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The influence of tibiofemoral kinematics and knee extensor mechanics on patellar tendon stress: a comparison of persons with and without patellar tendinopathy

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The influence of tibiofemoral kinematics and knee extensor mechanics on patellar tendon stress: a comparison of persons with and without patellar tendinopathy

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THE INFLUENCE OF TIBIOFEMORAL KINEMATICS
AND KNEE EXTENSOR MECHANICS ON PATELLAR TENDON STRESS:
A COMPARISON OF PERSONS
WITH AND WITHOUT PATELLAR TENDINOPATHY

By

Kyung-Mi (Jasmine) Park

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(BIOKINESIOLOGY)

August 2022

Copyright 2022 Kyung-Mi Park
ii
DEDICATION

To my family

My parents for their unconditional support and love
My husband, Taehyun Park, who always accompanies me on all my journeys
My daughter, Juha Park, who is another motivation for being a better person

iii
ACKNOWLEDGEMENTS
I spent 8 years for pursuing PhD at USC Division of Biokinesiology and Physical
Therapy, which is way longer than I and my advisor expected. During this long journey, I have
experienced many ups and downs, and at the same time, I have had lots of opportunities to
know and interact with amazingly brilliant, passionate and creative people. Without the
guidance and supports from them, I would not have completed my PhD dissertation.
First, I would like to thank my dissertation committee. I am honored to meet my advisor,
Dr. Christopher Powers who was my dream advisor from my undergraduate. He has been the
best example as a researcher, clinician, educator, writer, and presenter for me to look up to.
Also, I will be forever grateful for his continuous support and encouragement throughout the
many changes I have encountered while pursuing my PhD. I am also thankful for the guidance
from Dr. Kornelia Kulig who always gives me chances to explore concepts more deeply. I have
been inspired by her unending energy and scientific curiosity. I really appreciate Dr. Joyce
Keyak, who assisted with the development of the finite element modeling aspects of this project
and who has been passionate and generous with her time to help me for learning the basic
concepts for finite element analysis from scratch. I will never forget our meetings at the
Anaheim station as well as our conversations via more than 300 email threads. And, I would
like to thank Dr. George Salem for asking many questions to expand my thoughts and giving
me great comments on my dissertation. Lastly, I am extremely grateful for having Dr. Patrick
Colletti on my committee. He was the one of the few people who gave me compliments and
boosted my confidence by reminding me how novel and great my dissertation project is in the
field.
It would not have been possible to complete my dissertation without support from
faculty, staff, and colleagues in the USC Division of Biokinesiology and Physical Therapy. I
would like to thank our program dean, Dr. James Gordon, for all his academic and financial
iv
support, and the staff, especially Tasha Hsu, Matthew Sandusky, Ramraj Singh, Janet
Stevenson, Lydia Vazquez, Troy Lord, and Oshawa Smith, for always supporting all of the
students in our division. I would like to express my gratitude to the former and present BKN
students and especially MBRL members for their support and friendship. I am lucky to have
been able to learn and grow with amazing researchers and labmates like Jennifer (Tzu-Chieh)
Liao, Yo Shih, Steffi (Hai-Jung) Shih, Jia Liu, Jane (Jeongah) Kim, Jonathan Lee, Sungwoo
Park, Aram Kim, Sujin Kim, Sara Almansouri, Jordan Cannon and David Ortiz. And, I would
like to thank Dr. Seol Park for her volunteer assistance as well as warm encouragement.
I would like to express my deepest gratitude to my family and friends. My parents have
showed unconditional love and support, and they have encouraged all of my endeavors.
Taehyun, my husband, has been my constant source of encouragement, entertainment, and love.
I cannot thank him and our daughter, Juha, enough for making every day a privilege. And, my
Korean friends met in LA, they have made me feel at home and have been best cheerleaders
during my journey.
Finally, I want to acknowledge the financial support for my dissertation from the USC
Division of Biokinesiology and Physical Therapy, the International Society of Biomechanics,
the American Society of Biomechanics, USC Department of Radiology, Southern California
Clinical and Translational Science Institute (SC CTSI) Voucher Program. And, lastly, I would
like to recognize my research participants for making this research possible.

v
TABLE OF CONTENTS
DEDICATION ...........................................................................................................................ii
ACKNOWLEDGEMENTS ..................................................................................................... iii
LIST OF TABLES ..................................................................................................................... v
LIST OF FIGURES .................................................................................................................. ix
ABSTRACT ............................................................................................................................... x
CHAPTER I: OVERVIEW .................................................................................................... 1
CHAPTER II: BACKGROUND AND SIGNIFICANCE .................................................... 3
STATEMENT OF THE PROBLEM ...................................................................................... 3
DO PERSONS WITH PATELLAR TENDINOPATHY EXHBIT ELEVATED PATELLAR
TENDON LOADING? .......................................................................................................... 3
BIOMECHANICAL CONTRIBUTORS TO PATELLAR TENDINOPATHY .................. 5
Sagittal Plane Kinematics and Kinetics ............................................................................. 5
Frontal and Transverse Plane Kinematics .......................................................................... 6
STRUCTURUAL CAUSES OF PATELLAR TENDINOPATHY ...................................... 7
SUMMARY AND GAPS IN EXISTING LITERATURE ..................................................... 8
CHAPTER III: THE INFLUENCE OF ISOLATED FEMUR AND TIBIA ROTATIONS
ON PATELLAR TENDON STRESS: A SENSITIVITY ANALYSIS ............................... 9
INTRODUCTION ............................................................................................................... 10
METHODS .......................................................................................................................... 11
Subjects ............................................................................................................................ 11
Procedures ........................................................................................................................ 12
FE Model Development ................................................................................................... 14
Model Output & Post-processing ..................................................................................... 18
Statistical Analyses .......................................................................................................... 18
vi
RESULTS ............................................................................................................................ 19
Transverse plane .............................................................................................................. 19
Frontal plane .................................................................................................................... 20
DISCUSSION ...................................................................................................................... 22
SUMMARY ......................................................................................................................... 24
CHAPTER IV: PERSONS WITH PATELLR TENDINOPATHY EXHIBIT GREATER
PATELLAR TENDON STRESS DURING A SINGLE LIMB LANDING TASK: A
FINITE ELEMENT ANALYSIS STUDY: ......................................................................... 26
INTRODUCTION ............................................................................................................... 27
METHODS .......................................................................................................................... 28
Subjects ............................................................................................................................ 28
Procedure ......................................................................................................................... 30
Data Analysis ................................................................................................................... 33
FE Model Development ................................................................................................... 33
Model Output & Post-processing ..................................................................................... 37
Statistical Analysis ........................................................................................................... 37
RESULTS ............................................................................................................................ 38
DISCUSSION ...................................................................................................................... 40
SUMMARY ......................................................................................................................... 43
CHAPTER V: DIFFERENCE IN KNEE EXTENSOR MECHANICS BETWEEN
PERSONS WITH AND WITHOUT PATELLAR TENDINOPATHY ............................ 44
INTRODUCTION ............................................................................................................... 45
METHODS .......................................................................................................................... 47
Subjects ............................................................................................................................ 47
Procedure ......................................................................................................................... 49
vii
Statistical Analysis ........................................................................................................... 51
RESULTS ............................................................................................................................ 52
DISCUSSION ...................................................................................................................... 53
SUMMARY ......................................................................................................................... 56
CHAPTER VI: SUMMARY AND CONCLUSIONS ......................................................... 57
REFERENCES ....................................................................................................................... 63

viii
LIST OF TABLES
Table 4-1 Participants characteristics. ............................................................................... 29
Table 4-2 Comparison of the variables of interest for the patellar tendinopathy and
healthy control groups at the time of peak adjusted knee extensor moment
during a single-leg landing task ....................................................................... 38
Table 5-1 Participants characteristics. ............................................................................... 48
Table 5-2 Knee extensor mechanics for the patellar tendinopathy and healthy control
groups ............................................................................................................... 52
Table 5-3 Knee joint kinematics and kinetics for the patellar tendinopathy and healthy
control groups during a single-leg landing task ............................................... 53

ix
LIST OF FIGURES
Figure 3-1 Finite element modeling pipeline ...................................................................... 15
Figure 3-2 Maximum principal stress distribution in the patellar tendon of a
representative participant at 2 , 4 , 6 , 8  and 10  of tibia internal rotation and
femur adduction ................................................................................................. 17
Figure 3-3 Influence of femur and tibia rotation in the transverse plane on peak maximum
principal stress in the patellar tendon ................................................................ 20
Figure 3-4 Influence of femur and tibia rotation in the frontal plane on peak maximum
principal stress in the patellar tendon. ............................................................... 21
Figure 4-1 Finite element modeling pipeline ...................................................................... 35
Figure 4-2 Maximum principal stress distribution in the patellar tendon from a
representative participant with patellar tendinopathy and a control participant
........................................................................................................................... 36
Figure 4-3 Association between peak maximum principal stress in the patellar tendon
with tibiofemoral rotation in the transverse plane ............................................. 39
Figure 5-1 Comparison of quadriceps moment arm and patellar tendon moment arm between
normal patellar position and patella alta. ............................................................ 46

x
ABSTRACT
Patellar tendinopathy is a common condition among athletes who participate in sports
that involve repetitive jumping/landing movements. Excessive patellar tendon loading is
considered to be the primary cause of patellar tendinopathy. Causes of increased patellar tendon
loading can be characterized as biomechanical (e.g. lower extremity kinematics and kinetics)
and/or structural (e.g. patellar height). The objective of this dissertation was to examine the
interrelationships among tibiofemoral kinematics/kinetics, knee extensor mechanics, and
patellar tendon stress in persons with and without patellar tendinopathy. To accomplish this
objective, three studies were undertaken.
The purpose of Chapter III was to determine to determine the influence of frontal and
transverse plane rotations of the femur and tibia on peak maximum principal stress in the
patellar tendon. Using finite element (FE) modeling, patellar tendon stress profiles of 8 healthy
individuals were developed during a simulated squatting task (45° of knee flexion). Input
parameters for the FE model included joint geometry and quadriceps muscle forces. The femur
and tibia of each model were then rotated 10° (in 2° increments) along their respective axes
beyond that of the natural degree of rotation. This process was repeated for the transverse plane
(internal and external rotation) and frontal plane (adduction and abduction). Quasi-static
loading simulations were performed to quantify peak maximum principal stress in patellar
tendon. Internal and external rotations of the femur and tibia that exceeded 4 degrees beyond
that of the natural rotation resulted in progressively greater patellar tendon stress (p < 0.05).
Incremental femur and tibia adduction and abduction resulted in an increase in patellar tendon
stress, but only at the end range of motions evaluated.
The purpose of Chapter IV was to determine whether persons with patellar
tendinopathy exhibit greater peak maximum principal stress in patellar tendon compared to
healthy individuals. A secondary purpose was to determine the kinematic predictors of peak
xi
patellar tendon stress during a single-leg landing task. Using FE modeling, patellar tendon
stress profiles of 28 individuals (14 with patellar tendinopathy and 14 pain-free controls) were
created at the time of the peak knee extensor moment during a single-leg landing task. Input
parameters to the FE model included subject-specific patellofemoral joint geometry,
quadriceps muscle forces, and tibiofemoral kinematics in the frontal and transverse planes.
Independent t-tests were used to compare peak maximum principal stress in patellar tendon
between groups. In addition, independent t-tests were used to compare biomechanical variables
used as input variables to the FE model (knee flexion, knee rotation in the frontal and transverse
planes and the peak knee extensor moment). A stepwise regression model was used to
determine the best biomechanical predictor(s) of peak maximum principal stress in patellar
tendon for both groups combined. Compared to the healthy individuals, those with patellar
tendinopathy exhibited significantly greater peak maximum principal stress in the patellar
tendon (mean ± SD, 77.4 ± 25.0 MPa vs 60.6 ± 13.6 MPa, p < 0.05) and greater tibiofemoral
internal rotation compared to the control group (Mean ± SD, 4.6 ± 4.6 degrees vs 1.1 ± 4.2
degrees, p < 0.05). Transverse plane tibiofemoral rotation was the best predictor of peak
maximum principal stress in the patellar tendon (26.2% of variance, r = 0.51, p < 0.05).
The purpose of Chapter V was to compare the knee extensor mechanics in persons with
and without patellar tendinopathy. Twenty-eight individuals participated (14 with patellar
tendinopathy and 14 pain-free controls). Sagittal magnetic resonance (MR) images of the knee
were acquired at the knee flexion angle that corresponded to the knee flexion angle at the time
of peak knee extensor moment during a single-leg landing. Measurements of patellar
tendon/quadriceps tendon force (Fpl/Fq) ratio, quadriceps moment arm, patellar tendon
moment arm, and patellar height (Insall-Salvati ratio) were obtained. Independent t-tests were
used to compare the variables of interest between groups. When compared to the control group,
the patellar tendinopathy group exhibited a significantly greater Fpl/Fq ratio (Mean ± SD, 1.0
xii
± 0.1 vs 0.8 ± 0.1, p < 0.05), a larger quadriceps moment arm (Mean ± SD, 23.9 ± 2.0 mm vs
22.1 ± 2.9 mm, p < 0.05), a smaller patellar tendon moment arm (Mean ± SD, 24.2 ± 1.7 mm
vs 26.3 ± 2.4 mm, p < 0.05) and a greater Insall-Salvati ratio (Mean ± SD, 1.2 ± 0.1 vs 1.1 ±
0.1, p < 0.05).
The findings of Chapters III and IV suggest that patellar tendon stress is influenced by
tibiofemoral kinematics in the frontal and transverse planes. In addition, the findings of Chapter
V indicate that persons with patellar tendinopathy exhibit differences in the knee extensor
mechanics that may expose these individuals to higher patellar tendon loading. Specifically,
the higher Fpl/Fq ratio observed in persons with patellar tendinopathy suggests that these
individuals may experience greater forces in the patellar tendon for a given level of quadriceps
force. Taken together the findings of this dissertation highlight the interplay among
biomechanical and structural factors that may be contributory to development and progression
of patellar tendinopathy.

