given the average medial translation found in the current
study (six percent of patellar width), the estimated medial excursion would be
about 3 mm. This value corresponds closely to the data of van Kampen and
Huiskes,141 and Nagamine et al.,121 for knee flexion angles less than 45
degrees.
The tendency for the patella to translate medially during knee extension,
appears to be related to the geometry of the femoral trochlear groove. As the
lateral femoral condyle is larger and projects further anteriorly than the medial
condyle, the trochlear surface is angled slightly medial when viewed from distal
to proximal.181 As the knee extends, the patella is displaced anteriorly as a
99
result of the increasing protrusion of the anterior femoral condyles which reach
maximum prominence at approximately 15 degrees of flexion.130 Since the
patella is obliged to follow within the confines of the trochlear surface, it is logical
that the larger lateral patellar surface would have a tendency to guide the patella
medially.
The shift from medial to lateral patellar displacement starting at 18
degrees of flexion, can be explained by the screw-home mechanism. During
terminal extension of the non weight-bearing knee, external rotation of the tibia
occurs, as the result of the unequal curvature between the femoral condyles.168
As demonstrated by van Kampen and Huiskes,181 patellar motion is highly
influenced by rotation of the tibia with external rotation inducing a lateral patellar
shift. Additionally, as demonstrated by the sulcus angle data of the present
study, stability offered by the trochlear groove steadily decreases as the knee
extends. Given the combination of increased lateral force vector acting on the
patella as a result of the tibial rotation associated with the screw-home
mechanism, and the decreasing bony stability, it is not surprising that lateral
translation occurred at this point in the range.
The normal results for patellar displacement based on the bisect offset
were not supported by previous studies which used the sam e index.19* 171
Although the data of Brossmann et al.19 was consistent with the present study
at 30 degrees of flexion (approx. 55 percent of the patella lateral to the midline),
these authors observed that lateral shifting steadily increased with further knee
extension, with the magnitude of lateral displacement being substantially greater
than that currently reported (65 vs. 54 percent of the patella lateral to the
midline). Similarly, Stanford et al.171 reported a greater degree of lateral patella
displacement, however, the overall movement pattern was one of medial
100
translation (76 percent of the patella lateral to midline at 45 degrees, which
decreased to 70 percent of the patella lateral to the midline at full extension). It
should be noted however, that Stanford and colleagues171 only reported data at
two points within the range of motion (45 and 0 degrees knee flexion), and
therefore the true kinematic pattern of the patella was not fully assessed.
One reason for the larger amount of lateral patellar displacement
observed by both Brossmann et al.19 and Stanford et al.171 may be related to
the fact that these authors analyzed unresisted knee extension, while the current
study used a significant load. The different results obtained under resisted and
unresisted conditions indicate that increased quadriceps contraction has a
centering effect on the patella. This may be the consequence of increased
quadriceps tendon tension pulling the patella deeper with the trochlear groove,
therefore improving stability. By applying a resistive force which required
substantially more quadriceps force, a better representation of patellar
kinematics was most likely obtained.
Likewise, the loaded condition used in the present study m ost likely had a
significant effect on patellar tilt. On the average, five degrees of medial tilt
occurred from 45 to 0 degrees of flexion. Compared to the data of Brossmann
et al.,19 this pattern was similar in magnitude, but not in direction, as an average
of five degrees of lateral tilt was reported as the knee extended. Although a
different technique was used by these authors to measure patellar tilt, it is
probable that this discrepancy was the result of the varying degrees of
quadriceps contraction employed by the two studies. As dem onstrated by
Nagamine et al.,121 as well as Reider and colleagues,141 loading of the
extensor mechanism in cadaver specimens resulted in medial patellar tilting as
the knee moved from 90 degrees of flexion to full extension. These findings are
101
consistent with the data from the present investigation, and underscore the
contribution of quadriceps contraction to patellar kinematics. In general, it is
apparent that contraction of the vastus medialis is adequate in counteracting the
tendency of the vastus lateralis to laterally tilt the patella in normal subjects.
The patient examples presented in this study demonstrate the wide
variability in patellar tracking that may occur. Although these subjects had
similar symptoms and complaints, the patellar kinematics ranged from normal to
severe malalignment. It is interesting to note that the degree of patellar
maltracking appeared to be related to the depth of the femoral trochlea. For
example, Patient # 3 had the most extreme tracking abnormalities and a convex
femoral groove, whereas Patient #1 had both normal tracking and a normal
femoral sulcus depth (Figures 3-12, 3-13, 3-14). Patient # 2 presented with
substantial lateral tilt and displacement, as well as a modest degree of sulcus
shallowing compared to normal (Figures 3-12, 3-13, 3-14). These observations
are similar to those of Malghem and Maldague,107 who noted that subjects with
shallow trochlear grooves were most likely to demonstrate patellar subluxation.
This is logical as the bony support provided by the anterior femoral condyles
would be compromised. Further research looking at this relationship under
dynamic conditions is warranted, and may provide information regarding the
influence of trochlear morphology on patellar kinematics.
102
FACTORS CONTRIBUTING TO ABNORMAL PATELLAR TRACKING
PATTERNS IN SUBJECTS WITH PATELLOFEMORAL PAIN
Results
Reliability
The day to day reliability of KMRI data using the procedures described
above, has been previously reported to be satisfactory.'135 Results of the
reliability testing in obtaining the VLVMO and VL:VML ratios showed moderate
consistency across all angles of knee flexion (ICC values of .61 and .64
respectively). In addition, it was determined that averaging the data obtained
from three trials produced the most consistent results (Table 3-3).
Table 3-3
EMG Ratio Reliability: Intraclass Correlation Coefficients
(averaged across ail angles of knee flexion)
# of averaged
measurements VLVMO VL:VML
IljSlllSlllllilll
lillllillilllBl
l l i l l l l l l l l l l l l l l
■■■■■■III
llillllllllllllll!
■ ll illlil!
l l l l l l l l l l i S l l l l l B l l I l I B
103
Kinematic Magnetic Resonance Imaging
A statistically significant difference was found in patellar tilt between the
two groups (significant group effect, no interaction). Compared to normal, the
PFP subjects demonstrated a greater degree of lateral patellar tilt when
averaged across all angles of knee flexion (10.7 degrees vs. 5.5 degrees;
p< .02) (Figure 3-16). The largest difference between the two groups was seven
degrees (PFP: 11.7 degrees vs. normal: 4.7 degrees) which occurred at 27
degrees of knee flexion.
w
O
< D
O)
a>
■o
14
1 2 -
10--
8 -
6-
3-
60 46 40 38 30 26 20
NORMAL
3“ P F P *
K N E E F L E X IO N (degrees)
Figure 3-16. Comparison of patellar tilt between the patellofemoral pain (PFP)
and normal groups from 45 to 0 degrees of knee flexion. Positive values indicate
lateral tilt, in d icates lateral patellar tilt for the PFP group greater than the normal
group when averaged across all knee flexion angles (p<.02). Error bars
indicate one standard error.
104
In contrast, there was no significant difference in the bisect offset between
the PFP and normal groups (no group effect or interaction; Figure 3-17). When
averaged across all knee flexion angles, the bisect offset for the PFP group was
57.9 percent of the patella lateral to midline, compared to 53.8 percent of the
patella lateral to midline in the normal group.
u i
Z
_j
Q
1
2
_ j
£
ui
§
5
£
63'
68 - -
6 6 -
63-
60--
46
60 46 40 36 30 26 20 16 10
J— NORMAL
3— PFP
K N E E F L E X IO N (degrees)
Figure 3-17. Comparison of bisect offset between the patellofemoral pain (PFP)
and normal groups from 45 to 0 degrees of knee flexion. Error bars indicate one
standard error.
There was an overall trend of an increased sulcus angle (ie. decreased
depth) in the PFP group when averaged across all angles of knee flexion (149.4
105
degrees vs. 144.6 degrees; Figure 3-18), however, this was not statistically
significant (p=.06). The largest difference between the two groups was 8.5
degrees, which was evident at 18 degrees of flexion (PFP: 154.4 degrees vs.
normal: 145.9 degrees).
O ) 166--
160- -
141--
'is'
> — NORMAL
PFP
K N E E F L E X IO N (degrees)
Figure 3-18. Comparison of sulcus angle between the patellofemoral pain
(PFP) and normal groups from 45 to 0 degrees of knee flexion. Error bars
indicate one standard error.
Dynamic EMG
When averaged across all knee flexion angles, there was an overall trend
towards a greater VLVMO ratio in the PFP group (1.85 vs. 1.17; Figure 3-19),
but, this was not statistically significant (p=.08). The largest mean difference for
106
this ratio was 1.19 which occurred at 45 degrees of flexion (PFP: 2.35 vs.
normal: 1.16).
3. Or
2 . 8--
2 . 0 -
o
5 1 . 4
>
> — NORM AL
3— PFP
l.O" -L
G- . 6- 1--------- 1 --------- 1 --------- 1 ---------1 --------- 1 --------- 1 --------- 1 --------- 1 -------- 4 -------- 4
80 48 40 36 30 26 20 16 10 g f i
K N E E F L E X IO N (degrees)
Figure 3-19. Comparison of the vastus lateralis:vastus medialis oblique
(VLVMO) electromyographic ratio between the patellofemoral pain (PFP) and
normal groups from 45 to 0 degrees of knee flexion. Error bars indicate one
standard error.
There was no significant difference in the VL:VML ratio between the PFP
and normal subjects groups (no group effect or interaction; Figure 3-20). When
averaged across all angles of knee flexion, the mean VL:VML ratio for the PFP
group was .78 compared to .94 for the normal subjects.
107
1.4-r
1. 2*-
0
1 ' - 0-
— I
«E 0.8-
>
■ *
— I
^ 0 . 6 -
0 • ^ 'f | | -f— | j - - i - I t i _ i
60 48 40 3 6 3 0 28 30 16 lb 3 6
K N E E F L E X IO N (degrees)
Figure 3-20. Comparison of the vastus lateralis:vastus medialis longus
(VL:VML) electromyographic ratio between the patellofemoral pain (PFP) and
normal groups from 45 to 0 degrees of knee flexion. Error bars indicate one
standard error.
Relationship between EMG ratios, sulcus angle and patellofemoral joint
congruency
All r-values reported below represent partial correlations, as the grouping
variable had been forced into all regression equations. In general, the partial
correlations between the dependent and independent variables ranged from -.48
to .74 for bisect offset (Table 3-4) and patellar tilt (Table 3-5).
— NORMAL
— PFP
108
T able 3-4
Partial Correlations for Bisect Offset
(after controlling for group)
Independent
Variable 45
Knee flexion angle
36 27 18 9 0
VLVMO ratio ,03 .10 -.04 .14 -.23
VL:VML ratio “.02 -.22 -.48* -.25 ■ -,31 .30
Sulcus angle -.09 -.04 .10 ,25 ,41* .74*
VL=vastus lateralis, VMO=vastus medialis oblique, VML=vastus
longus. in d icates significant predictor of bisect offset (p< .05)
medialis
Table 3-5
Partial Correlations for Patellar Tilt
(after controlling for group)
Independent
Variable 45
Knee flexion angle
36 27 18 9 0
VLVMO ratio ,06 .14 .17 .02 -.22 13 :!
VLVML ratio -.23 -.40* -.48* -.42* -.45* .37*: f
Sulcus angle -.16 -.18 .13 .45 .56*. <74* : :
VL=vastus lateralis, VMO=vastus medialis oblique, VML=vastus medialis
longus. in d icates significant predictor of patellar tilt (p< .05)
The only significant EMG predictor of the bisect offset index was the
VL:VML ratio at 27 degrees of flexion (r=-.48) which accounted for 24 percent
(R2) of the variability (Figure 3-21). The sulcus angle was a significant predictor
of the bisect offset at nine degrees of knee flexion (r=.41), however, it was a
109
stronger predictor of bisect offset at zero degrees (r=.74; Figure 3-22). These
correlations accounted for 21 percent and 55 percent of the variability
respectively.
2.5
o
2
i
_ i
>
■ ■
— I
>
1.5 -
□ PFP
a NORMAL
r= -.48
0 .5 -
40 50 80 30 70 60
Bisect offset
Figure 3-21. Relationship between the vastus lateralisivastus medialis longus
(VL:VML) electromyographic ratio and bisect offset (percent of patella lateral to
midline) for both patellofemoral pain (PFP) and normal subjects at 27 degrees of
knee flexion. (r=.-48; p< .05)
no
130 j
120 -
110
100 -
9 0 - o PFP
a NORMAL
a
o
c o
C D
7 0 -
5 0 -
r=.74
130 140 150 160 170 180 190 200 210
Sulcus angle
Figure 3-22. Relationship between the sulcus angle (decrees) and bisect offset
(percent of the patella lateral to midline) for both patellofemoral pain (PFP) and
normal subjects at zero degrees of knee flexion, (r=.74; p < .05)
The VL:VML ratio was the only significant predictor of patellar tilt at 36
degrees (r=-.40, R2 =.16) and 27 degrees of flexion (r=-.48, R2 =.24). At 18
degrees of flexion, both the sulcus angle (r=.45) and the VLVML ratio (r=.42)
were significant predictors of patellar tilt, and together with the grouping variable,
improved the overall regression coefficient to .68 (R2 =.46). Similarly, these
sam e variables were predictive of the bisect offset at nine degrees of flexion
(sulcus angle: r=.56, VLVML ratio: r=-.45), and combined with the grouping
factor, significantly strengthened predictability (r=.75, R2 =.56). At full
i n
extension, the sulcus angle was the only significant predictor of patellar tilt
(r=.74), and accounted for 55 percent of the variance (Figure 3-23).
I d
( 3
CL
3 5 -
30 -
25
2 0 -
1 5 -
1 0 -
5 -
□ PFP
a NORMAL
□□
- 5 -
- 10 -
r=.74
130 140 160 170 180
Sulcus angle
Figure 3-23. Relationship between the sulcus angle (degrees) and patellar tilt
(degrees) for both patellofemoral pain (PFP) and normal subjects at zero
degrees of knee flexion. (r=.74; p<.05). Positive values of patellar tilt indicate
lateral tilting.
