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A cone beam-CT evaluation of the availability of space for complete arch retraction in the mandible with TADs
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A cone beam-CT evaluation of the availability of space for complete arch retraction in the mandible with TADs
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Content
A CONE BEAM-CT EVALUATION OF THE AVAILABILITY OF SPACE FOR
COMPLETE ARCH RETRACTION IN THE MANDIBLE WITH TADs
by
Daniel Han
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)
May 2009
Copyright 2009 Daniel Han
ii
DEDICATION
To my loving family and friends that supported me throughout this whole process.
iii
ACKNOWLEDGEMENTS
A special thank you to:
Dr. Glenn Sameshima
My Parents
My co-residents
My LHCC
iv
TABLE OF CONTENTS
Dedication ii
Acknowledgements iii
List of Tables v
List of Figures vi
Abstract vii
Chapter 1: Introduction 1
Chapter 2: Review of Literature 7
Chapter 3: Hypothesis 38
Chapter 4: Materials and Methods 39
Chapter 5: Results 51
Chapter 6: Discussion 60
Chapter 7: Conclusion 68
Bibliography 70
v
LIST OF TABLES
Table 1: Stages of Development 29
Table 2: Normal Mandibular Measurements 30
Table 3: Space Distal to Mandibular Second Molars 53
Table 4: Space Distal to Mandibular Third Molars 53
Table 5: Space Distal to Mandibular Second Molars by Sex 54
Table 6: Space Distal to Mandibular Third Molars by Sex 55
Table 7: Median (Interquartile Range) for the Space Behind the
Most Posterior Tooth in the Lower Arch 57
Table 8: Median (Interquartile Range) for the Space Behind the
Second Molar in the Lower Arch 58
Table 9: Median (Interquartile Range) for the Space Behind the
Third Molar in the Lower Arch 59
vi
LIST OF FIGURES
Figure 1: Example of Mini-Implant 9
Figure 2: Intraoral Example of 3 mini-implants 13
Figure 3: Topographic Map of the Mandible showing areas of Remodeling 23
Figure 4: Measurement of Ganss Factor on Panoramic Radiograph 25
Figure 5: Outline of Skull Showing Method of Measuring Dimension
Representing Position of Face 26
Figure 6: Skull of White Infant 27
Figure 7: NewTom QR-DVT 3G 35
Figure 8: Volume Render Screen 43
Figure 9: Super Pano Screen 44
Figure 10: Construction of Pano Image 45
Figure 11: Occlusal Plane “Cut” 46
Figure 12: How to Make Linear Measurement on 3-D Pano 47
Figure 13: Close-Up of Measurement 47
Figure 14: Method of Measurement 48
Figure 15: Box and Whisker Plot for the Space Behind the Most Posterior
Tooth in Lower Jaw 57
Figure 16: Box and Whisker Plot for the Space Behind the Second Molar
in Lower Jaw 58
Figure 17: Box and Whisker Plot for the Space Behind the Third Molar
in Lower Jaw 59
Figure 18: Various Slices Through Mandible 63
Figure 19: Envelope of Discrepancy 65
vii
ABSTRACT:
The purpose of this study was to determine the amount of space available distal to
the most posterior tooth in the mandibular arch for complete arch distalization. Methods:
CBCT scans from 146 (110 after exclusion criteria) patients were rendered using InVivo
Dental
TM
Software. A 1:1 ratio panoramic image was reconstructed from each scan. A
horizontal measurement, parallel to the functional occlusal plane, was recorded from the
distal height of contour of the most posterior tooth to the ascending ramus. The
advantages of using a CBCT reconstruction are: (1) A traditional panoramic radiograph
uses an arbitrary focal trough; (2) There is inherent error and distortion due to the
magnification in the traditional panoramic radiograph; (3) The magnification from
traditional radiographs makes measurements very unreliable; (4) Contrast is improved
with CBCT and blurring and overlapping is eliminated. Results: There was an average of
7.27 mm [7.24 mm (left) and 7.29 mm (right)] distal to lower second molars and 2.80
mm [2.75 mm (left) and 2.85 mm (right)] distal to lower third molars. Overall, males
averaged more space: distal to mandibular second molars: (1) males: 7.32 mm (right),
7.41 mm (left); (2) females: 7.26 mm (right), 7.08 mm (left); Distal to mandibular third
molars: (1) males: 3.08 mm (right), 3.05 mm (left); (2) females: 2.59 mm (right), 2.34
mm (left). Conclusion: The space available for retraction distal to the second molars in
the mandibular arch was 7.27 mm; however, the thickness of the soft tissue must be
accounted for. With the average mucosal depth of 3.02 mm in the retromolar region, the
available space for retraction is reduced to 4.25 mm per side. This amount would be
enough to correct 8.5 mm of anterior crowding barring curve of spee, correct half step
class III, or possibly a change from extraction to non-extraction treatment plan. The space
viii
distal to the third molars was 2.80 mm which leaves no space for retraction on account of
the soft tissue.
1
Chapter One: Introduction
Anchorage in orthodontics is defined as resistance to unwanted dental movement.
To achieve quality results, the clinician must understand the anchorage needs and account
for it in the mechanics. Proffit stated “it is simply not possible to consider only the teeth
whose movement is desired. Reciprocal effects throughout the dental arches must be
carefully analyzed, evaluated, and controlled. An important aspect of treatment is
maximizing the tooth movement that is desired, while minimizing undesirable side
effects.”
[42,43]
Newton’s Third Law states that for every action, there must be an equal and
opposite reaction and this law applies to orthodontia as well. With traditional mechanics,
teeth are moved at the expense of other teeth; when force is applied to one or a segment
of teeth in one direction, opposite reactive force is applied to another segment. To
maximize anchorage and movement in the desired direction, more teeth or teeth with
more anchorage units can be used in the anchorage segment
[43,44]
. However, there are
countless situations where the anchorage requirements are difficult or impossible to
satisfy.
With increasing adult patients seeking orthodontic treatment, more ways to
maintain and supplement anchorage is needed. Compromised dentitions (e.g.
periodontally compromised and partially edentulous) require clever methods to maintain
anchorage and achieve quality results.
The concept of anchorage and its management has been critical since the
beginning of orthodontics. Understanding it separated the quality results from the
compromised. Edward Angle was the first to realize the limits of moving teeth against
2
other teeth for anchorage
[45]
. Since then, anchorage has been studied and classified in
many ways. Angle described anchorage as intraoral, extraoral, simple, stationary,
reciprocal, or intermaxillary
[45]
. Gianelly and Goldman
[54]
described anchorage needs as
“minimum”, “moderate”, and “maximum” and quantified the movement to be expected
of the segments being moved. Marcotte
[55, 56]
described anchorage in extraction cases as:
Group A—maintenance of posterior tooth position during anterior retraction, Group B—
reciprocal space closure, and Group C—maintenance of anterior tooth position during
posterior protraction.
Regardless of its classification, anchorage was an integral part of treatment
planning. Compounding the issue of anchorage control was the fact that even when
anchorage could be managed, a component of it usually had to be controlled by the
patient. Patient compliance became another issue that needed to be addressed.
For almost three decades, there have been many devices created to counter this
issue and reduce the need for compliance. Appliances such as magnets that repel each
other
[1-5]
, superelastic nickel-titanium (NiTi) arch wires
[6]
, coil springs on different arch
wires [continuous arch wire
[7,8]
, sectional arch wire (Distal Jet
[9-11]
, Jones Jig
[12-16]
), and
beta titanium arch wire (pendulum
[17-23]
, K-loop
[24]
)].
Conventionally, distal molar movement was only possible by two methods,
extraoral appliances such as headgear, or intraoral appliances, such as those mentioned
above. The advantage of the extraoral appliance was that it did not stress the anchorage
unit, but it depended solely on the cooperation of the patient. On the other hand, the
intraoral appliances required no cooperation, but stressed the anchorage unit which
increased treatment time.
3
Traditional intraoral distalization appliances were designed to create continuous
distal force on the molars by being anchored to the bicuspids. This caused a reactive
mesial force to the anterior segment. Thus, the desirable distal molar movement came at
the cost of protraction of the anterior teeth resulting in increased overjet and protrusion.
Other negative sequelae of conventional intraoral distalization appliances were anchorage
loss, distal tipping
[10,11,13-16, 18-21, 23,25]
and extrusion
[9,10,13-15, 18,21,22,25]
of molars.
Of late, there has been a resurgence of the nonextraction treatment and many are
advocating distalization of the arch instead of extractions. In patients with a low
mandibular plane angle and crowding in the lower arch, extraction of teeth may be
considered, but this may lead to deepening of the anterior overbite and make treatment
more difficult. Alignment of teeth without extractions tends to cause flaring and may
negatively affect the facial profile.
With the advent of temporary anchorage devices (TADs), the paradigm of
extraction treatment may be changing. In many cases, distalizaton may be done with
TADs. Maxillary retraction with TADs has been studied by numerous groups. Byloff et
al
[26]
designed a Graz implant-supported pendulum appliance. Carano et al
[27]
devised a
distal-jet combined with TADs anchorage system. Karaman et al
[28]
and Gelgor et al
[29]
combined distal force mechanics using a compressed coil spring with an intraosseus
screw. Kircelli et al
[30]
used a pendulum appliance with an intraosseus screw. In the
mandibular arch, there have not been a lot of studies done on retraction using
conventional methods besides lip bumper studies. Lip bumpers were shown to distalize
molars while proclining the incisors
[31,32]
.