1
CHAPTER I

OVERVIEW
Patellar tendinopathy is highly prevalent among athletes who participate in sports that
involve repetitive jumping/landing movements.
1
Excessive patellar tendon loading has been
considered to be the primary cause of patellar tendinopathy.
2,3
Causes of increased patellar tendon
loading can be characterized as biomechanical (e.g. lower extremity kinematics and kinetics)
and/or structural (e.g. patellar height). It has been reported that greater hip adduction and knee
internal rotation were exhibited in persons with asymptomatic patellar tendon ultrasonographic
abnormality (PTA) compared to healthy controls.
2
In addition, several studies have shown that the
knee extensor mechanics such as force transmission from the quadriceps muscle to the patellar
tendon could be influenced by patellar height.
4,5
Persons with patellar tendinopathy have been
reported to have a high riding patella compared to persons without patellar tendinopathy
suggesting that these individuals could experience greater patellar tendon loading per unit of
quadriceps force.
6,7

To date, a comprehensive analysis of the biomechanical and structural factors in the context
of patellar tendon loading has not been performed. The overall purpose of this dissertation was to
examine the interrelationships among lower extremity biomechanics, knee extensor mechanics,
and patellar tendon stress in persons with and without patellar tendinopathy. To accomplish this
objective, three studies with the following specific aims were performed:

SPECIFIC AIM 1: To determine the influence of frontal and transverse plane rotations of the
femur and tibia on peak maximum principal stress in the patellar tendon (sensitivity analysis).
2
SPECIFIC AIM 2: To determine whether persons with patellar tendinopathy exhibit greater peak
patellar tendon stress compared to pain-free individuals. A secondary purpose was to determine
the kinematic predictors of peak patellar tendon stress during a single-leg landing task.

SPECIFIC AIM 3: To compare the knee extensor mechanics in persons with and without patellar
tendinopathy.

3
CHAPTER II

BACKGROUND AND SIGNIFICANCE

STATEMENT OF THE PROBLEM
Patellar tendinopathy is characterized by pain originating from the patellar tendon and/or
its bony attachment. This condition is often called “jumper’s knee”, as it is commonly observed
among athletes who participate in the sports involving repetitive and explosive jump/landing tasks.
Specifically, the prevalence of the patellar tendinopathy has been reported to be as high as 30%
and 45% among elite basketball and volleyball players, respectively.
8
Patellar tendinopathy often
becomes a chronic condition and can have detrimental effect on an athletic career.
9

Excessive patellar tendon loading is considered to be the primary cause of patellar
tendinopathy.
10
The patellar tendon can be exposed to a force as high as about 6 times an
individual’s body weight with a loading rate of about 70 body weights per second during landing
movements.
2
Causes of increased patellar tendon loading can be characterized as biomechanical
(e.g. lower extremity kinematics and kinetics) and/or structural (e.g. patellar height). The following
sections briefly review the pertinent literature in this area.

DO PERSONS WITH PATELLAR TENDINOPATHY EXHBIT ELEVATED PATELLAR
TENDON LOADING?
To date, studies comparing patellar tendon loading between persons with and without
patellar tendinopathy are few and a clear relationship between mechanical loading and patellar
tendinopathy has yet to be established. A few studies have used the peak vertical ground reaction
4
force as a predictor of the load in the patellar tendon.
11,12
However, Finni, et al.
13
reported different
peak patellar tendon forces during two different tasks that generated similar peak vertical ground
reaction forces suggesting that the peak vertical ground reaction force may not be an accurate
variable reflecting the force sustained by the patellar tendon.
Additional studies have estimated patellar tendon stress in persons with and without
patellar tendinopathy. For example, patellar tendon stress has been quantified by simply dividing
the estimated tendon force during a maximal voluntary isometric contraction (MVIC) by the distal
patellar tendon cross-sectional area as measured from MR images. However, findings among
studies have been inconsistent.
10,14
Couppé, et al.
10
reported that elite badminton players with
patellar tendinopathy exhibited 52% higher patellar tendon stress compared to those without
patellar tendinopathy. In contrast, Wiesinger, et al.
14
reported lower patellar tendon stress in
persons with patellar tendinopathy compared to those without patellar tendinopathy. It should be
noted that neither group considered the influence of lower extremity kinematics in their tendon
stress estimates.
To date, only one study has reported the maximum principal stress in the patellar tendon
using a three-dimensional (3D) finite element (FE) model generated from a healthy participant.
15

Results demonstrated that peak maximum principal stresses are region-specific in the patellar
tendon during 0° to 90° knee flexion and slow and fast level-ground walking. Specifically, central
proximal posterior region of the patellar tendon exhibted the highest peak stress during knee
flexion and walking activities. However, a limitation of Wang, et al.
15
was that a tendinopathy
comparison group was not evaluated. A study in which patellar tendon stress profiles obtained
using subject-specific 3D FE models that account for tendon morphology, quadriceps force, and
tibiofemoral kinematics in persons with and without patellar tendinopathy has yet to be undertaken.
5

BIOMECHANICAL CONTRIBUTORS TO PATELLAR TENDINOPATHY
It has been hypothesized that altered lower extremity biomechanics during
jumping/landing movements can be contributory to excessive patellar tendon loading and the
development of patellar tendinopathy. Previous studies have reported differences in tibiofemoral
biomechanics in sagittal, frontal and transverse planes between persons with and without patellar
tendinopathy. In particular, the degree of knee flexion and the magnitude of the knee extensor
moment are variables that have been evaluated to gain insight into the patellar tendon loading
during various types of jumping/landing tasks.
2,11,12,16-19
In contrast, little attention has been paid
to the potential influence of lower extremity kinematics in frontal and transverse planes.
2,17

Although there is mixed evidence to suggest that persons with patellar tendinopathy may exhibit
differences in frontal and transverse plane kinematics at the hip and knee, it is not clear if such
motions influence patellar tendon loading.

Sagittal Plane Kinematics and Kinetics
A greater maximum knee flexion angle during landing is commonly used as an indicator
of higher loads on the patellar tendon. Richards, et al.
11
reported that the greater maximum knee
flexion angle during landing from a spike jump was significantly related with the likelihood of the
patellar tendinopathy. Edwards, et al.
2
also reported that athletes with asymptomatic patellar
tendon ultrasonographic abnormality (PTA), the precursor of patellar tendinopathy, exhibited
significantly greater knee flexion angles at initial contact and relatively greater knee flexion angles
during landing compared to healthy control group. The influence of elevated knee flexion on
patellar tendon loading can be explained by the results of a study conducted by Lavagnino, et al.
20
.
6
These authors investigated the maximal principal strain in the patellar tendon in response to a
decrease in patella-patellar tendon angle (corresponding to increase in knee flexion angle) using
FE analysis. Results revealed that patellar tendon strain increased as the knee flexion angle
increased, implying a higher risk of development of patellar tendinopathy.
Apart from knee kinematics, the magnitude of the knee extensor moment also has been
hypothesized as contributing to patellar tendon loading. However, several studies have reported
that persons with patellar tendinopathy exhibit lower knee extensor moments,
12
or no differences
in the knee extensor moment
11,19
during landing compared to pain-free controls. Given that a
higher knee extensor moment would be expected to result in greater patellar tendon stress, the
results of previous studies in this area suggest that persons with patellar tendinopathy may employ
compensatory strategies to avoid the loading of the patellar tendon.

Frontal and Transverse Plane Kinematics
Greater hip adduction and knee internal rotation during landing tasks have been reported
in persons with asymptomatic PTA compared to control group.
2
However, it is not clear if such
motions influence patellar tendon loading. Given that the patellar tendon inserts on the tibia, it is
reasonable to speculate that frontal and transverse plane motions of the tibia could have a
significant influence on patellar tendon stress. Similarly, frontal and transverse plane motions of
the femur have been reported to influence patellofemoral joint kinematics,
21,22
which in turn could
influence patellar tendon stress. It has been suggested that tibiofemoral kinematics in the transverse
plane is related with patellar tendon loading by altering patella and/or patellar tendon orientation
during quadriceps contraction.
21-23
Varadarajan, et al.
23
found a significant positive correlation
between tibia internal rotation and patellar tendon twist during weight-bearing knee flexion. In
7
addition, Li, et al.
21
found that patellofemoral joint kinematics are influenced by transverse plane
femur rotations. Specifically, femoral rotation was significantly and strongly correlated with
patellar tilt during weight-bearing knee flexion.
Taken together, the limited literature in this area suggests that altered knee joint kinematics
in the frontal and transverse planes may contribute to higher loading in patellar tendon. However,
the relationships among frontal and transverse knee joint kinematics and patellar tendon loading
needs further investigation.

STRUCTURUAL CAUSES OF PATELLAR TENDINOPATHY
It has been suggested that altered force transmission from the quadriceps muscle group to
the patellar tendon may be contributory to patellar tendinopathy. The ratio of patellar tendon force
to quadriceps force (Fpl/Fq ratio), is one of the key variables that has been considered in the context
of patellar tendon loading. The Fpl/Fq ratio has been shown to be influenced by anatomical factors
related to the patellofemoral joint including the shape of the distal femur and patella, the changing
point of contact between the patella and femur as the knee flexes and extends, and patellar
height.
4,24
Specifically, patellar height is a structural characteristic that may be influential in the
context of patellar tendinopathy. Persons with a high riding patella (ie. patella alta) have been
shown to have a significantly higher Fpl/Fq ratio compared to persons with normal patellar height.
5

There is evidence that persons with patellar tendinopathy may exhibit a higher patellar
position compared to those without tendinopathy.
6
In addition, Dan, et al.
7
reported that the
moment arms for the quadriceps tendon relative to the patellar tendon was greater in persons with
apparent patellar tendinopathy compared to those without patellar tendinopathy. As such, it is
possible that persons with patellar tendinopathy exhibit anatomical features that expose these
8
individuals to higher patellar tendon forces for a given level of quadriceps force. To date, it not
known if increased patellar height leads to higher patellar tendon stress during dynamic activities
associated such as jumping and landing.

SUMMARY AND GAPS IN EXISTING LITERATURE
Based on previous literature, elevated patellar tendon stress can be influenced by various
factors including altered lower extremity biomechanics and/or altered knee extensor mechanics.
Although studies have explored the differences in tibiofemoral kinematics/kinetics in persons with
and without patellar tendinopathy, most have focused on sagittal plane variables. In contrast, little
attention has been paid to motion in the frontal and transverse planes. Also, it is not known which
tibiofemoral kinematic and/or kinetic factors are most predictive of elevated patellar tendon stress
in persons with patellar tendinopathy.
Apart from biomechanical factors, there is evidence at altered knee extensor mechanics
may be contributory to elevated patellar tendon loading in this population. However, a
comprehensive evaluation of specific structure factors that may contribute to excessive patellar
tendon loading has not been undertaken. The overall purpose of this dissertation is to quantify the
interrelationship between tibiofemoral kinematics and kinetics, knee extensor mechanics and
patellar tendon stress in persons with and without patellar tendinopathy. Through a better
understanding of underlying factors contributing to patellar tendinopathy, more effective
intervention programs can be developed.

9
CHAPTER III

THE INFLUENCE OF ISOLATED FEMUR AND TIBIA ROTATIONS ON PATELLAR
TENDON STRESS: A SENSITIVITY ANALYSIS USING FINITE ELEMENT
ANALYSIS

The purpose of this study was to determine the influence of frontal and transverse plane
rotations of the femur and tibia on peak maximum principal stress in the patellar tendon. Using FE
modeling, patellar tendon stress profiles of 8 healthy individuals were obtained during a simulated
squatting task (45° of knee flexion). The femur and tibia of each model were rotated 10° (in 2°
increments) about their respective axes beyond that of the natural degree of rotation, defined from
MRI. This process was repeated for the transverse plane (internal and external rotation) and frontal
plane (adduction and abduction). Quasi-static loading simulations were performed to quantify peak
maximum principal stress in patellar tendon. To test the hypothesis that peak patellar tendon stress
varied with femur and tibia rotation angles, a 2 x 6 analysis of variance (ANOVA) with repeated
measure was performed (segment x rotation angle). Post-hoc paired t-tests were performed if there
were significant interactions.