D iscussion
The bisect offset data obtained for both groups indicated that the patella
w as lateral to the midline throughout the range of motion. On the average, the
PFP subjects demonstrated greater lateralization at all angles of flexion,
however, this was not statistically significant. The normal kinematic pattern for
112
the bisect offset was characterized by slight medial displacement between 45 to
18 degrees of knee flexion, followed by subtle lateral displacement as the knee
extended from 18 to 0 degrees. This pattern of movement is consistent with that
previously described as a frontal plane "C" curve.68 Although, the average
bisect offset pattern of the PFP group was similar to that of the normal subjects
in the 45 to 27 degree range, as the knee continued to extend, there was a
reversal to a progressively more lateral alignment. The largest difference
between groups was evident at zero degrees (62 vs. 54 percent of the patella
lateral to the midline), which coincides with the clinical findings of Fulkerson and
Hungerford41 who reported that patellar subluxation typically occurs during
terminal knee extension.
The bisect offset data of the PFP group demonstrated large variability at
18, 9, and 0 degrees of flexion. At these angles, the standard deviations were
approximately two to three times that of normal, indicating that these subjects
exhibited a wide range of horizontal patellar displacement (Figure 3-17). At zero
degrees for example, 21 percent of the PFP subjects had a bisect offset greater
than two standard deviations of the normal group, while 61 percent had a value
within one standard deviation of normal. These results are in agreem ent with the
data of Schutzer et al.,152 who reported that patellar lateralization was evident in
only 50 percent of their subjects. Similarly, these findings support the work of
Shellock et al.155 who reported only 26 percent of their patient population
demonstrated lateral subluxation of the patella. Although the data of Shellock
and colleagues165 were based on qualitative MRI assessm ent, the results of
these previous studies, as well as the data of the present investigation, indicate
that excessive lateral displacement of the patella is not a universal finding in this
113
population. Given as such, the role of patellar subluxation as the primary cause
of PFP may be questioned.
Compared to the bisect offset data of Brossmann et al.,19 the magnitude
of lateral displacement of the current PFP group was substantially less. Using
motion triggered MR imaging, these authors noted that lateral patellar
displacement in their patient population increased linearly during knee
extension, with mean values ranging from 60 percent of the patella lateral to the
midline at 30 degrees, to 80 percent of the patella lateral to the midline at full
extension. In contrast, the maximum bisect offset for the PFP group in the
present study was only 62 percent of the patella lateral to the midline. One
explanation for this discrepancy may have been the difference in the two patient
populations. For example, Brossmann and colleagues19 only studied
individuals with recurrent patellar dislocation and subluxation, while the present
study evaluated a much broader range of subjects, using reproducible
symptoms as the primary inclusion criteria. By limiting their patient group to
those with documented abnormalities, it is not surprising that the bisect offset
values were much larger.
Another potential reason for the difference in the bisect offset data
reported by Brossmann et al.19 may have been related to the type of load used.
These authors assessed active knee extension with no resistance, while the
current investigation used an external load, which placed a substantial demand
on the extensor mechanism. It is possible that the resisted quadriceps action
had more of a centering effect on the patella, seating it deeper within the femoral
sulcus. Given these methodological differences, the bisect offset data of
Brossmann and colleagues.19 may not be representative of the entire spectrum
114
of PFP and caution must be made in generalizing these results to this
population.
The patellar tilt data showed that the patella was laterally tilted throughout
the range of motion in both groups, with the PFP subjects demonstrating
significantly greater magnitudes when averaged across all knee flexion angles.
This result indicates that excessive lateral tilt is a more frequent radiological
finding in PFP compared to lateral displacement or subluxation.
The normal subjects demonstrated an overall pattern of decreasing lateral
tilt as the knee extended, which is consistent with findings obtained with cadaver
specim ens.64-88 The average tilt values for the PFP subjects however,
remained fairly consistent across all knee flexion angles. This is in contrast to
the patient data of Brossmann and colleagues, 19 which showed an overall
tendency towards progressive lateral tilt as the knee extended. Such a trend
was evident in only 27 percent of the current PFP subjects, which suggests that
this should not be considered the dominant motion pattern. This kinematic
discrepancy could have been the result of the varied subject populations
between these two studies, as well as the different measurement technique
employed to determine patellar tilt.
The sulcus angle, as measured in this study, was representative of the
depth of the femoral trochlea at the mid-section of the patella. In general, there
w as a trend towards a more shallow groove in the PFP group when averaged
across all knee flexion angles. It is evident from these data however, that
although the two groups had similar sulcus angles at 45, 36 and 27 degrees of
flexion, a substantial increase (loss of depth) was observed in the PFP group as
the knee extended beyond 27 degrees. This increase in the sulcus angle is
similar to that reported by Schutzer et al.,152 as well as Kujala et al.,94 and
115
suggests that bony stability at end range extension is compromised in this
population.
The EMG data obtained from the normal group were fairly consistent, with
the VL:VMO and VLVML ratios averaging 1.16 and .95 respectively across all
knee flexion angles. This finding is consistent with previous studies which have
reported that activity of the VMO relative to the VL in normal subjects is
approximately 1 :1 .22,169
The EMG data obtained from the PFP population, however, showed
much greater variability. Although not statistically significant, the average
VL:VML ratios for the PFP group were lower than that of the normal subjects
(indicative of greater VML activity relative to the VL), across all knee flexion
angles. In contrast, the VL:VMO ratios of the PFP group were greater than
normal between 45 and 18 degrees of knee flexion, with the activity of the VL
being twice that of the VMO. As the knee extended beyond 18 degrees
however, there was a trend towards increasing VMO activity relative to the VL,
with the average ratio at zero degrees being 1.2. Despite the large differences in
the VL:VMO ratio at the higher knee flexion angles, there was not a significant
difference between groups when averaged across all knee flexion angles. This
w as the result of the high variability demonstrated by the PFP subjects
throughout the range of motion (approximately two to three times greater than
normal).
Stepwise regression analysis of the EMG, sulcus angle and
patellofemoral joint kinematics found that the sulcus angle w as the best
predictor of patellar tilt at 18, 9 and, 0 degrees, as well as the bisect offset at 9
and 0 degrees. Although the VL:VMO ratio was not predictive of patellar motion
at any point in the range of knee flexion, the VL:VML ratio was found to be a
116
significant predictor of patellar tilt at 36, 27, 18, and 9 degrees, as well as the
bisect offset at 27 degrees of flexion.
All significant correlations involving the VLVML ratio were negative. For
example, subjects with lower VL:VML ratios (increased VML activity) were found
to have greater degrees of lateral patellar displacement and tilt, while subjects
with higher VLVML ratios (decreased VML activity) had less severe
abnormalities. Although not statistically significant, a similar relationship was
observed between the pattern of VMO activity and the bisect offset. In general,
a s the average VMO EMG was increasing relative to the VL, the patella was
demonstrating progressive lateralization. These EMG results suggest an active,
but inadequate effort to correct patellar alignment.
The finding of VML activity being significantly related to patellar
malalignment, indicates that this structure was recruited more readily than the
VMO in an attempt to control patella position. This was reflected by the finding
that VML activity remained consistent throughout knee extension, while the
activity of the VMO became more pronounced only at the end range. This
observation may be related to the fact that the VML is primarily a knee extensor,
while the VMO is much less efficient in this role as a result of its more oblique
fiber alignm ent." Since knee extension was the primary movement performed,
it is logical that the VML would be recruited more readily in order to accomplish
this task. Furthermore, this increased recruitment would indicate that the VML is
a more significant counterforce to the VL during this type of activity. On the
other hand, the fact VMO EMG became more pronounced at the end range of
extension, emphasizes the function of this structure in assisting the VML in
providing patella stability, as it was at this point in the range were maximum
lateral patella displacement occurred. The difference in the neuromuscular
117
recruitment patterns of these two muscles stresses the varied roles these
structures play in contributing to patellofemoral joint mechanics.
These results also illustrate the limitations associated with the use of EMG
ratios as indictors of patellofemoral joint kinematics. Although normalized EMG
data are useful in measuring relative levels of activation between muscles (ie.
intensity of effort), such information is not indicative of muscular force or
"muscular balance" as is commonly assum ed. This was clearly reflected by two
findings: 1) decreased activity of the VML or VMO, was not an indicator of
patellar malalignment, and 2) despite increased recruitment, the VML was not
effective in stabilizing the patella. Without considering factors such as muscle
cross-sectional area and angle of insertion of the various muscle fibers, it would
appear that EMG has limited use in determining the effective muscle force on the
patella. In light of these findings, it is apparent that EMG ratios cannot be used
to predict patellar stability, and, therefore, their use in the assessm ent of
patellofemoral disorders appears to have little value.
The fact that the sulcus angle was the strongest predictor of lateral
patellar displacement and lateral patellar tilt at end range underscores the
importance of the bony anatomy in contributing to patellar stability, and would
appear to explain the clinical manifestation of patellar subluxation during terminal
knee extension. This association was evident in the PFP data, where it was
observed that the point at which the sulcus angle began to deviate from normal
(approximately 27 degrees) was at the sam e point in which the lateral
subluxation becam e more pronounced (Figures 3-17, 3-18). The finding that
more than half of the variability in patellar tilt and displacement could be
explained by the sulcus angle at zero degrees (R2=.55), further em phasizes this
relationship, and supports the observation of Brattstrom17 that a shallow
118
femoral sulcus is the predominate predisposing factor with regard to patellar
malalignment.
During knee extension, the sulcus angle of the normal group increased
an average of 10 degrees, indicating that the patella was moving to a more
shallow portion of the femoral trochlea. Since the patella migrates superiorly as
the knee extends,46> 154 this observation would suggest that the bony stability
afforded by the cranial portion of the trochlear groove is less than that provided
by the caudal portion. This hypothesis is supported by the findings of Malghem
and Maldague1°7 who reported that the depth of the proximal trochlear groove
(as determined by lateral radiographs) was less than the middle portion in
normal subjects.
In the PFP group, the average increase (flattening) of the sulcus angle
during extension was almost twice that of normal (19 degrees). While this is
clearly indicative of compromised patellar stability, the etiological factor behind
this finding is not entirely evident. For example, there are two possible
explanations for the increase in the sulcus angle: 1) dysplasia of the cranial
portion of the femoral trochlea, and 2) superior migration of the patella to a
position above the trochlear groove (ie. patella alta). Although both of these
alternatives are possible, it is difficult to separate the effects of each with regard
to patellar tracking. In fact, data presented by Hvid and colleagues7'* suggest
that both findings are typically found in conjunction with each other. However,
without knowing the vertical position of the patella within the femoral trochlea, it
would be difficult to ascertain whether an increased sulcus angle was the result
of dysplasia, patella alta or a combination of both. This would require further
radiological evaluation, using lateral view techniques that have been described
119
to asse ss trochlear dysplasia51-107 and patella alta,13-26-77 or serial axial views
to determine the exact position of the patella within the tochlear groove.155
Despite the fact that the KMRI data collected in this study were limited in
assessing the exact vertical position of the patella, some general information
was gained. For example, it was apparent in a number of cases (22 percent)
that the patella was superior to the femoral trochlea, which would be indicative of
patella alta. As seen in Patient #3 (Figure 3-24), the patella is situated on the
shaft of the femur, well above the level of the femoral condyles. In contrast,
Patient # 2 (Figure 3-24) demonstrates a relatively shallow trochlear groove,
however the posterior femoral condyles are still visible, suggesting that this
image section was not above the level of the femoral trochlea. Therefore, an
argument could be made that the diminished sulcus depth in this subject was
more likely the result of trochlear dysplasia.
It is apparent from the results of this study that the position of the patella
within the femoral trochlea is an important factor with regard to patellar tracking
and stability. These findings may have significant clinical implications as
restoring normal patellar tracking is a primary goal of both conservative and
surgical treatment of PFP. For example, if patellar tracking is primarily dictated
by bony structure, then treatment procedures that address only soft-tissue
components (such quadriceps strengthening or a lateral retinacular release)
may have limited success. Likewise, the long term success of a procedure such
as a distal realignment may depend on whether or not the patella can be
relocated within the bony confines of the trochlea.
120
Normal (18 degrees)
Patient #1 (0 degrees!
Patient #2 (18 degrees)
Patient #3 (0 degrees)
Figure 3-24. Axial plane images obtained from one normal subject and three
patients with patellofemoral pain. The normal subject and Patient #1 present
with a centered patella within the trochlear groove. Patient # 2 dem onstrates a
moderate degree of lateral displacement ana lateral tilting as well as a relatively
shallow trochlear groove. In Patient #3, the patella is positioned well above the
trochlear groove and there is extreme lateral displacement and lateral tilting of
the patella.
121
SUMMARY
The results of these investigations demonstrate that resisted KMRI can
give reliable information regarding patellar tracking patterns. In addition, the
normal data obtained through these methods correspond to previously reported
results, and appear to give a more physiologic representation of patellofemoral
joint kinematics compared to other radiological techniques.
Overall, the PFP group displayed wide variability in patellar kinematic
patterns. When compared to normal, the PFP group demonstrated a significant
increase in lateral patellar tilt, however, only a trend towards increased lateral
patellar displacement was found. This finding indicates that excessive lateral
patellar tilt was a more frequent finding in this population, and suggests that
lateral subluxation of the patella may not be as strongly associated with PFP as
previously believed.
In addition, it was determined that the sulcus angle was a strong predictor
of both patellar tilt and displacement during terminal knee extension, which
supports the premise that bony structure is an important determinate of patellar
stability. Furthermore, VL:VMO and VLVML EMG ratios are indicators of
relative muscular effort (not muscular force), and are, therefore, not
representative m easures of dynamic patellar control. Thus, the use of such data
in the study of PFP must be questioned.
122
CHAPTER IV
PATELLOFEMORAL JOINT DYSFUNCTION AND ITS INFLUENCE
ON GAIT
Knee mobility and stability are major factors in the normal pattern of
gait.129 For the patient with patellofemoral joint dysfunction, walking may
present a significant challenge. Clinically, subjects with patellofemoral pain
(PFP) report limitations in ambulation, especially ascending and descending
stairs, and inclines. The discomfort associated with these activities usually
results in gait modifications which attempt to reduce muscular dem ands and
subsequent pain. These compensatory mechanisms, however, may be
deleterious to the patient.