4
With the use of the TADs, all the negative sequelae experienced with
conventional mechanics are eliminated. Dental implants
[33]
, miniplates
[34]
, and TADs
[35-
37]
, do away with the reactive mesial movement of the anterior segment (anchorage loss)
during distal movement of molars. Among these devices, the TADs (miniscrews) have
the advantage of easy placement and removal, with minimal anatomical limitations due to
their small size and low cost
[35]
. Therefore, their clinical applications have been
expanded, and they have been adopted for distalization of mandibular molars
[38]
.The
nature of absolute anchorage allows for retraction of the anterior teeth with simultaneous
distal movement of posterior teeth.
The treatment effects of TADs on distalization of the maxillary and mandibular
molars were quantified by Park et al
[38]
. The maxillary first premolar and first molars
showed significant distal movement, with no significant distal movement of the anterior
teeth. The mandibular first premolar and first and second molars showed significant distal
movement, but no significant movement of the mandibular incisor was observed. The
TADs success rate was 90% over a mean application period of 12.3 ± 5.7 months. These
results support the use of the TADs as an anchorage for group distal movement of the
teeth.
Park et al
[39]
reported that the efficacy and potency of the TADs aid mechanics in
the nonextraction treatment of both labial and lingual treatments. Anterior crowding can
be resolved without deleterious effects on facial profile and there were no adverse
reciprocal effects of conventional mechanics such as premolar extrusion and flaring of
the incisors. Not only can TADs move single or segments of teeth, it can also move entire
arches
[40]
. In patients that demonstrate class II or class III dentition, the movement of an
5
entire arch simultaneously can reduce treatment time as opposed to moving one tooth or
one segment at a time.
With the new found interest in TADs being used for retraction, a guideline needs
to be established for the amount of retraction that is anatomically possible. In order to
determine exactly how much space there is distal to the most posterior tooth in the
mandibular arch for complete arch retraction, CBCT data of 146 patients from an archival
database at the University of Southern California, Department of Orthodontics was
analyzed. The data was acquired from the NewTom (DVT9000) Volume Scanner QRsr1
Verona CBCT, which provides accurate 1:1 images that are not subject to distortion seen
in traditional digital panoramic radiographs. By presenting this data, this study will help
set a guideline for the amount of arch retraction that is possible. This can potentially
translate to how much crowding relief is possible, how much class III molar correction is
possible, and moreover, a change from an extraction to a non-extraction treatment plan.
6
The purpose of the study
-To evaluate the space distal to the most posterior tooth (second or third molar) in the
mandibular arch to establish a safe limit for distalization.
-Determine whether significant differences exist between gender groups.
-Determine whether significant differences exist between left and right lower sites.
7
Chapter Two: Review of Literature
Skeletal anchorage was first attempted in dentistry in 1945 by Gainsforth and
Higley who placed vitallium screws in the mandibles of dogs and applied orthodontic
forces with elastics. All of these screws were unsuccessful by 31 days
[42,45]
. The material
proved to be non-biocompatible. The original titanium dental implants were those created
by Branemark in 1964 and they were designed to replace missing teeth. He noticed that
these implants had bone-to-implant contact under microscopy and the concept of
“osseointegration” was born
[42]
. Research on titanium implants soon expanded and its
uses in orthodontics soon followed.
In 1969, blade implants were successfully used an anchors for elastics to retract
the incisors
[42,57]
. In 1984, Roberts et al
[58]
successfully placed titanium implants into
rabbit femurs and loaded it with force comparable to orthodontics. The screws remained
stable and osseointegration was observed. This led to dental implants and its use for
skeletal anchorage.
The disadvantages of traditional dental implants, as a source for absolute
anchorage in orthodontics, far outweighs its advantages: they are limited in its location of
placement and the direction of force that can be applied, require more invasive surgery
for placement and removal, healing time is required for osseointegration, and is
expensive
[59]
. These disadvantages make the dental implants not a good option,
especially in patients without edentulous spaces in which to place the implants.
Palatal implants were developed to reinforce anchorage in patients without
edentulous spaces, but this too required a traumatic surgery for placement and removal
and stayed out of favor.
8
The search for a convenient, compliance-free, less invasive form of anchorage led
to a miniaturized form of titanium screws. These mini-screws were first introduced by
Brons and Boering for orthognathic surgery fixation. This laid the framework for the
development of orthodontic mini-screws as temporary anchorage devices
[60]
. In 1997,
Kanomi
[61]
described the mini-implants specifically made for orthodontics and in 1998,
Costa et al
[62]
developed a mini-screw with a bracket head. Now there are many different
designs made for the mini-screw.
A TAD is a device that is temporarily fixed to the bone for the purpose of
enhancing orthodontic anchorage and is removed after its use. TADs have become the
focus of much research lately. The popularity can be attributed to their ease of insertion,
ability to be immediately loaded, patient comfort, ease of removal and versatility in
usage
[45]
. The wealth of information that is now available on mini-screws has increased
clinician and patient acceptance. The TADs offers a cost-effective way of increasing
orthodontic anchorage, while eliminating patient compliance, decreasing treatment time
and occasionally permitting orthodontic treatments that were previously thought
impossible without surgical intervention
[44]
.
Orthodontic TADs are made of medical type IV or type V titanium and range
from 1.2-2.0 mm in diameter and 4.0-14.0 mm in length
[42]
. Their miniature size makes
them ideal for the oral cavity, especially in interradicular spaces. Histological studies
have shown that TADs do not completely osseointegrate, allowing for easy removal,
while serving as an adequate source for skeletal anchorage.
Recently, the application of TADs had expanded dramatically. It has been
documented in assisting in the treatment of many difficult cases. They are particularly
useful in patients with compromised dentitions, such as the partially edentulous and the
periodontally involved
[63]
. Their use has been indicated for patients requiring maximum
anchorage, en-masse retraction of anterior teeth, upper and/or lower molar distalization,
molar uprighting, retraction or protraction of one arch, closure of extraction spaces,
asymmetric tooth movement, alignment of dental midlines, intermaxillary anchorage for
the correction of sagittal discrepancies, anchorage for rapid palatal expansion in older
patients, correction of single tooth crossbites, correction of deep overbites, intrusion and
extrusion of teeth, correction of occlusal cants, and correction of other vertical skeletal
discrepancies that would otherwise require orthognathic surgery
[44,45]
. Whatever the
situation may be, TADs are increasing treatment options while eliminating patient
compliance.
Figure 1. Example of a mini-implant temporary anchorage device. The Mondeal
miniimplant. (left) is 7mm in length and 2mm in diameter. The Orlus mini-implant (right)
is 6mm in length and 1.8mm in diameter.
9
10
Surgical Procedure
Preoperative Examination Stage
[88]
The insertion site should be examined thoroughly through inspection, palpation,
and panoramic radiography. The surface topography should be established by palpation
to determine the insertion angle. With full retraction of the soft tissue, infiltration
anesthetic is administered on the mucosa. After anesthesia is obtained, the bone quality is
evaluated with a periodontal probe. If the bone quality appears to be soft and is easily
penetrated with a probe, the site of insertion should be changed.
Marking Stage
[88]
After the placement area is cleaned with povidone-iodine, the insertion site should
be marked with a periodontal probe. The vertical reference line that bisects the interdental
area parallel to the axes of the proximal teeth should be marked. The horizontal reference
line should then be marked according to the position of the alveolar crest and the required
amount of vertical force.
A separate incision is usually not required. However, when is to be inserted in the
area of a frenum, a frenectomy should accompany the procedure to prevent possible
mechanical irritation around the implant during function. Frenectomies can be performed
before or after implantation. It can be advantageous to perform before implantation
because it eliminates extra soft tissue, although the disadvantage of that sequence is that
it requires bleeding control before implant placement. For implantation on unattached
gingival, minimal retraction of soft tissue is generally adequate. If soft tissue gets
entangled during insertion, it should be loosened through counter-rotation of the driver,
11
after which the procedure can continue. Depending on the preference of the operator, 3.0
mm of stab incision may be performed on the mucosa.
Perforating Stage
[88]
There are two ways by which to perforate through cortical bone: use of surgical
drill and use of an implant. In the perforating stage, insertion perpendicular to the surface
is recommended to prevent slippage on the surface. The slope of osseus tissue should be
determined by palpation at an earlier stage.
To perforate cortical bone, an adequate amount of vertical force should be applied
and a palm rest should be used to firmly establish the path and to turn the screw. Lateral
fore should be avoided at this time to prevent fracture. Operations should be performed
by virtue of the function of the screw rather than by vertical force. The cortical bone
should be perforated with the use of a turning motion. To reduce the risk of root injury
and minimize surgical trauma, it is desirable that a manual drill system be used.
The new type of implant has a screw that penetrates cortical bone without pre-
drilling. To avoid slippage of the implant, the operator should make an approach
perpendicular to the surface of the cortical bone from the beginning of insertion to a
depth of 1.0 to 1.5 mm in cortical bone.