10
INTRODUCTION
Patellar tendinopathy is characterized by pain originating from the patellar tendon and/or
its bony attachment.
25
This condition is often called “jumper’s knee” as it is commonly observed
among athletes who participate in the sports involving repetitive and explosive jump/landing
tasks.
8,26
The prevalence of patellar tendinopathy has been reported to be as high as 30% and 45%
among elite basketball and volleyball players, respectively.
8

Excessive patellar tendon loading has been considered to be the primary cause of patellar
tendinopathy.
10
In particular, the degree of knee flexion and the magnitude of the knee extensor
moment are variables that have been evaluated to gain insight into loading of the patellar tendon
during various types of jumping/landing tasks.
2,11,12,16-19
In contrast, little attention has been paid
to the potential influence of lower extremity kinematics in frontal and transverse planes.
2,17

Edwards, et al.
2
reported significantly greater hip adduction and knee internal rotation during a
landing task in persons who exhibited asymptomatic (pain-free) PTA as quantified using
ultrasound compared to those without documented PTA. However, Rosen, et al.
17
reported no
significant differences in frontal and transverse plane knee kinematics between persons with and
without patellar tendinopathy during a vertical jump.
Although there is mixed evidence to suggest that persons with patellar tendinopathy may
exhibit differences in frontal and transverse plane kinematics at the hip and knee, compared with
those without patellar tendinopathy, it is not clear if such motions influence patellar tendon loading.
Given that the patellar tendon inserts on the tibia, it is reasonable to speculate that frontal and
transverse plane motions of the tibia could have a significant influence on patellar tendon stress.
Similarly, frontal and transverse plane motions of the femur have been reported to influence
patellofemoral joint kinematics
21,22
which in turn could influence patellar tendon stress.
11
To date, no study has systematically explored the influence of isolated tibia and femur
rotations on patellar tendon stress. Evaluating isolated femur and tibia rotations in the frontal and
transverse planes (as opposed to relative tibiofemoral joint rotations) is important as each segment
motion has the potential to have a distinct influence on the patellar tendon loading. The purpose
of the current study was to investigate the influence of femur and tibia rotations in the frontal and
transverse planes on peak maximum principal stress in the patellar tendon using 3D FE modeling.
It was hypothesized that rotations of both the femur and tibia in the frontal and transverse planes
would result in elevated peak maximum principal stress in the patellar tendon. Given that the
patellar tendon attaches to the tibial tuberosity, we also hypothesized that tibia rotations in either
plane would result in greater peak maximum principal stress in the patellar tendon when compared
to femur rotations.

METHODS

Subjects
Eight healthy, active individuals (3 males and 5 females) between the ages of 18 and 28
participated. Participants were excluded if any of the following were present: 1) previous history
of knee pathology or surgery, 2) current knee pain or effusion, 3) neurological involvement that
would influence balance control, 4) contraindications to MR imaging: implanted electronic devices
(i.e., pacemaker), metallic implants (i.e., aneurysm clips, fixation screws), or claustrophobia. Prior
to the beginning of the study, participants were informed as to the nature of the study and signed
a human subject’s consent form approved by the Health Sciences Institutional Review Board of
the University of Southern California.

12
Procedures
Participants underwent two data collection sessions on separate days. The first session
consisted of biomechanical testing, whereas the second session consisted of MR assessment of the
tibiofemoral and patellofemoral joint.

Biomechanical Testing
Biomechanical data were collected at the Jacquelin Perry Musculoskeletal Biomechanics
Research Laboratory at University of Southern California. Participants were instrumented for 3D
motion and electromyography (EMG) analyses as described in a previous publication.
27
Lower
extremity kinematics were captured at 150 Hz using 11-camera QUALISYS motion analysis
system (Qualisys Inc., Gothenburg, Sweden). Three molded thermoplastic clusters with rigid
reflective tracking markers were securely placed laterally on the thigh, leg, and heel counter of the
shoes on each side. Additional tracking markers were placed bilaterally on the iliac crests and the
L5/S1 junction. In addition to tracking markers, calibration markers were placed bilaterally on the
greater trochanters, anterior superior iliac spines, medial and lateral femoral epicondyles, medial
and lateral malleoli, and 1st and 5th metatarsal heads. Ground reaction forces were collected at
1500 Hz using a force plate (AMTI Model #OR6-6-1, Newton, MA).
EMG signals of selected lower extremity muscles were recorded at 1500 Hz using surface
EMG electrodes (Noraxon, Scottsdale, AZ). EMG data were used for the estimation of quadriceps
muscle forces (see below for details). The electrodes of the medial and lateral hamstrings were
placed midway between the ischial tuberosity and the medial and lateral sides of the popliteal fossa,
respectively.
28
The electrodes for the medial and lateral gastrocnemius were placed at one third of
the distance between the medial and lateral sides of the popliteal fossa, respectively, and the
13
Achilles tendon insertion, starting from the popliteal fossa.
28
EMG data were normalized to the
activity acquired during a MVIC.
A standing calibration trial was obtained to define the anatomical coordinate systems.
Following a standing calibration trial, subjects were asked to perform a bilateral squat to 90˚ of
knee flexion with each foot positioned on a separate force plate. The speed of the squat was
controlled with the use of a metronome such that the descending and ascending phases were 3s
each. Kinematic, kinetic and EMG data were collected simultaneously and synchronized during a
squatting motion. Biomechanical data for input parameters for FE modeling were obtained at 45˚
of knee flexion during the descending phase of the squat.

MR Assessment
Subject-specific bone geometry of the knee and the patellofemoral joint, and morphology
of the cartilage and patellar tendon were obtained using a 3.0T MR scanner (General Electric
Healthcare, Milwaukee, WI, USA). Images were acquired with an 8-channel knee coil using a 3D,
high-resolution, fat-suppressed, fast spoiled gradient-recalled echo sequence (SPGR) (repetition
time, 14.5 ms; echo time, 2.8 ms; flip angle, 10°; matrix, 320 × 320; field of view, 16 cm; slice
thickness, 1.0 mm; scan time, 8:58 min). During this scan, subjects were positioned supine with
45˚ of knee flexion.
Quadriceps muscle morphology was assessed from sagittal plane MR images of the thigh
using a 3D SPGR protocol (repetition time, 9.4 ms; echo time, 4.1 ms; flip angle, 20°; matrix, 384
× 384; field of view, 46 cm; slice thickness, 2 mm; scan time, 8:03 min). Sagittal plane images of
the thigh were subsequently reconstructed in the coronal and axial planes and were used to estimate
the 3D fiber orientation of each of the quadriceps muscles.
29
Axial images of the thigh were utilized
14
to measure the cross-sectional area of the quadriceps muscles which was subsequently used as an
input variable for the biomechanical model to estimate the magnitude of muscle forces.
29

FE Model Development
Subject-specific input parameters entered into the modeling pipeline included joint
geometry and quadriceps muscle forces (Figure 3-1).
30
Using a commercial software package
(Sliceomatic, Tomovision, Montreal, Quebec), the high resolution, sagittal plane MR images of
the knee were manually segmented and the 3D surfaces of the femur, tibia, patella, patellar tendon,
and articular cartilage covering of the femur and patella were created. Surfaces created for the
femur, tibia, and patella were subsequently used to create a rigid body shell of each bony structure
using a proprietary FE pre-processor (Hypermesh, Altair Engineering Inc., Troy, MI). The articular
cartilage of the patella and femur was modeled as homogeneous and isotropic using tetrahedral
continuum elements with an elastic modulus of 25 MPa
31
and a Poisson ratio of 0.47.
32
The patellar
tendon was modeled as homogeneous with transversely isotropic elastic properties using
tetrahedral continuum elements. Nine elastic constants for the patellar tendon included transverse
moduli (E1=E2= 46.13 MPa), longitudinal modulus (E3=966.58 MPa), Poisson’s ratios
(V12=0.25; V13=V23=0.03), and shear moduli (G12=11.66 MPa; G13=G23=60.75 MPa)
whereby direction 3 was parallel to the fibrils.
33

The methods used to estimate the individual quadriceps muscle forces from the
biomechanical testing session have been described previously.
29
Briefly, a subject-specific
representation of the extensor mechanism was created using SIMM modeling software
(MusculoGraphics, Santa Rosa, CA). Subject-specific biomechanical data (kinematics, kinetics,
15
EMG) were used to drive the model (via an optimization routine) and 3D quadriceps muscle forces
were computed.
29

Figure 3-1. Finite element modeling pipeline. Reproduced with permission (Liao, et al.
30
).

The elements representing the quadriceps muscles were separated into four functional
groups comprised of uniaxial force actuators (the rectus femoris, vastus intermedius, vastus
medialis, and vastus lateralis muscles). Six actuators were used to represent the forces of the vastus
medialis and vastus lateralis, while three actuators were used to represent the forces of the rectus
femoris and vastus intermedius. The direction of muscle lines of pull for the rectus femoris and
vastus intermedius group were set parallel to the long axis of the femur.
34
The most lateral and
medial borders of the quadriceps line of pull (i.e., the borders of the vastus lateralis and vastus
16
medialis) were determined from the fiber orientation of each muscle in the sagittal and frontal
planes, as measured from MR images of the subject's thigh.
29
Connector elements with stiffness
of 10 N/mm representing each muscle group were then distributed uniformly from the medial-to-
lateral borders.
35

Simulations were performed using a hard contact algorithm with a surface coefficient of
friction of 0.02.
32
The surface consisted of articular surfaces of femur and patella cartilages. Quasi-
static loading simulations were performed using a nonlinear FE solver (Abaqus, SIMULIA,
Providence, RI). The initial (natural) orientations of the patella, femur, and tibia were determined
from the MR images at 45° of knee flexion. To examine the influence of femur and tibia rotations
on patellar tendon stress, the femur and tibia were rotated along their respective axes from the
initial (natural) position to 2°, 4°, 6°, 8°, and 10°. Since the soft tissues controlling the rotation of
the tibiofemoral and patellofemoral joint were not included in the models, the three rotational
degrees of freedom of the patella were constrained during the re-alignment procedure and the
simulations. However, the patella was allowed to translate within trochlear groove.
The re-alignment process was repeated for the transverse plane (internal and external
rotation) and frontal plane (abduction and adduction). The medial-lateral axis of the femur was
established by the midpoints of medial and lateral femoral condyles and the center of rotation was
defined as the midpoint between the medial and lateral condyles. The superior-inferior axis of the
femur was established to be parallel to the femur shaft, while the anterior-posterior axis was
established to be perpendicular to the other two axes. Similarly, the medial-lateral axis of the tibia
and center of rotation were established by the midpoints of the centroids of medial and lateral tibial
plateau. The superior-inferior axis of the tibia was established to be parallel to the superior-inferior
axis of MR global coordinate system, and the anterior-posterior axis was then established to be
17
perpendicular to the other two axes. A total of 40 re-aligned models were created for each
participant (beyond that of the natural position).
The quadriceps muscles were allowed to rotate with the segments, femur and tibia
respectively. The three rotational degrees of freedom of the patella were constrained during the re-
alignment procedure and the simulation since the soft tissues controlling the rotation of the
tibiofemoral and patellofemoral joint were not included in the models. Model output from a
representative participant for tibia internal rotation and femur adduction is presented in Figure 3-
2.

Figure 3-2. Maximum principal stress distribution in the patellar tendon of a representative
participant (anterior view) at 2 , 4 , 6 , 8  and 10  of tibia internal rotation (A) and femur
adduction (B).
18
Model Output & Post-processing
The maximum principal stress, which approximates the tensile stress that tends to elongate
the patellar tendon,
20
was calculated at each node. Elements representing the bone-tendon interface
(ie. proximal and distal ends of the tendon) were not considered because the material properties of
the bone-tendon interface are unknown owing to a gradual transitional zone from tendon to bone,
including a combination of uncalcified and calcified layers of tissues.
36
In addition, elements
within 3 mm (2 times the slice thickness of MR Images) from the ends of the remaining tendon
were not considered because the material properties of the adjacent bone-tendon interface could
potentially influence the stress values by an unknown amount.
36
The remaining patellar tendon
was examined to identify the peak value of the maximum principal stress. The peak value of the
maximum principal stress within the patellar tendon was determined as follows.
The peak value of the maximum principal stress within the patellar tendon was determined
using a process designed to avoid spurious peak stress values. First, the maximum principal stress
within 5 nodes (2.5 mm) in each direction of the node that exhibited the peak maximum principal
stress were screened. If the maximum principal stresses at any of these adjacent nodes were less
than 30% of the peak value of maximum principal stress, the peak maximum principal stress at
this particular location was considered an artifact and disregarded. This vetting process was built
into an algorithm in MATLAB (MathWorks, Natick, MA) and repeated automatically until a “true”
peak maximum stress value was identified.

Statistical Analyses
To test the hypothesis that the peak maximum principal stress in the patellar tendon differed
depending on femur and tibia rotation angles, a 2 × 6 (segment × rotation angle) analysis of
19
variance (ANOVA) with repeated measures was performed. This analysis was repeated for internal
rotation, external rotation, adduction, and abduction. If significant main effects (no interaction) for
segment were identified, collapsed (combined) means across rotation angles were reported. If a
significant interaction was identified, post-hoc paired t-tests were employed. For all main effect
and post-hoc analyses, only comparisons to the natural rotation were made. For all statistical
analyses, the significance level was set as α = 0.05.