While understanding abnormal gait patterns in this population is important
in restoring efficient movement and providing a basis for treatment, only a few
studies have explored this topic in depth 53 This chapter will provide a
comprehensive view of the three major components of gait (kinetics, kinematics,
and muscle activation patterns) in this population. The introduction, results and
discussion sections will be divided into three different topics: 1) Timing and
intensity of vasti contraction during gait activities, 2) Loading characteristics in
subjects with PFP, and 3) The influence of quadriceps strength and PFP on gait
characteristics and joint motion. These three topics will share a common
methods section.
123
INTRODUCTION
TIMING AND INTENSITY OF VASTI CONTRACTION DURING GAIT
ACTIVITIES
The most widely accepted mechanism of PFP is abnormal patellar
tracking, which can lead to excessive strain on both peripatellar retinacular
supports and the patellar articular cartilage.73 One hypothesis suggests that
patients with PFP have an imbalance between the primary dynamic patellar
stabilizers which results in lateral tracking and malalignment.105 In this model,
the lateral pull of the vastus lateralis (VL) is not adequately counteracted by the
medial pull of the vastus medialis oblique (VMO) and vastus medialis longus
(VML). Several studies have addressed this dynamic imbalance theory by
examining the electromyographic (EMG) activity of the VMO and VL in patients
with PFP.10,103,109,'169 while some previous investigations have found
significant differences in VMO and VL activity in this population,109* 169 others
have not.103* 116 This conflicting evidence implies that the magnitude of motor
unit activity may not be the sole contributor to dynamic patellar imbalance,
however direct comparisons of these studies are difficult owing to differences in
experimental technique and methods of assessing EMG data.
Asynchronous timing of vasti contraction has also been postulated as
contributing to patellar instability. In patients with PFP, the VL is hypothesized to
contract earlier than the VMO rather than simultaneously.132 This premise has
been incorporated into clinical treatment of PFP with the use of biofeedback and
muscle re-education. It is the purpose of such treatment to alter the timing of the
VMO and VL, focusing on initiating VMO contraction before the VL to counteract
any early laterally directed force on the patella.112
124
Evidence in support of the asynchronous timing hypothesis was
presented by Voight and Wieder,182 who found that activation of the VMO in
subjects with PFP was delayed compared to the VL during a monosynaptic
reflex (patellar tendon tap). These findings however, have been recently
disputed by Karst and Willett87 who reported no vasti timing differences in their
patient group. Although the conflicting data presented in these studies can be
explained by methodological differences, as well as the inherent variability of
EMG data, continued research is necessary to establish whether timing
differences do actually exist in this population. In addition, investigation of vasti
timing during functional activities is essential, as it is during such tasks that PFP
is typically reproduced. This information would contribute to the knowledge
base regarding the etiology of PFP, and would validate clinical assumptions
upon which current treatment protocols are based.
The purpose of this phase of the study was to test the hypothesis that
subjects with PFP would demonstrate EMG patterns consistent with that
proposed for compromised patellar stability (ie. delayed timing or reduced
intensity of the VMO relative to the VL). To accomplish this task, vasti activity
was assessed during various functional activities (level walking, stairs and
ramps). Knee joint motion also was recorded in order to document potential
compensatory gait mechanisms in this population.
LOADING CHARACTERISTICS IN SUBJECTS WITH PATELLOFEMORAL
PAIN
Studies on normal knee kinetics have indicated that the flexed posture
during initial loading represents the greatest demand on the joint as indicated by
an increased knee flexion moment and corresponding muscular
125
response.85-163 This knee flexion and subsequent eccentric quadriceps
contraction during loading is considered to be a primary shock absorbing
mechanism,129 as it is at this point of the gait cycle where the peak vertical
ground reaction forces occur.188 For the patient with PFP, the forces and
muscular response of loading present a significant challenge. A kinematic
analysis conducted by Dillon and colleagues33 found that subjects with PFP
demonstrated reduced stance phase knee flexion during level walking,
descending a 15 degree ramp. This compensatory behavior is logical as the
patellofemoral joint reaction force increases with knee flexion, and the
magnitude of quadriceps activity.28-69-169
Radin et al.139 reported that subjects with intermittent tibio-femoral pain
demonstrated decreased loading response knee flexion as well as a very rapid
initial rise of the vertical ground reaction force. It was postulated that this
reduction of knee motion contributed to an increased loading rate, which
indicated that the primary shock absorbing mechanism in these subjects was
muted. This is of significant concern, as it has been reported that the rate of
loading appears to be more predictive of osteoarthritic changes than the peak
loading force.138
Since subjects with PFP limit the amount of knee flexion in order to
minimize patellofemoral joint forces, this population may be at risk for increased
rates of loading and subsequent tibio-femoral joint pathology. This would be
particularly true if the knee flexion during loading response was reduced in this
population compared to normal persons. The purpose of the second phase of
this investigation was to test the hypothesis that subjects with PFP would
demonstrate differences in loading rates and force plate characteristics
126
compared to a group of normal controls during a self selected, free walking
velocity.
THE INFLUENCE OF QUADRICEPS STRENGTH AND PATELLOFEMORAL
PAIN ON GAIT CHARACTERISTICS AND JOINT MOTION
During the stance phase of gait, the knee is the principle determinant of
limb stability.129 The quadriceps muscles act as the primary stabilizer of the
knee, especially during loading response, when the knee flexion moment is the
greatest.85 Activity of this musculature is necessary to support the flexed knee
posture.61
Reduced knee flexion during loading response is generally a substitutive
action to limit joint forces, and is evidence of intrinsic knee pathology.129 For
example, pain and weakness are commonly associated with patellofemoral joint
pathology,38 and the avoidance of knee flexion during stance has been
previously reported in this population.33 Berchuck et al.12 utilized the term
"quadriceps avoidance pattern" in subjects with anterior cruciate ligament (ACL)
deficient knees to describe a gait pattern that minimized the knee flexion
moment during loading response, and therefore the demand of the knee
extensors. It appears that subjects with PFP may adopt a similar gait strategy by
reducing the patellofemoral joint reaction forces associated with increased knee
flexion and quadriceps activity. A quadriceps avoidance gait pattern may prove
to be deleterious to the patient with PFP however, as further quadriceps atrophy
may result from disuse. This may contribute to patellar instability which is
commonly believed to be at least partly the result of weakened dynamic
stabilizers.38,41,53
127
Andriacchi4 proposed that the quadriceps avoidance gait pattern was the
result of a subconscious protective mechanism to avoid excessive stresses to
the knee and that there was a reprogramming of the locomotor process. These
adaptations were believed to be the consequence of early experiences following
dysfunction, and not the result of repetitive stimulus that occurs during each gait
cycle.4
While gait patterns have been described for various knee pathologies
such as degenerative joint disease,1 5 2 > 172 rheumatoid a r th r itis ,5 2 ,8 9 ,1 7 2 a n c |
ACL insufficiency,12 little is known about subjects with PFP and the relationship
between pain and weakness. For example, are compensatory gait patterns a
result of pain, weakness, or both? Do functional gait adaptations associated
with patellofemoral joint pathology differ between subjects with varying degrees
of pain?
The purpose of the third phase of this study was to correlate the degree
of PFP and muscle weakness with stride characteristics and the amount of
loading response knee flexion during different gait conditions (level walking,
stairs, and ramps). Subjective functional assessm ents were also correlated to
actual gait characteristics. It was hypothesized that there would be a significant
correlation between either pain and weakness and the functional gait limitations
associated with PFP. This information will aid in guiding treatment programs to
better restore the function of these patients and provide insight to the ability and
adaptation of this population.
128
MATERIALS AND METHODS
SUBJECTS
Two groups of female subjects were recruited for the gait portion of this
dissertation. Only female subjects were studied owing to the higher incidence in
the general population (2:1; females vs. males), and the inherent biomechanical
differences between the sexes 80,98
Thirty-five female subjects between the ages of 14 and 46 years with a
diagnosis of PFP served as the experimental group, while 27 females between
the ages of 18 and 38 years with no history of knee pain or dysfunction served
a s the control group. Subjects for the PFP group were recruited from the
Southern California Orthopaedic Institute as well as local physical therapy
clinics, and were screened to rule out ligamentous instability, internal
derangement, patellar tendinitis, or large knee joint effusion. Only those
subjects meeting the following inclusion criteria were admitted to the
experimental group of this study:
1) pain (vague or localized) originating from the patellofemoral joint
articulation (only patient histories relating to overuse or indsidious onset were
accepted)
2) readily reproducible pain with at least two of the following functional
activities commonly associated with PFP:
a. stair ascent or descent
b. squatting
c. kneeling
d. prolonged sitting
e. isometric quadriceps contraction
129
PFP subjects were excluded from the study if they reported:
1) previous knee surgery
2) acute traumatic patellar dislocation
3) neurological impairment that would influence gait
The control group was recruited from the University of Southern
California, and Rancho Los Amigos Medical Center. These subjects were
selected based on the same criteria as the experimental group except that
subjects had no:
1) previous history or diagnosis of knee pathology or trauma
2) current knee pain or effusion
3) pain or discomfort with any of the activities previously listed
4) limitations that would influence gait
Of the 35 PFP subjects that met the criteria for gait analysis, 15 underwent
force plate testing for the loading characteristics segment, while 19 participated
in the portion of the study which assessed the relationship of pain and strength
on gait characteristics. Twenty-six PFP subjects participated in the
electromyographic (EMG) section of the study. Of the 27 control subjects that
were recruited, 10 underwent force plate analysis, while 19 participated in the
EMG and stride characteristics/strength portions of the study.
INSTRUMENTATION
Dynamic EMG
Dynamic EMG utilizing sterile, indwelling wire electrodes was employed to
record the timing and intensity of vasti muscle activity. The electrodes were
130
bipolar in configuration, and were made of nylon-insulated 50 micron wire
(nickel-chromium alloy). The wires were passed through the cannula of a 25
gauge hypodermic needle with the distal ends staggered and folded over the
needle tip as described by Basmajian and DeLuca7
After insertion into the muscle of choice, the wires were secured to a
reference (ground) electrode and the signals were fed directly into a differential
amplifier/FM radio transmitter unit.3 The differential amplifier had a common
mode rejection ratio of 60 dB. The EMG signals then were telemetered from the
transmitter to the receiver unit where the signal was band pass filtered (150-1000
Hz) and amplified again. The raw signal was sampled and digitized by a DEC
11 /23 data acquisition computer, b The sampling rate was 2500 Hz. The overall
gain of the system was 1000.
Motion Analysis
Motion analysis was performed using a computer aided video motion
analysis system (VICON).c This system utilized six cameras, each containing
infrared light emitting diodes (wavelength 940 nm). Two of the cam eras
contained lenses with a focal length of 12.5 mm, while the other four had lenses
with a focal length of 8 mm. The measurement volume of the system was 1.8
meters in height, 1.2 meters in width, and 4 meters in length. A schematic of the
cam era placement with respect to the measurement volume is presented in
Figure 4-1.
a. Biosentry Telemetr/ Inc., 20720 Earl St., Torrance, CA 90503
b. Digital Equipment Corp., 146 Main St., Maynard, MA 01754-2571
c. Oxford Metrics Ltd., Unit 14, 7 West Way Botley, Oxford OX20JB
131
Data Acquisition Field
------------6m----------------
Vfcon Acquisition Ftelc
i 4m------ — 3
1.2m
10m w alkw ay
Figure 4-1. Camera placement with respect to the data acquisition field and the
VICON system measurement volume.
Reflective markers (20 mm diameter spheres) placed at specific
anatomical landmarks were used to determine sagittal plane motion of the
pelvis, hip, thigh, knee and ankle. Determination of marker centroid and location
(x,y,z coordinates) was accomplished using the VICON/AMASS software.
Markers were identified manually and then digitized automatically using the
sam e software. The placement of a minimum of three markers on each body
segm ent allowed segment embedded coordinate system s to be defined. The
angular displacement of the distal segment was determined relative to the
proximal segment using Eulerian angles. Only the sagittal plane rotations were
reported in this study. The motion data were sampled at 50 Hz and recorded
digitally on a DEC 11 /73 computer,0 1
d. Digital Equipment Corp., 146 Main St., Maynard, MA 01754-2571
132
Force Plate Analysis
A Kistler0 force plate (Type 9281 -B) was used to record the ground
reaction forces during level walking. This multi-component force platform (41 x
61 cm) was a piezoelectric transducer capable of measuring the medial-lateral
(Fx), fore-aft (Fy), and vertical (Fz) components of the ground reaction force, as
well as the three components of the resulting moment vector (Mx, My, and Mz)
related to the origin of the coordinate system. The force plate measuring range
for Fz was -10 kN to 20 kN, while the measuring ranges for Fx and Fy were -10
kN to 10 kN. The frequency response of the force platform was 800 Hz.
The force plate was situated within the middle of the walkway with the
variegated pattern of tile flooring camouflaging its location. This reduced the
possibility of subject targeting during the gait trials.
Stride Analysis
Stride characteristics were recorded with a microprocessor based
Footswitch Stride Analyzer System.* This system consisted of compression
closing footswitch insoles, FM-FM telemetry and a memory/calculator unit. The
footswitches contained sensors at the heel, first and fifth metatarsal heads, and
the great toe which responded to compression equal to or greater than three
psi. The signals were conducted by a small cables to a battery powered
transmitter which telemetered the signals to the recording/calculator unit.
Automatic calculation of the stride variable were attained with the
microcomputer-based Stride Analyzer. Stride characteristics calculated from
this system included: velocity, stride length, cadence, single and double limb
e. Kistler Instruments Corp., 2475 Grand island Blvd., Grand Island, NY 14072
f. B & L Engineering, 8807 Pioneer Blvd., Suite C, Santa Fe Springs, CA 90670
133
support times, stance duration, swing duration, and gait cycle duration. The
quantified stride data were expressed in absolute values and as a percentage of
normal, with values being recorded in analog and digital printout form.
A 10 meter walkway was used for free and fast walking trials with data
being collected over the middle six meters. Photoelectric cells delineated the six
meter interval and were used to initiate and terminate data collection. Stair
analysis was done with a four step staircase with a slope of 33.7 degrees, a step
height of 20.3 cm and a tread depth of 30.5 cm. Ramp walking was assessed
with a 12 degree incline, 6.1 meters in length.