After the implant is placed to a depth of approximately 1.0 mm into the cortical
bone, a change in angle can be attempted. Because of the risk of implant fracture, use of a
narrower implant with a center diameter of less than 1.6 mm is not recommended,
particularly in the mandible.
12
Guiding Stage
[88]
After perforation of cortical bone, an implant should be inserted up to about two-
thirds of the full length according to the planned angle of insertion. During this stage,
minimal vertical force should be applied as long as the insertion angle is maintained, and
a palm rest should be used once again to provide firm basis for securing the path. To
avoid root fracture and to increase the cortical bone contact area, the insertion angle
against the surface of the cortical bone should be approximately 30 degrees.
As in the perforating stage, the implant should be inserted by virtue of the screw
function, not vertical force.
Finishing Stage
[88]
The placement is finished with maximal support from cortical bone, because the
insertion path is established through the recapitulation procedure. After approximately
two thirds of the full length of the screw is inserted and its bone engagement is secured,
implant placement should be finished with only rotational motion by a finger grip to
maximize support from cortical bone. When the screw has engaged bone, rotational
motion is enough to finish the procedure, because the screw will transform this rotation
into the required translational movement. Forces in any direction can cause vibration and
compromise intimate contact between bone and implant.
Figure 2. Intraoral example of three mini-implants. The mini-implants are placed in the
interradicular space between the maxillary right canine and first premolar, the maxillary
first premolar and second premolar and the mandibular right second premolar and first
molar.
Limitations
Failure is a part of any new development and TADs are no exception. Failures
occur if the implant loosens or fractures. Mucosal overgrowth can derail the treatment.
Damage to roots or the sinus is a risk of inexperience. Park et al
[87]
cited failure rates:
Maxilla 12.5%, Mandible 16.3%, and Palate 4.2 %, however these may be reduced with
careful technique.
Host factors
[87]
:
1.) The thickness of the cortical plate important: thicker increases success.
a. In the mandible, buccal bone is thicker than lingual.
b. In the maxilla, the cortical bone is thickest on the palate. The thickest
is 4 mm lingual to the incisal papilla. The rest of the roof is uniformly
1 mm except in the mid palate (6-7mm).
2.) A band in the midpalate is typically minimum 4 mm deep.
13
14
a. The midpalate of younger patients may not have a closed suture, and
may not support TADs.
Failure rate is not correlated with age, with the exception of a partially closed
palatal suture, in which case the screw is placed off to one side of the midline.
Screw factors
[87]
:
1.) Most of the stress on the screw is in the neck.
2.) Variables are the diameter, the length, the sharpness of the thread edge, and
the screw pitch.
3.) A larger diameter (>1.2mm) has better stress distribution.
4.) It is best to have a smooth surface (no thread) in the soft tissue area. The core
of the screw is tapered to condense the bone. The surface is sand blasted and acid etched.
The thread edge is sharp, which means you do not have to drill a pilot hole.
Loading
[87]
:
1.) Each TAD can support approximately 200-300g. Overloading of the TADs
may cause a stress fracture, especially in the neck area.
a. If desire more force, more TADs may be placed.
2.) There has been a debate as to when to start loading
Ohmae et al
[105]
, demonstrated that loading did not affect the success rate, but
could have an effect on the amount of bone-to-implant contact that occurs at the interface.
His findings suggested that immediate loading increased bone remodeling around the
TADs, thereby enhancing its stability. Freudenthaler et al
[106]
showed that immediate
loading of the TAD can be achieved without affecting success rate, which can allow for
shorter treatment times and higher patient acceptance.
15
Forces can be applied either directly or indirectly to the TADs, with no detrimental
effects, as long as the screw remains stable. Screw stability should be verified
periodically by the clinician.
One note on location of TAD when attempting complete arch retraction may be
that it would be beneficial to place the TAD where a reposition will not be necessary. If a
TAD is placed interproximal to the bicuspids or molars, there is the risk of damaging the
roots as the dentition is retracted. However, if the TAD is placed distal to the entire
dentition, such as in the retromolar area, that risk is eliminated. The TADs placed in
retromolar area should be longer (10+ mm) than those placed interradicularly (6-8 mm)
to get through the thick soft tissue.
Complications and failure
Screw failure is perhaps the most troublesome complication because it results in
compromised anchorage. If a TAD is mobile and no longer provides absolute anchorage,
it should be removed and a new site for placement should be selected. Miyawaki et al
[107]
conducted a study on TAD failure rates and concluded that three factors were associated
with increased failure of TAD implants placed in the buccal alveolar bone of the posterior
region for orthodontic anchorage:
1) The diameter of a screw of 1.0 mm or less.
2) Inflammation of the peri-implant tissue.
3) A high mandibular plane angle (which is associated with thinner cortical bone
than in low mandibular plane angle patients).
16
He found that TADs with inflammation of the peri-implant tissue showed
significantly lower success rates than those without inflammation. Therefore he
concluded that the prevention of inflammation is extremely important to prevent mobility
and subsequent failure of the TADs.
The most obvious complication of TAD placement is iatrogenic damage to dental
or other vital structures such as the maxillary sinus, nerves, or blood vessels
[108]
. Careful
selection of sites for TAD placement, visualization of the site with radiographs, and
insertion with minimal force can help to minimize such complications.
Fortunately, severe damage resulting in tooth loss has not been documented in
orthodontic TAD placement, probably because of better tactile feel during placement
which allows the operator to redirect the TAD before causing serious irreversible damage
to adjacent teeth.
Fabbroni et al,
[109]
evaluated complications following contact of screws placed for
fixation of fractured mandibles. They observed that even when screw contact occurred,
the incidence of clinically significant damage was very low. From these findings, they
concluded the contact of sterile titanium on the root surface of a healthy tooth was of
little consequence and resulted in very few complications.
In situations where iatrogenic damage of adjacent teeth has been caused, the
periodontal ligament space is usually reestablished upon removal of the TADs, and the
created lesion is repaired with a narrow zone of cellular cementum on the root surface,
with few other complications.
[110]
17
Growth and Development
[131]
:
The cranial base
In contrast to the calvarium, the bones of the cranial base are initially formed in
cartilage (the chondrocranium) on the ventral surface of the brain. This endochondral
osteogenesis then leads to the development of the ethmoid, the body of the sphenoid, the
basiocciput, and the petrous temporal bones, although cartilaginous tissue remains
between the body of the sphenoid and ethmoid anteriorly and the occipital posteriorly, in
addition to intersphenoid synchondrosis between the two parts of the sphenoid bone.
These synchondroses tend to fuse by birth, except for the intersphenoidal and
sphenooccipital synchondroses; they persist until the age of 7 years and late adolescence
respectively, therefore contributing significantly to skull growth.
The length and growth of the cranial base has an important impact on craniofacial
development. For instance, the anterior cranial fossa connects to the upper part of the
facial skeleton, whereas the mandible articulates with the middle component of the
cranial fossa through the temporomandibular joint. This is how growth at the
synchondroses affects their morphogenetic development. For instance, the maxilla is
carried upwards and forwards through the growth at the spheno-occipital synchondrosis
and thereby contributes to increased facial height and depth. This also means that any
deficiencies in growth of the cranial base lead to retarded maxillary growth.
18
The mandible
[131]
Migrations of neural crest cells are not the only factor in initial mandibular development;
growth factors derived from Meckel’s cartilage control early development of both the
mandible and maxilla. Mandibular growth is principally attributed to intramembranous
osteogenesis, augmented by areas of endochondral ossification at the condylar head,
mandibular angle, and coronoid process. Hellman
[131]
describes the subsequent complex
growth patterns as “synchronized ‘cortical drifting’ at the lateral and medial periosteal
surfaces leading to forward and downward mandibular rotation and expansion.”
Mandibular growth is also dependent on muscle attachments, where tooth development
and eruption influence alveolar development.
These dynamic influences are superimposed on differential surface remodeling
patterns. For instance, growth in mandibular length results from surface apposition on the
posterior surface of the ramus and resorption at the anterior surface (i.e. the mandible
essentially grows longer as the ramus moves away from the chin).
The classic view holds that that the mandible grows downwards and forwards
relative to the cranial base, although more recent evidence indicates rather downward and
forward rotational movements dictated by surface remodeling at different rates in
different locations. The fact is that both forms of morphogenetic change occur, with the
anterior mandible having more pronounced rotation than the posterior mandible. This
subsequently requires the maxilla and mandible to compensate for each other to maintain
correct occlusal relationships. The mandible then does not grow as an isolated entity,
since normal skeletal and dental relationships area a part of the development of the face,
lips, and tongue.
19
Growth-related bony changes in the mandible (corpus width) is accomplished by
apposition and resorption of bone.
[132, 123]
Width increases of the corpus measured at
gonion (bigonial width) have been shown throughout adolescence.
[133, 134, 135]
Width
increases measured along the lateral surface of the mandibular body anterior to
attachments of the masseter muscle have also been reported through adolescence
[137]
and
into adulthood.
[136]
Increases in mandibular width at the level of the alveolar process have
not been reported.
Hesby et al
[138]
reported that mandibular molars upright transversely with age by
tipping buccally, but without the lateral translation of the roots seen in the maxillary
molars.