RESULTS

Transverse plane
With respect to internal rotation, the ANOVA revealed no significant rotation angle ×
segment interaction (p = 0.70), and no significant main effect for segment (p = 0.62). However, a
significant main effect for rotation angle was found (p < 0.05). When combined across the femur
and tibia segments, peak maximum principal stress in the patellar tendon was significantly greater
at 4°, 6°, 8°, and 10° of internal rotation (13.9  3.3, 18.1  3.9, 22.8  5.5 and 27.6  7.2 MPa,
respectively) when compared to the natural position (8.1  1.5 MPa, p < 0.05) (Figure 3-3).
For external rotation, the ANOVA revealed no significant rotation angle × segment
interaction (p = 0.05), and no significant main effect for the segment (p = 0.37). However, a
significant main effect for rotation was found (p < 0.05). When combined across the femur and
tibia segments, peak maximum principal stress in the patellar tendon was significantly greater at
4°, 6°, 8°, and 10° of external rotation (13.7  3.3, 17.1  4.6, 21.7  5.3 and 26.0  6.4 MPa,
respectively) when compared to the natural position (8.1  1.5 MPa, p < 0.05) (Figure 3-3).
20

Figure 3-3. Influence of femur and tibia rotation in the transverse plane on peak maximum
principal stress in the patellar tendon. 0° represents the natural degree of rotation. *combined
means for the tibia and femur were significantly different from natural rotation.

Frontal plane
With respect to adduction, the ANOVA revealed no significant rotation angle × segment
interaction (p = 0.07). However, statistically significant main effects for rotation and segment were
found (p < 0.05) (Figure 3-4). When combined across the femur and tibia segments, peak
maximum principal stress in the patellar tendon was significantly greater at 10° of adduction when
compared to the natural position (16.2  8.5 MPa vs 8.1  1.5 MPa, p < 0.05) (Figure 3-4). When
averaged across rotation angles, peak maximum principal stress in patellar tendon was
significantly greater with tibia adduction compared to femur adduction (14.0  6.5 MPa vs 8.7 
2.6 MPa, p < 0.05) (Figure 3-4).
21
For abduction, the ANOVA revealed no significant rotation angle × segment interaction (p
= 0.12). However, statistically significant main effects for rotation and segment were found (p <
0.05) (Figure 3-4). When combined across the femur and tibia segments, peak maximum principal
stress in the patellar tendon was significantly greater at 6°, 8° and 10° of abduction (13.1  3.0,
14.8  3.7, and 16.6  4.8 MPa, respectively) when compared to the natural position (8.1  1.5
MPa, p < 0.05) (Figure 3-4). When averaged across rotation angles, peak maximum principal stress
in the patellar tendon was significantly greater with tibia abduction compared to femur abduction
(13.9  5.0 MPa vs 10.7  2.2 MPa, p < 0.05) (Figure 3-4).

Figure 3-4. Influence of femur and tibia rotation in the frontal plane on peak maximum principal
stress in the patellar tendon. 0° represents the natural degree of rotation. *combined means for the
tibia and femur were significantly different from natural rotation. #combined means for tibia
adduction and abduction were significantly different from femur adduction and abduction.

22
DISCUSSION
The purpose of the current study was to determine the influence of femur and tibia rotations
in the frontal and transverse planes on patellar tendon stress. Consistent with our primary
hypothesis, we found that rotations of both the femur and tibia in the frontal and transverse planes
resulted in elevated peak maximum principal stress in the patellar tendon (Figure 3-3 and 4). With
respect to our secondary hypothesis, rotations of the tibia resulted in greater peak maximum
principal stress compared to the femur, but only for rotations in the frontal plane. Taken together
our findings suggest that the patellar tendon is susceptible to elevated stress with greater degrees
of frontal and transverse plane femur and tibia rotation.
Transverse plane rotation of the femur and tibia had a more pronounced influence on
patellar tendon stress compared to frontal plane rotation. As visualized in Figure 3-3, a linear
increase in patellar tendon stress was observed with incremental rotation in either direction.
Specifically, for every degree of femur or tibia internal rotation beyond the natural position there
was a corresponding increase of 2.0 MPa (24.1%) in patellar tendon stress. Similarly, patellar
tendon stress increased by 1.8 MPa (22.1%) for every degree of femur or tibia external rotation.
This finding may be clinically relevant as Edwards, et al.
2
reported that persons with asymptomatic
PTA exhibited 7 degrees greater tibiofemoral joint internal rotation compared to a healthy control
group. Given that tibiofemoral internal rotation could be the result of internal rotation of the tibia
relative to the femur or femur external rotation relative to the tibia, our results suggest that small
amounts of rotation motion beyond that required for normal tibiofemoral joint function could be
relevant from a tissue pathology perspective.
Isolated femur and tibia abduction also resulted in a progressive increase in patellar tendon
stress, but statistical significance only was achieved at the end range of the motions evaluated.
23
Significant differences only were observed at 10 of adduction, while significant increases in peak
maximum principal stress were observed at 6  of abduction and beyond. In addition, frontal plane
motion of the tibia had a greater influence on patellar tendon stress than frontal plane motion of
the femur. Linear increases in peak maximum principal stress were observed for rotations in either
direction albeit to a lesser degree than transverse plane rotations. Specifically, we observed a 1.5
MPa (18.7%) and 1.2 MPa (15.0%) increase in peak maximum principal stress for every degree of
tibia adduction and abduction, respectively, beyond the natural position. The finding that frontal
plane motion of the tibia had a greater influence on patellar tendon stress than frontal plane motion
of the femur may be explained by the fact that the patellar tendon has its boney attachment on the
tibia.
The findings of the current study are consistent with reports for the patellofemoral joint.
Liao, et al.
35
found that frontal and transverse plane motion of the tibia and femur had a significant
influence on patella cartilage stress. Interestingly, Liao, et al.
35
reported that patellofemoral stress
was most susceptible to femur rotation in the transverse plane, while the current study found that
patellar tendon stress was more susceptible to transverse plane tibia rotation. This difference
between studies is logical as the patellar tendon attaches to the tibia, while the patellofemoral joint
involves articulation of the patella and the trochlear surface of the femur. Regardless, the results
of the current study and that of Liao, et al.
35
suggest that the extensor mechanism is highly
susceptible to transverse plane rotations and, to a lesser extent, frontal plane motions. Future
biomechanical studies involving persons with patellar tendinopathy should consider both frontal
and transverse plane kinematics to gain a complete picture of the potential mechanisms underlying
patellar tendinopathy.
24
It should be noted that the current study only examined isolated femur and tibia rotations.
Combined rotations of the femur and tibia in multiple planes may have a compounding influence
on patellar tendon stress. In addition, structural features such as trochlear morphology and patellar
height (ie. patella alta) may have a possible influence on patellar tendon stress as these variables
are known to influence patellofemoral joint alignment and patellar tracking.
22,37,38
Future studies
should consider both kinematic and structural variables to gain a complete picture of the potential
contributors to elevated patellar tendon stress.
The results of the current study need to be interpreted in light of several limitations. First,
data were obtained from young and healthy individuals which limits the generalizability of the
results to other populations (i.e. persons with patellar tendinopathy). Second, the results from our
model were obtained during simulated squatting to 45 degrees of knee flexion. It is not clear if
femur and tibia rotations would be as influential at higher or lower knee flexion angles. Third, soft
tissues that surround and support the patellofemoral joint were not included in our model. It is
possible that omission of the stabilizing soft tissues of the patella could have influenced patella
motion and therefore patellar tendon stress. Lastly, the patellar tendon was modeled as
homogeneous, linearly elastic and transversely isotropic. Additional studies are necessary to
measure the parameters that will allow the computational model to incorporate the precise
mechanical behavior of the patellar tendon and bone-tendon junction.

SUMMARY
Rotations of the femur and tibia in the transverse and frontal planes have a significant
influence on patellar tendon stress. Specifically, transverse plane rotations of the tibia and femur
have a larger influence on patellar tendon stress compared to rotations in the frontal plane. In
25
general, rotations of the tibia were found to have a greater influence on patellar tendon stress
compared to rotations of the femur. These findings highlight the importance of frontal and
transverse plane knee kinematics as potentially contributing to the development of patellar
tendinopathy.
26
CHAPTER IV

PERSONS WITH PATELLAR TENDINOPATHY EXHIBIT GREATER PATELLAR
TENDON STRESS DURING A SINGLE-LEG LANDING TASK:
A FINITE ELEMENT ANALYSIS STUDY

The purpose of this study was to compare peak maximum principal stress in the patellar
tendon between persons with and without patellar tendinopathy during simulated single-leg
landing. A secondary purpose was to determine the biomechanical predictor(s) of peak maximum
principal stress in the patellar tendon. Using FE modeling, patellar tendon stress profiles of 28
individuals (14 with patellar tendinopathy and 14 pain-free controls) were created at the time of
the peak knee extensor moment during a single-leg landing task. Input parameters to the FE model
included subject-specific patellofemoral joint geometry, quadriceps muscle forces, and lower
extremity kinematics. Independent t-tests were used to compare the peak maximum principal stress
in the patellar tendon between groups. In addition, independent t-tests were used to compare
biomechanical variables used as input variables to the FE model (knee flexion, knee rotation in the
frontal and transverse planes and the peak knee extensor moment). A stepwise regression model
was used to determine the best biomechanical predictor(s) of peak maximum principal stress in
the patellar tendon for both groups combined.

27
INTRODUCTION
Patellar tendinopathy is one of the most common conditions in physically active
populations. Specifically, athletes who participate in sports that require repetitive and explosive
jump/landing activities exhibit a high prevalence rate of patellar tendinopathy (30% and 45% of
elite basketball and volleyball players, respectively).
8,26
The etiology of patellar tendinopathy is
multifactorial, with a variety of risk factors having been proposed (i.e. training volume, quadriceps
muscle tightness, altered lower extremity biomechanics, etc.).
39-41

From a biomechanical standpoint, altered sagittal plane kinematics and kinetics have been
proposed to contribute to patellar tendinopathy. In particular, elevated knee flexion angles and
knee extensor moments have been considered to be indicative of the higher patellar tendon loading
during landing activities. Richards, et al.
11
reported that the likelihood of the patellar tendinopathy
was statistically related to higher knee flexion angles during landing from a spike jump.
11
In
addition, Edwards, et al.
2
observed that athletes with patellar tendon abnormalities as quantified
using ultrasound, exhibited significantly greater knee flexion angles at initial contact and during
landing compared to a healthy control group. However, studies comparing knee extensor moments
between persons with and without patellar tendinopathy during landing activities have reported
that persons with patellar tendinopathy actually exhibit similar
11,19
or lower knee extensor
moments
12,18
compared to those without patellar tendinopathy.
Apart from sagittal plane biomechanics, it has been proposed that abnormal frontal and
transverse plane kinematics may be associated with higher patellar tendon loading. A recent study
by our group demonstrated that patellar tendon stress is highly influenced by frontal and transverse
plane motions of the tibia and femur.
42
In particular, transverse plane motions of the tibia and
femur had the greatest influence on patellar tendon stress. Studies examining motions of the knee
28
joint in frontal and transverse planes are few, however there is evidence to suggest that persons
with asymptomatic patellar tendon abnormalities based on ultrasound findings (either hypoechoic
regions or fusiform swelling) exhibit greater transverse plane knee rotation compared to those
without asymptomatic PTA.
2

Although persons with patellar tendinopathy have been shown to exhibit abnormal lower
limb kinematics and kinetics during jumping and landing compared to healthy persons, it is not
clear how these biomechanical differences influence patellar tendon stress. In addition, it is not
known which kinematic variables are most predictive of patellar tendon stress. Using a 3D subject-
specific FE modeling approach, the purpose of the current study was to determine whether persons
with patellar tendinopathy exhibit greater peak maximum principal stress in the patellar tendon
compared to pain-free controls during a simulated single-leg landing task. A secondary purpose
was to determine the kinematic predictors of peak maximum principal stress in the patellar tendon.
We hypothesized that persons with patellar tendinopathy would exhibit greater peak maximum
principal stress in the patellar tendon compared to persons without patellar tendinopathy during
single-leg landing. In addition, we hypothesized that tibiofemoral rotation in the transverse plane
would be the best predictor(s) of elevated maximum principal stress in the patellar tendon in
persons with and without patellar tendinopathy.

METHODS

Subjects
Twenty-eight persons participated in this study: 14 with patellar tendinopathy and 14 who
were pain-free (Table 4-1). All participants were between the ages of 18 and 36 and were
29
physically active. Participants’ physical activity levels were examined based on the World Health
Organization’s Global Physical Activity Questionnaire (GPAQ). The GPAQ has been widely used
for a valid and reliable estimate of physical activity.
43

Table 4-1. Participant characteristics (mean ± standard deviation).

PT group
(n=14)
Control group
(n=14)
p value
Sex (female/male, n) 8/6 8/6 NA
Age (years) 25.0 ± 4.9 25.6 ± 4.5 0.75
Height (m) 1.71 ± 0.7 1.71 ± 0.7 0.81
Weight (kg) 66.3 ± 11.0 66.6 ± 10.7 0.92
Activity Level (MET. mins/week) 1124 ± 548 1104 ± 384 0.56
VISA-P 65.6 ± 11.2 NA NA
Duration of PT symptoms (mos) 22.7 ± 13.3 NA NA
PT: patellar tendinopathy; mos: months; MET: Metabolic equivalents; VISA-P: Victorian Institute
of Sports Assessment-Patellar Tendinopathy; NA: not applicable.