Torque Testing
Isometric knee extension torque was recorded using a LIDO isokinetic
dynamometer.9 The dynamometer contained a hydraulic unit controlled by a
computer, and was capable of torque assessm ent at speeds up to 400
degrees/seconds.
The dynamometer contained a variable length resistance arm, which fed
the effective lever arm of the patient back to the controller so a true biological
torque could be measured. Prior to testing, compensation for limb weight and
the effects of gravity were made automatically by the computer software
program. Torque data were recorded by the DEC 11 /23 computer at a rate of
2500 Hz.
Visual Analog Pain Scale
Subjective knee pain was recorded using the Visual Analog Scale (VAS).
The VAS was comprised of a 10 cm horizontal line, the ends of which defined
g. Loredan Biomedical Corp., P.O. Box 1154, Davis, CA 95617
134
the minimum and maximum of perceived pain. The patient placed a mark on the
line to indicate the intensity of pain. The amount of pain indicated on the line
was converted to a numerical value based on the distance (in centimeters) from
the minimal possible pain to the mark on the line. An example is given in Figure
4-2. The VAS has been shown by Chesworth23 to be a valid indicator of clinical
change in patients with PFP.
Figure 4-2. Example of Visual Analog Scale scoring. The "X " indicates the
subjects subjective level of pain which is 7 cm from the no pain base line. A pain
score of seven would be recorded for this subject.
Functional Assessment Questionnaire
To evaluate subjective symptoms and functional limitations in
patellofemoral disorders, a questionnaire developed by Kujala et al.92 was used
(Fig. 4-3), This functional assessm ent questionnaire (FAQ) contained 13
multiple choice questions, relating specifically to patellofemoral joint symptoms.
Scoring was based on a numerical scale depending on question response, with
som e items being weighted more than others. The maximum possible score
was 100, which represented no subjective pain and full functional ability. This
scoring system has been demonstrated to differentiate between different
classifications of anterior knee pain.92
7 cm
NO
PAIN
EX T R E M E
135
FUNCTIONAL ASSESSMENT SCALE
Name___________ Dale___
Age Knee: R/L Duration of Symptoms
1.Ump
a) None (5)
b Slight or periodical (3)
c) Constant (6)
2. Support
a) Full support without pain (5)
b) Pa(nfu((3)
c) Weight bearing impossible (0)
3. Walking •
a) Unlimited (5}
b) More than 2 km (3)
c t-2 km (2)
d Unable (0}
4. Stalre
a) No difficulty (10)
b) Slight pain when descending (8)
c) Pain when descending and asceti
d) Unable (0)
5. Squatting
aj No difficulty (5)
b) Repeated squatting paintu) (4)
c | Painful each time (3)
d) Possible with partial weight bear!
ef Unable (0)
6. Running
a) No difficulty (10)
b) Pain after more than 2 km (8)
c) Slight pain from start (6)
d) Severe pain (3)
e) Unable (0)
7. Jumping
a) No difficulty (10)
b) Slight difficulty (7)
c) Constant pain (6)
d) Unable (0)
I. Prolonged sitting with knees flexed
a
b
d
e;
9. Pain
No difficulty (10)
Pain after exercise (8)
Constant pain (6)
Pain forces to extend knees temporarily (4)
Unable (0)
a
b
c
d
e
10.Swa
a'
b
c
d
e
None (10)
Slight snd occasional (8)
Interferes with sleep (6)
Occasionally severe (3)
Constant and severe (0)
ling
None (10)
After severe exertion (8)
After dally activities (6)
Every evening (4)
Constant (0)
11. Abnormal painful kneecap movements
None (0)
Occasionally In sports activities (6)
Occasionally In daily activities (4)
At least one documented dislocation (2)
More than two dislocations (0)
12. Atrophy of thigh
None (5)
Slight (3)
Severe (0)
on deficiency
None (5)
Slight (3)
Severe (0)
c
13. Flex
Figure 4-3. Functional assessm ent questionnaire. Numbers in parentheses
indicate numerical score for a given response. From Kujala et al. Scoring of
patellofemoral disorders. J Arthroscopic ReiSurg, 1993; 9:159-163.
136
PROCEDURES
Dynamic EMG and comprehensive gait analysis was performed at the
Pathokinesiology Laboratory, Rancho Los Amigos Medical Center. Before
testing, all procedures were explained to each subject and informed consent
was obtained (all procedures for this portion of the study were approved for
human subjects by the Los Amigos Research and Education Institute of Rancho
Los Amigos Medical Center). PFP subjects were then asked to complete the
FAQ based on their current symptoms and limitations.
Subjects were appropriately clothed, allowing the hip, pelvis and thigh of
the involved limb to be exposed. This was done to accomm odate the
instrumentation and allow for the accurate recording of joint motion during gait.
Prior to gait analysis, maximal isometric knee extension torque and
subjective knee pain was recorded. Subjects were seated on the LIDO
dynamometer chair with the hips flexed to 90 degrees, and the knee flexed to 60
degrees. The axis of rotation of the dynamometer was then positioned in line
with the axis of rotation of the knee, with the resistance arm cuff placed proximal
to the malleoli. A velcro strap was placed across the pelvis to ensure proper
stabilization. Sixty degrees of knee flexion was used as this position has been
found to result in the greatest torque output in normal females,100 and provides
greater patellar stabilization within the trochlear groove.41 This would potentially
minimize quadriceps inhibition resulting from the pain associated with patellar
instability.
Isometric torque obtained during a five second maximal contraction was
then recorded. Verbal encouragement was given to all subjects during the trial.
At the conclusion of the torque test, the PFP subjects were asked to rate the
degree of knee pain experienced during the maximal contraction using the VAS.
137
Following the strength and pain assessm ent, subjects were instrumented
for dynamic EMG and gait analysis. Sterilized, fine wire electrodes were inserted
into the VMO, VML, VL, and the vastus intermedius (VI) using the technique
described by Basmajian and Deluca.7 After insertion of the electrode into the
muscle of choice, the needle was immediately removed. Electrodes were
secured within the muscle by having the subject go through full passive range of
motion followed by a maximal contraction. Wires were taped to the skin with a 2
cm loop to prevent dislodging during testing. The insulation of the exposed
wires was removed using tweezers coated with sandpaper, and the wires were
then wrapped around ground plates taped to the subjects lateral thigh.
Electrode placement was confirmed by local muscle contraction and the pull of
the patella induced by mild electrical stimulation.
After confirmation of electrode placement and signal integrity, five
seconds of EMG data was collected with the subject resting supine to determine
background noise at the time of recording. The EMG activity during a five
second maximal isometric contraction was then recorded with the subject
positioned on the LIDO as described above. This served as the maximal EMG
for normalization.
Prior to the gait trials, the footswitches were taped to both of the subject’s
bare feet. Reflective markers used to determine sagittal plane motion were
placed at the sacrum, anterior superior iliac spine bilaterally, greater trochanter,
anterior thigh, medial and lateral femoral condyles, medial and lateral malleoli,
anterior tibia, dorsum of the foot, fifth metatarsal head and the posterior heel
(Fig. 4-4).
One practice trial of both free and fast walking allowed the subject to
become familiarized with the instrumentation. EMG, joint motion and stride
138
characteristics were then assessed during free and fast level walking, ascending
and descending stairs and ascending and descending ramps. Force plate data
were collected for free walking only. A successful free walking trial was one in
which the foot of interest landed fully on the force plate. EMG, joint motion and
stride data were recorded simultaneously for each condition. One trial of data
was collected for level and ramp walking, while two trials of data were collected
for stair ambulation.
Following gait testing, the maximal isometric muscle test on the LIDO
dynamometer was repeated, with the maximal EMG being recorded. This was
done to ensure that the intramuscular electrodes had not been displaced during
the testing procedure.
Figure 4-4. Anatomical marker placement for motion analysis; frontal view (a)
and lateral view (b)
a
b
139
DATA MANAGEMENT
Dynamic EMG
Digitally acquired EMG data for all gait conditions were full wave rectified
and integrated over 0.01 second intervals. To allow for comparison of EMG
intensity between subjects and muscles, and to control for the variability of
electrode placement, EMG data were normalized to the EMG acquired during
the maximal isometric muscle test. Intensities were reported as a percentage of
maximum muscle test (%MMT).
In order to assess whether electrode displacement occurred during
testing, all EMG data collected during the end isometric muscle test were
screened for discernible drops or rises in intensity. If a noticeable change was
evident for a particular insertion, then all acquired runs were examined to
determine where the drop or rise occurred. A ll trials subsequent to that point
were then normalized by the end muscle test EMG.
Assessm ent of EMG timing (onset and cessation) was accomplished
through the EMG Analyzer software.*1 To average EMG data from multiple
strides and to facilitate comparison between subjects, the software program
normalized the stance phase of gait to 62 percent of the total gait cycle. This
value is considered to be representative of normal walking,- *29 and was
consistent with the average stance phase demonstrated by the subjects in this
study for all conditions.
The EMG analyzer software determined the onsets and cessations for all
packets of EMG that exceeded an amplitude of 5 %MMT. Packets of EMG
separated by an interval of less than five percent of the gait cycle were
combined. Only the initial packet of EMG (late swing to loading response) was
h. B & L Engineering, 8807 Pioneer Blvd., Suite C, Santa Fe Springs, CA 90670
140
assessed for onset, cessation and mean intensity for the six gait conditions.
Activity during this phasing is considered to be the normal timing of the vasti
during gait,129 and has been shown to be the most consistent, as well as
dominant EMG packet for the vasti during preliminary study. A ll EMG onsets
and cessations were reported as a percentage of the gait cycle (%GC).
Motion
Sagittal joint motion of the ankle, knee, hip, thigh, and pelvis was
calculated for all conditions tested. Raw motion data were filtered at 6 Hz using
a 4th order, Butterworth recursive filter. The data was then digitized and then
linearly interpolated to 0.01 second intervals. The stance phase of each stride of
motion collected was normalized to 62 percent of the gait cycle in order to
average data from multiple strides and different subjects. Maximum and
minimum motion for each of the phases of gait were analyzed for each joint.
Footswitch data were used to determine the phases of gait.
Force Plate Data
Force plate data were integrated at 0.001 second intervals. Figure 4-5
shows param eters chosen from the digital vertical ground reaction force data
used for analysis.
141
160
120
LU
150 0 600 450 300
TIME (m s)
Figure 4-5. Parameters used for force plate analysis. Fx represents the peak of
the heel strike transient, while Fj signifies the initial peaK of the vertical ground
reaction force. T1 indicates the time from initial contact to Fj.
The initial peak of the vertical ground reaction force (F-j) was identified
and normalized to body weight in order to facilitate comparison between
subjects. The time to peak vertical force (T^) was calculated by finding the time
between initial contact and the peak vertical force of weight acceptance.
The peak loading rate (PLR) was calculated by taking the slope of a
series of points along the initial upswing of the vertical force trace between heel
strike and the peak of F-p The number of data points (0.001 time intervals)
included for the peak slope calculation were those whose slopes were greater
than 50 percent of the maximum slope found. This standard was used in order
to obtain data points that were representative of the loading rate, while omitting
values that constituted slope discontinuity.
142
Typically 5 to 10 data points were used for each subject. Regression
analysis was performed on the series of data points to determine the actual
overall slope of the data. This overall slope was reported a s the PLR (Fig. 4-6).
LU
o
o
I L
—I
2
I "
o c
LU
>
' PEAK SLOPE
5 10 15 20 25
T IM E (ms)
Figure 4-6. Interval selection for the determination of peak loading rate. Figure
depicts the portion of the vertical ground reaction force analyzed (initial contact
to peak of the heel strike transient which is indicated by Fj). The interval with
the maximum slope is indicated by the small arrow. * indicates intervals whose
slopes are within 50 percent of the maximum slope. Vertical lines indicate 0.001
second intervals.
T orque
Torque data were integrated at 0.1 second intervals. The torque
produced by the limb weight (as determined by the gravity compensation test)
was added to the raw torque to account for the effects of gravity. The greatest
value over the five second trial was recorded for each subject. To control for the
effects of subject size, all torque data were normalized by body weight and
expressed in Nm/kg.
143
DATA ANALYSIS
BMDP statistical software* was used for all data analysis. Prior to
analysis, descriptive statistics for all data was performed to determine the
means, standard deviations and normality of all variables. The data were tested
for normality of distribution using the Wilks-Shapiro W statistic. All significance
levels were set at p < .05.
Subject characteristics (age, height, and weight) were compared between
groups using two sample t-tests. Since different groups of subjects were utilized
during this portion of the study, this analysis was repeated as necessary.
To determine whether EMG timing varied between groups or muscles, a 2
X 4 (group x muscle) two-way analysis of variance (ANOVA) with repeated
m easures on one variable (muscle) was performed. This analysis was repeated
for each condition for both EMG onset, cessation and mean intensity.
To determine whether stride characteristics differed between groups and
conditions, a 2 X 6 (group x condition) two-way ANOVA with repeated m easures
on one variable (condition) was performed. This analysis was repeated for each
stride characteristic. Stride length and cadence during stair ambulation were
omitted from the analysis owing to the limitation imposed on these param eters
as a result of the fixed stair height and depth.
Motion plots for each joint were screened visually to determine the
greatest discrepancies between groups, and to determine which phase of gait
these differences occurred. The joint motion for that particular gait phase was
then analyzed between groups and conditions, using a 2 X 6 (group x condition)
two-way ANOVA with repeated m easures on one variable (condition).
i. BMDP Statistical Software Inc., 1440 Sepulveda Blvd., Suite 316, Los Angeles,
CA 90025
144
For each of the two factor ANOVA tests described above, if a significant
interaction was identified, the analysis was repeated with the main factors being
considered independently of each other using either one-way ANOVA’s or
multiple two-sample t-tests. A Tukey’s post-hoc test was employed if
significance was found to determine the significant comparisons.
A one-way analysis of covariance (ANCOVA) was used to com pare the
peak vertical forces, time to peak and peak rate of loading between groups.
Walking velocity was used as the covariate. The appropriateness of using
velocity as a covariate was determined by assessing if a linear relationship
between the covariate and the dependent variables existed, and if the slope of
the regression line was similar for both groups.
Comparison of walking velocity, peak loading response knee flexion and
the position of the knee at initial contact between groups was done using a two
sample t-test. This analysis was performed for the loading characteristics
portion of this study.