[139]
Hesby found “the alveolar crest measurement points had mean increases in
width of 1.60 mm (right buccal surface to left buccal surface) and 1.02 mm (right lingual
surface to left lingual surface), carrying the alveolar surfaces away from the midsagittal
plane. In contrast, the more apical measurement points at the mandibular midalveolar
process had no significant change in width. This pattern is consistent with the mean
change in mandibular first molar intermolar width (2.02 mm) and suggests transverse
uprighting that occurs by rotation in the coronal plane around a point in the body of the
tooth root. However, these mandibular width increases in the dentoalveolar region,
proximal to the first molars, were far less than the mean changes in the basilar width
measured at the angle of the mandible, representing less than 15% of mean basilar width
increase measured at antegonion (10.81 mm).”
Hesby et al
[138]
also reported that mandibular basal bone, measured at gonion or
antegonion, increased in width approximately twice as much as maxillary basal bone,
over the first 2 decades of life, growing approximately 20 mm wider in the transverse
dimension which agrees with many other studies.
[137, 133, 134]
In some studies, with
implants in the buccal cortical plate of the mandibular body in the region apical to the
first molar, the authors reported lateral displacement of the right and left corpora of the
mandible during growth.
[137, 140]
Hesby’s measurements, made at a more occlusal level in
the alveolar bone, do not support these findings. Hesby concluded that there is a pattern
of width changes in the maxilla, the maxillary alveolar process, the maxillary first molars,
the mandibular first molars, and the mandibular alveolar process. The width changes
occur as a gradient in the vertical dimension (jugale-mandibular alveolar process). The
greatest width change occurs more superior (jugale point). The smallest width change
occurs inferiorly (midalveolar point of the mandible).
Mechanisms of craniofacial growth control
[131]
At birth, the volume of the cranial vault is greater than that of the face due to
differential growth pattern of the nervous system and the face. This continues until
approximately the age of six when craniofacial growth catches up and becomes faster
than that of the neurocranium. Such different growth patterns may reflect forces derived
from:
• skeletal (bone) tissues, with passive reactions of the associated cartilaginous
and soft tissues
• cartilage with passive reactions from the skeletal and soft tissues
• soft tissues with passive reactions for both skeletal and cartilaginous
tissues.
The main difference between these forces relates to their precise genetic
20
21
control. The periosteal tissues and sutures are unlikely to be the primary determinants
of craniofacial growth, since they primarily react to extraneous forces.
Endochondral osteogenesis at the mandibular condyles, cranial base
synchondroses, and nasal septum contribute to craniofacial growth. Whereas the cranial
base synchondroses and nasal septum contribute to craniofacial growth by acting as
independent growth centers, the mandibular condyles are reactive to external forces, in
common with the cranial sutures.
No single process predominantly controls craniofacial growth, which not only
depends on functional demands but also on the variable skeletal cartilaginous responses
that in turn are also influenced by differential reactions to both local and systemic growth
factors.
The currently accepted concept of upward and backward growth of the mandible
was first described by John Hunter in 1771
[111]
. Prior to that, it was thought that the
mandible grew primarily by deposition of bone at the chin. Researchers studied the ramus
and condyle to determine the changes in the mandibular growth or position because of
this concept of upward and backward growth. The condyle became the focus of research
as the determining factor in mandibular growth. Early theorists proposed that the condyle
acted like an epiphyseal plate for the mandible and controlled growth of the entire bone
based on the studies of long bones
[112]
. However, surgical removal of the condyle in
animals failed to dramatically change the growth of the mandible
[113]
. The notion of the
“master center” was rejected by modern craniofacial biologists in favor of the concept of
the condyle as a “growth site” able to adjust to changes occurring in other parts of the
face
[114]
. Johnston furthered proposed that the “condyle acts as a ratchet to hold the
22
growth achieved by tissue-separating forces that occur in areas remote to the condyle
itself”
[115]
. Because the condyle can not be considered the control center, variations in
mandibular size and shape must be achieved by remodeling of the ramus as it’s
osteogenic and chondrogenic connective tissues receive epigenetic signals from other
parts of the face and neurocranium.
The concept of patterns of bony change was brought into focus by this shift in
emphasis from condyle to the whole ramus. The idea of pattern as it relates to facial
growth was made popular by Brodie
[116]
. Further evidence that different individuals
exhibit different patterns of mandibular growth was offered by Björk
[117-120]
. Moss
[121]
further proposed that “mandibular growth followed a logarithmic spiral and not a linear
pattern.” Ricketts supplemented this idea and proposed that mandibular growth followed
an arc
[122]
. The logarithmic spiral and arcial concepts suggest that the mandible
undergoes changes in size and shape with respect to some temporal sequence.
Biologically, hard tissues such as bone can only grow by apposition, laying down
new layers on top of old ones. Deposition and resorption, two components of appositional
growth, must account for the dramatic changes in size and shape. In 1964, Enlow and
Harris mapped the histologic characteristics of the entire inner and outer surfaces of a
sample of human mandibles
[123]
. This study resulted in a topographic map of the
developing mandible, showing areas where bone was remodeling out or growing in.
Figure 3. Topographic map of the mandible showing areas of bone remodeling.
Developed by Enlow and Harris.
Given that other authors have proposed that the mandible does not follow one
characteristic pattern throughout life, it is likely that the map of mandibular growth varies
with age of the individual. Hans et al
[124]
demonstrated histologic reconstructions of
remodeling variations of the mandibular ramus, which is significant because
morphogenic relationships between the ramus and corpus establish mandibular arch
position.
Bishara et al found arch length increases in the maxillary and mandibular arches
occurred until the age of 13 and 8, respectively
[126]
. These changes were attributed to the
eruption of the permanent incisors. The greatest incremental increases occurred during
23
24
the first two years of life. Following the increases, there were significant and consistent
decreases in arch lengths mesial to the permanent first molars in both the maxillary and
mandibular arches. These decreases continued until the age of 45. Normally, the teeth do
not change significantly in crown size except through interproximal attrition. As a result,
the decrease in arch length is translated as an increase in the tooth size-arch length
discrepancy, unless interproximal attrition keeps pace with the decrease in arch lengths.
As stated earlier, between 13 and 45, maxillary arch length decreased an average of 5.7
mm in males and 4.6 mm in females and mandibular arch length decreased 5.0 mm in
both sexes. Bishara et al
[126]
further discovered a sexual dimorphism whereby the males
have significantly greater total arch length in both arches than females.
Niedzielska et al
[125]
discovered that the lower arch length, width and the dental
arch segments varied under certain conditions. There was an increase in length, width and
arch segments in the extraction groups, and a decrease when the third molars were
retained, in which case the Ganss ratio was lower (0.60 to 0.65). Medium to high ratios
(0.85 to 0.89) correlated with a lack of change in the lower arch measurements (see Fig
4.).
In comparing groups where one third molar was extracted with groups with
retained third molars, the length of the arch decreased by –1.15 to –0.25 mm on the side
where the third molar was retained. In contrast, in groups where one or both third molars
were extracted, arch length increased by 0.40 mm to 0.70 mm. No changes in dental arch
length were observed in third molar agenesis subjects. The Ganss ratio was relatively low
(0.61) in those patients in whom the length of the lower right quadrant either increased
following third molar extraction or decreased when the third molar was left in situ. In the
majority of patients there was no change in the length of the segment and the mean value
of the Ganss ratio was higher at 0.89. These relationships were statistically significant at
P < 0.001 for the right side, and P < 0.05 for the left side.
Figure 4. Measurement of Ganss factor (ratio of accessible retromolar space to the width
of the crown of the third molar (A:B)) on the panoramic radiographs.
Hellman
[131]
evaluated the development of the face pertaining to the position of
the face in its relation to the brain case, and the position of the teeth in relation to the face.
The position as it concerns the face and teeth was ascertained by means of measurements
taken from the external auditory meati on both sides to nasion, prosthion superior,
prosthion inferior, and menton, in the sagittal plane, as indicated in Fig. 5. Hellman
devised a classification system for development of the dentition.
25
Figure 5. Outline of skull, showing method of measuring dimension representing
position of face.
Hellman stated that growth occurred posteriorly in the A-P direction. This fact
was well illustrated by the growth centers of the jaw bones. These centers are situated in
the tuberosity of the maxilla and in the retro-molar triangle of the mandible. Fig. 6
presents the dental arches of an infant with the deciduous dentition completed. Behind the
last deciduous molars are seen the areas where the first and second permanent molars are
developing. There is as yet insufficient room to accommodate these teeth. But as growth
takes place, these areas increase in size. The increase, however, occurs as the portion of
the maxilla and mandible anterior to the growth centers move forward; i.e., they depart
from the pterygoid processes above and from the ramus below. When this is
26
accomplished, and the first molar takes its position, the growth center in the same way
begins to show the development of the third molar in addition to that of the more
advanced state of the second.
Figure 6. Skull of White Infant. Showing occlusal view of deciduous dentition, posterior
to which are seen the areas for future accommodation of the three permanent molars. In
the tuberosities of the upper jaw, the position of the permanent first molar is visible. In
the retromolar triangle of the lower jaw, two openings on each side indicate the position
for the permanent first and second molars.