To be included in the patellar tendinopathy group, participants had to have reported a
previous history of patellar tendon pain (greater than 30 out of 100 on a visual analog pain scale)
and focal tenderness upon palpation of the patellar tendon for greater than 3 months. Individuals
with patellar tendinopathy were excluded if any of the following were present: 1) previous history
of knee surgery, 2) neurological involvement that would influence gait, 3) contraindications to MR
imaging: implanted electronic devices (i.e., pacemaker), metallic implants (i.e., aneurysm clips,
30
fixation screws), or claustrophobia, or 4) evidence of patellar tendinosis. The presence of patellar
tendinosis was determined using ultrasound using previously described methods.
44,45
Specifically,
potential participants who presented with a hypoechoic area greater than 2 mm in diameter,
46
or a
spatial frequency lower than 1.3,
44
were excluded from participation.
To avoid the potential influence of age, weight, height, activity level on patellar tendon
stress, participants in the control group were matched for these variables to those in the patellar
tendinopathy group. Individuals without patellar tendinopathy were excluded if any of the
following were present: 1) previous history of knee pathology or surgery, 2) current knee pain or
effusion, 3) neurological involvement that would influence balance control, 4) contraindications
to MR imaging: implanted electronic devices (i.e., pacemaker), metallic implants (i.e., aneurysm
clips, fixation screws), or claustrophobia. Prior to participation, all participants were informed as
to the nature of the study and signed a human subject’s consent form approved by the Health
Sciences Institutional Review Board of the University of Southern California.
Our sample size was estimated based on data from Couppé
47
who compared patellar tendon
stress in persons with and without patellar tendinopathy, and the findings of Edwards, et al.
2
who
compared 3D knee kinematics between persons with and without asymptomatic PTA during
various landing tasks. It was estimated that 16 participants (8 per group) would be sufficient to
achieve a statistical power of 80% for a moderate effect size for patellar tendon stress and knee
kinematic variables of interest using an alpha level of 0.05.

Procedure
Participants underwent two data collection sessions on separate days. The first session
consisted of biomechanical testing, whereas the second session consisted of MR assessment of the
31
tibiofemoral and patellofemoral joints. Biomechanical testing was performed first to obtain the
knee flexion angle at the time of peak knee extensor moment during a single-leg landing task. For
participants in the patellar tendinopathy group, testing was performed on the symptomatic side.
For participants who had bilateral patellar tendinopathy, the more symptomatic side was evaluated.

Biomechanical Testing
Biomechanical data were collected at the Jacquelin Perry Musculoskeletal Biomechanics
Research Laboratory at University of Southern California. Subjects were instrumented for 3D
motion and electromyographic (EMG) analyses. Lower extremity kinematics were collected at 150
Hz using an 11-camera QUALISYS motion analysis system (Qualisys Inc., Gothenburg, Sweden).
Three molded thermoplastic clusters with rigid reflective tracking markers were securely placed
laterally on the thigh, leg, and heel counter of the shoes on each side. Additional tracking markers
were placed bilaterally on the iliac crests and the L5/S1 junction. In addition to tracking markers,
calibration markers were placed bilaterally on the greater trochanters, anterior superior iliac spines,
medial and lateral femoral epicondyles, medial and lateral malleoli, and 1st and 5th metatarsal
heads. Ground reaction forces were collected at 1500 Hz using a force plate (AMTI Model #OR6-
6-1, Newton, MA).
EMG signals of knee flexors (medial and lateral hamstrings, and medial and lateral
gastrocnemius) were recorded at 1500 Hz using surface EMG electrodes (Noraxon, Scottsdale,
AZ). The electrodes for the medial and lateral hamstrings were placed midway between the ischial
tuberosity and the medial and lateral sides of the popliteal fossa, respectively.
28
The electrodes for
the medial and lateral gastrocnemius were placed at one third of the distance between the medial
and lateral sides of the popliteal fossa, respectively, and the proximal aspect of the Achilles
32
tendon.
28
EMG data were normalized to the activity acquired during a MVIC and were used for
the estimation of quadriceps muscle forces (see below for details).
A standing calibration trial was obtained to define the anatomical coordinate systems.
Following the standing calibration trial, participants were asked to perform a single-leg landing
task by stepping off a 0.3-m platform. Participants landed with the symptomatic limb onto the
force plate and were instructed to jump upward as high and fast as possible. Participants were
instructed to employ their natural landing style and were allowed to practice the task as needed.
Five minutes of rest was provided prior to data collection. Data were acquired during consecutive
3 trials of single-leg landing with at least a 1-minute resting period between trials. Data from the
average of 3 trials were used for analysis. EMG, kinematic, and kinetic data were collected
simultaneously and synchronized.

MR Assessment
Subject-specific bone geometry of the tibiofemoral and patellofemoral joints, and
morphology of the cartilage and patellar tendon were obtained using a 3.0 T MR scanner (General
Electric Healthcare, Milwaukee, WI, USA). Images were acquired with an 8-channel knee coil
using a 3-dimensional (3D), high-resolution, fat-suppressed, fast SPGR (repetition time, 14.5 ms;
echo time, 2.8 ms; flip angle, 10°; matrix, 320 × 320; field of view, 16 cm; slice thickness, 1.0 mm;
scan time, 8:58 min). During the scan, participants were positioned supine with the knee flexed to
the angle that corresponded to the knee flexion angle at the time of peak adjusted knee extensor
moment during the single-leg landing task.
Quadriceps muscle morphology was assessed from sagittal plane MR images of the thigh
using a 3D SPGR protocol (repetition time, 9.4 ms; echo time, 4.1 ms; flip angle, 20°; matrix, 384
33
× 384; field of view, 46 cm; slice thickness, 2 mm; scan time, 8:03 min). Sagittal plane images of
the thigh were subsequently reconstructed in the coronal and axial planes and were used to estimate
the 3D fiber orientation of each of the quadriceps muscles (see below for details). Axial images of
the thigh were utilized to measure the cross-sectional area of the quadriceps muscles which was
subsequently used as an input variable for the biomechanical model to estimate the magnitude of
muscle forces.

Data Analysis
3D kinematics and kinetics of the tibiofemoral joint were quantified using Visual 3D
software (C-Motion, Rockville, MD). The kinetic variable of interest was peak adjusted knee
extensor moment during single-leg landing. Briefly, the net knee extensor moment was computed
using inverse-dynamic equations while knee flexor moment was estimated through forward-
dynamic equations using SIMM modeling software (MusculoGraphics, Santa Rosa, CA). The
adjusted knee extensor moment was calculated as the sum of net knee extensor moment and knee
flexor moment. Kinematic variables of interest included knee flexion angle, as well as the knee
rotation angles in the frontal and transverse planes at the time of peak adjusted knee extensor
moment.

FE Model Development
Subject-specific input parameters entered into the modeling pipeline included: joint
geometry, tibiofemoral kinematics and quadriceps muscle forces (Figure 4-1). Using a commercial
software package (Sliceomatic, Tomovision, Montreal, Quebec), the high resolution, sagittal plane
MR images of the knee were manually segmented and the 3D surfaces of the femur, tibia, patella,
34
patellar tendon, and articular cartilage covering of the femur and patella were created. Surfaces
created for the femur, tibia, and patella were subsequently used to create a rigid body shell of each
bony structure using a proprietary FE pre-processor (Hypermesh, Altair Engineering Inc., Troy,
MI). The articular cartilages of the patella and femur were modeled as homogeneous and isotropic
using tetrahedral continuum elements with an elastic modulus of 25 MPa
31
and a Poisson ratio of
0.47
32
. The patellar tendon was modeled as homogeneous with transversely isotropic elastic
properties using tetrahedral continuum elements. Nine elastic constants for the patellar tendon
included the transverse moduli (E1 = E2 = 46.13 MPa), longitudinal modulus (E3 = 966.58 MPa),
Poisson’s ratios (V12 = 0.25; V13 = V23 = 0.03), and shear moduli (G12 = 11.66 MPa; G13 =
G23 = 60.75 MPa) whereby direction 3 was parallel to the fibrils of the patellar tendon.
33
The methods used to estimate the individual quadriceps muscle forces from the
biomechanical testing session have been described previously.
29
Briefly, a subject-specific
representation of the extensor mechanism was created using SIMM modeling software
(MusculoGraphics, Santa Rosa, CA). Subject-specific biomechanical data (kinematics, kinetics,
EMG) were used to drive the model (via an optimization routine) and 3D quadriceps muscle forces
were computed.
29
The elements representing the quadriceps muscles were separated into four
functional groups comprised of uniaxial force actuators (the rectus femoris, vastus intermedius,
vastus medialis, and vastus lateralis muscles). Six actuators were used to represent the forces of
the vastus medialis and vastus lateralis respectively, while three actuators were used to represent
the forces of the rectus femoris and vastus intermedius respectively. The direction of muscle lines
of pull for the rectus femoris and vastus intermedius group were set parallel to the long axis of the
femur.
34
The most lateral and medial borders of the quadriceps line of pull (i.e., the borders of the
vastus lateralis and vastus medialis) were determined from the fiber orientation of each muscle in
35
the sagittal and frontal planes, as measured from MR images of the subject's thigh.
29
Connector
elements with stiffness of 10N/mm representing each muscle group were then distributed
uniformly from the medial-to-lateral borders.
35

Upon model completion, the femur and tibia were rotated to varying degrees depending on
each participant’s landing kinematics. Specifically, relative segment motion of tibia and femur in
the frontal and transverse planes at the time of peak adjusted knee extensor moment during single-
leg landing were assigned to the model. Only one segment (either femur or tibia) was rotated in
each plane while the other segment remained fixed. The rotated segment was the one that exhibited
the greater rotation angle in that plane. For instance, if the femur and tibia internal rotation angles
at the time of peak adjusted knee extensor moment were 10° and 5° respectively, the femur would
be internally rotated 5° on the tibia.

Figure 4-1. Finite element modeling pipeline. Reproduced with permission (Liao, et al.
30
).
36
To rotate the femur and tibia, local coordinate systems for each segment were established.
The medial-lateral axis of the femur was established by the midpoints of medial and lateral femoral
condyles and the center of rotation was defined as the midpoint between the medial and lateral
condyles. The superior-inferior axis of the femur was established to be parallel to the femur shaft,
while the anterior-posterior axis was established to be perpendicular to the other two axes.
Similarly, the medial-lateral axis of the tibia and center of rotation were established by the
midpoints of the centroids of medial and lateral tibial plateau. The superior-inferior axis of the
tibia was established to be parallel to the superior-inferior axis of MR global coordinate system,
and the anterior-posterior axis was then established to be perpendicular to the other two axes.
The quadriceps muscles were allowed to rotate with the femur. Since the soft tissues
controlling the rotation of the tibiofemoral and patellofemoral joint were not included in the
models, the three rotational degrees of freedom of the patella were constrained during the
simulations. However, the patella was allowed to translate within trochlear groove. Model output
from representative participants in patellar tendinopathy and control groups is presented in Figure
4-2.

Figure 4-2. Maximum principal stress distribution in the patellar tendon from a representative
participant with patellar tendinopathy (A) and a control participant (B).
37
Model Output & Post-processing
The maximum principal stress, which approximates the tensile stress that tends to elongate
the patellar tendon,
20
was calculated at each node. Patellar tendon stress was calculated at each
node. Elements representing the bone-tendon interface (ie. proximal and distal ends of the tendon)
were not considered as the material properties of the bone-tendon interface are unknown given the
gradual transitional zone from tendon to bone, including a combination of uncalcified and calcified
layers of tissues.
36
In addition, elements within 3 mm (2 times the slice thickness of MR Images)
from the ends of the remaining tendon were not considered as the material properties of the
adjacent bone-tendon interface could potentially influence the stress values by an unknown
amount.
36
The remaining patellar tendon was examined to identify the peak value of the maximum
principal stress.
The peak value of the maximum principal stress within the patellar tendon was determined
using a process designed to avoid spurious peak stress values. First, the maximum principal stress
within 5 nodes (2.5 mm) in each direction of the node that exhibited the peak maximum principal
stress was screened. If the maximum principal stresses at any of these adjacent nodes was less than
30% of the peak value of maximum principal stress, the peak maximum principal stress at this
particular location was considered an artifact and disregarded. This vetting process was built into
an algorithm in MATLAB (MathWorks Inc.) and repeated automatically until a “true” peak
maximum stress value was identified.

Statistical Analysis
Independent t-tests were used to test the hypothesis that peak maximum principal stress in
the patellar tendon differed between persons with and without patellar tendinopathy. In addition,
38
independent t-tests were used compare the biomechanical input variables to the FE model (knee
flexion angle, knee rotation angles in the frontal and transverse planes and the peak knee extensor
moment) between persons with and without patellar tendinopathy. Stepwise regression was
performed with both groups combined to determine the best kinematic predictor(s) of peak
maximum principal stress in patellar tendon.

RESULTS
The means and standard deviations (SD) of peak maximum principal stress and
tibiofemoral kinematics and kinetics for both groups are presented in Table 4-2. When compared
to the control group, the patellar tendinopathy group exhibited significantly greater peak maximum
principal stress in the patellar tendon (77.4 ± 25.0 vs 60.6 ± 13.6, Mean ± SD, p < 0.05).

Table 4-2. Comparison of the variables of interest for the patellar tendinopathy and healthy control
groups at the time of peak adjusted knee extensor moment during a single-leg landing task.
PT group
(n=14)
Control group
(n=14)
p value
Peak Maximum Principal Stress (MPa) 77.4 ± 25.0 60.6 ± 13.6 0.04
Knee Flexion (degrees) 44.2 ± 5.7 43.5 ± 4.9 0.74
Knee Rotation (degrees)* -4.6 ± 4.6 -1.1 ± 4.2 0.04
Knee Abduction (degrees) 1.9 ± 3.3 -0.7 ± 5.0 0.11
Adjusted Knee Extensor Moment (Nm/kg) 3.2 ± 0.4 3.1 ± 0.3 0.50
PT: patellar tendinopathy. * Negative values indicate internal rotation and adduction.

39
In terms of biomechanical variables, persons with patellar tendinopathy exhibited
significantly greater tibiofemoral internal rotation compared to the control group (4.6 ± 4.6 vs 1.1
± 4.2, Mean ± SD, p < 0.05). No between group differences were observed for the other
biomechanical variables of interest. The stepwise regression analysis revealed that transverse plane
tibiofemoral rotation was the only variable to enter the model, explaining 26.2% of the variance in
peak maximum principal stress (r=0.51, p < 0.05) (Figure 4-3).

Figure 4-3. Association between peak maximum principal stress in the patellar tendon with
tibiofemoral rotation in the transverse plane.

40
DISCUSSION
The purpose of the current study was to evaluate whether persons with patellar
tendinopathy exhibit elevated patellar tendon stress, and to evaluate the potential kinematic
contributors to elevated peak maximum principal stress in the patellar tendon during a simulated
single-leg landing task. In regards to the primary purpose, results revealed that peak maximum
principal stress in the patellar tendon was on average 28% higher in the patellar tendinopathy group
compared to the control group. In terms of the secondary purpose, tibiofemoral joint rotation in
the transverse plane was found to be the best predictor of peak maximum principal stress for both
groups combined.
The finding of elevated patellar tendon stress in persons with patellar tendinopathy is
consistent with the results of Couppé, et al.
10
who reported that elite badminton players with
patellar tendinopathy exhibited 52% higher patellar tendon stress compared to those without
patellar tendinopathy. However, our findings are in contrast with those of Wiesinger, et al.
14
who
reported lower patellar tendon stress in persons with patellar tendinopathy compared to those
without patellar tendinopathy during ramp. It should be noted however that the studies of Couppé,
et al.
10
and Wiesinger, et al.
14
quantified patellar tendon stress by simply dividing the estimated
tendon force during MVICs by the distal patellar tendon cross-sectional area as measured from
MR images. In addition, neither group considered the influence of lower extremity kinematics in
their tendon stress estimates. In contrast, the patellar tendon stress values obtained in the current
study were obtained using a subject-specific 3D models which accounted for tendon morphology,
quadriceps force, and tibiofemoral kinematics.
Tibiofemoral rotation in the transverse plane was the only variable that differed between
groups. On average, persons with patellar tendinopathy exhibited 3.5 degrees greater tibiofemoral
41
internal rotation compared to those without patellar tendinopathy. This finding is consistent with
Edwards, et al.
2
who reported persons with asymptomatic PTA exhibited 7 degrees greater
tibiofemoral internal rotation compared to a control group at the time of peak patellar tendon force
during a horizontal landing task. While the average group difference in tibiofemoral internal
rotation would appear to be small, previous work from our group has shown that the patellar
tendons stress is highly susceptible to transverse plane rotations of the tibia or femur.
42
Specifically,
it was reported that 4 degrees of tibiofemoral femoral rotation beyond that of the natural degree of
rotation can increase patellar tendon stress by as much as 70%. The fact that transverse plane
tibiofemoral rotation was the best predictor of peak maximum principal stress in the patellar tendon,
suggests that small degrees of tibiofemoral rotation could be viewed as being highly relevant from
a tissue injury perspective.
In terms of sagittal plane kinematics and kinetics, no between group differences were
observed. This is somewhat surprising as abnormal sagittal plane mechanics has been thought to
contribute to excessive patellar tendon loading. Previous authors have reported that persons with
patellar tendinopathy exhibit greater knee flexion angles compared to those without patellar
tendinopathy during landing tasks,
2,11,17
however this finding has not been consistent among
studies.
16,48
In terms of the knee extensor moment, our findings are consistent with others who
have reported no differences in sagittal plane knee kinetics between groups during landing
tasks.
2,19,48
Although it is intuitive that higher knee extensor moments would result in higher
quadriceps muscle forces and greater patellar tendon stress, this variable did not discriminate
between those with and without patellar tendinopathy in the current study. This finding is
consistent with the premise that persons with patellar tendinopathy may employ compensatory
42
movement behavior resulting in lower than normal knee extensor moments,
12,18
presumably as a
response to avoid pain.
Although the current study provides insight into a potential kinematic contributor to
patellar tendon loading in persons with patellar tendinopathy, it should be noted that we were only
able to explain 26% of the variance in peak maximum principal stress. Nonetheless, our findings
suggest that multiplane loading of the knee may contribute to the development of patellar
tendinopathy. This assertion is similar to what has been reported for the development of
patellofemoral pain in which the combination of frontal and transverse plane knee rotations has
been shown to be contributory to patella cartilage stress.
49
Our findings highlight the need for more
comprehensive assessments of 3D kinematics and kinetics in persons with patellar tendinopathy
to better understand underlying causes of this condition. In addition, the results of the current study
highlight the need for clinicians to identify and correct altered hip and knee kinematics in the
transverse plane (if present) in this population.
There are several limitations that need to be considered when interpreting the results of this
study. First, our study design was cross-sectional in nature. As such, cause and effect cannot be
determined. Second, the current FE model did not take into consideration the inhomogeneity, non-
linearity, and viscoelastic properties of the tendon. Further study is necessary to completely define
the specific parameters that will allow the computational model to accurately simulate the material
and functional characteristics of the patellar tendon. This will help better understanding of the
biomechanical function of patellar tendon. Third, the results from our model did not consider the
stress in the area of the bone-tendon junction. The bone-tendon junction, especially the inferior
pole of the patella, is a common area of symptoms in the patellar tendinopathy population. Fourth,
soft tissues that surround and support the patellofemoral joint were not included in our model. To
43
minimize the influence of the soft tissues, the 3 rotational degrees of freedom of the patella was
constrained. Lastly, we investigated the peak patellar tendon stress only at a single knee joint angle
during a single-leg landing task. Results could vary depending on degree of knee flexion or task
evaluated.

SUMMARY
Persons with patellar tendinopathy demonstrated greater peak maximum principal stress in
the patellar tendon during simulated single-leg landing compared to pain-free individuals.
Transverse plane tibiofemoral rotation was found to be the best predictor of the elevated peak
stress during landing. These findings highlight the importance of non-sagittal plane kinematics as
potentially being contributory to the development of patellar tendinopathy.
44
CHAPTER V

DIFFERENCES IN KNEE EXTENSOR MECHANICS BETWEEN PERSONS WITH
AND WITHOUT PATELLAR TENDINOPATHY

The purpose of this study was to compare the knee extensor mechanics in persons with and
without patellar tendinopathy. Twenty-eight individuals participated (14 with patellar
tendinopathy and 14 pain-free controls). Sagittal MR images of the knee were acquired at the knee
flexion angle that corresponded to the knee flexion angle at the time of peak knee extensor moment
during a single-leg landing task. Measurements of the quadriceps moment arm, patellar tendon
moment arm, patellar tendon/quadriceps tendon force ratio, as well as patella height (Insall-Salvati
ratio) were obtained. Independent t-tests were used to compare the variables of interest between
groups.

45
INTRODUCTION
Patellar tendinopathy, also known as jumper’s knee, is a common condition among athletes
who participate in sports that involve repetitive jumping and landing movements.
1,26
The incidence
of patellar tendinopathy in basketball and volleyball has been reported to be as high as 12-14% in
recreational players,
26
and even higher 32-45% in elite athletes.
1
Patellar tendinopathy can cause
symptoms that decrease sport performance or long-term sport cessation.
9

It has been postulated that excessive patellar tendon stress is a primary cause of patellar
tendinopathy,
2,3
however underlying causes of abnormal tendon loading are not clearly understood.
From a biomechanical standpoint, previous studies have compared knee extensor moments
between persons with and without patellar tendinopathy during various jumping/landing activities.
Interestingly, studies in this area have reported that persons with patellar tendinopathy exhibit
similar.
19,48
or lower knee extensor moments compared to persons without tendinopathy.
12,18

One factor that has not been considered in previous biomechanical studies of persons with
patellar tendinopathy is how force is transmitted from the quadriceps muscle group to the patellar
tendon. Apart from being a pulley, the patella also acts as a lever creating a force differential
between the quadriceps tendon and the patellar ligament.
4,24
This force differential, quantified as
the ratio of patellar tendon force to quadriceps force (Fpl/Fq ratio), has been shown to be
influenced by anatomical factors related to the patellofemoral joint including the shape of the distal
femur and patella, the changing point of contact between the patella and femur as the knee flexes
and extends, and patellar height.
4,24

One structural characteristic that may be influential in the context of patellar tendinopathy
is patella height. Persons with a high riding patella (ie. patella alta) have been shown to have a
significantly higher Fpl/Fq ratio compared to persons with normal patellar height.
5
From a
46
biomechanical perspective, a high riding patella effectively increases the moment arm of the
quadriceps tendon while simultaneously decreasing the lever arm of the patellar tendon (Figure 5-
1). As such, greater force would need to be developed within the patellar tendon to balance the
moment created by the quadriceps tendon. There is evidence that persons with patellar
tendinopathy may exhibit a higher patellar position compared to those without tendinopathy,
6

however this finding has not been consistent across all studies.
7

Figure 5-1. Comparison of quadriceps moment arm (Mq) and patellar tendon moment arm (Mpl)
between (A) normal patellar position and (B) patella alta.

It is possible that persons with patellar tendinopathy exhibit anatomical features that expose
these individuals to higher patellar tendon forces for a given level of quadriceps force. Evidence
in support of this premise was provided by Dan, et al.
7
who reported that the moment arm for the
quadriceps tendon relative to the patellar tendon was greater in persons with apparent patellar
tendinopathy compared to those without patellar tendinopathy. A limitation of Dan, et al.
7
was the
retrospective nature of the study in that a clinical diagnosis of patellar tendinopathy could not be
47
made and the images used to quantify the moment arm ratios were obtained at highly variable knee
flexion angles. The latter is important as the Fpl/Fq ratio varies as a function of knee flexion angle.
5

The purpose of the current study was to extend the work of Dan, et al.
7
by comparing
Fpl/Fq ratio between persons with and without patellar tendinopathy using subject-specific
biomechanical data obtained during a single limb landing task. In doing so, we sought to quantify
the potential impact of the Fpl/Fq ratio during a task that commonly reproduces symptoms in this
population. In addition, we evaluated the specific variables that potentially would influence the
Fq/Fpl ratio such as the moment arms of the patellar tendon and quadriceps tendon and patellar
height (Insall-salvati ratio). It was hypothesized that individuals with patellar tendinopathy would
exhibit a greater Fpl/Fq ratio than persons without patellar tendinopathy at a point in time during
a single limb landing task where the knee extensor moment was greatest. In addition, it was
hypothesized that persons with patellar tendinopathy would have relatively higher patellar position
and a subsequent lower patellar tendon and higher quadriceps moment arm compared to a pain-
free control group. Information gained from this study is an important step in identifying potential
intrinsic risk factors for associated with the development of patellar tendinopathy.

METHODS

Subjects
Twenty-eight persons participated in this study: 14 with patellar tendinopathy and 14 who
were pain-free (Table 5-1). All participants were between the ages of 18 and 36 and were
physically active. Participants’ physical activity levels were examined based on the GPAQ. The
GPAQ has been widely used for a valid and reliable estimate of physical activity.
43

48
Table 5-1. Participants characteristics (mean ± standard deviation).

PT group
(n=14)
Control group
(n=14)
p value
Sex (female/male, n) 8/6 8/6 NA
Age (years) 25.0 ± 4.9 25.6 ± 4.5 0.75
Height (m) 1.71 ± 0.7 1.71 ± 0.7 0.81
Weight (kg) 66.3 ± 11.0 66.6 ± 10.7 0.92
Activity Level (MET. Mins/week) 1124 ± 548 1104 ± 384 0.56
VISA-P 65.6 ± 11.2 NA NA
Duration of PT symptoms (mos) 22.7 ± 13.3 NA NA
PT: patellar tendinopathy; m/o: months; MET: Metabolic equivalents; VISA-P: Victorian Institute
of Sports Assessment-Patellar Tendinopathy; NA: not applicable.