To determine the relationship between PFP, quadriceps strength, and
FAQ score, the Pearson correlation coefficient was employed. Separate
analyses were used to assess the association between PFP and strength, PFP
and the FAQ score, as well as strength and the FAQ score.
Stepwise regression analyses were performed to determine if any of the
independent variables (pain, quadriceps strength or FAQ score) were predictive
of any of the stride characteristics or the amount of loading response knee
flexion (dependent variables). This analysis was performed for the PFP subjects
only and was repeated for all six of the walking conditions.
145
RESULTS AND DISCUSSION
TIMING AND INTENSITY OF VASTI CONTRACTION DURING GAIT
ACTIVITIES
Results
Subject Characteristics
No statistically significant differences were found between the
experimental and control groups for mean age or height (p> .05; Table 4-1).
There was a trend towards a greater average weight for the PFP group
compared to the normals (63.9 kg vs. 59.2 kg), however this also was not
significant (p=0.09; Table 4-1).
Table 4-1
Subject Characteristics
Ags.te?
mean
sd
range
Height (pm)
mean
sd
range
W e i g b L & g )
mean
■sd
range
25.6
7.1
(14-46)
165.1
10.4
(151.1-
63.9
9.8
(42.0-82.7)
27.5
4,7
(23-38)
165,3
7.7
(149.9-183.5)
59.2
7.5
(46.8-74.1)
.32
.92
.09
PFP= patellofemoral pain
146
Dynamic EMG
Onset
No statistically significant differences were evident in the onset of EMG
activity between vasti within either group (no muscle effects). This was
consistent for each gait condition tested (Figures 4-7, 4- consistent for each
across all conditions, the VMO and VL onset for the PFP group were very similar
(VMO: 90.6 %GC vs. VL: 90.2 %GC; Table 4-2). The sam e trend was seen in the
normal group (VMO: 88.3 %GC vs. VL: 87.7 %GC; Table 4-2).
Table 4-2
EMG Onset (% Gait Cycle)
Vastus Medialis Oblique (VMO) and Vastus Lateralis (VL)*
mean (sd)
Normal Patellofemoral Pain
VMO VL VMO VL
FR 87.8 (4.6) 87.3(4.8) 89.7 (5.7) m 3 (5.9) ; ; ■ ■
FT 84.6 (6.8) 83.3(5,5) 84,6 (7.5) (85.2(5.0) : :
AS 92.1 (8,6) 92.7(5.1) 95.3(1.9) \ • :i;8£3:{3;4) C
DS 86.2 (8.7) 85.5(7.7) . 88.7 (9.0) j;-8& 4;(9>6}: :
AR 92.3 (S.9) 90.7 (6.1) 96.0 (4.9) .9.3-4 (6,3). . ”,
DR 86.4 (7.7) 86.8(4.2) 89.8 (4.9) 93.4 (3,4);
; Mean
88.3 (7.0) 87.7 (5.6) 90.6 (5.7) 9 0 . 2 (6.g)
FR= free walking, FT= fast walking, AS= ascend stairs, DS= descend stair,
AR= ascend ramp, DR= descend ramp
*No significant differences were found between the VMO and VL within either
group for any of the conditions tested.
147
FREE
VI
VL
VML
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VL
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so 70 BO 90 10 20 30 40 SO 60
! " J '! I ! J I I I I 1 1 J
9tane«(i2%OC)
a t m r a
% Galt Cycla
| J Control [ 3 PataBoternoral Pain
Figure 4-7. Onset and cessation of vasti activity during free (top) and fast
(bottom) walking (expressed as a percentage of the gait cycle) for patellofemoral
pain and control subjects. Onsets are indicated by the right edge of the
horizontal bars. Error bars indicate one standard deviation from the mean. 0%
of the gait cycle indicates initial contact. (VI = vastus intermedius, VL= vastus
lateralis, VML= vastus medialis longus, VMO= vastus medialis oblique. *
indicates EMG onset for the patellofemoral pain group delayed compared to
normal in these muscles (p<.05).
148
ASCEND STAIR
VI
VL
VML
VMO
% Gall Cycla
|H Control PateOofamora) Pain
DESCEND STAIR
VI*
VL
VML*
VMO*
80 70 60 90 0 10 20 30 4 0 50 60
,= i b h i
% Galt Cycla
^ Control Patellofemoral Pain
Figure 4-8. Onset and cessation of vasti activity during ascending (top) and
descending (bottom) stairs (expressed as a percentage of the gait cycle) for
patellofemoral pain and control subjects. Onsets are indicated by the right edge
of the horizontal bars. Error bars indicate one standard deviation from the
mean. 0% of the gait cycle indicates initial contact. (V I = vastus intermedius,
VL= vastus lateralis, VML= vastus medialis longus, VMO= vastus medialis
oblique. * indicates EMG cessation for the patellofemoral pain group delayed
compared to normal in these muscles (p< .05).
i r
I l
. J f ~
S i WWiViVi
I I— ■ I I
i i I i i i r 1 i i i i
60 70 80 90 10 20 30 4 0 50 60
i Iwlng ( 3 * K I SUnoa(«2%OC)
149
ASCEND RAMP
VI
VL
VML
VMO
% Galt Cycla
^ Control PaleNofemoral Pain
DESCEND RAMP
VI
VL
VML
VMO
60 70 80 90 0 10 20 30 4 0 60 60
% Galt Cycla
H Control [22 Patellofemoral Pain*
Figure 4-9. Onset and cessation of vasti activity during ascending (top) and
descending (bottom) ramp (expressed as a percentage of the gait cycle) for
patellofemoral pain and control subjects. Onsets are indicated by the right edge
of the horizontal bars. Error bars indicate one standard deviation from the
mean. 0% of the gait cycle indicates initial contact. (VI = vastus intermedius,
VL= vastus lateralis, VML= vastus medialis longus, VMO= vastus medialis
oblique. * indicates EMG onset for the patellofemoral pain group delayed
compared to normal when averaged across all muscles for descend ramp
condition (p<.05).
T “ r T“ r“ r " r “ ! “r “ r“ r * F T " n
;l: nutM(«a%oc)
150
During ramp descent, a significant group effect (no interaction) was
found. The vasti onset of the PFP group was significantly delayed compared to
the normal group when averaged across all muscles (86.0 %GC vs. 90.5 %GC;
p < .01)(Figure 4-9). In addition, a significant group effect and group x muscle
interaction was found during fast walking. Breakdown of the muscles
individually by group revealed a significantly delayed onset in the PFP subjects
for the VML (86.9 %GC vs. 81.7 %GC; p<.01), and VI (88.5 %GC vs. 82.6 %GC;
p<.02) compared to normal (Figure 4-7). No other significant group effects
were found for EMG onset.
Cessation
No statistically significant differences were evident in the cessation of
EMG activity between vasti within either group (no muscle effects) regardless of
condition (Figures 4-7, 4-8, 4-9). As with EMG onset, there was very little
difference between the VMO and VL when averaged across all conditions for
both the PFP (VMO: 30.1 %GC vs. VL: 31.1 %GC) and normal groups (VMO:
30.6 %GC vs. VL: 31.7 %GC)(Table 4-3).
A significant group effect and group x muscle interaction was evident
during stair descent. Breakdown of the muscles individually by group revealed a
significantly later cessation of the VMO, VML and VI compared to the normal
controls (VMO: 55.1 %GC vs. 52.7 %GC, p<.04; VML: 56.1 %GC vs. 50.8 %GC,
p<.02; VI: 56.3 %GC vs. 50.5 %GC, pc.01)(Figure 4-8). No other significant
group effects were found for EMG cessation.
151
Table 4-3
EMG Cessation (% Gait Cycle)
Vastus Medialis Oblique (VMO) and Vastus Lateralis (VL)*
mean (sd)
Normal Patellofemoral Pain
VMO VL VMO VL
FR 14.8 (8.7) 17,6(9.7) 13.1 (5.7) 15,1 (6.9)
FT 20.1 (12.9) 23.1(14,0) 19,2 (12.0) 21.0(12.2)
AS 40,4 (7.5) 40.7(5,5) 39.1 (8.7) 40,5(7.8)
: DS 52.7 (3.0) 52.4(3.7) 55.2(4.1)
53.9(5,2)
AR 25.4 (12.5) 27.7(12.3) . 21.6 (12.9) 25,3(11,7)
DR 30.2(18.2) . 29.0(15.8) 32,2(14.7) 30,9(15,4)
Mean 30.6 (10.5) 31.7(10.2) 30.1 (9.6) 31,1 (9.9)
FR= free walking, FT = fast walking, AS= ascend stairs, DS= descend stair,
AR= ascend ramp, DR= descend ramp
*No significant differences were found between the VMO and VL within either
group for any of the conditions tested.
Mean Intensity
There were no significant differences between the mean intensity of the
different vasti within each group (no muscle effects). As with EMG onset and
cessation, the average intensities of the VMO and VL for each condition were
very similar within both groups (Table 4-4).
152
Table 4-4
EMG Intensity (% Gait Cycle)
Vastus Medialis Oblique (VMO) and Vastus Lateralis (VL)*
mean (sd)
Normal Patellofemoral Pain
VMO VL VMO VL
FR 16,8 (9.1) ■ 18.9(10.7) 11.6(6.7) •^14.1: (7.6) j ‘
PT
■ r i 30.0 (9.1) 30.7(16.7) 23.3(10.1)
; 23.7(7.8) ' ■
AS 27.5 (10.7) 29.4(9.9) . 30.7 (14.8) ^ . 2 (14.3)
OS 20.1 (13.0) 19.6 (10.2) 18.8 (8.3) - 12,4 (5.5) .
AR 20.8 (7,0) 19.4(6,4) 15.4(7.0) ,13.9 (7.5)
OR 16.7 (10,6) 16.8:{7.5) 11.8(5.0) ' 11 ,8(4*0) I J
Mean 22.0(9.9) 22.5 (10.2) 18.6(8.7) 17.5 (7,8).:; ; ;
FR= free walking, FT = fast walking, AS= ascend stairs, DS= descend stair,
AR= ascend ramp, DR= descend ramp
*No significant differences were found between the VMO and VL within either
group for any of the conditions tested.
Significant group effects (no interactions) were found however, for the
free, fast, ascend ramp, and descend ramp conditions. In general, the vasti
activity of the PFP group was significantly less than the normal group when
averaged across all muscles (free: 12.5 %MMT vs. 18.1 %MMT, p<.02; fast:
22.1 %MMT vs. 30.2 %MMT, p< .01; ascend ramp: 13.9 %MMT vs. 19.7 %MMT,
p< .01; descend ramp: 12.2 %MMT vs. 16.8 %MMT, p< .01)(Figure 4-10).
153
Condition
■ Control g j Paieitofemoral Pah
Figure 4-10. Mean intensity of vasti contraction (expressed as a percentage of
MMT (manual muscle test) between patellofemoral pain and control subjects for
all conditions tested (fr= free walking, ft= fast walking, a s= ascending stairs,
ds= descending stairs, ar= ascend ramp, d s= descend ramp. Error bars
indicate one standard deviation from the mean. * indicates EMG intensity of the
patellofemoral pain group less than normal for these conditions (p < .05)
Knee Motion
No significant differences in knee motion were found between groups for
any of the conditions tested. This was consistent for all phases of the gait cycle.
Although there was decreased loading response knee flexion in the PFP group
when averaged across all conditions (20.3 degrees vs. 23.6 degrees), this
finding was not statistically significant (p=. 10) (Figure 4-11). The largest
154
difference in loading response knee flexion between the two groups was evident
during descending stairs (4.0 degrees) and descending ramp (3.8
degrees) (Figure 4-11).
■ Control @ 3 PtURotamoral Pain
Figure 4-11. Comparison of maximum knee flexion during the loading response
phase of the gait cycle between patellofemoral pain and control subjects for all
conditions tested. (fr= free walking, ft= fast walking, as= ascending stairs, ds=
descending stairs, ar= ascend ramp, ds= descend ramp. Error bars indicate
one standard deviation from the mean.
Discussion
Results of this study have demonstrated no statistically significant muscle
differences within either group for the onset or cessation of EMG activity,
regardless of the task. These findings have significant clinical implications, since
155
it has been postulated that the VMO may contract later than the VL in subjects
with PFP, and thereby contribute to lateral patellar tracking.112> 182
The VMO has been given the distinction as being the primary medial
patellar stabilizer, owing to its oblique fiber orientation (55 degree angle of
insertion into the patella). In a mechanical study, Lieb and P e rry " found that
the function of the VMO was to counterbalance the lateral pull of the VL. The
longer fibered VML also provides medial patellar support, however, the angle of
fiber orientation into the patella is only 15 to 18 degrees from the midline of the
femur. This anatomical difference makes it a less effective medial stabilizer
compared to the VMO. Theoretically, the function of either of structure would be
compromised if the neuromuscular activation was delayed or diminished.
The possibility of a temporal feedforward mechanism in which the VMO
contracts before the VL to counteract the larger force capacity of the VL has
been discussed in the literature.49 This hypothesis is supported by the work of
Voight and Wieder182 who noted VMO activation prior to that of the VL in
normal individuals during a patellar tendon tap. Similarly, Grabiner et al.49
reported VMO activity preceded that of the VL during maximal isometric
contractions in normal subjects, however, despite statistical significance, the
temporal difference between these two muscles was only 5.6 milliseconds.
These authors concluded that these results were not clinically significant, and
without greater differences the feedforward activation hypothesis should be
contested.
In contrast to the results obtained from their normal subjects, Voight and
Wieder182 reported that reflex activity of the VL preceded that of the VMO in
subjects with extensor mechanism disorders. Despite failure to report the
magnitude of this timing difference, and the lack of evidence indicating that this
156
phenomenon would be present in voluntary contractions, these authors
hypothesized that this finding was indicative of a neurophysiologic motor control
imbalance, and therefore contributory to patellofemoral joint dysfunction. In a
subsequent study, Karst and Willett87 found no evidence of timing differences
between the VMO and VL. Using techniques which improved upon the
procedures of Voight and Wieder182 (such as increasing the temporal
resolution of the reflex latency measurement, and controlling for subject height),
these authors refuted the hypothesis of timing differences during reflex
conditions. In addition, these authors reported that there were no onset timing
differences during voluntary knee extension.