27
28
In senility the dentition is either worn down or lost, in consequence of which the
size of the face in the total height is reduced, but this is accompanied by a complete
change in position of the mandible. It should, however, be noted that despite the change
in position of the mandible, the mandibular angle is not changed materially. It is changed
in position only because of the greater approximation of the mandible to the maxilla. The
flexion of the mandibular angle, it should be noted, is associated with the relative
proportions of the ramus to the body of the mandible. The shorter the former in relation
to the latter, the more obtuse the mandibular angle. Thus, as growth progresses and the
ramus increases in height relatively more than does the body of the mandible in length,
the angle becomes more and more acute. In senility the angle is not greatly modified, it
only undergoes a marked change in position. Table 1 clearly illustrates the relationship of
ramus height and body length of the mandible to the mandibular angle. It will be noticed
that as the mandible grows larger, the ramus increases its proportionate height more and
more. Thus, at the beginning, the ramus height is only slightly over one-half of the body
length of the mandible; at the end, it is more than two-thirds of that length. The only drop
in this relative increase occurs during childhood, when the growth of the body of the
mandible is very much accelerated. Coincident with the relative increase of the ramus
height is a decrease in the mandibular angle until old age. In senility, the angle increases
again, but only by 1.7°.
It was further corroborated by Keen et al in London that the angle of the mandible
does not increase provided that sufficient teeth remain in the jaws to ensure a “bite”.
Keen collected data from 3 age groups, 6-21 years, 25-45 years, and 50-76 years, limiting
this series to individual with good teeth or sufficient number to hold the jaws apart when
the mouth was closed. The results are shown in Table 1.
Table 1. Stages of development. Showing relationship of ramus height and body length
of mandible to mandibular angle. Developed by Hellman and Keen et al in London. Stage
I: The period of early infancy: designated by the state of development before the
deciduous dentition is completed. Stage II: The period of late infancy: designated by the
state of development when the deciduous dentition is completed. Stage III: The period of
childhood: designated by the state of development when the permanent first molars are
erupting or have erupted, in addition to which some or all of the deciduous incisors have
been lost and are being replaced by the permanent successors. Stage IV: The period of
pubescence: designated by the state of development when the second permanent molars
are erupting or have erupted, in addition to which some or all of the deciduous canines
and molars have been lost and are being replaced by their permanent successors. Stage V:
The period of adulthood: designated by the state of development when the third molars
are erupting or have erupted. Stage VI: The period of old age: designated by the state of
development when the occlusal surfaces of the molars are worn off to the extent of
obliterating the pattern of grooves. Stage VII: The period of senility: designated by the
state of development when at least half of the crowns of the teeth are worn off,
accompanying or following which some, most, or all of the teeth are lost.
29
In 1995, Losken et al
[127]
revised the Bolton standards created by Broadbent and
Holden
[128]
and devised normal lengths of the vertical ramus, body, and angle of the
mandible at different ages.
Table 2. Normal Mandibular measurements. Developed by Losken in 1995.
30
31
This table shows that with age, the vertical ramus and the body of the mandible
increases in length, whereas, the gonial angle decreases. This study did not evaluate
patients over 18 and therefore, any conjectures to the lengths in older patients would be
mere guesses. However, it can be inferred from the trend that as people age, the vertical
ramus and body length would increase and consequently, the space distal to the most
posterior tooth to the ascending ramus would also increase. Moreover, the fact that teeth
migrate mesially and not distally also support the notion that this space would increase
with age.
Harris et al
[130]
determined that some variables- particularly those between arches
(overbite and overjet, molar relationship) and mandibular intercanine width- remained
age invariant. In contrast, all other measures of arch width and length changed
significantly: Arch widths increased over time, especially in the distal segments, whereas
arch lengths decreased. These changes significantly altered arch shape toward shorter-
broader arches. Harris suggested that the changes during adulthood occur most rapidly
during the second and third decades if life, but do not stop thereafter.
32
Gender differences in JAW SIZE
The examination of skull sexual dimorphism has been the subject of numerous
morphologic and craniometric studies. Many authors have concentrated on the skull for
determining the sex of an individual. Several methods have been refined. Initially,
morphologic examinations were developed. These were qualitative methods making use
of descriptive criteria
[90-92]
. The disadvantage of such methods is that they lack
objectivity and are dependent on the experience of the operator
[93]
.
Craniometric examination was also developed, and this is a quantitative
examination involving taking direct measurements of the skull
[94,95]
. In 1958, Céballos
and Rentschler
[96]
were the first to work on teleradiography for determining sex from the
skull. Other authors succeeded them
[97-101]
using both posteroanterior and lateral
teleradiography.
In 1996, Hsiao et al
[102]
conducted lateral teleradiography on a sample of 50
males and 50 females from Taiwan. Using 18 variables from cephalometric plots
obtained from the teleradiography plates, they claimed to be able to determine the sex of
an individual with 100% accuracy. Of the numerous points evaluated, the linear
measurements of the cranium as well as the jaw were larger for males than females.
In 2004, Patila et al
[103]
used lateral cephalograms to determine a method to
establish gender. A discriminant function derived from 10 cephalometric variables
provided 99% reliability in sex determination. The formulae obtained from regression
analysis using the maximum length of skull showed very high degree of reliability for
33
estimation of stature in males as well as females. A finding from this study also
recognizes that males have larger skulls than females.
In 2008, Franklin et al
[104]
undertook a comprehensive analysis of sexual
dimorphism in the mandible of Black South Africans, incorporating individuals from a
selection of the larger local population groupings; the primary aim is to produce a series
of metrical standards for the determination of sex. The sample analyzed comprises 225
non-pathological mandibles of Black South African individuals drawn from the R.A. Dart
Collection. Nine linear measurements, obtained from mathematically transformed three-
dimensional landmark data, are analyzed using basic univariate statistics and discriminant
function analyses. All of the measurements examined are found to be sexually dimorphic;
the dimensions of the ramus and corpus lengths are most dimorphic.
While differences in tooth size between different ethnicities and gender are well
documented, inconsistencies between studies could be attributed to other factors that
affect tooth size, thereby making clear cut distinctions difficult. While teeth are subject to
sexual dimorphism, there has been a substantial decrease in sexual dimorphism in
modern day. This trend for decreased sexual dimorphism has been associated with the
overall trend for dental reduction. The evidence suggests that reduction in sexual
dimorphism is more related to changes in male dentitions than to changes in female tooth
size.
34
Cone Beam Computed Tomography.
In 1972, Hounsfeld
[64]
developed the first CT and put it to use. Over time, it has
been improved to increase scanning speed, reduce exposure to the patient and improve
image quality. This device allows 3-D imaging without the distortion caused by
magnification. Superimposition errors form this distortion have subsequently been
eliminated as well. Contrast is improved from the traditional radiography while blurring
and overlapping is eliminated
[65]
. Its limitations include its high cost, increased levels of
exposure and complexity
[66]
.
When CBCT was introduced, 3-D imaging of craniofacial structures for dentistry
became much more practical. Costs were reduced along with patient exposure
[65,66]
.
CBCT allows dimensionally accurate, 1:1 ratio imaging of craniofacial structures. A
single scan can construct images in any plane of space, and a 3-D graphic rendering of
the object allows the technician to zoom, rotate, and pan through voxels of the image
[68]
.
Implantology quickly accepted its use for its ability to identify precise location. The cone
beam produces a more focused beam and considerably less scatter than fan shaped CT
devices therefore increasing the X-ray utilization and reducing X-ray capacity required
for volumetric scanning
[69]
. Recent advancements in image production allowed for
secondary reconstructions of the CBCT data into panoramic, lateral, and A-P
cephalometric images, making it a highly useful imaging modality in orthodontics
[67]
.
Figure 7. NewTom QR-DVT 3G (Quantitative Radiology, srl, Verona, Italy).
The NewTom 3G (Fig. 7) operates similarly to CT. The patient is placed in
the supine position and the head and neck are scanned. CBCT can provide 3D images
with up to 4 times less radiation than a conventional CT
[70]
. An effective absorbed
radiation dose from an imaging session with the NewTom 9000 is approximately
50.2µSv, that of a typical full mouth series
[68]
. This is approximately 1.4 % or 5 days of
the annual per capita background radiation dose of 3600µSv in the USA
[69]
. This is
considerably less than conventional CT which results in an effective radiation dose of
17.6µSv to 656.9µSv for a maxillary examination and 124.9µSv to 528.4µSv for a
mandibular examination depending upon the volume of the jaw and the operational
35
36
settings on the CT
[68]
. The effective dose of a panoramic examination is 2.9µSv to 9.6
µSv and for a full-mouth series is 33µSv to 84 µSv depending on the operational setting
such as kVp, mA, film speed, and collimation. For reference, each passenger on a single
flight from Paris to Tokyo has an effective radiation dose of 139 µSv
[71]
. Ultimately, the
effective dose of CBCT is significantly smaller than conventional CT and is within the
range of traditional dental imaging modalities
[72]
. At present there are a number of
clinical applications of CBCT being reported
[73]
. Currently CBCT is being used in oral
diagnoses, surgery, and to locate incidental oral abnormalities
[67]
. Three-dimensional
volumetric imaging is being used routinely in 3D airway analysis and for visualization of
temporomandibular joint morphology. CBCT data can very helpful for the imaging of the
mandibular canal prior to surgical removal of mandibular 3rd molars
[74]
. For the
implantologist conventional radiographs often do not supply the necessary diagnostic
information regarding alveolar bone quality and height
[75]
. CBCT overcomes many of
the limitations of conventional radiography and can be used to accurately assess bone
dimensions, bone quality and alveolar height
[68]
. Implant planning is currently one of the
most common uses for CBCT
[76,77]
.
The CBCT technology can also be applied in the field of orthodontics. Instead of
taking the usual panoramic, lateral cephalogram, occlusal films, and a TMJ series
separately, a single scan from the CBCT technology is able to provide the same
information
[68]
. The 3D orientation of impacted canines is very important for the surgical
and orthodontic management of impacted canines. Cone beam volumetric imaging
provides invaluable information about impacted canines and assists in the treatment and
understanding of these cases surgically and orthodontically
[79]
.
37
Recently, researchers have studied the geometric and linear accuracy of the
digital volume tomograms obtained with the NewTom QR-DVT 9000 (Quantitative
Radiology, srl, Verona, Italy), the predecessor to the NewTom QR-DVT 3G. It was found
that the volume produced by the NewTom 9000 has geometric distortion below the
resolution power of the tomograms
[80]
. These results indicate that the images presented
by the NewTom 9000 are geometrically correct and, from a geometric standpoint, are
suitable for 3D diagnosis and treatment planning
[80]
. The accuracy of the linear
measurements obtained in CBCT images generated by the NewTom 9000 was evaluated
by Lascala
[72]
. Lascala showed that the actual measurements were always larger than
those obtained from the CBCT images, but these differences were only significant for
structures outside of the dentomaxillofacial area, therefore validating the accuracy of
linear measurements on CBCT images
[72]
. Danforth and Peck
[132]
found similar findings
in 2003 where they concluded the cone beam volume tomography would be a viable
imaging option for dental uses and useful especially with pretreatment evaluation and in
decision-making process for impactions in the mandible.
Finally, the accuracy and the low radiation dose of the NewTom make CBCT a
useful technique for imaging of the dentomaxillofacial complex. Researchers around the
world are currently utilizing CBCT technology. As databases at imaging facilities grow
these data become a great source of information for dental research.
38
Chapter Three: Hypothesis
Null Hypothesis
1. A norm can not be established to provide a guideline for the amount of complete arch
retraction possible in the mandibular arch.
2. There is no significant difference in the space available for complete arch retraction
between males and females.
3. There is a significant difference in the amount of complete arch retraction possible
between the left and right sides.
39
Chapter Four: Materials and Methods
The availability of bone for complete arch distalization with TADs was evaluated
for the mandible. Space distal to the lower second and third molars was measured in adult
orthodontic patients. All measurements were analyzed from pretreatment NewTom Cone
Beam Computerized Tomography (CBCT) scan data from the patients of the University
of Southern California Graduate Orthodontic Clinic. These records were from the first
146 consecutive adult CBCT scans from 2005 to 2008. The inclusion and exclusion
criteria are as follows:
Inclusion Criteria:
(1) Adults (age 18 and over) who have completed their growth. Growth cessation was
established by use of the cervical vertebrae maturation (CVM) method
[41]
. Their CVMI
(cervical vertebrae maturation index) was measured to be 6, indicating that growth
modification was not plausible.
(2) Patients with both lower second molars and third molars fully erupted or
(3) Patients with second molars fully erupted only.
(4) High quality CBCT volumetric data.
Exclusion Criteria:
(1) Patients exhibiting any jaw malformations or evidence of pathology within the
mandible.
(2) Patients with extreme differences from left and right.
(3) Patients with extreme tipping of molar teeth.
40
(4) Patients with impacted lower second or third molars.
(5) Patients with missing any lower second molars.
(6) Poor quality CBCT volumetric data with indistinct cortical borders, artifacts, or
blurring.
The protocol for this study was submitted to the University of Southern
California, Health Sciences Campus, Institutional Review Board (IRB) for the review and
approval prior to the initiation of data collection. The study protocol was approved and an
approved proposal number, HS-08-00420, was assigned.
41
CBCT data were acquired using NewTom QR-DVT 3G (Quantitative Radiology,
srl, Verona, Italy). Scans were obtained with a 12” sensor providing a reconstruction
volume of 110 X 150 mm. The device acquires 360 images at 1 image per angular degree
intervals and imaging time is approximately 75 seconds. The image resolution was 512 X
512 pixels and 12 bits per pixel. The reconstruction matrix voxel was 0.25 X 0.25 X 0.3
mm
[65]
.
Once a patient was selected, they were assigned an anonymous identification
number and gender was recorded into a spreadsheet. InVivo Dental
TM
Software supplied
by Anatomage was used to produce a reconstruction of the volumetric data.
The computer software InVivo Dental
TM
was used to reconstruct 3-D volume
renderings (Fig.8) and panoramic images (Fig.9). 3-dimensional images were sliced into
2-dimensions for accurate measurements. The upper and lower borders of the panoramic
x-ray were manipulated and are denoted by the green lines. The red line indicates the
mandibular occlusal plane (Fig. 10), which also dictates the vertical component of future
measurements. This will manifested as a horizontal “cut” through the entire image at that
level. From this “cut” a customized focal trough (Fig. 11) was made so that the trough
slice goes through the distal height of contour and the ascending ramus. The trough
designates the sagittal plane of the future measurement. The outline of the crown and the
posterior limitation was used to assess the amount of bone distal to the second molars of
the mandible parallel to the functional occlusal plane. From these designations, a
panoramic image was fabricated (Fig. 12). A linear horizontal measurement was acquired
from the distal height of contour of the lower second molar crowns to the ascending
ramus. The ascending ramus (posterior limit of retraction) is visible in the horizontal slice
42
(Fig. 11,12). A measurement guide was used to measure the distance from distal height of
contour of the most posterior tooth to the ramus (Fig. 12,13). This novel technique
provides a more accurate method of establishing distance compared to traditional digital
panoramic radiographs because:
1. A traditional panoramic radiograph uses an arbitrary focal trough where
the distance from the object to the trough can vary unpredictably at any
point in the image.
2. There is an inherent error and distortion due to the magnification in a
traditional panoramic radiograph
3. Traditional radiographs have a coefficient of magnification which makes
measurements from the images very unreliable.
The system used here with the NewTom CBCT scan eliminates all these problems.
1. It uses a customized focal trough- can choose the points in 3-dimensions
(distal height of contour and the point on the ascending ramus-
representing the location where the retracting crown will touch first).
2. There is no magnification error. This technology provides a 1:1 ratio
accurate representation of the object and allows for precise measurements.
3. The horizontal slice through the occlusal plane allows for accurate
measurements.
In our study, we used the ascending ramus along the occlusal plane as the limit. It
is apparent in Fig. 18 that the body of the ramus transitions from a kidney bean shape to a
more teardrop configuration more posteriorly along the mandible. The superior portion of
this teardrop represents the ascending portion of the ramus which is more cortical in
nature. The cortical bone would be much too difficult to move teeth through and the
surface area of this region makes it not suitable for teeth to be stable. This was the
rationale for the use of the ascending ramus as our hypothetical posterior limit of
retraction.
Figure 8. Volume Render Screen. 3-Dimensional view of skull is visible
43
Figure 9. Super Pano Screen: Customized 1:1 ratio panoramic x-ray can be constructed.
44
Figure 10. Construction of panoramic image: To construct a customized panoramic
image, upper and lower borders must be delineated. Green lines: upper and lower borders.
Red line: occlusal plane “cut”. This “cut” is depicted as an occlusal view in the next
screen (Fig. 16).
45
Figure 11. Occlusal plane “Cut”. From this screen, the focal trough is customized for
each patient. The distal height of contour of the most posterior teeth and ramus just distal
to it can be visualized.
46
Figure 12. How to make measurement. A guideline is constructed along the functional
occlusal plane and the measurement is made parallel to this line.
Figure 13. close up of measurement.
47
Figure 14. Method of measurement. A line is made that represents the functional
occlusal plane. The linear measurement is made parallel to this line from the distal of the
most posterior tooth to the ascending ramus.
48
49
The space measurements were recorded in the same manner as those of third
molar eruption space measured by Kim et al
[49]
and others
[51-53]
(Fig 12, 13). The
distance was measured parallel to the occlusal plane
[46]
. The available space was defined
as the distance from the anterior border of the ramus
[47,48]
to the distal surface of the
mandibular second molar crown parallel the occlusal plane (Fig 12, 13). Subjects with
incomplete eruption of the mandibular second molars were not measured by Kim.
Similarly, Hattab et al
[129]
measured the space for third molar eruption using a panoramic
radiograph and measuring the space distal to the second molar.
50
Analysis of Data
Descriptive statistics were calculated for the sample. Wilcoxon tests (Wilcoxon
rank sum and Wilcoxon signed rank test) were used to assess the difference of space
availability between genders and different sides. For all the tests, α< .05 is defined as
the significance level. Mean, range, standard deviation have been calculated for data,
however, due to the nonparametric nature of the data, the statistical analysis was
completed with corresponding tests.
All measurements were made by one examiner. Method error was assessed by
statistically analyzing the difference between duplicate measurements made by the same
examiner collected 1 week apart on 20 different patients randomly selected from the
sample. Because the data does not hold the assumption of normality, Spearman’s Rank
Correlation test was used to examine the reliability of the repeated measurements.
51
Chapter Five: Results
Patient Demographics
The sample consisted of 146 consecutively scanned adult patients. The ages
ranged from 18 to 61 at the time of the scan. Of the 146 patient scans evaluated, 36 met
the exclusion criteria and were omitted. The remaining 110 patients were evaluated for
space distal to the most posterior tooth in the mandibular arch [second molar (n=73) and
third molar (n=37)]. The measurements were assessed by evaluating panoramic images
reconstructed from 3-D CBCT data. The gender distribution was almost equal. The males
accounted for 51% (56/110) while females accounted for 49% (54/110).
Comparison of Sites Measured: Left v. Right
The measurements showed insignificant differences between the left and right
side. There was an average of 7.27 mm distal to lower second molars [(left) 7.24 ± 2.10
mm and range of 2.08-13; (right) 7.29 ± 2.02 mm and range of 1.92-12.32] and 2.80 mm
distal to lower third molars [(left) 2.75 ± 2.11 and range of 0-8.24; (right) 2.85 ± 2.11 and
range of 0-10.56].
The measurements recorded were that of hard tissue. It was found by Costa et al
[89]
that the average mucosal depth at the retromolar area is 3.02 mm. This leaves an
average of 4.25 mm distal to the second molars and insufficient space distal to third
molars. However, the third molars can easily be extracted is extra space is required.
52
Comparison of Sites Measured: Male v. Female
The measurements indicated a sexual dimorphism. Males tended to have more
space distal to both second and third molars.
Average space distal to mandibular second molars: (1) males: 7.32 mm with S.D.
of 2.21 mm and range of 1.92 – 11.63 mm (right), 7.41 mm with S.D. of 2.43 mm and
range of 2.08 – 13 mm (left); (2) females: 7.26 mm with S.D. of 1.86 mm and range of
4.12 – 12.32 mm (right), 7.08 mm with S.D. of 1.76 mm and range of 3.88 – 10.3 mm
(left). Average space distal to mandibular third molars: (1) males: 3.08 mm with S.D. of
2.55 mm and range of 0 – 10.56 mm (right), 3.05 mm with S.D. of 2.35 mm and range of
0-8.24 mm (left); (2) females: 2.59 mm with S.D. of 1.35 mm and range of 0 – 4.12 mm
(right), 2.34 mm with S.D. of 1.73 mm and range of 0 – 5.33 mm (left).
As with previous measurements, the soft tissue thickness must be accounted for.
Reliability of Measurements
The reproducibility of the measurements was assessed by statistically analyzing the
difference between double measurements made by one examiner taken 1 week apart on
20 randomly selected patients. Because the data does not hold the assumption of
normality, Spearman’s Rank Correlation test was used to examine the reliability of the
repeated measurements.
The correlation coefficient between the two measurements was very high. The
Spearman correlation coefficient was r =.997 (p<.001); As a r > .8 is generally regarded
as the threshold for reliability, the measurements of this study were found to be fairly
reliable.
53
Average Space
Left (mm) Right (mm)
Second Molars 7.24 7.29
Standard Deviation 2.1 2.02
Range 2.08-13.0 1.92-12.32
Table 3. Space Distal to Mandibular Second Molars. Average space distal to mandibular
second molars: 7.29 mm (right) and 7.24 mm (left). Range: 1.92-13.00 mm; standard
deviation of 2.02 mm (right) and 2.10 mm (left).
Average Space
Left (mm) Right (mm)
Third Molars 2.75 2.85
Standard Deviation 2.11 2.11
Range 0-8.24 0-10.56
Table 4. Space Distal to Mandibular Third Molars. Average space distal to mandibular
third molars: 2.85 mm (right) and 2.75 mm (left). Range: 0-10.54 mm. standard deviation
of 2.11 mm (right) and 2.11 mm (left).
54
Average Space
males left (mm) right (mm)
Second Molars 7.41 7.32
Standard Deviation 2.42 2.21
Range 2.08-13 1.92-11.63
females
Second Molars 7.08 7.26
Standard Deviation 1.76 1.86
Range 3.88-10.3 4.12-12.32
Table 5. Space Distal to Mandibular Second Molars by Sex. Results separates by sex:
Average space distal to mandibular second molars: (1) males: 7.32 mm with S.D. of 2.21
mm and Range of 1.92 – 11.63 mm (right), 7.41 mm with S.D. of 2.43 mm and Range of
2.08 – 13 mm(left); (2) females: 7.26 mm with S.D. of 1.86 mm and Range of 4.12 –
12.32 mm(right), 7.08 mm with S.D. of 1.76 mm and Range of 3.88 – 10.3 mm (left).
55
Average Space
left (mm) right (mm)
males
Third Molars 3.05 3.08
Standard Deviation 2.35 2.55
Range 0-8.24 0-10.56
females
Third Molars 2.34 2.59
Standard Deviation 1.73 1.35
Range 0-5.33 0-4.12
Table 6. Space Distal to Mandibular Third Molars by Sex. Results separated by sex:
Average space distal to mandibular third molars: (1) males: 3.08 mm with S.D. of 2.55
mm and Range of 0 – 10.56 mm(right), 3.05 mm with S.D. of 2.35 mm and Range of 0-
8.24 mm(left); (2) females: 2.59 mm with S.D. of 1.35 mm and Range of 0 – 4.12 mm
(right), 2.34 mm with S.D. of 1.73 mm and Range of 0 – 5.33 mm(left).
56
Descriptive Statistics:
After evaluation, it was discovered that the space behind the most posterior tooth
in the lower jaw is not a normally distributed variable (it is skewed to the right).
Therefore, nonparametric tests (using medians, interquartile ranges, and Wilcoxon tests
instead of means, standard deviations and t-tests) were used in these analyses. Since
there was a significant difference between the spaces behind the second vs. the third
molars, these groups were analyzed separately in tables 8 and 9.
N Median (IQR) p-value
Total sample 220 teeth 5.880 (3.715-7.995)
Gender .94
1
Males 56 patients 5.595 (3.555-8.07)
Females 54 patients 5.85 (3.84-7.99)
Side .38
2
Left 110 teeth 5.915 (3.68-8.11)
Right 110 teeth 5.865 (3.90-7.85)
Molar <.0001
1
2 73 patients 7.31 (5.81-8.60)
3 37 patients 2.545 (1.24-4.12)
Table 7. Median (interquartile range) for space behind the most posterior tooth in the
lower jaw
1. Wilcoxon rank sum test of mean space ((right+left)/2).
2. Wilcoxon signed rank test of space behind tooth.
Because the data was not normally distributed, appropriate nonparametric statistical tests
were used.
male
(56 pts)
female
(54 pts)
left
(110 teeth)
right molar 2
(73 pts)
molar 3
(37 pts)
0
2
4
6
8
10
12
Space
Figure 15. Box and Whisker plots for the space behind the most posterior tooth in the
lower jaw
57
N Median (IQR) p-value
Gender 73 patients .64
1
Males 35 patients 7.17 (5.54-9.355)
Females 38 patients 7.195 (5.655-8.55)
Side 146 teeth .49
2
Left 73 teeth 6.88 (5.73-8.78)
Right 73 teeth 7.33 (5.84-8.38)
Table 8. Median (interquartile range) for space behind the second molar in the lower jaw
1. Wilcoxon rank sum test of mean space ((right+left)/2).
2. Wilcoxon signed rank test of space behind tooth.
Figure 16. Box and Whisker plots for the space behind the second molar in the lower jaw
58
N Median (IQR) p-value
Gender 37 patients .62
1
Males 21 patients 2.455 (1.47-4.40)
Females 16 patients 2.42 (1.40-3.57)
Side 74 teeth .67
2
Left 37 teeth 2.54 (1.08-4.12)
Right 37 teeth 2.55 (1.50-4.11)
Table 9. Median (interquartile range) for space behind the third molar in the lower jaw
1. Wilcoxon rank sum test of mean space ((right+left)/2).
2. Wilcoxon signed rank test of space behind tooth.
Figure 17. Box and Whisker plots for the space behind the third molar in the lower jaw
59
60
Chapter Six: Discussion
Orthodontists have been concerned with anchorage and its management since the
beginning of the specialty. The conventional appliances (e.g. LLHA, Nance, headgear,
TPA, etc.) designed to enhance anchorage are not able to eliminate the tooth movement
caused by the reactive force
[81-83]
. TADs provide absolute skeletal anchorage that is
compliance free and can prevent undesirable tooth movement.
TADs in orthodontics are placed in the maxilla and mandible. Unlike traditional
endosseous implants, they do not osseointegrate. The TADs rely on the mechanical
interdigitation between the cortical bone and implant surface for its retention and stability
[84-86]
.
TADs allow the orthodontist to achieve results that were not possible before.