To be included in the patellar tendinopathy group, participants had to have reported a
previous history of patellar tendon pain (greater than 30 out of 100 on a visual analog pain scale)
and focal tenderness upon palpation of the patellar tendon for greater than 3 months. Individuals
with patellar tendinopathy were excluded if any of the following were present: 1) previous history
of knee surgery, 2) neurological involvement that would influence balance control, 3)
contraindications to MR imaging, or 4) evidence of patellar tendinosis. The presence of patellar
tendinosis was determined using ultrasound using previously described methods.
44,45
Specifically,
potential participants who presented with a hypoechoic area greater than 2 mm in diameter,
46
or a
spatial frequency lower than 1.3,
44
were excluded from participation.
To avoid the potential influence of age, weight, height, activity level on patellar tendon
stress, participants in the control group were matched for these variables to those in the patellar
49
tendinopathy group. Individuals without patellar tendinopathy were excluded if any of the
following were present: 1) previous history of knee pathology or surgery, 2) current knee pain or
effusion, 3) neurological involvement that would influence balance control, or 4) contraindications
to MR imaging. Prior to participation, all participants were informed as to the nature of the study
and signed a human subject’s consent form approved by the Health Sciences Institutional Review
Board of the University of Southern California.

Procedure
Participants underwent two data collection sessions on separate days. The first session
consisted of biomechanical testing, whereas the second session consisted of MR assessment of the
tibiofemoral and patellofemoral joints. Biomechanical testing was performed to obtain the knee
flexion angle at the time of the peak knee extensor moment during a single-leg landing task. This
angle was subsequently replicated for the MR assessment to quantify the imaging variables of
interest. For participants in the patellar tendinopathy group, testing was performed on the
symptomatic side. For participants who had bilateral patellar tendinopathy, the more symptomatic
side was tested.

Biomechanical Testing
Biomechanical data were collected at the Jacquelin Perry Musculoskeletal Biomechanics
Research Laboratory at University of Southern California. Participants were instrumented for 3D
motion analyses. Lower extremity kinematics were collected at 150 Hz using an 11-camera
QUALISYS motion analysis system (Qualisys Inc., Gothenburg, Sweden). Three molded
thermoplastic clusters with rigid reflective tracking markers were securely placed laterally on thigh,
50
leg, and heel counter of the shoes on each side. Additional tracking markers were placed on
bilateral iliac crests and the L5/S1 junction. In addition to tracking markers, calibration markers
were placed on bilateral greater trochanters, anterior superior iliac spines, medial and lateral
femoral epicondyles, medial and lateral malleoli, and 1
st
and 5
th
metatarsal heads. Ground reaction
forces were recorded at 1500 Hz using an AMTI force plate (Model #OR6-6-1, Newton, MA). A
standing calibration trial was obtained to define the anatomical coordinate systems.
Following the standing calibration trial, participants performed a single-leg landing task by
stepping off a 0.3-m wooden platform, landing onto the force plate with the leg being tested, and
then jumping upward as high and fast as possible. Participants were instructed to employ their
natural landing style and were allowed to practice the task as needed. Kinematic and kinetic data
were acquired simultaneously and synchronized during 3 trials of single-leg landing task with at
least a 1-minute resting period between trials. Data from the 3 trials were averaged of analysis.
The biomechanical variable of interest was the knee flexion angle at the time of peak knee extensor
moment during a single-leg landing task.

MR Assessment
Subject-specific bone geometry of the tibiofemoral and patellofemoral joints, and
morphology of the cartilage and patellar tendon were obtained using a 3.0 T MR scanner (General
Electric Healthcare, Milwaukee, WI, USA). Images were acquired with an 8-channel knee coil
using a 3-dimensional (3D), high-resolution, fat-suppressed, fast spoiled gradient recalled echo
sequence (repetition time, 14.5 ms; echo time, 2.8 ms; flip angle, 10°; matrix, 320 × 320; field of
view, 16 cm; slice thickness, 1.0 mm; scan time, 8:58 min). During this scan, participants were
positioned supine with the knee flexed to the angle corresponding to the time of the peak knee
51
extensor moment during single-leg landing task as described above. The knee flexion angle was
established using a goniometer. The sagittal MR images were screened to determine which image
contained the midsection of the knee. This was determined by identifying the image containing
the intersection of the cruciate ligaments. This image was used to measure all four indices of knee
extensor mechanics according to the methods described by Yamaguchi and Zajac
4
. Quadriceps
moment arms (Mq) and patellar tendon moment arm (Mpl) were measured as the perpendicular
distances from the patellofemoral contact point to the respective tendons (Figure 5-1). When the
patellofemoral contact point could not be identified by a single point, the line of contact between
the patella and femur was bisected and the midpoint of this line was used as the axis of rotation.
5

The Fpl/Fq ratio was then computed using the following equation
50
: Fpl / Fq =Mq / Mpl. (1)
To quantify patellar height, MR images were screened to determine which contained the
maximum patellar tendon length and patellar length. Measurements of patellar tendon length were
made along the posterior surface from the tibial tuberosity to the patellar apex, while measurements
of patellar length were made from the apex of the patella to the most posterior superior aspect of
the patellar base.
51
In all cases, the longest patellar ligament length and longest patellar length were
used even if they were on separate images in a series, as previous research has suggested that this
method produces the strongest reliability and validity with measurements obtained from lateral
radiographs of the knee.
52
The length of the patellar tendon was then divided by the length of the
patella to yield the Insall-Salvati ratio.
51

Statistical Analysis
Independent t-tests (one-tailed) were used to compare the Fpl/Fq ratio between groups.
Similarly, the variables contributing to the Fpl/Fq ratio (quadriceps moment arm, patellar tendon
52
moment arm and the Insall-salvati ratio) also were compared between groups using independent
one-tailed t-tests. Lastly, the peak knee extensor moment at the knee flexion angle at the time of
the peak knee extensor moment were compared between groups using independent t-tests. All
statistical analyses were performed using SPSS statistical software (SPSS Inc., Chicago, IL) with
a significance level of p < 0.05.

RESULTS
The means and standard deviations of the dependent variables of interest for both groups
are presented in Table 5-2. On average, the patellar tendinopathy group exhibited a significantly
higher Fpl/Fq ratio than the control group. In addition, persons with patellar tendinopathy exhibited
significantly lower patellar tendon moment arms, greater quadriceps tendon moments arms and a
greater Insall-Salvati ratio when compared to the control group (Table 5-2). No between group
differences were observed for the peak knee extensor moment or the knee flexion angle at the time
of the peak knee extensor moment during the single limb landing task (Table 5-3).

Table 5-2. Knee extensor mechanics for the patellar tendinopathy and healthy control groups.

PT group
(n=14)
Control group
(n=14)
p value
Fpl/Fq ratio 1.0 ± 0.1 0.8 ± 0.1 0.00*
Moment Arm of Patellar Tendon (mm) 24.2 ± 1.7 26.3 ± 2.4 0.01*
Moment Arm of Quadriceps (mm) 23.9 ± 2.0 22.1 ± 2.9 0.03*
Insall-Salvati ratio 1.2 ± 0.1 1.1 ± 0.1 0.00*
Data are presented as mean ± standard deviation. Fpl: patellar tendon force; Fq: quadriceps force.
53
Table 5-3. Knee joint kinematics and kinetics for the patellar tendinopathy and control groups at
the during single-leg landing.
PT group
(n=14)
Control group
(n=14)
p value
Knee Flexion Angle at the time of Peak
Knee Extensor Moment (degrees)
44.2 ± 5.7 43.5 ± 4.9 0.74
Peak Knee Extensor Moment (Nm/kg) 2.5 ± 0.4 2.5 ± 0.3 0.60
Data are presented as mean ± standard deviation. Fpl: patellar tendon force; Fq: quadriceps force.

DISCUSSION
Although it has been postulated that excessive patellar tendon stress may be contributory
to the development of patellar tendinopathy, intrinsic risk factors have not been clearly elucidated.
The purpose of the current study was to compare the mechanics of force transmission from the
quadriceps muscle group to the patellar tendon between persons with and without patellar
tendinopathy. The primary finding of our investigation was that persons with patellar tendinopathy
exhibited a higher Fpl/Fq ratio (25% on average) compared to control group. Importantly, this ratio
difference was quantified at the point in the single-leg landing task where the knee extensor
moment was the greatest. This result suggests that persons with patellar tendinopathy would
experience a significantly greater force in the patellar tendon per unit quadriceps force compared
to control participants.
The greater Fpl/Fq ratio observed in the patellar tendinopathy group was the result of the
moment arm differential, in which the quadriceps moment arm and patellar tendon moment arms
were larger and smaller than that of the control participants, respectively. This finding agrees with
54
Dan, et al.
7
who reported that the persons with patellar tendinopathy exhibited a higher moment
arm ratio (Mq/Mpl) than that of the controls. Interestingly our finding of a 25% higher Fpl/Fq ratio
in persons with patellar tendinopathy mirrors the 25% greater Mq/Mpl ratio reported by Dan, et
al.
7
. Taken together, the findings of the current study and that of Dan, et al.
7
highlight the potential
influence of anatomical features in contributing to elevated patellar tendon loading in this
population.
The observed differential in quadriceps and patellar tendon moment arms in the patellar
tendinopathy group can be explained by the higher patellar position. On average, the Insall-Salvati
ratio in the patellar tendinopathy group was 9% greater than the control participants. This finding
is consistent with Garms, et al.
6
_ENREF_12 who reported that persons with patellar tendinopathy
demonstrated a higher patellar position as well as a 3 times higher prevalence rate of patella alta
compared to pain-free individuals. A post hoc analysis of the variables contributing to the Insall-
Salvati ratio (patella length and patellar tendon length) revealed that the primary contributor to the
higher riding patella in the patellar tendinopathy group was a longer patellar tendon. Similar to
what has been reported for persons with patella alta,
11
a longer patellar tendon would effectively
lower the moment arm of the patellar tendon while simultaneously increasing moment arm of the
quadriceps (Figure 5-1). Although the average Insall-Savati ratio in the patellar tendinopathy did
not exceed the 1.2 threshold typically used to designate patella alta, any increase in patellar height
would be expected to alter the Fpl/Fq ratio as a result of the patellofemoral contact point being
closer to the patella apex.
4,5,24

It is interesting to note that peak knee extensor moment during a single-leg landing was
similar between groups. This finding is consistent with others who have reported no significant
differences in knee extensor moments between persons with and without patellar tendinopathy.
19,48

55
However, the results of the current study suggest that persons with patellar tendinopathy still may
be exposed to higher patellar tendon loading despite a similar or even a lower knee extensor
moment compared to healthy individuals. As such, care should be taken when inferring patellar
tendon loading based on biomechanical studies in which the net knee join moment is the variable
of interest.
The findings of the current study have clinical implications for the treatment of patellar
tendinopathy. Although the structural risk factors cannot be changed with conservative approaches,
modification of jumping/landing strategies aimed at lowering the force required by quadriceps
muscle group may be indicated. Evidence in support of this premise was provided by Silva, et al.
53
,
who reported that an 8-week movement retraining intervention aimed at lowering the knee
extensor moment during jumping/landing resulted in abolishment of patellar tendinopathy
symptoms. Such an approach would appear to be especially be beneficial for persons with
documented patella alta or other structural features that would expose the patellar tendon to
excessive loading.
The results of the current study need to considered in light of several limitations. First, all
MRI variables were measured at a single knee flexion angle. Although the angle examined
matched the time of peak knee extensor moment during a single-leg landing task, care should be
taken in extrapolating these results to other knee flexion angles. Second, the investigator taking
the MRI measurements was not blinded to group assignment. As such, the possibility of
investigator bias cannot be ruled out. Lastly, we only examined a relatively small sample of
persons with patellar tendinopathy. Although the study was adequately powered for the variables
of interest, examination of a larger cohort would be needed to adequately characterize the impact
of knee extensor mechanics in contributing to patellar tendinopathy.
56
SUMMARY
This current study examined differences in knee extensor mechanics between persons with
and without patellar tendinopathy. On average, persons with patellar tendinopathy exhibited a
significantly greater Fpl/Fq ratio compared to persons without patellar tendinopathy. The higher
ratio observed in the patellar tendon group could be explained by the greater differential in the
quadriceps and patellar tendon moment arms owing to a higher riding patella. Taken together, our
findings suggest that persons with patellar tendinopathy may experience greater forces in the
patellar tendon for a given quadriceps force.

57
CHAPTER VI

SUMMARY AND CONCLUSIONS
Previous studies have suggested that the patellar tendon loading may be dependent on
biomechanical factors such as tibiofemoral kinematics and kinetics and/or structural factors
affecting knee extensor mechanics including patellar height.
2,6,7,11,12,17
However, it is not known
which tibiofemoral kinematic and/or kinetic variables are most predictive of patellar tendon stress.
Furthermore, the differences in knee extensor mechanics in persons with and without patellar
tendinopathy has not been comprehensively studied. The purpose of this dissertation was to
quantify the interrelationship between tibiofemoral kinematics and kinetics, knee extensor
mechanics and patellar tendon stress in persons with and without patellar tendinopathy. To
accomplish this objective, three studies were undertaken.
Chapter III examined the influence of frontal and transverse plane rotations of the femur
and tibia on peak maximum principal stress in the patellar tendon. In the transverse plane, internal
and external rotations of the femur and tibia that exceeded 4 degrees beyond that of the natural
rotation resulted in progressively greater patellar tendon stress (22-24% increase per degree of
rotation beyond the natural position). In the frontal plane, femur and tibia abduction also resulted
in a progressive increase in patellar tendon stress, but statistical significance only was achieved at
the end range of the motions evaluated. In terms of abduction, significant differences were
observed at 6  of abduction and beyond, while significant increases in patellar tendon stress were
observed only at 10  of adduction. Frontal plane motion of the tibia in either direction resulted in
linear increases in patellar tendon stress (15-19% increase in stress per degree of rotation beyond
the natural position). Rotations of the tibia resulted in greater patellar tendon stress compared to
58
the femur, but only in the frontal plane.
_ENREF_9Chapter IV examined whether persons with patellar tendinopathy exhibit greater
peak maximum principal stress in the patellar tendon compared to pain-free controls during a
single-leg landing task. A secondary purpose was to determine the kinematic and/or kinetic
predictor(s) of peak patellar tendon stress. Compared to the pain-free controls, those with patellar
tendinopathy exhibited greater peak patellar tendon stress (77.4  25.0 MPa vs 60.6  13.6 MPa,
p < 0.05) and greater knee internal rotation (4.6  4.6 degrees vs 1.1  4.2 degrees, p < 0.05).
Transverse plane rotation of the tibiofemoral joint was the best predictor of peak patellar tendon
stress (26.2% of variance, r = 0.51, p < 0.05). Persons with patellar tendinopathy exhibited greater
peak patellar tendon stress compared with pain-free individuals during a single-leg landing. The
degree of tibiofemoral rotation in the transverse plane best predicted the peak patellar tendon stress.
Chapter V examined whether knee extensor mechanics vary among persons with and
without patellar tendinopathy. In particular, this study was interested in evaluating the structural
factors that increase force transmission from the quadriceps to the patellar tendon. Results revealed
that the patellar tendinopathy group exhibited a significantly higher Fpl/Fq ratio (25% on average)
than the control group (p < 0.05). In addition, persons with patellar tendinopathy exhibited
significantly smaller patellar tendon moment arms, larger quadriceps tendon moment arms and a
greater Insall-Salvati ratio (9% on average) when compared to the control group (p < 0.05). No
between group differences were observed for the peak knee extensor moment or the knee flexion
angle at the time of the peak knee extensor moment during a single-leg landing task.
Taken together the three studies that comprise this dissertation suggest that altered
tibiofemoral kinematics and altered knee extensor mechanics are associated with elevated patellar
tendon loading, and therefore, may play a role in the development/progression of patellar
59
tendinopathy. With respect to biomechanical factors associated with patellar tendinopathy, the
findings of Chapters III and IV highlight the importance of tibiofemoral rotation in the transverse
plane as being contributory to patellar tendon stress. Specifically, the patellar tendon stress values
reported in Chapter III suggest that the patellar tendon is highly susceptible to tibiofemoral rotation
in the transverse plane. Therefore, the small knee transverse rotation differences between groups
found in Chapter IV could be considered meaningful from a tissue injury perspective.
Apart from the biomechanical factors, the finding of a higher Fpl/Fq ratio in persons with
patellar tendinopathy in Chapter V highlights the potential influence of anatomical features
contributing to elevated patellar tendon loading in this population. Although peak knee extensor
moments did not differ between groups, peak patellar tendon stress and the Fpl/Fq ratio were
significantly greater in persons with patellar tendinopathy when compared to pain-free controls.
This suggests that persons with patellar tendinopathy may be exposed to higher patellar tendon
loading despite a similar or even a lower knee extensor moment compared to pain-free individuals.
As such, care should be taken when inferring patellar tendon loading based on biomechanical
studies in which the net knee joint moment is the variable of interest.
The Fpl/Fq ratio was influenced by the height of the patella relative to the trochlear groove.
An increase in patellar height would effectively lower the moment arm of the patellar tendon while
simultaneously increasing moment arm of the quadriceps by moving the patellofemoral contact
point closer to the patella apex.
4,5,24
Given that the prevalence of patella alta has been reported to
be higher in persons with patellar tendinopathy,
6
and the fact that persons with patella alta have
been reported to exhibited an increased Fpl/Fq ratio
5
, suggests that this variable should be
considered as potential risk factor for patellar tendinopathy. Future research should evaluate the
60
influence of additional structural factors that may contribute to a greater Fpl/Fq ratio in persons
with patellar tendinopathy (i.e., femoral trochlear morphology, patellar cartilage morphology, etc).
The results of this dissertation have implications for clinical practice. Importantly, Chapters
III and IV draw attention to the importance of identifying and addressing abnormal motions in the
frontal and transverse planes in persons with patellar tendinopathy. Similar to what has been
described for patellofemoral pain, impaired strength and/or control of the hip musculature may
underlie faulty hip/femur kinematics which in turn would influence frontal and transverse plane
knee kinematics.
53,54
As such, training of the hip musculature,
54,55
_ENREF_61 and/or implementing
real-time biofeedback training,
56,57
may be helpful in correcting faulty lower extremity kinematics
in persons with patellar tendinopathy.
Apart from the influence of hip/femur rotation, patellar tendon stress also was found to be
highly influenced by rotation of the tibia. The fact that tibia internal rotation is coupled with
subtalar joint pronation and varus foot alignment,
58
suggests that excessive rearfoot pronation may
contribute to patellar tendon stress. Given that an increased varus foot alignment is associated with
patellar tendinopathy in volleyball and basketball players,
59,60
interventions such as foot orthoses
or foot/ankle strengthening should be considered in the presence of abnormal rotation of the tibia.
The findings from Chapter V highlights the fact that structural factors also may influence
patellar tendon loading. In particular patellar height may be a useful indicator of persons who may
be exposed to excessive patellar tendon loading. As structural changes cannot be made
conservatively, modification of jumping/landing strategies aimed at lowering the force required
by quadriceps muscle group may be indicated. Movement retraining aimed at lowering the knee
extensor moment during jumping/landing has been reported to be effective for reducing patellar
tendinopathy symptoms.
53
Such an approach would appear to be especially be beneficial for
61
persons with documented patella alta or other structural features that would expose the patellar
tendon to excessive loading.
A popular conservative standard treatment approach for tendinopathies has been eccentric
exercise training.
61,62
The rationale behind this approach is that eccentric loading of the muscle-
tendon unit assists in the remodeling of collagen tissue.
63
However several randomized controlled
trials have found that this form of treatment is largely ineffective for treating athletes with patellar
tendinopathy during their competitive season.
64
Based on the findings of Chapter V, it could be
argued that application of eccentric quadriceps training in the presence of an elevated Fpl/Fq ratio
could have the unintended effect of contributing to patellar tendon overloading, thus exacerbating
pain and further tissue injury. Either way, it could be argued that eccentric quadriceps training may
not be addressing underlying cause(s) of patellar tendinopathy.
Based on the findings of this dissertation, an argument could be made that the treatment of
patellar tendinopathy should be individualized based on identified movement faults and/or
underlying structural factors. Although the identification of optimal treatment approaches was not
an aim of this dissertation, the findings presented provide the basis for continued work in this area.
Clinical trials are needed to identify optimal treatment approaches for persons with patellar
tendinopathy.

CONCLUSIONS
The results of the dissertation suggest that altered tibiofemoral kinematics and altered knee
extensor mechanics are associated with elevated patellar tendon stress. Importantly, the primary
findings of the three studies that comprised this dissertation highlight the interplay among
biomechanical and structural factors that may be contributory to development and progression of
62
patellar tendinopathy. Findings from this work emphasize the need for clinicians to identify the
underlying causes of the patellar tendinopathy to provide more effective and individualized
treatment approaches.

63
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Abstract (if available)

Abstract Patellar tendinopathy is a common condition among athletes who participate in sports that involve repetitive jumping/landing movements. Excessive patellar tendon loading is considered to be the primary cause of patellar tendinopathy. Causes of increased patellar tendon loading can be characterized as biomechanical (e.g. lower extremity kinematics and kinetics) and/or structural (e.g. patellar height). The objective of this dissertation was to examine the interrelationships among tibiofemoral kinematics/kinetics, knee extensor mechanics, and patellar tendon stress in persons with and without patellar tendinopathy. To accomplish this objective, three studies were undertaken.
The purpose of Chapter III was to determine to determine the influence of frontal and transverse plane rotations of the femur and tibia on peak maximum principal stress in the patellar tendon. Using finite element (FE) modeling, patellar tendon stress profiles of 8 healthy individuals were developed during a simulated squatting task (45° of knee flexion). Input parameters for the FE model included joint geometry and quadriceps muscle forces. The femur and tibia of each model were then rotated 10° (in 2° increments) along their respective axes beyond that of the natural degree of rotation. This process was repeated for the transverse plane (internal and external rotation) and frontal plane (adduction and abduction). Quasi-static loading simulations were performed to quantify peak maximum principal stress in patellar tendon. Internal and external rotations of the femur and tibia that exceeded 4 degrees beyond that of the natural rotation resulted in progressively greater patellar tendon stress (p < 0.05). Incremental femur and tibia adduction and abduction resulted in an increase in patellar tendon stress, but only at the end range of motions evaluated.
The purpose of Chapter IV was to determine whether persons with patellar tendinopathy exhibit greater peak maximum principal stress in patellar tendon compared to healthy individuals. A secondary purpose was to determine the kinematic predictors of peak patellar tendon stress during a single-leg landing task. Using FE modeling, patellar tendon stress profiles of 28 individuals (14 with patellar tendinopathy and 14 pain-free controls) were created at the time of the peak knee extensor moment during a single-leg landing task. Input parameters to the FE model included subject-specific patellofemoral joint geometry, quadriceps muscle forces, and tibiofemoral kinematics in the frontal and transverse planes. Independent t-tests were used to compare peak maximum principal stress in patellar tendon between groups. In addition, independent t-tests were used to compare biomechanical variables used as input variables to the FE model (knee flexion, knee rotation in the frontal and transverse planes and the peak knee extensor moment). A stepwise regression model was used to determine the best biomechanical predictor(s) of peak maximum principal stress in patellar tendon for both groups combined. Compared to the healthy individuals, those with patellar tendinopathy exhibited significantly greater peak maximum principal stress in the patellar tendon (mean ± SD, 77.4 ± 25.0 MPa vs 60.6 ± 13.6 MPa, p < 0.05) and greater tibiofemoral internal rotation compared to the control group (Mean ± SD, 4.6 ± 4.6 degrees vs 1.1 ± 4.2 degrees, p < 0.05). Transverse plane tibiofemoral rotation was the best predictor of peak maximum principal stress in the patellar tendon (26.2% of variance, r = 0.51, p < 0.05).
The purpose of Chapter V was to compare the knee extensor mechanics in persons with and without patellar tendinopathy. Twenty-eight individuals participated (14 with patellar tendinopathy and 14 pain-free controls). Sagittal magnetic resonance (MR) images of the knee were acquired at the knee flexion angle that corresponded to the knee flexion angle at the time of peak knee extensor moment during a single-leg landing. Measurements of patellar tendon/quadriceps tendon force (Fpl/Fq) ratio, quadriceps moment arm, patellar tendon moment arm, and patellar height (Insall-Salvati ratio) were obtained. Independent t-tests were used to compare the variables of interest between groups. When compared to the control group, the patellar tendinopathy group exhibited a significantly greater Fpl/Fq ratio (Mean ± SD, 1.0 ± 0.1 vs 0.8 ± 0.1, p < 0.05), a larger quadriceps moment arm (Mean ± SD, 23.9 ± 2.0 mm vs 22.1 ± 2.9 mm, p < 0.05), a smaller patellar tendon moment arm (Mean ± SD, 24.2 ± 1.7 mm vs 26.3 ± 2.4 mm, p < 0.05) and a greater Insall-Salvati ratio (Mean ± SD, 1.2 ± 0.1 vs 1.1 ± 0.1, p < 0.05).
The findings of Chapters III and IV suggest that patellar tendon stress is influenced by tibiofemoral kinematics in the frontal and transverse planes. In addition, the findings of Chapter V indicate that persons with patellar tendinopathy exhibit differences in the knee extensor mechanics that may expose these individuals to higher patellar tendon loading. Specifically, the higher Fpl/Fq ratio observed in persons with patellar tendinopathy suggests that these individuals may experience greater forces in the patellar tendon for a given level of quadriceps force. Taken together the findings of this dissertation highlight the interplay among biomechanical and structural factors that may be contributory to development and progression of patellar tendinopathy.

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Asset Metadata

Creator Park, Kyungmi (author)

Core Title The influence of tibiofemoral kinematics and knee extensor mechanics on patellar tendon stress: a comparison of persons with and without patellar tendinopathy

School School of Dentistry

Degree Doctor of Philosophy

Degree Program Biokinesiology

Degree Conferral Date 2022-08

Publication Date 07/22/2022

Defense Date 05/10/2022

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

Tag finite element analysis,knee extensor mechanics,knee kinematics,OAI-PMH Harvest,patellar tendinopathy,patellar tendon,patellar tendon stress

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Powers, Christopher (committee chair), Colletti, Patrick (committee member), Keyak, Joyce (committee member), Kulig, Kornelia (committee member), Salem, George (committee member)

Creator Email Kyungmi87@gmail.com,parkkyun@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC111373985

Unique identifier UC111373985

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Document Type Dissertation

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Type texts

Source 20220722-usctheses-batch-961 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

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