The current EMG results during functional gait activities support the
conclusions of Karst and Willett87 that timing differences between the vasti do
not exist in patients with PFP, and therefore do not play a role in contributing to
this disorder. Therefore, the clinical rationale behind the use of biofeedback and
muscle re-education techniques in an attempt to alter the onset of VMO relative
to the VL must be questioned.
In this PFP population, all vasti had decreased mean intensities compared
to the normals in four of the six conditions tested (free and fast waking, ascend
and descend ramp). This decreased activity is suggestive of a quadriceps
avoidance pattern, which is similar to the response seen in subjects with to
anterior cruciate ligament tears.12 Perry129 stated that subjects with weak
quadriceps or painful knees avoid loading response knee flexion as it is at this
point of the gait cycle where the knee joint reaction forces and muscular
dem ands are the greatest. Although this premise was supported by the work of
Dillon et al.33 who reported a significant reduction in stance phase knee flexion
in eight subjects with PFP, the knee motion results of current investigation do not
157
adequately explain the differences in vasti EMG between groups. Instead the
reduced EMG in the PFP subjects was most likely the result of a decreased
external knee flexion moment (not measured), which could have been
accomplished through a subtle positioning of body weight over the knee joint
axis or a conscious effort to reduce walking velocity.
Mean vasti intensity was not significantly different between groups for
both ascending and descending stairs, indicating that the higher muscular
dem and associated with these activities was unavoidable. This is logical since
larger ranges of knee flexion are required to accomplish these tasks. Since
patellofemoral joint reaction forces are directly related to amount of knee flexion
and quadriceps force,63 it is not surprising as to why ascending or descending
stairs commonly reproduces PFP symptoms.
Within the PFP group there was no differences in vasti intensity for any of
the conditions tested. Although EMG ratios were not calculated in this study, the
VMO, VML and VL had similar mean intensities, indicating that recruitment of the
medial quadriceps was not compromised. This finding is in contrast to that
reported by Mariani and Caruso169 as well as Souza and Gross'*63 who found
decreased VMO activity compared to the VL in this population. These conflicting
findings can be attributed to methodological differences as these authors based
conclusions of VMO insufficiency on non-normalized EMG obtained from small
sample sizes. Given the failure to use normalized EMG in order to control for
other variables not related to muscle function (ie. electrode placement), the
validity of these previous results must be challenged.
The EMG timing for both groups during level walking are in agreem ent
with the data of Adler et al.1 who analyzed the timing patterns of the vasti in
normal individuals during free and fast walking. According to Perry,129 the
158
onset of vasti activity in terminal swing functions to reverse the swing phase
knee flexion and prepare the limb for initial contact. Continued activity to the
beginning of midstance controls the external knee flexion moment in loading
response and provides limb stability into single limb support. In general, our
results indicate that the onset of vasti activity during free walking occurred from
85 to 89 percent of the gait cycle (terminal swing) with termination of activity
evident from 13 to 21 percent of the gait cycle (midstance). Overall, the timing of
the vasti during fast walking for the normal group demonstrated a slightly earlier
onset of vasti activity and later cessation compared to free walking (Figure 4-7).
The PFP group demonstrated a delayed onset for the VML and VI during
fast walking compared to the normals, suggesting that preparation phase for
initial contact may have been compromised. The sam e finding was evident
during descending ramp trials as all four vasti were delayed compared to
normal. It is possible that this delayed activity was the result of the PFP subjects
anticipating a decreased muscular demand during loading response, owing to
the employment of a quadriceps avoidance gait pattern. Conversely, normal
individuals would anticipate the increased muscular demand and would be more
likely to ensure adequate knee stability at initial contact.
The prolonged vasti activity evident in both groups during stair and ramp
ambulation reflects the increased muscular demand necessary to support the
flexed knee posture associated with such activities. The reason why the PFP
group had significantly prolonged EMG of the VMO, VL, and VI during
descending stairs is not entirely clear, however, it is possible that these subjects
were more deliberate in controlling the rate or descent rather than "skipping" to
the next step. This was observed in a number of patients who were obviously
cautious in descending the staircase.
159
LOADING CHARACTERISTICS IN SUBJECTS WITH PATELLOFEMORAL
PAIN
Results
Subject Characteristics
The mean age and height of the PFP group were not significantly different
from the normal subjects (Table 4-5). The mean weight of the PFP group
however, was significantly greater than the mean weight of the control group
(65.1 kg vs. 57.8 kg; pc.05)(Table 4-5).
Table 4-5
Subject Characteristics
PFP (n=15)__________Normals (n = 10) -value
Aae fvrs)
mean
sd
range
26.5
7.2
(1441)
25.4
6.1
(18-37)
.69
mean 164.4
5.6
(153,7477.2) (154.9-182.9)
.Weight (kg)
65.1*
8.0
(53.1-79.4)
57,6
9.5
.04 mean
sd
range (44.2-72.6)
PFP= patellofemoral pain; *PFP > Normal (p<.05)
Stride Characteristics
The average walking velocity of the PFP group was 7.9 m/min less than
that for the control group (77.8 m/min vs. 85.7 m/min, p<.03; Table 4-6). The
cadence in the PFP group (117.3 steps/min) was not significantly different from
the control group (122.1 steps/min)(Table 4-6). There was a trend towards
decreased stride length in the PFP group compared to the control group (1.32
meters vs. 1.41 meters), however this comparison was not statistically significant
(p=.06; Table 4-6).
Table 4-6
Stride Characteristics
PFP Normals p-value
mean
sd
range
77.8*
12,0
(63.2-102.0)
, Cadence_(s,tep/m?n)
mean 117,3
sd 12.3
(97.6-143.2)
1.32
0.11
(1.13-1.55)
mean
sd
. range
85.7
5.4
(75.8-95.5)
122.1
7 8
(109.1-130,4)
Slilliillili
0.10 .
(1.31-1.59)
.03
.28
,06
PFP= patellofemoral pain; *PFP < Normal (p<.05)
Walking velocity was found to be linearly related to the peak vertical
ground reaction force (r=.73, p < .05); Figure 4-12), time to peak vertical ground
reaction force (r=-.55, p<.05; Figure 4-13), and peak loading rate (r=.74,p< .05;
Figure 4-14) with the slope being similar across both groups. This justified the
use of walking velocity as a covariate for the three force plate variables.
161
155 t
C O
gS 145
V./
IL
at
O 135 ■ ■
o P F P
□ NORMAL
2S 125 ■
115
90 100 110 70 80
60
VELOCITY (M/MIN)
Figure 4-12. Relationship between peak vertical ground reaction force (VGRF)
expressed as a percentage of body weight (%BW) and walking velocity. (r=.73;
p<.01). PFP= patellofemoral pain.
o
0.18t
LU
< />
v - /
L L .
0.16*
at
0
■ ■
>
0 .1 4 "
<
LU
O.
0.12"
o
■ 1
1"
IXJ
0 .1 0 "
. s
H- 0.08-1-
&
70 80 90 100
VELOCITY (M/MIN)
o PFP
□ NORMAL
Figure 4-13. Relationship between time to peak vertical ground reaction force
(VGRF) and walking velocity (r=-.55; p<.01). PFP= patellofemoral pain.
162
260 t
220 -
gj 180"
| 140
“ 100 "
CL
60
o P F P
□ NORMAL
70 60 80 90 100 110
VELOCITY (M/MIN)
Figure 4-14. Relationship between peak loading rate (PLR) expressed as body
weight per second (BW/SEC) and walking velocity, (r=.74; p < .01). PFP=
patellofemoral pain
Vertical Ground Reaction Force
The average initial peak vertical ground reaction force for the PFP
subjects, after controlling for walking velocity was not statistically different from
the control group (131.9 %bw and 131.0 %bw, respectively)(Table 4-7). In
addition, there was no significant difference between groups for the time to peak
vertical ground reaction force after controlling for velocity (PFP: .110 sec vs.
control: .103 sec;)(Table 4-7).
Peak Loading Rate
The correlation coefficient and the coefficient of determination of the
linear regression used to determine the overall peak loading rate from the
selected data points was .99 or higher for ail subjects. On the average, the peak
163
rate of loading for the PFP group was 82 percent of that for the control group,
after controlling for velocity (102.1 bw /sec vs. 125.3 bw /sec; Table 4-7).
Although these averages were not significantly different, there was a definite
statistical trend towards decreased mean peak loading rate in the PFP group
(p=.08).
Table 4-7
Force Plate Characteristics
(after controlling for velocity)
PFP_______________ Normals____________ p-value
■ w m m m
m ean 131.9 131.0 .79
Sd 7.7 7.6
. range (117-148) (117-152)
Time to PVGRFfsecI
..mean
sd
range
,Plfi,.ibw/S3£)
0.110 0.103 .88
0.02 . 0.02
(97.6-143.2) (109.1-130.4)
mean : 102.1 • ; ; 125.3- ■ ... . ' • ■ • .08'.: :
sd 30.1 30.6
range (37.5-150-7) (103.7-260.1)
PFP= patellofemoral pain; PVGRF= peak vertical ground reaction force; PLFT
peak loading rate
Knee Flexion
The maximum loading response knee flexion of the PFP group (12.9
degrees) was not significantly different from the normal controls (13.4 degrees;
p>.05)(Table 4-8). Similarly, the knee flexion angle just prior to initial contact
w as not significantly different between groups (PFP: -0.7 degrees vs. normal: -
2.9 degrees; p>.05).
164
Table 4-8
Knee Flexion
(initial contact and loading response)
PFP Normals pjAjalue
.te itjf l,g a tin g
Smtgc U d egrgesJe ^n)
-0.7
3.3
(-5.3-4.S)
moan
sd
range
.42
Knee Flexion (degrees*
m e a n
sd
range
12.9 .
& & Q .3 )
1 3 :4
4 .6
( 7 .3 - 2 0 .6 )
PFP= patellofemoral pain; Negative values indicate hyperextension
Discussion
The subjects with PFP demonstrated a trend towards decreased peak
rate of loading compared to normal, however, this difference was not significant.
This finding indicates that the PFP subjects did not demonstrate excessive
impulsive loading, and therefore would not be subjected to increased tibio
femoral joint compressive stress. In addition, there were no differences in the
magnitude of the initial vertical ground reaction force (F^) and the time to peak
vertical force (T-j), implying that the peak forces experienced by these two
groups during loading were similar.
The lack of significant differences in these force plate param eters can
m ost likely be explained by the similarity in loading response mechanics.
Kinematic data did not show any differences between groups for either
maximum loading response knee flexion or the position of the knee just prior to
initial contact. This suggests that the normal shock absorbing mechanism was
not compromised in the PFP subjects. Typically, subjects with PFP will avoid or
165
report pain with activities that require high quadriceps demand (ie. stairs),98
suggesting that compensatory gait deviations, such as limiting loading response
knee flexion, are most likely to occur during conditions that are pain producing.
The finding of similar loading response kinematics at the knee is not entirely
surprising, as these subjects did not report any pain with free walking, despite
the fact that patellar pain could be readily reproduced with provocative tests.
Although the PFP subjects did not adopt a strategy of reducing the
amount of loading response knee flexion during free walking, joint forces may
have been minimized by altering walking velocity. This was supported by the
significant difference in walking velocity between the two groups. Although
direct correlations between pain and velocity were not m ade in this study, the
greater standard deviation for velocity in the PFP group (more than twice that of
the normals) demonstrates that these subjects exhibited a wider range of
speeds. The variability may have been the result of anticipated knee pain, and is
consistent with previously reported data indicating that the speed of ambulation
ptays a key role in creating demands at the knee. 187 This concept also is
supported by prior studies that have found significant reductions in walking
velocity in patients with degenerative knee p a t h o l o g y .5 2 ,8 9 ,1 7 2 a reduced
walking velocity would also minimize the demands imposed by a significantly
greater body weight which was evident in the PFP group. By reducing the
speed of ambulation instead of limiting loading response knee flexion, joint
forces could be minimized with the preservation of normal shock absorption.
The difference in walking velocity between groups and the greater
variability in walking speed evident in the PFP subjects necessitated using
walking speed as a covariate. In addition, the finding of significant correlations
between velocity and all force plate variables supported this decision. Peak
166
loading rate demonstrated the strongest association with velocity (r=.74), which
is consistent with data previously reported by Skinner et al.164 By factoring out
the influence of velocity, the true effects of pathology on force plate parameters
could be discerned. Failure to control for this variable may result in biased force
plate parameters, especially when studying populations that demonstrate large
variations in walking speed.
The protective joint mechanisms include both passive (material
deformation) and active (neuromuscular control) com ponents,139 and may play
different roles in the attenuation of loading forces. For example, the heel strike
transient, which was the result of floor impact, occurred within 10 to 15 ms after
initial contact for all subjects. Given the fact that the maximum loading response
knee flexion did not occur until 100 to 150 ms after initial contact, suggests that
this motion may not have contributed much in the attenuation of the peak
loading rate. During the brief interval following heel strike, deformation of
articular cartilage and subchondral bone was most likely be the primary source
of shock absorption, as the impact peak preceded most of the eccentric muscle
control of loading response. Such repetitive deformation, if excessive, would be
of concern as both of these structures are viscoelastic in nature, and
demonstrate degenerative changes at high loading rates.137.133*!62
Jefferson et al.,62 as well as Radin and colleagues,139 suggested that
quadriceps activity in terminal swing decelerates the effect of both gravity and
hamstrings contraction in lowering the shank and foot to the ground. Such
neuromuscular control was theorized to reduce impulsive forces upon initial
contact. While this premise is possible, these authors did not discuss the
potential influence of walking speed on shank velocity just prior to floor contact.
This is important as Clark et al.24 has reported that the vertical heel velocity prior
167
to heel strike increased with progressively increasing running velocities. The
findings of these authors support the concept that ambulation velocity plays a
key role determining the impact forces at initial contact. Since loading impact
and subsequent shock wave propagation is most likely uncomfortable, it is
understandable as to why persons with knee pathology may choose to adopt a
slower gait velocity.
The maximum vertical ground reaction force that occurs during initial
stance represents the loading peak, and occurs during the sam e time frame as
the maximum loading response knee flexion (10 to 15 percent of the gait cycle).