Force can be delivered to one tooth, segment of teeth, or to the whole arch without the
reactive force encountered with traditional mechanics
[36-40]
. Complete Arch retraction, to
correct class II or class III, has been achieved on a regular basis with this new technology.
Orthodontists can now achieve this result without patient compliance and without any
reactive movement.
Park et al
[36-40, 87]
was able to achieve retractive movement of 4.15 mm and
protractive movement of 9 mm of the lower molars and fully correct an arch length
discrepancy of 5.9 mm. Our results indicate that there is an average of 7.27 mm of space
distal to the lower second molars, however, the thickness of the soft tissue must be
accounted for. With the average mucosal depth of 3.02 mm in the retromolar region
[89]
,
the available space for retraction is reduced to 4.25 mm. This amount would be enough to
correct 8.5 mm of anterior crowding, correct half step class III, or possibly change from
61
extraction to non-extraction treatment plan. The implications of this finding may change
a treatment plan of extraction to that of non-extraction.
One finding of note is the fact that females tended to have less space available for
retraction even with the anthropological evidence that females tend to have smaller
dentition. All the craniometric studies undertaken show that females have smaller faces
along with the individual parts of the face, such as the mandible. It could be logically
surmised that the space measurement would even out with the male measurement since
both the jaw and the dentition is smaller compared to the males. However, it is our
finding that females consistently had less space distal to the most posterior tooth in the
mandible than males.
Clinical Limitations
As with any new technology, there are limitations and other factors that need to
be considered. First is the facial profile. If the face will not be improved from this amount
of retraction, then other means of correction should be considered. Secondly, if the host
factors
[87]
are not adequate to maintain stability of the TADs, this should not be
attempted. In cases where retraction is needed, there are also limits based on the quantity
of bone. If there is not enough in the symphyseal area, the anterior segment can be
retracted out of the bone and cause dehiscence. Also with posterior retraction, the teeth
can not move straight back. If the dentition were to be brought straight back, the bite
would open and a brody bite situation would arise. Therefore, the vertical and transverse
planes must be considered. In order to maintain the pretreatment mandibular plane angle
62
and transverse condition, the posterior molars must be intruded and moved slightly
buccally as the teeth are retracted.
Our measurements were recorded from the distal height of contour of the most
posterior tooth to the ascending ramus. However, in clinical use, the thickness of the soft
tissue must be taken into consideration. Costa et al
[89]
measured the mucosal thickness of
the retromolar region and found it to be approximately 3.02 mm (Range: 2.3-4.0 mm).
This approximates the available space at 4.25 mm, but normal variations must be
considered.
Growth must also be taken into account and adjustments of space assessments
must be made accordingly. Our measurements were done on patients who had no more
growth expected (according to age ≥18 and CVMI). If more growth is still expected such
as in adolescents, there should be less space available for arch retraction. Studies done by
Losken
[127]
(Table 2) and Broadbent may be used to evaluate normal and abnormal
amount of horizontal and vertical growth of the mandible and to approximate amount of
growth left.
Another clinical limitation was the posterior border of retraction. In our study, we
used the ascending ramus along the occlusal plane as the limit. It is apparent in Fig. 24
that the body of the ramus transitions from a kidney bean shape to a more teardrop
configuration more posteriorly along the mandible. The superior portion of this teardrop
represents the ascending portion of the ramus which is more cortical in nature. The
cortical bone would be much too difficult to move teeth through and the surface area of
this region makes it not suitable for teeth to be stable. This was the rationale for the use
of the ascending ramus as our hypothetical posterior limit of retraction.
Figure 18. Various slices through the mandible. Illustrates the change in shape of the
body as it transitions from a more kidney bean shape to a more teardrop configuration as
the mandible progresses posteriorly.
63
64
Orthodontics has come a long way since its inception, but there are still
indications where surgery is called for. Now it is possible to be at least semi-quantitative
about the limits of orthodontic treatment, in the context pf producing a normal occlusion.
As Fig. 19 indicates, the limits vary both by the tooth movement that would be needed
and by the patient’s age. Because growth modification in children enables greater
changes than are possible by tooth movement alone in adults, some conditions that could
have been treated by orthodontics alone in children become surgical problems in adults.
The envelope of discrepancy outlines the limits of hard tissue change toward ideal
occlusion, if other limits due to the major goals of treatment do not apply. In fact, soft
tissue limitations not reflected in the envelope of discrepancy often are a major factor in
the decision for orthodontic or surgical-orthodontic treatment
[43]
.
Figure 19. Envelope of Discrepancy
[43]
. The ideal position of the upper and lower
incisors are shown by the origin of the x and y axes. This envelope of discrepancy shows
65
66
the amount of change that could be produced by orthodontic tooth movement alone (inner
envelope of each diagram; orthodontic tooth movement combined with growth
modification (middle envelope); and orthognathic surgery (outer envelope). There is
more potential to retract than procline teeth and more potential for extrusion than
intrusion. With the use of TADs, the potential to retract with orthodontic movement alone
has increased over 1 mm (shown by red dot).
With this new 3-D technology, a superior method of assessing and measuring the
space distal to the most posterior tooth can be established. The measurements from this
method are as close to measuring on cadavers as possible due to the lack of distortion and
the 1:1 size ratio. The traditional panoramic radiograph is inherently inferior to the
panoramic reconstruction from the CBCT scans due to the arbitrary focal trough and
magnification problems. Overlapping of images are also eliminated with the CBCT
method.
Study Limitations
The optimum method for measuring the available space distal to the molars in the
mandible would be to use a cadaver sample. This would have eliminated the distortion
due to patient movement, however, the impracticality of such a study outweigh its
usefulness. It would also be difficult to procure enough samples to derive any conclusive
data.
It is reasonable to assume that the stature and size of the mandible may have an
effect on the available space distal to the most posterior tooth in the mandibular arch.
However, it was not possible to account for the size of the body in this study because the
database did not contain that information. At present, there have not been any studies that
show that stature does have an effect on the available space, but if it does, then a
67
difference between gender groups would also be expected. Further investigation needs to
be done on the effect of stature and jaw size.
Another limitation is the voxel size of the Newtom inage. CBCT data acquired
using NewTom QR-DVT 3G scans have an image resolution of 512 X 512 pixels and 12
bits per pixel. The reconstruction matrix voxel was 0.25 X 0.25 X 0.3 mm
[79]
. This
means that the measurements are accurate to within 0.3 mm.
Future Studies:
With the increasing popularity of TADs in orthodontics, there needs to be more
investigations into the limits. This study examined the available space for retraction in the
mandibular arch. This information will benefit the treatment of class III patients as well
as those with severe lower anterior crowding, but not those who require class II
correction. More studies need to be done on the maxilla and its limits to establish a
guideline for its retraction.
68
Chapter Seven: Conclusion
1.) There was enough space distal to the second molars in the mandible to correct
approximately 8.5 mm of anterior crowding barring curve of spee, correct half step class
III, or possibly a change from extraction to non-extraction treatment plan.
2.) There is significant difference in available space distal to the most posterior tooth in
the mandibular arch between males and females.
a. Mean space distal to lower second molars for males was 7.37 mm.
b. Mean space distal to lower second molars for females was 7.17 mm.
c. Mean space distal to lower third molars for males was 3.07 mm.
d. Mean space distal to lower third molars for females was 2.47 mm.
3.) There was not a significant difference between the right (7.29 mm) and left (7.25
mm) sides. There may be significant variations from person to person, but not from the
patient pool as a whole.
4.) The space distal to the third molars was 2.80 mm which leaves no space for retraction
on account of the soft tissue. However, third molars can easily be extracted to provide
sufficient space.
69
5.) The use of CBCT reconstructions for intraoral measurements is a viable option due to
its many advantages:
A traditional panoramic radiograph uses an arbitrary focal trough where the
distance from the object to the trough can vary unpredictably at any point in the
image.
There is an inherent error and distortion due to the magnification in a traditional
panoramic radiograph.
Traditional radiographs have a coefficient of magnification which makes
measurements from the images very unreliable.
Contrast is improved from the traditional radiography while blurring and
overlapping is eliminated
[65]
.
70
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Abstract (if available)
Abstract
The purpose of this study was to determine the amount of space available distal to the most posterior tooth in the mandibular arch for complete arch distalization. Methods: CBCT scans from 146 (110 after exclusion criteria) patients were rendered using InVivo DentalTM Software. A 1:1 ratio panoramic image was reconstructed from each scan. A horizontal measurement, parallel to the functional occlusal plane, was recorded from the distal height of contour of the most posterior tooth to the ascending ramus. The advantages of using a CBCT reconstruction are: (1) A traditional panoramic radiograph uses an arbitrary focal trough
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Creator
Han, Woo Daniel
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Core Title
A cone beam-CT evaluation of the availability of space for complete arch retraction in the mandible with TADs
School
School of Dentistry
Degree
Master of Science
Degree Program
Craniofacial Biology
Publication Date
04/27/2009
Defense Date
03/02/2009
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class III,CT,distalization,mandible,OAI-PMH Harvest,retraction,TAD
Language
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Sameshima, Glenn T. (
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), Moon, Holly (
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), Paine, Michael (
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)
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danielhandds@gmail.com,whan@usc.edu
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Han, Woo Daniel
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Tags
class III
CT
distalization
retraction
TAD