It is logical therefore, to assum e that neuromuscular control (ie. eccentric control
of knee flexion during loading response) would most likely play a key role in the
dampening of peak forces, and not necessarily the peak loading rate.
Radin and colleagues139 demonstrated increased loading rates in
patients with intermittent tibio-femoral pain and hypothesized that this impulsive
loading was a cause rather than effect of joint pain. Although subjects in the
study by Radin et al.139 walked at similar walking speeds com pared to normal
controls, no effort was made to control for the effects of velocity, thus potentially
limiting the results.
While the experimental group of Radin et al.139 consisted of subjects with
tibial-femoral pain of non-specified etiology, the subjects in the current
investigation were specifically screened for pain originating from the
patellofemoral joint. The fact that peak loading rates in the subjects with PFP
were similar to that of normal controls after controlling for velocity, suggests that
subjects with different knee disorders may exhibit other loading characteristics.
The effects of increased rate of loading would probably be manifested as tibio
femoral pain as this joint is more susceptible to axial compression by nature of
168
its horizontal orientation. On the other hand, the patellofemoral joint has a more
vertical alignment, with joint compression being dependent on the magnitude of
quadriceps forced08 It is iikely therefore, that the issue of increased rate of
loading as being contributory to knee pathology may be diagnosis dependent.
In the current study, the peak loading rates for both groups were
substantially higher than that reported by Radin et al.’ * 39 This was most likely the
result of differences in the determination of peak loading rate and the sampling
rate of the force plate data, as the walking velocities between these two studies
were similar. By establishing a stringent criteria for the inclusion of data points
for the regression analysis (accepting only those whose slopes were within 50
percent of the maximum slope), a more representative assessm ent of the peak
loading rate was most likely made. In addition, the sampling rate (2500 Hz) and
integration interval (1 ms) ensured that the data were not excessively sm oothed
and were representative of the vertical force pattern. This method ensured that
data points representing slope discontinuity at either end of the initial upswing of
the vertical force trace were able to be identified and therefore omitted. The very
high correlation coefficients and coefficients of determination found with
regression analysis for each subject (.99 or greater) indicates that the individual
data points used did indeed have similar slopes.
Although the current study did not demonstrate increased loading rates in
the PFP population, further research is needed to clarify the role of loading rates
in the development of joint pain, especially during pain evoking activities. In
addition, the cause and effect relationship between peak loading rates with
regard to various knee diagnoses and pathology needs to be further defined.
This information would assist in identifying patients that may be at risk for the
development of degenerative joint changes.
169
THE INFLUENCE OF QUADRICEPS STRENGTH AND PATELLOFEMORAL
PAIN ON GAIT CHARACTERISTICS AND JOINT MOTION
Results
Subject characteristics
There were no significant differences between the PFP and control
groups for either age, height or weight (Table 4-9).
Table 4-9
Subject Characteristics
PFP (n = 19) Normals (n=19) p-value
_ Acte fvrs)
m ean 25.4 27.5 i35: '- '- v : ■
sd 8.2 m m m m im m m m m m rn i
range (14-46) (23-38)
Ifcteightte)
. m ean 165.1 165.3
sd 7,6 7.7
range (151.1-177.2) (149.9-183.5)
M ejghtjtg)
62,4
; p i i i ! i i i p p i i | | i i i i s i i i p
:':.25 ; ! mean 59.2
sd 9.3 7.5
; ■ r - 1 : : : = ■ - :
range (42.0-82.7) (46.8-74.1)
PFP= patellofemoral pain
Relationship between knee extension torque, pain and functional assessm ent
score
After normalizing by body weight, the maximum knee extension torque of
the PFP group was significantly less than the normal group (2.35 Nm/kg vs. 3.04
Nm/kg; pc.05)(Table 4-10). During the maximal isometric test, the PFP
subjects reported an average pain level of 4.4 out of 10 on the VAS (Table 4-11).
170
The mean score on the FAQ for the PFP subjects was 67.5 out of a possible 100
(Table 4-11).
Table 4-10
Maximum Knee Extensor Torque
(normalized by body weight)
PFP Normals |>value
.Tb?m3..(Nro/frQ3
mean 2,35 3,04 ,03
sd 0.78 0,69
range (1,28-3.92) (1.96-4.02)
PFP= patellofemoral pain
Table 4-11
Scores for the Visual Analog Pain Scale and Functional Assessment
Questionnaire
(PFP subjects only)
VAS FAQ
:mean : : : : : 4.4 ' ■ M & : j ; ; : •
, :=gd.:=:- = = : : :3.1 = = : : : , .. . = ' 18.1
range .. (0-9.6) . . ■
PFP= patellofemoral pain; VAS= Visual Analog Pain Scale (10=maximum pain);
FAQ= Functional Assessment Questionnaire (10=maximum function)
The VAS pain score was not significantly correlated to knee extensor
strength in the PFP subjects (r=.03). In addition, knee extensor strength was
not correlated to the FAQ score (r=,25). The VAS pain score, however,
demonstrated a significant correlation to the FAQ score in the PFP group (r=.72;
p < .001) (Figure 4-15).
171
100 m
60
40
20
2 0 10 8 4 6
VISUAL ANALOG PAIN SCORE
Figure 4-15. Correlation between Functional Assessment Questionnaire (FAQ)
score and Visual Analog Pain score (patellofemoral subjects only). r=.72,
p<.001.
Stride characteristics
There was a statistically significant difference between the PFP and
control group (significant group effects, with no interactions) for velocity,
cadence, and stride length when averaged across all conditions (Figures 4-16,
4-17,4-18). In general, the PFP group demonstrated decreased values for these
stride characteristics compared to the normal controls.
The average walking velocity of the PFP group (for all conditions) was 81
percent of the average walking velocity of the normal subjects (56.5 m/min vs.
69.7 m/min; p < .001)(Figure 4-16), while the average stride length across all
conditions was 88 percent of normal (1.22 m vs. 1.38 m; pc.001)(Figure 4-17).
Cadence was 91 percent of normal when averaged across all conditions (114.1
steps/m in vs. 125.2 steps/min; pc.001)(Figure 4-18). There were no significant
differences between groups for time spent in single limb support, double limb
support, swing, and stance.
172
FH FT AS DS AB OR
Condftkxi
■ Co ntra? H PatellolomomlPaln*
Figure 4-16. Mean velocity between patellofemoral pain and normal subjects
for all conditions tested. * indicates average walking velocity for the
patellofemoral pain group less than normal when averaged across all conditions
(p<.001). FR=free walking, FT=fast walking, A S=ascend stairs, D S^descend
stairs, AR=ascend ramp, DR= descend ramp.
I |
• 2 4 X - K -
■ Control Q PttsHofamorBl P*Jn*
Figure 4-17. Mean stride length between patellofemoral pain and normal
subjects for level and ramp conditions (stair data omitted owing to the limitation
imposed as a result of the fixed stair height and depth). * indicates average
stride length for the patellofemoral pain group less than normal when averaged
across all conditions (p<.001). FR=free walking, FT=fast walking, AR=ascend
ramp, DR=descend ramp.
173
CondHtan
■ Control H PtHiotemor* P iln *
Figure 4-18. Mean cadence between patellofemoral pain and normal subjects
for level and ramp conditions (stair data omitted owing to the limitation imposed
as a result of the fixed stair height and depth). * indicates average cadence for
the patellofemoral pain group less than normal when averaged across all
conditions (p<.001). FR=free walking, FT=fast walking, AR=ascend ramp,
DR= descend ramp.
Of the three independent variables m easured (pain, strength and
functional score) knee extension torque was the only significant predictor of
velocity, with increased strength resulting in faster speeds. This w as evident for
five out of the six conditions (free walking: r=.59, p<.05; fast walking: r=.59,
p<.05; ascend stairs: r=.50, p<.05; ascend ramp: r=.62, p<.05; descend
ramp: r=.67, pc.05)(Table 4-12). In addition, knee extension torque also was
the only significant predictor of stride length for four out of the six conditions
(free walking: r=.73, p<.05; fast walking: r=.61, p<.05; ascend ramp: r=.62,
p<.05; descend ramp: r=.76, p<.05)(Table 4). No other significant
associations were found between any of the independent variables and the
remaining stride characteristics.
174
Table 4-12
Stepwise Regression Results for Predicting Velocity,
Stride Length, and Cadence
Condition
Stride
Characteristic
Independent variable
(predictor) r-value*
fr velocity knee ext. torque
ft vefocity knee ext. torque ■ .59
a s velocity knee ext. torque
i - m
ds velocity . . none
> . V i
ar velocity knee ext. torque ; : ;.62 s ::-
dr velocity . knee ext. torque 67.
lllllilll stride length knee ext. torque ► 7 8 :::
■ ft stride length knee ext. torque : .61
ar stride length knee ext. torque .62 .
dr stride length knee ext. torque ;:,76:
lllBllllll cadence none
*............
ft
cadence none l l i i i l i l i i i
ar cadence. none
. dr cadence none
fr= free walk, ft= fast walk, as= ascend stair, ds= descend stair, a s= ascend
ramp, dr= descend ramp. Stride length and cadence for ascending and
descending stairs ommitted owing to the limitation imposed by the fixed stair
height and depth. *all r-values significant at the p< .05 level.
Joint motion
There was a significant group effect and significant interaction for ankle
joint motion during the terminal stance phase of gait. When conditions were
analyzed separately between the two groups, the PFP subjects demonstrated
significantly greater ankle dorsiflexion compared to normal for fast walking (9.9
degrees vs. 7.0 degrees; p<.05), descending stairs (27.6 degrees vs. 18.9
degrees; pc.001), and descending ramp (15.8 degrees vs. 11.9 degrees;
p<.01)(Figure 4-19). No other differences for ankle motion were found.
175
ANKLE MOTION
r> ■
« o p o « o
%Galt cycfo
• o
• to - -
o 0 0 m wo 40 • 0 TO * 0 00
fr
%Galt cycle
ft1
a s
%Gott cycle
ds*
I
I
40 M 00
%Gatt cycle
• a
ar
lo o
Figure 4-19. Ankle motion for patellofemoral pain (light line) and normal
subjects (dark line) for all conditions tested. Dotted vertical line delineates the
division between stance and swing phases (62 percent of the gait cycle).
Dashed vertical line indicates terminal stance. * indicates the mean ankle
dorsiflexion during terminal stance was significantly greater in the patellofemoral
pain group compared to normal (p<.05). FR= free walking, FT= fast walking,
AS= ascend stairs, DS= descend stairs, AR= ascend ramp, DR= descend
ramp.
176
KNEE MOTION
TO-
1 0 • 0 100
%Gatf cycle
fr
■ o -
l t > -
10 M 0 0 TO
%Gatt cycle
as
• O r
• O •
10
• 0 .
TO
• o - -
40
1 » •
- 10-
ID H 40 ■ 0
%Gatt cycle
f t
• o M 1 0 0
ds
to.
TO
40
•0 100
%Gatt cycle
ar
%Ga!t cycle
dr
Figure 4-20. Knee motion for patellofemoral pain (light line) and normal
subjects (dark line) for all conditions tested. Dotted vertical line delineates the
division between stance and swing phases (62 percent of the gait cycle). FR =
free walking, FT= fast walking, AS= ascend stairs, DS= Descend stairs, AR=
ascend ramp, DR= descend ramp.
177
HIP MOTION
S O - ■
l O - 10- -
o • o - •
- 10-
H --- 1 ----1
t o 0 0 IO O
1 0 * 9 0 0 4 0 0 0 t o VO PO 0 0 1 0 0
%Gaft cycle
fr
ft
S O ■
1 0 - -
1 0 0 0 9 0 4 0 t o 6 0 TO 9 0 9 0 1 0 0
%Galt cycle
as
6 0 -
10 -
1 0 0 0 9 0 4 0 0 0 6 0 VO 0 0 0 0 1 0 0
%Galt cycle
ds
4 0 - -
9 0 - -
« 0 "
o
10- • 1 0 - •
- 10-
-1 0 -
1 0 9 0 0 0 4 0 6 0 6 0 T O t o OO 1 0 0
1 0 0 0 9 0 4 0 6 0 t o VO t o 0 0 1 0 0
ar dr
Figure 4-21. Hip motion for patellofemoral pain (light line) and normal subjects
(dark line) for all conditions tested. Dotted vertical line delineates the division
between stance and swing phases (62 percent of the gait cycle). FR= free
walking, FT= fast walking, AS= ascend stairs, DS= Descend stairs, AR =
ascend ramp, DR= descend ramp.
178
There were no significant differences in knee motion between the PFP
and normal subjects for any phase of the gait cycle regardless of the condition.
There was however, a trend towards increased knee flexion during terminal
stance for the PFP group when averaged across all conditions (p = .08)(Figure 4-
20).
Similarly, hip joint motion was not statistically different between groups
regardless of the condition for any phase of the gait cycle. There was a trend
towards decreased hip flexion during terminal swing and loading response for
the PFP group when averaged across all conditions, however, this was not
significant (p = .09)(Figure 4-21).
Neither pain, quadriceps strength nor functional score were significant
predictors of the amount of loading response knee flexion. This was consistent
for all conditions tested.
Discussion
Muscle weakness and joint pain can impose substantial limitations on
normal gait mechanics.'129 For the individual with PFP, disturbance of the
extensor mechanism (ie. abnormal patellar tracking) has been reported to have
an influence on normal knee kinematics and stride c h a r a c te r is tic s .3 3 ,1 3 3 T h e
exact cause of these gait modifications (ie. pain or weakness) however, is not
entirely clear.
The relationship between knee pain and quadriceps inhibition has
previously discussed in the literature. Reflex inhibition has been demonstrated in
subjects with knee pathology, 17§ and is reported to occur when afferent stimuli
from receptors in or around the knee joint prevent activation of alpha motor
neurons in the anterior horn of the spinal cord.174 Although pain and inhibition
179
have been associated clinically,11,1 decreased motor unit recruitment of the
quadriceps appears to be linked empirically to knee joint
effusion,27.170.174.175 and has been reported to be independent of
pain.174* 175 Young et al.,100 reported that afferent block by local anesthesia
was not effective in reducing quadriceps inhibition despite a complete reduction
in pain. Additionally, Stratford175 did not observe any potential relationship
between pain and inhibition that would explain reduced quadriceps
electromyography during a maximal isometric contraction in subjects with
acutely effused knees. In contrast, deAndrade and colleagues27 presented
evidence that pain reduction through lidocaine injection delayed quadriceps
inhibition in knees that were artificially distended. These observations however,
were m ade on only four subjects.
The results of this study found a significant decrease in knee extensor
torque in the PFP group (77 percent of normal), as well as an average pain
score of 4.4 out of a possible 10 during testing. These associated findings might
suggest that pain may have played a role in reducing quadriceps torque, but,
when pain was correlated to knee extensor torque, this inference did not hold
true. In fact, these two variables were completely unrelated (r=.03). This would
imply that knee extensor torque was not affected by pain which is consistent
with the observations of Young et al.190 and Stratford.175
The lack of a significant association between knee extensor strength and
pain suggests that the reduction in torque output was probably the result of
muscular atrophy, although this was not assessed in this study. Quadriceps
atrophy is a common clinical finding in this population,38 and is m ost likely
caused by the avoidance of activities that require quadriceps force. Quadriceps
inhibition as a result of effusion may have also contributed to the reduction in
180
quadriceps torque, however the amount swelling was not assessed in this study.
It should be noted though, that none of the PFP subjects demonstrated gross
joint effusion.
Another possible reason for the lack of a significant correlation between
pain and quadriceps strength may have been related to the testing position used
to elicit knee pain. In this study, maximum isometric knee extension torque was
assessed at 60 degrees of flexion, which placed the quadriceps muscle group at
its greatest length tension advantage,100 but may have been inadequate in
reproducing the amount of patellar pain that would inhibit normal function.
Although the high quadriceps forces produced at this knee flexion angle would
have also resulted in substantial patellofemoral joint reaction forces,108 the
relatively modest pain scores reported by our subjects suggests that this
compression was reasonably tolerated. This may have been the result of the
increase in contact surface area between the patella and femur which has been
reported by Mathews et al.110 to be approximately 40 percent more than that at
15 degrees of flexion. Increased contact area would reduce the joint contact
pressure as the joint forces would have been distributed over a greater area. In
addition, since PFP has been clinically linked to patellar subluxation,59 and it has
been shown radiographically that patellar subluxation occurs at angles less than
30 degrees of knee flexion,19 it is possible that testing the subjects at less
flexion (ie. 0 to 30 degrees) would have yielded greater pain scores. This
position, however, would have placed the quadriceps at a mechanical
disadvantage, and therefore would have resulted in lower torque values as
previously demonstrated by Lieb and Perry.100 Given this paradox between
testing position and the pain/strength relationship, as well as the need to assess
181
both simultaneously for correlation purposes, it is not entirely surprising that no
relationship was found.
A significant inverse linear association however, was found between pain
and the FAQ score, indicating this instrument is sensitive to individual pain
levels. This finding corroborates the work of Kujala et al.92 who reported that a
low summed score (using the same questionnaire) best correlated with
increased lateral patellar tilt as determined by magnetic resonance imaging.
Although these authors did not record pain levels, this finding lends support to
the use of this questionnaire for pain assessm ent in this population. In addition,
our average questionnaire score of 67.5 was very close to the average score of
68.2 found by Kujala et al.92 for subjects with patellar subluxation, indicating that
this instrument may be reliable. The lack of a significant association between
quadriceps torque and the FAQ score was not surprising since most of the
possible responses to items in the questionnaire pertained primarily to pain
during functional activities.
It has been previously reported that subjects with PFP reduced the
amount of stance phase knee flexion during gait.39 During loading response,
this compensation appears logical as knee flexion would increase the
patellofemoral joint reaction forces and possibly pain. Contrary to the findings of
Dillon and colleagues,33 the PFP subjects in this study did not demonstrate
reduced stance phase knee flexion, but in fact demonstrated a trend towards
increased knee flexion during terminal stance, although this finding was not
statistically significant. The increase in terminal stance knee flexion was
associated with significant increases in terminal stance ankle dorsiflexion in four
of the six conditions tested (free and fast walking, descending stairs, descending
ramp). As described by Perry,129 the association between excessive ankle
182
dorsiflexion and knee flexion in terminal stance is indicative of weakness of the
gastrocnemius and soleus. Although calf strength was not tested in this study, it
is possible that the failure to control the anterior alignment of the tibia would
result in excessive knee flexion and the continued need for quadriceps support.
This concept is supported by previous work that demonstrated prolonged vasti
EMG during stair descent in patients with PFP compared to n o r m a l. 132
Prolonged quadriceps activity would increase the patellofemoral joint reaction
forces during this phase of the gait cycle, which would appear to be deleterious
for the patient with PFP.
The results of this study did not find a significant reduction in loading
response knee flexion when averaged across all conditions, indicating that these
subjects did not alter the normal knee joint kinematics during early stance. This
suggests that quadriceps strength in the PFP subjects, although reduced, was
capable of providing stability during this phase of the gait cycle. The fact that
pain or quadriceps strength was not a significant predictor of the amount of
loading response knee flexion, suggests that avoidance of this motion should
not be considered the principle gait deviation in this population.
The primary gait adaptation in the PFP population however, was a
reduction in walking velocity, which was consistent across all conditions. The
greatest differences between groups occurred during the higher demand
activities of fast walking and ascending ramp, which suggests that the higher
dem and activities required greater speed attenuation. Winter"*®7 has
demonstrated that this gait adaptation reduces the demand of the quadriceps
during initial stance, by decreasing the flexion moment. The reduction of the
knee flexion moment during slower walking is most likely the result of the
reduced vertical component of the ground reaction force, which is the
183
predominant external force contributing to the knee flexion moment. This is
consistent with previous findings in which the peak vertical ground reaction force
was shown to be linearly related to velocity.134 Therefore, a decrease in
walking velocity could allow for a reduction of muscular demand, without a
compromise in knee kinematics, and is concordant with previous findings of
decreased electromyographic activity of the vasti in subjects with PFP.132
While it would appear that subjects with knee pain will adopt a slower gait
velocity to reduce joint forces, there was no relationship between the amount of
knee pain and walking velocity for any of the conditions. There was however, a
significant relationship between quadriceps strength and walking velocity for five
out of the six conditions (descending stairs excepted), with increased
quadriceps strength resulting in faster walking velocities. This association
suggests that strength of this muscle group is essential in accepting the higher
dem ands of increased ambulation speed which is consistent with the findings of
Winter.187
The reduction in gait velocity in the PFP subjects was a function of
reduced stride length and cadence, both of which were significantly less in the
PFP group compared to the control subjects across all conditions. The trend
towards decreased terminal swing hip flexion in the PFP group contributed to
this decreased stride length by limiting the forward position of the limb at initial
contact.
As with walking velocity, quadriceps strength was the only significant
predictor of stride length in four of the six conditions tested, further supporting
the concept that increased strength is essential in promoting normal stride
characteristics. Although the FAQ score was linearly related to patellar pain,
there was no significant correlation between any of the stride characteristics,
184
and therefore, its use in terms of assessing true functional status may be limited.
Further research determining the validity of this questionnaire is warranted.
The fact that knee extensor torque was predictive of velocity and stride
length, indicates that stride characteristics will likely vary from patient to patient.
This would suggest that gait adaptations in this population are not the result of a
generalized reprogramming of the locomotor process, as proposed by
A n d r ia c c h i1 6 0 for subjects with ACL deficient knees.
The results of this study may have significant clinical implications as
conservative care of PFP typically involves both pain management and
strengthening of the extensor mechanism. These findings suggest that
increased quadriceps strength is essential in restoring normal gait function in
this population. It would appear therefore, that the reduction of symptoms
without an associated strengthening program would not be adequate in
returning an individual to a normal functional status.
SUMMARY
The results of these investigations indicated that the primary gait
adaptation for the patient with PFP was a decreased walking velocity. This was
significant for all conditions tested and was a function of both reduced stride
length and cadence.
The finding of decreased vasti EMG in the PFP subjects for four of the six
conditions was indicative of a quadriceps avoidance pattern and suggested that
the compensation of a slower gait velocity was effective at reducing the demand
at the knee. Although there was reduced vasti EMG in the PFP subjects, there
185
were no timing or intensity differences between the VMO and VL that were
suggestive of dynamic patellar instability.
There was a trend towards decreased loading response knee flexion in
the PFP subjects compared to normal, however, this result was not statistically,
significant. This finding would indicate that subjects with PFP do not significantly
alter knee joint kinematics in order to reduce the demand at the knee. By
reducing gait velocity instead of limiting knee flexion, the normal shock
absorbing mechanics of loading were preserved. This was verified by findings of
similar force plate parameters (peak vertical forces and peak rates of loading)
between the two groups during free walking.
The primary factor contributing to compensatory speed reduction in the
PFP subjects was quadriceps strength, as this parameter was significantly
correlated to walking velocity for five out of the six conditions. This finding
indicates that quadriceps strength is essential in restoring normal gait function,
and should be addressed in the rehabilitation of the patient with PFP.
186
CHAPTER V
SUMMARY AND CONCLUSIONS
Although activation differences between the medial and lateral vasti have
been implicated as being associated with patellar instability and patellofemoral
pain (PFP), there is little objective evidence to support this premise. Given as
such, this dissertation has sought to examine two primary issues regarding the
function of the vasti in subjects with PFP. The purpose of the first chapter was to
establish the role of the vasti in contributing to patellar kinematics, while the
second chapter addressed the function of the vasti during gait.
When considering the hypothesis of abnormal vasti contraction as being
contributory to PFP, two aspects of neuromuscular recruitment must be
considered: the timing of activity (ie. onset and cessation) as well as the
magnitude (or intensity) of muscular contraction. These issues were addressed
in both components of this study under a variety of conditions. During gait,
there was no difference in vasti timing or intensity that was indicative of dynamic
patellar instability in subjects with PFP. In fact, the motor unit activity of the
vastus medialis oblique (VMO), vastus medialis longus (VML) and the vastus
lateralis (VL) was remarkably similar across all conditions. Similarly, during knee
extension, there was no difference between the PFP and normal groups for
either the VL:VMO or VL:VML electromyographic (EMG) ratios when averaged
across all angles of knee flexion. These data indicate that the various vasti are
recruited equally in this population, which is consistent with the fact that all share
a common function and innervation. Additionally, the lack of association
187
between abnormal activation patterns and PFP suggests that this entity cannot
be considered as being a cause or effect of deficient neuromotor control.
Another significant finding was the fact that the VL:VML intensity ratio was
a significant predictor of patella motion for both groups, however this was an
inverse relationship. For example, as the VLVML ratio decreased (increased
relative VML activity compared to the VL), the patella demonstrated increasing
lateral tilt and displacement. The sam e overall trend was observed with the
VL:VMO ratio, however its relationship to patellar kinematics was not statistically
significant. These findings emphasize the fact that EMG ratios cannot be used
with the assumption that such data are an indicator of patellar motion. To the
contrary, increased recruitment of the medial vasti appeared to be a response to
meet the increased demand of providing patellar stability. Thus, the inability of
the medial vasti to control the patella despite increased EMG, underscores the
limitation of using the level of neuromuscular activation as a predictor of the
effective muscle force acting on the patella. Without considering factors such as
muscle cross-sectional area and angle of insertion of the various muscle fibers, it
would appear this measure has little value in the assessm ent of patellofemoral
joint mechanics.
In contrast to the EMG results, patella malalignment was strongly
associated with the depth of the femoral trochlea, with greater tracking
abnormalities being exhibited by subjects with a shallow intercondylar groove.
The fact that the sulcus angle was a strong predictor of patellar kinematics,
indicates that bony structure is an important determinant of patellofemoral joint
stability, and that the force exerted on the patella by the vasti is normally
counteracted by the bony confines of the femoral trochlea. For example, vasti
contraction in the presence of trochlear groove insufficiency (ie. patella alta or
188
trochlear dysplasia) was found to have a lateralizing effect on the patella.
Although patella subluxation was not a predominate finding in our PFP
population, the tendency of the patella to track laterally can be explained by the
fact that the line of pul) of the vasti is orientated 15 to 20 degrees laterally.
Conversely, vasti contraction in conjunction with adequate trochlear depth
resulted in normal patellar kinematics. This would suggest that a lateral tracking
patella is not necessarily the result of medial vasti insufficiency, but the net effect
of the entire vasti group exerting a force on the patella without adequate bony
stabilization.
The results regarding patellar kinematics also explain the functional
adaptations patients with PFP exhibit during functional activity, namely the
quadriceps avoidance gait pattern. By adopting a slower walking velocity, the
PFP subjects were able to minimize the demand at the knee and, therefore, the
recruitment of the vasti. Such a compensation would reduce the potential
lateralizing forces on the patella as there would be a reduction in the force
requirement of the vasti. This would appear to be particularly beneficial to the
patient with a shallow intercondylar groove as the amount of shear and
compressive forces associated with malalignment would be limited. Reducing
walking velocity was the more logical adjustment for these subjects (compared
to reducing stance phase knee flexion), as this adaptation preserved the normal
loading response mechanics, and allowed for normal attenuation of the ground
reaction forces.
In conclusion, the similarity in motor unit activity between the vasti in
subjects with PFP and normal individuals indicates that these muscles work in
concert to provide knee extension force and stability during functional tasks. In
addition, there is little evidence to support the concept of patellar malalignment
189
being the result of medial vasti insufficiency (as defined by EMG). Instead,
patellar instability appears to be the result of deficient anatomical structure.
Furthermore, increased recruitment of the medial patellar stabilizers in subjects
with patellofemoral malalignment demonstrates an attempt by the
neuromuscular system to overcome a structural limitation, and stresses the
influence of joint mechanics on muscle activation.
190
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Asset Metadata

Creator Powers, Christopher Michael (author)

Core Title The role of the vasti in patellar kinematics and patellofemoral pain

School Graduate School

Degree Doctor of Philosophy

Degree Program Biokinesiology

Degree Conferral Date 1996-05

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag health sciences, medicine and surgery,health sciences, radiology,health sciences, rehabilitation and therapy,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-501760

Unique identifier UC11352130

Identifier 9636366.pdf (filename),usctheses-c17-501760 (legacy record id)

Legacy Identifier 9636366-0.pdf

Dmrecord 501760

Document Type Dissertation

Rights Powers, Christopher Michael

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA