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AN EVALUATION OF THE MANDIBULAR SYMPHYSIS AS IT RELATES TO
LONG-TERM POST-ORTHODONTIC CROWDING
AND FACIAL DIVERGENCE
Joseph M. Mess, D.D.S.
An Abstract Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2012
Abstract
Introduction:
A long-term stable alignment of mandibular
anterior teeth remains a persistent problem in contemporary
orthodontics which has become commonly addressed by a
philosophy of permanent fixed or lifetime retention.
Although several studies have investigated what has been
believed to cause relapse, few individual factors have been
able to explain the wide variability of long-term
malalignment.
Adverse risks and consequences are reported
in the literature for patients with a thin mandibular
symphysis, but little is known about its influence on
crowding.
Purpose:
In an attempt to study a variable not
fully explored in the literature, this investigation
proposes to assess the correlation of mandibular crowding
with symphysis dimensions in treated individuals.
Materials and Methods:
A sample of 89 randomly selected
patients with pre-treatment, immediate post-treatment, and
long-term post-retention dental models and cephalograms
were utilized.
Scanned images of the mandibular occlusion
were used to assess incisor irregularity.
Mandibular
symphysis landmarks were plotted on each cephalogram so
that several dimensional measurements could be collected.
Symphysis dimensions and irregularity were compared.
Additionally, the sample was subdivided based upon
1
mandibular plane angle to assess the relationship between
vertical facial types and symphysis characteristics.
Results:
Significant but weak negative correlations
between long-term crowding and symphysis width were found.
Individuals with greater facial divergence exhibited a
significantly narrower and taller symphysis, as well as
more posteriorly positioned landmarks.
These subjects also
displayed more incisor irregularity at the start of
treatment and during the long-term interval.
Conclusions:
The morphology of the symphysis is significantly different
among individuals with different vertical growth patterns.
While a more narrow symphysis may demonstrate a weak yet
significant association to long-term incisor crowding, it
is more commonly observed in individuals with increased
facial divergence.
The narrowness of the symphysis may be
a contributing factor for the development of crowding over
time.
2
AN EVALUATION OF THE MANDIBULAR SYMPHYSIS AS IT RELATES TO
LONG-TERM POST-ORTHODONTIC CROWDING
AND FACIAL DIVERGENCE
Joseph M. Mess, D.D.S.
A Thesis Presented to the Graduate Faculty of Saint
Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2012
COMMITTEE IN CHARGE OF CANDIDACY:
Professor Eustaquio A. Araujo,
Chairperson and Advisor
Professor Rolf G. Behrents
Associate Clinical Professor Donald R. Oliver
i
DEDICATION
This work is dedicated to my wife, Nicole.
Thank you
for your constant love and support during my orthodontic
residency.
Without you, none of this was possible.
To my parents, Jane and Steve, whose unwavering
commitment offered me the pursuit of a higher education.
am exceedingly grateful for your love and assistance.
And lastly, to the faculty of Saint Louis University,
it is with great esteem to have been trained under your
exceptional guidance.
Collectively, your instruction will
forever serve as the foundation of my professional career.
ii
I
ACKNOWLEDGEMENTS
This project could not have been completed without the
help and support of the following individuals:
Dr. Eustaqio Araujo.
Thank you for your guidance
during my thesis preparation and for enriching my education
with your tremendous dedication and enthusiasm for
orthodontics.
Dr. Rolf Behrents.
Thank you for your contributions
to my thesis and allowing me to obtain an orthodontic
education at Saint Louis University.
Dr. Donald Oliver.
Thank you for your attention to
detail during my thesis preparation and revisions.
You
have provided immeasurable value to my clinical education.
Dr. Steven Harrison.
Thank you for this thesis topic
suggestion and meaningful contribution to my clinical
education.
Dr. R.G. “Wick” Alexander.
Thank you for the use of
your long-term records in this study.
It is an inspiration
to examine such remarkable treatment results.
Dr. Heidi Israel.
Thank you for your assistance with
the statistical analysis for this thesis.
iii
TABLE OF CONTENTS
List of tables............................................v
List of figures..........................................vi
CHAPTER 1: INTRODUCTION...................................1
CHAPTER 2: REVIEW OF THE LITERATURE
Growth at the mandibular symphysis...................3
Morphology of the symphysis..........................6
Limitations of tooth movement........................8
Irregularity index..................................12
Crowding in untreated and treated individuals.......13
Factors related to crowding.........................15
Statement of thesis.................................16
Literature cited....................................17
CHAPTER 3: JOURNAL ARTICLE
Abstract............................................21
Introduction........................................23
Materials and methods...............................25
Sample.........................................25
Cephalometric technique and analysis...........26
Model analysis.................................29
Error study....................................30
Statistical analysis...........................30
Results.............................................31
Discussion..........................................39
Conclusions.........................................43
Literature cited....................................45
Vita Auctoris............................................48
iv
LIST OF TABLES
Table 2.1
Width before and after retraction of
mandibular incisors (modified from
Lippincott)...............................11
Table 2.2
Summarized data for treated and untreated
samples based on treatment setting
(adapted from Goldberg)...................14
Table 3.1
Sample demographics.......................26
Table 3.2
Cephalometric variables
and abbreviations.........................31
Table 3.3
Descriptive statistics for incisor
irregularity..............................32
Table 3.4
Descriptive statistics for cephalometric
measures..................................32
Table 3.5
Pearson correlation coefficients between
symphysis dimensions and long-term
changes in irregularity...................33
Table 3.6
Dental comparisons between high angle
and low angle groups......................34
Table 3.7
Lingual, labial, and total widths
at the greatest points of curvature
of the symphysis..........................35
Table 3.8
Symphysis widths and heights..............36
Table 3.9
Horizontal symphysis landmarks............38
v
LIST OF FIGURES
Figure 2.1
Remodeling of the mandible.................3
Figure 2.2
Bone remodeling of the
mandibular symphysis.......................4
Figure 2.3
Symphysis width and height dimensions in
individuals with short, normal, and long
lower face heights (modified from
Beckman)...................................7
Figure 2.4
Mandible of a 19 year old female evaluated
during autopsy (modified from Wehrbein)....8
Figure 2.5
Width measurements at different root
levels(modified from Lippincott)..........11
Figure 2.6
Increases in annual irregularity decreases
over time (modified from Buschang and
Shulman)..................................13
Figure 3.1
Cephalometric tracing for each time point
in each series............................27
Figure 3.2
Close-up views of the various measurements
taken from the mandibular
symphysis tracings........................28
Figure 3.3
Scanned images of the occlusal surfaces
of a complete series T1-T3 ................29
Figure 3.4
A close-up scanned image showing incisor
irregularity..............................29
Figure 3.5
Comparison of symphysis thickness and
horizontal landmark position between
high and low angle groups.................37
Figure 3.6
Comparison of overall symphysis shape
between high and low angle groups.........39
vi
CHAPTER 1: INTRODUCTION
While the primary goal of orthodontics is to create an
esthetically pleasing and functional occlusion, patients’
motivation and expectations must be addressed to ensure
treatment success.
An improvement in dental appearance and
in particular the correction of crowded anterior teeth is
the most frequently mentioned reason orthodontic therapy is
desired.1,2
Along with obtaining the aforementioned objectives,
achieving patient satisfaction is vital to a successful
practice.
In this referral-based dental specialty, the
decision of a general dentist to refer a patient to a
particular orthodontist is based partly on patient and
parental satisfaction.3
This is important because over the
long term, patient satisfaction is associated with the
stability of the orthodontic treatment.4,5
Considering that most individuals seek treatment for
irregular anterior teeth, it should be the goal of
orthodontics to correct and maintain a properly aligned
anterior dentition over time.
Despite the technological
advancements in delivering treatment, the long-term stable
alignment of the anterior teeth remains a persistent
problem in contemporary orthodontics.
Researchers from the
University of Washington argue that the only way to ensure
1
long-term proper alignment of the incisors is to use fixed
or removable retainers for life.6–8
Considering their struggles with maintaining
mandibular anterior alignment, many of today’s
orthodontists are more resolved to use a form of fixed
retention.9
This exposes practitioners to potential issues
of increased calculus deposits, greater marginal recession,
and increased probing depths, especially among patients
with poor oral hygiene.10
While stability has been extensively studied, few
individual factors have been shown to display strong
significant correlations to incisor irregularity over time.
It is the purpose of this study to evaluate the potential
relationship between various dimensions of the mandibular
symphysis and long-term post-orthodontic crowding.
2
CHAPTER 2: REVIEW OF THE LITERATURE
Growth at the Mandibular Symphysis
The mandible undergoes a considerable amount of bone
remodeling to reach its adult shape and size.
Using vital
staining over two centuries ago, Hunter first showed that
the mandible grows in a posterior direction with bone
deposition at the posterior border of the ramus and
resorption at its anterior surface.11
Contrary to the
popular belief that the mandible grows down and forward
primarily by deposition at the chin, Hunter showed the
region of the symphysis displays only modest activity with
little contribution to the overall growth of the mandible.
Figure 2.1.
Harris.12
Remodeling of the mandible.
3
Modified from Enlow and
Studying archeological remains of human infants,
Becker showed that the mandibular symphysis fuses
approximately 6-9 months following birth.13
Although the
symphysis fuses at this early age, the anterior mandible
continues to develop through childhood and adolescence.
Implant14 and histological12 studies of the mandible
precisely located the active growth sites of the symphysis
as depicted in Figure 2.2.
A substantial amount of
periosteal deposition occurs on the lingual surface of the
symphysis with moderate deposition on the inferior surface
and in the area of the mental protuberance or chin, a
feature recognized as uniquely human among primates.
Figure 2.2. Bone remodeling of the mandibular symphysis. Deposition
of bone occurs on all surfaces except for a resorptive area superior to
the chin.
4
The area superior to the mental protuberance exhibits
bone resorption and enhances chin prominence, but as Enlow
and Harris note, this region shows considerable individual
variation.12
Some subjects exhibit negligible resorption
while others display considerable remodeling with minimal
bone remaining over the anterior teeth.
Enlow later
speculated that the resorptive nature of the mandibular
incisor region may represent an adjustment to the growth
process and function to stabilize the occlusion.15
While implant and histological studies provided a
qualitative description of growth at the mandibular
symphysis, Buschang et al. attempted to quantitatively
assess growth changes during childhood and puberty.16
Symphyseal landmarks showed significant vertical and
horizontal movement with the greatest growth changes
occurring in the upper half of the symphysis.
Horizontal
growth changes of most landmarks of the symphysis exhibit a
lingual drift with B-point moving the most.
Collectively, these studies describe the growth
changes of the symphysis in an upward and backward
direction with bone apposition on the lingual surface and
bone resorption above pogonion on the labial surface.
5
Morphology of the symphysis
Variability in the size and shape of the mandibular
symphysis is greatly influenced by the amount and direction
of growth.
As was previously mentioned, individual
differences in symphysis morphology exist, and several
studies have examined the relationship between growth and
symphysis shape.
Ricketts first stated that the morphology
of the symphysis can be used to predict the direction of
mandibular growth.17
Aki et al. tested this hypothesis and
found that individuals with an anterior growth direction
exhibited a symphysis of short height and large depth while
a large height and small depth are associated with a
posterior growth direction.18
Using lateral cephalometric
films, Handelman found that although a narrow symphysis
could be found among any of the facial types, it is more
commonly observed in individuals with steep mandibular
plane angles (SN-MP ≥ 39°).19
Furthermore, Beckmann et al.20
used lower face height to examine the symphysis, and they
concluded that as lower face height increases, the
symphysis becomes more elongated and narrow as seen in
Figure 2.3.
6
Figure 2.3. Symphysis width and height dimensions in individuals with
short (A), normal (B), and long (C) lower face heights. Modified from
Beckmann et al.20
More recently, cone-beam computed tomography studies
of untreated individuals have supported these claims that
the total thickness of the mandibular symphysis is greater
in short-face subjects as opposed to their long-face
counterparts.21,22
While there may be sufficient bony
support for the mandibular incisors in short-faced
individuals, there is sufficiently less in long-faced
individuals.
Thus, there is more freedom for tooth
movement in short-faced individuals with a wide symphysis.
The relationship between growth direction and symphysis
morphology can be useful to the practitioner when
diagnosing and planning treatment and can provide valuable
information on the potential treatment implications that
may be encountered.17
7
Limitations of tooth movement
Effective and safe tooth movement is limited to the
confines of the bone that makes up the alveolus.
this boundary, iatrogenic damage may occur.
Beyond
The lingual
and labial cortical bone of the symphysis places a physical
restriction on movement of the mandibular incisors.
Orthodontic patients who possess both a narrow symphysis
and require considerable sagittal movement or derotation of
mandibular incisors are at increased risk of bone loss and
root resorption.23
Figure 2.4 illustrates the small amount
of bone support for the mandibular incisors in a deceased
female with a narrow symphysis.24
Figure 2.4. Mandible of a 19 year old female evaluated during autopsy.
Note the narrow and high symphysis with a pronounced deficiency of
lingual and labial bone. Photo modified from Wehrbein et al.24
8
Similar findings were observed among active patients
with a narrow symphysis concluding that orthodontic tooth
movement may increase the risk of bony fenestrations and
dehiscences, root resorption, and gingival recession.25
These findings emphasize the importance of assessing the
bone quantity around the mandibular incisors prior to
planning orthodontic treatment.
Artun and Krogstad evaluated the mandibular symphysis
following mandibular incisor proclination and discovered a
significant correlation between the width of the symphysis
and increased crown height following proclination.26
Their
findings suggested that periodontal changes occur in these
patients resulting in an overall decrease in the crestal
bone height and consequently a loss of alveolar support.
Sarikaya et al. used computed tomography and
cephalograms to evaluate the symphysis following the
extraction of four bicuspids and retraction of the incisors
in patients with dentoalveolar bimaxillary protrusion.27
Measurements of the symphysis width in response to incisor
retraction were collected.
The amount of bone between the
labial cortical plate and the incisors decreased
significantly only at the most coronal measurement level.
The thickness of bone labial to the incisors remained
relatively unchanged at all other levels.
9
The overall
width, however, decreased significantly because as the
incisors were retracted, the lingual cortical plate did not
move accordingly in a lingual direction.
The amount of
bone lingual to the incisors decreased significantly at all
levels, resulting in a decreased overall bone width and
dehiscences in some patients where the roots contacted the
cortical plate.
Sarikaya et al. concluded that
compensatory remodeling of the bone does not always match
tooth movement, and the long-term consequences of the
narrowed bone support for the incisors is unknown.27
Lippincott performed a similar study using miniscrews
to assess if the addition of absolute anchorage would
increase the amount of cortical plate remodeling as the
mandibular incisors were retracted.28
The labial cortical
plate was found to remodel at a 1:1 ratio to the amount of
incisor retraction.
However, only the most coronal 25% of
the lingual cortical plate exhibited significant remodeling
at a 1:1.4 ratio (Figure 2.5).
Thus, the lingual cortical plate serves as an anatomic
barrier to tooth movement because it exhibits minimal
remodeling ability.
The incisors moved mostly by
controlled tipping with minimal retraction of the lingual
cortical plate, thus decreasing the overall width of the
alveolus (Table 2.1).
10
Figure 2.5. Width measurements at different root levels.
from Lippincott.28
Modified
Table 2.1. Width before and after retraction of mandibular incisors.
Modified from Lippincott.28
Root
Level
Pre-tx width (mm)
25%
50%
75%
100%
6.80
7.30
7.38
8.02
±
±
±
±
0.82
1.33
1.38
1.56
Post-tx width (mm)
5.71
5.83
6.19
7.54
±
±
±
±
0.96
1.39
1.69
2.03
Δ in
width (mm)
-1.09
-1.48
-1.18
-0.49
±
±
±
±
0.60
0.50
0.58
0.88
As previously discussed, the symphysis tends to
naturally drift in a superior and posterior direction with
most growth changes occurring in the superior third of the
symphysis.16
However, the study performed by Lippincott
evaluated changes in non-growing individuals.
Practitioners may be able to take advantage of these growth
changes in adolescent patients if mandibular incisors need
retracted, although to what extent this is possible is not
11
clearly reported in the literature.
There appears to be an
increased risk for adverse consequences when patients
present with a narrow symphysis, and the mandibular
incisors need to be retracted to correct a malocclusion.
Irregularity Index
As mentioned previously, boundaries in the mandible
are much more restrictive, and the literature suggests that
incisor crowding is significantly larger in the mandibular
arch.29
Because of these challenges to correct mandibular
crowding without infringing on cortical bone, traditional
orthodontics places considerable importance on assessing
the initial and final position of the mandibular incisors.30
Measuring the amount of irregularity or “crookedness” of
the incisors is important to determine the severity of each
malocclusion.
The Irregularity Index (II), devised by
Little,31 measures the linear displacement of the adjacent
anatomic contact points of the mandibular incisors in
millimeters.
The sum of the five measurements represents
the total irregularity score.
According to Little, a score
less than 3.5 mm is clinically acceptable, and a score
greater than 7 mm is considered clinically severe
malalignment.
12
Crowding in Untreated and Treated Individuals
From 1988 to 1994 the United States Public Health
Service conducted the third National Health and Nutrition
Examination Survey (NHANES III), collecting a sample of
30,000 individuals.32
Using this data, Proffit et al.
showed a general trend among untreated individuals where
incisor irregularity worsens from childhood to adolescence
to adulthood.33
The tendency for increases in irregularity
with age was further described by Buschang and Shulman.34
They showed that crowding increases most during early
adulthood, but that the rate of annual irregularity
decreases over time as shown in Figure 2.6.
Figure 2.6. Increases in annual irregularity decreases over time.
Modified from Buschang and Shulman.34
13
Studies of treated individuals display similar longterm changes in incisor irregularity.
Data collected at
the University of Washington have evaluated incisor
irregularity at post-treatment intervals of 10 and 20 years
post-retention.35,36
Incisor irregularity increased an
average of 3.59 mm and 0.77 mm during these two intervals.
Satisfactory alignment (II < 3.5 mm) was found in less than
30% at 10 years post-retention and 10% at 20 years postretention.
The worsening of irregularity over time appears to
occur at similar rates for both treated and untreated
individuals.
Goldberg discussed these findings.37
He
amalgamated irregularity data from several longitudinal
studies and noted considerable similarities in treated and
untreated individuals (Table 2.2).
Table 2.2. Summarized data for treated and untreated samples based on
treatment setting. Modified from Goldberg.37
Δ II/yr
N
Δ Age (yrs)
Δ II (mm)
(mm/yr)
Total (Untreated)
690
12.5
+1.13
+0.09
Total (Treated)
1252
12.5
+1.55
+0.12
The data show similar rates of worsening of
irregularity in untreated (+0.09 mm/yr) and treated (+0.12
mm/yr) individuals, suggesting that additional factors
besides orthodontic treatment are involved in the
development of crowding over time.
14
Factors Related to Crowding
Several factors have been studied to determine what
causes crowding and irregularity of teeth following
orthodontic treatment.
Few factors, however, have been
able to display a strong correlation.
Mellion evaluated
several commonly believed causes of relapse (change in
lower incisor to mandibular plane angle (ΔIMPA), axial
inclination of buccal teeth, irregularity, intercanine
width, maxillary growth, differential mandibular growth).38
Five significant multiple regression equations were found,
but each showed a weak correlation.
This indicates that
treatment factors have a relatively small impact on longterm post-orthodontic crowding.
In a review of the long-term retention literature,
Blake and Bibby39 concluded that well-documented orthodontic
principles must be followed to prevent relapse although no
definite conclusion can be generated concerning the
relative contribution of each treatment factor to posttreatment irregularity.
Those principles include
maintaining the initial lower arch form and intercanine
width, minimizing advancement of the lower incisors, and
performing fibrotomies to prevent rotations.
Despite
following these basic principles, crowding still occurs at
15
fairly similar rates in treated and untreated individuals.
Thus, other factors must be considered.
Statement of Thesis
Crowding has been shown to develop at similar rates in
both treated and untreated individuals.
This implies that
a factor external to treatment may be contributing to
irregularity.
Because few factors have been strongly
correlated to crowding, future studies are needed to
understand why irregularity worsens following orthodontic
treatment.
The resolution for lifetime retention without
striving for long-term stability is unacceptable.
While adverse risks and consequences are reported in
the literature for patients with a thin mandibular
symphysis, little is known about long-term crowding in
these patients.
In an attempt to study a variable not
fully explored in the literature, this investigation
proposes to assess the correlation of mandibular crowding
with symphysis morphology in treated individuals.
16
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malocclusion and orthodontic treatment need in the
United States: estimates from the NHANES III survey.
Int J Adult Orthodon Orthognath Surg. 1998;13:97–106.
34.
Buschang PH, Shulman JD. Incisor crowding in untreated
persons 15-50 years of age: United States, 1988-1994.
Angle Orthod. 2003;73:502–508.
35.
Little RM, Wallen TR, Riedel RA. Stability and relapse
of mandibular anterior alignment-first premolar
extraction cases treated by traditional edgewise
orthodontics. Am J Orthod. 1981;80:349–365.
36.
Little RM, Riedel RA, Artun J. An evaluation of
changes in mandibular anterior alignment from 10 to 20
years postretention. Am J Orthod Dentofacial Orthop.
1988;93:423–428.
37.
Goldberg A. An evaluation of mandibular anterior
crowding as it relates to facial divergence in treated
and untreated subjects. [Master’s Thesis] St. Louis:
Saint Louis University; 2012.
38.
Mellion, N. A longitudinal, multivariate analysis of
orthodontic relapse. [Master’s Thesis] St. Louis:
Saint Louis University; 2011.
39.
Blake M, Bibby K. Retention and stability: a review of
the literature. Am J Orthod Dentofacial Orthop.
1998;114:299–306.
20
CHAPTER 3: JOURNAL ARTICLE
Abstract
Introduction:
A long-term stable alignment of mandibular
anterior teeth remains a persistent problem in contemporary
orthodontics which has become commonly addressed by a
philosophy of permanent fixed or lifetime retention.
Although several studies have investigated what has been
believed to cause relapse, few individual factors have been
able to explain the wide variability of long-term
malalignment.
Adverse risks and consequences are reported
in the literature for patients with a thin mandibular
symphysis, but little is known about its influence on
crowding.
Purpose:
In an attempt to study a variable not
fully explored in the literature, this investigation
proposes to assess the correlation of mandibular crowding
with symphysis dimensions in treated individuals.
Materials and Methods:
A sample of 89 randomly selected
patients with pre-treatment, immediate post-treatment, and
long-term post-retention dental models and cephalograms
were utilized.
Scanned images of the mandibular occlusion
were used to assess incisor irregularity.
Mandibular
symphysis landmarks were plotted on each cephalogram so
that several dimensional measurements could be collected.
Symphysis dimensions and irregularity were compared.
21
Additionally, the sample was subdivided based upon
mandibular plane angle to assess the relationship between
vertical facial types and symphysis characteristics.
Results:
Significant but weak negative correlations
between long-term crowding and symphysis width were found.
Individuals with greater facial divergence exhibited a
significantly narrower and taller symphysis, as well as
more posteriorly positioned landmarks.
These subjects also
displayed more incisor irregularity at the start of
treatment and during the long-term interval.
Conclusions:
The morphology of the symphysis is significantly different
among individuals with different vertical growth patterns.
While a more narrow symphysis may demonstrate a weak yet
significant association to long-term incisor crowding, it
is more commonly observed in individuals with increased
facial divergence.
The narrowness of the symphysis may be
a contributing factor for the development of crowding over
time.
22
Introduction
While the primary goal of orthodontics is to create an
esthetically pleasing and functional occlusion, patients’
motivation and expectations must be addressed to ensure
treatment success.
An improvement in dental appearance and
in particular the correction of crowded anterior teeth is
the most frequently mentioned reason orthodontic therapy is
desired.1,2
Along with obtaining the aforementioned objectives,
achieving patient satisfaction is vital to a successful
practice.
In this referral-based dental specialty, the
decision of a general dentist to refer a patient to a
particular orthodontist is based partly on patient and
parental satisfaction.3
This is important because over the
long term, patient satisfaction is associated with the
stability of the orthodontic treatment.4,5
Despite the technological advancements in delivering
treatment, the long-term stable alignment of the anterior
teeth remains a persistent problem in contemporary
orthodontics.
Researchers from the University of
Washington argue that the only way to ensure long-term
proper alignment of the incisors is to use fixed or
removable retainers for life.6–8
23
Considering their struggles with maintaining
mandibular anterior alignment, today’s orthodontists are
more resolved to use a form of fixed retention.9
This
exposes practitioners to potential issues of increased
calculus deposits, greater marginal recession, and
increased probing depths, especially among patients with
poor oral hygiene.10
Several studies have demonstrated in patients with a
thin symphysis that risks and consequences to orthodontic
treatment include bony fenestrations, dehiscences, root
resorption, and gingival recession.11–14
Although remodeling
of the symphysis occurs primarily during childhood and
adolescence,15,16 retraction and proclination of incisors in
adults has been shown to modify symphysis dimensions.17,18
While stability has been extensively studied, few
individual factors have been shown to display strong
significant correlations to incisor irregularity over time.
It is the purpose of this study to evaluate the potential
relationship between the dimensions of the mandibular
symphysis and long-term post-orthodontic crowding.
24
Materials and Methods
Sample
The sample consisted of 89 randomly selected former
patients of a single private-practice orthodontist, Dr.
R.G. “Wick” Alexander (Arlington, Texas).
All Angle
classifications were represented: 45 Class I, 40 Class II
(division 1), 2 Class II (division 2), and 2 Class III
malocclusions.
The sample was comprised of 34 extraction
and 55 nonextraction cases.
The retention protocol was a
fixed lingual canine-to-canine retainer.
All subjects were
out of retention at the long-term follow-up.
Subjects presented at their own discretion and
consented to having follow-up orthodontic records taken.
Because they returned at their own preference, the sample
was possibly biased toward successful orthodontic
treatment, but this bias does not imply that irregularity
or causative factors were not present.
All patients were
treated according to the Alexander Discipline.
Dental models and cephalograms were collected at three
time points: T1 (pre-treatment), T2 (immediate posttreatment/debond), and T3 (long-term post-retention).
subjects with complete records were included.
summarizes the sample’s demographics.
25
Only
Table 3.1
Table 3.1.
Sample demographics (yrs)
Time/Interval
Mean
S.D.
T1 (pre-tx)
12.97
3.49
T2 (immediate post-tx)
15.72
3.40
T1-T2 (total tx time)
2.75
0.90
T3 (post-retention)
32.08
8.34
T2-T3 (retention period)
16.35
7.43
Treatment
Posttreatment
Cephalometric Technique and Analysis
Using a tracing pencil and 0.003 matte acetate tracing
paper, the outlines of the inferior border of the mandible,
ascending ramus, and mandibular symphysis were traced
(Figure 3.1).
Additionally, the following cephalometric
landmarks were identified: sella, nasion, mandibular
central incisor tip and apex, infradentale, pogonion (Pg),
menton (Me), point B, and SL (the most posterior point on
the curvature of the outline of the lingual of the
symphysis).
All tracings were imported into Dolphin
Imaging software and calibrated with a 50 millimeter ruler.
The mandibular plane was constructed, according to
Downs, tangent through the gonial angle and through the
lowest point of the symphysis (menton).
were measured at all time points.
SN-MP and IMPA
Lines perpendicular to
the mandibular plane were drawn through pogonion, point B,
and SL.
The intersection of these lines with the
26
mandibular plane were marked as Pg’, B’, and SL’.
The
distances between these points were measured.
Figure 3.1. Cephalometric tracing for each time point in each series.
**Se(sella), N(nasion), SN(sella-nasion plane), SL(most posterior point
on the lingual curvature of the symphysis), Id(infradentale), B(point
B), Pg(pogonion), Me(menton), MP (mandibular plane)
Height was measured on each tracing as the
perpendicular distance between infradentale and the
mandibular plane.
This distance was divided into five
equal segments to allow for the construction of five lines
parallel to the mandibular plane.
The intersections of
these five lines was marked where each crossed the labial
and lingual contour of the symphysis and SL perpendicular.
The width of the symphysis (W1, W2, W3, etc.) and the
27
distance between SL perpendicular and the lingual contour
(P1, P2, P3, etc.) was measured (Figure 3.2.)
A
B
C
D
Figure 3.2. Close-up views of the various measurements taken from the
mandibular symphysis tracings.
(A) Lingual, labial, and total symphysis widths at the broadest
points of curvature: SL’-B’, B’-Pg’, SL’-Pg’
(B) Width of symphysis: W1, W2, W3, W4, W5
(C) Posterior position of symphysis: P1, P2, P3, P4, P5, PMe
(D) Anterior position of symphysis: A1, A2, A3, A4, A5
**SL(most posterior point on the lingual curvature of the symphysis),
B(point B), Pg(pogonion), MP(mandibular plane), Id(infradentale),
Me(menton), h(height)
28
Model Analysis
Mandibular occlusal scans of the dental models were
made using a flatbed scanner (EPSON Perfection 4990 Photo).
Images were imported into Dolphin Imaging and calibrated
with a 50 millimeter ruler (Figure 3.3)
Incisor
irregularity was calculated according to Little (Figure
3.4).
Figure 3.3. Scanned images of the occlusal surfaces of a complete
series T1-T3 (pre-treatment, post-treatment, post-retention).
Figure 3.4. A close-up scanned image showing mandibular incisor
irregularity (I.I.). Little’s irregularity index: the sum of the
displacements of the five lower anterior contacts A+B+C+D+E.
29
Error Study
In order to assess measurement error, approximately
10% of the total sample was chosen to be re-analyzed.
A
random number generator (from www.random.org) selected 9
series (27 radiographs and mandibular dental models) to be
measured for a second time.
Measurement consistency was
determined using Cronbach’s alpha.
As a general rule,
reliability is considered to be “adequate” when intra-class
correlations are greater than or equal to 0.80.
Statistical Analysis
Data was collected and organized into a spreadsheet
format using Microsoft Office Excel 2007.
Statistical
analysis was performed using SPSS statistical analysis
software (PASW Statistics Version 18.0, SPSS, Inc.).
Cephalometric variables and their abbreviations are
indicated in Table 3.2.
Means and standard deviations were calculated for all
cephalometric and model measurements.
Pearson correlation
coefficients (r) were calculated to evaluate the effect of
symphysis dimensions on long-term irregularity.
error was set at α=0.05 (2-tailed).
30
The type-I
The sample was further divided into two groups based
on pre-treatment (T1) facial divergence: SN-MP≥35° (n=26)
and SN-MP≤30° (n=27).
Independent t-tests were used to
test the null hypothesis that there is no difference in
irregularity and symphysis dimensions between the two
groups.
The type-I error was set at α=0.05 (2-tailed).
Table 3.2.
Cephalometric variables and abbreviations
Measure
Abbreviation
Sella-nasion to mandibular plane
SN-MP
Incisor to mandibular plane angle
IMPA
Symphysis width
W1, W2, W3, W4, W5
Posterior position of symphysis
P1, P2, P3, P4, P5, PMe
Anterior position of symphysis
A1, A2, A3, A4, A5
Lingual width
SL’-B’
Labial width
B’-Pg’
Total width
SL’-Pg’
Symphysis height
h
Results
All measurements were reported at three time points:
pre-treatment (T1), immediate post-treatment/debond (T2),
and long-term post-retention (T3).
Cronbach’s alpha intra-
class correlations were greater than 0.80 indicating that
reliability of measurements was “adequate.”
Descriptive
statistics for incisor irregularity and cephalometric
measures are described in Tables 3.3 and 3.4.
At the post-
retention follow up, subjects displayed an average of
1.85mm increase in incisor irregularity.
31
Table 3.3.
Descriptive statistics for incisor irregularity
Range
Std.
Mean
Deviation
Minimum
Maximum
T1
5.51
0.40
15.00
3.51
T2
0.32
0.00
2.50
0.45
T3
2.17
0.00
7.10
1.63
T3-T2
1.85
-1.10
7.10
1.64
n=89, T1(pre-treatment), T2(immediate post-treatment/debond), T3(postretention), T3-T2(long-term interval)
Table 3.4.
Descriptive statistics for cephalometric measures
Pre-Treatment
Post-treatment
Post-retention
(T1)
(T2)
(T3)
Angular measures
SN-MP
IMPA
Mean
32.33
94.41
S.D.
5.31
7.05
Mean
32.31
94.64
S.D.
5.75
7.07
Mean
31.04
94.02
S.D.
6.48
7.42
Symphysis width and
height
W1
W2
W3
W4
W5
h
Mean
6.48
7.46
9.85
13.40
14.73
28.44
S.D.
0.81
1.19
1.74
2.00
1.73
2.71
Mean
5.94
6.69
9.33
13.53
15.01
29.61
S.D.
0.95
1.41
2.10
2.27
1.89
2.84
Mean
6.10
6.54
9.66
14.19
15.38
30.75
S.D.
1.00
1.32
2.14
2.28
1.75
3.20
Width at the
extremes
SL' - Pg' (overall)
SL' - B' (lingual)
B' - Pg' (labial)
Mean
16.14
8.89
7.26
S.D.
1.76
1.93
1.81
Mean
16.67
8.06
8.61
S.D.
1.94
1.95
2.04
Mean
17.41
7.81
9.60
S.D.
2.01
1.97
2.27
Posterior landmarks
P1
P2
P3
P4
P5
P Me
Mean
2.92
1.42
0.69
0.51
1.27
12.57
S.D.
1.70
1.16
0.66
0.58
0.95
1.74
Mean
2.54
1.37
0.74
0.61
1.47
13.16
S.D.
1.70
1.06
0.76
0.85
1.13
1.86
Mean
2.52
1.42
0.74
0.62
1.80
13.76
S.D.
1.94
1.19
0.75
0.97
1.32
2.03
Anterior landmarks
A1
A2
A3
A4
A5
Mean
9.41
8.88
10.54
13.91
15.99
S.D.
2.13
1.82
1.83
1.86
1.79
Mean
8.49
8.06
10.07
14.15
16.48
S.D.
2.29
2.00
2.08
2.01
1.93
Mean
8.63
7.96
10.39
14.81
17.18
S.D.
2.31
2.03
2.14
2.02
1.97
32
Only those measurements showing a significant Pearson
correlation coefficient (p<0.05) are reported in Table 3.4.
These coefficients (r values) are based on a linear
relationship between symphyseal measurements (independent
variables) and the change in incisor irregularity from the
end of orthodontic treatment to the post-retention followup (dependent variable).
to -0.382.
Coefficients ranged from -0.210
This data showed that long-term irregularity
was more commonly found when the symphysis was narrow or
positioned more posteriorly.
Table 3.5. Significant Pearson correlation coefficients between
symphysis dimensions and long-term changes in mandibular incisor
irregularity (I.I. T3-T2)
Pre-treatment (T1)
Post-treatment (T2)
Post-retention (T3)
Symphysis width
Measure
R
W2
W3
-0.231*
-0.236*
Measure
W1
W2
W3
r
-0.310**
-0.353**
-0.311**
Measure
W1
W2
W3
r
-0.227*
-0.322**
-0.313**
Measure
P1
P2
r
-0.274**
-0.293**
Measure
A1
A2
A3
r
-0.329**
-0.382**
-0.369**
Posterior landmark position
Measure
P1
P2
r
-0.261*
-0.246*
Anterior landmark position
Measure
A1
A2
A3
R
-0.213*
-0.276**
-0.272**
Measure
A1
A2
A3
A4
r
-0.322**
-0.379**
-0.350**
-0.210*
Lingual width
Measure
R
Measure
r
Measure
SL'-B'
-0.255*
SL'-B'
-0.347**
SL'-B'
Note: See Figure 3.2 for illustrations of various measurements
*p<0.05
**p<0.001
33
r
-0.351**
The subjects were subdivided into two groups based
upon facial divergence: SN-MP≥35° (n=26) and SN-MP≤30°
(n=27).
Table 3.5 describes the differences between the
two groups in terms of incisor irregularity and incisor to
mandibular plane angle.
The group with SN-MP≥35° exhibited
significantly more upright mandibular incisors.
Incisor
irregularity was greater at pre-treatment (T1) and during
the long-term interval (T3-T2) in the higher divergent
group.
Thus, the group with greater facial divergence
displayed more long-term orthodontic relapse.
Table 3.6. Dental comparisons between groups based on facial divergence
SN-MP≥35°
SN-MP≤30°
Group Differences
T1
Mean
±SD
Mean
±SD
I.I.
6.82
4.19
4.69
2.33
0.029
IMPA
89.85
5.49
95.73
7.10
<0.001
T2
Mean
±SD
Mean
±SD
I.I.
0.25
0.35
0.50
0.45
NS
IMPA
90.31
6.00
97.75
6.83
<0.001
T3
Mean
±SD
Mean
±SD
I.I.
2.49
1.68
1.84
1.21
NS
IMPA
89.96
6.55
96.76
7.54
<0.001
T3-T2
Mean
±SD
Mean
±SD
I.I.
2.25
1.57
1.34
1.30
0.026
IMPA
-0.35
3.44
-0.99
5.68
NS
p<0.05, I.I. = mandibular incisor irregularity, NS = nonsignificant
As shown previously in Figure 3.2A, the overall width
of the symphysis was measured at its most labial (Pg) and
most lingual (SL) convexity.
The posterior (SL’-B’),
anterior (B’-Pg’), and total (SL’-Pg’) widths of the
34
symphysis are reported in Table 3.6.
The total overall
width is similar between the groups, but the group with SNMP≥35° has significantly less bone on the lingual aspect of
the symphysis (SL’-B’).
This is the result of B point
being situated more posterior in this group as illustrated
by Figures 3.5 and 3.6.
Table 3.7. Lingual, labial, and total widths at the greatest points of
curvature of the symphysis
SN-MP≥35°
SN-MP≤30°
Group Differences
T1
Mean
±SD
Mean
±SD
SL’-B’
7.72
1.75
9.36
1.48
<0.001
B’-Pg’
8.07
1.95
7.02
1.90
NS
SL'-Pg'
15.80
1.77
16.39
1.79
NS
T2
Mean
±SD
Mean
±SD
SL’-B’
6.86
1.97
8.79
1.66
<0.001
B’-Pg’
9.62
1.95
7.72
1.84
<0.001
SL'-Pg'
16.48
2.24
16.51
1.85
NS
T3
Mean
±SD
Mean
±SD
SL’-B’
6.66
1.71
8.54
1.73
<0.001
B’-Pg’
10.77
2.39
8.70
2.00
<0.001
SL'-Pg'
17.42
2.26
17.24
1.83
NS
p<0.05
A comparison of the widths at each level of the
symphysis is shown in Table 3.7.
The symphysis at levels
W2, W3, and W4 is significantly more narrow in the group
with greater facial divergence.
The height is also
significantly greater in this group.
Conversely, the group
with SN-MP≤30° is significantly shorter and wider (W2, W3,
W4).
Width and height differences are visualized in Figure
3.5 and Figure 3.6, respectively.
35
Table 3.8.
Symphysis widths and heights
SN-MP≥35°
SN-MP≤30°
Group Differences
T1
W1
W2
W3
W4
W5
Height
Mean
6.20
6.69
8.67
12.26
14.19
29.64
±SD
0.77
1.23
1.51
1.78
1.89
2.66
W1
W2
W3
W4
W5
Height
Mean
5.60
5.82
7.72
11.91
14.47
31.02
±SD
0.98
1.42
1.87
2.19
2.28
2.27
Mean
5.96
5.83
8.13
12.60
14.77
31.89
±SD
1.10
1.24
1.78
2.12
2.05
2.32
Mean
6.62
7.86
10.65
14.17
14.87
26.86
±SD
0.87
1.11
1.59
1.73
1.39
2.26
NS
<0.001
<0.001
<0.001
NS
<0.001
Mean
6.29
7.42
10.46
14.46
14.97
27.46
±SD
0.85
1.14
1.80
1.85
1.41
2.32
0.009
<0.001
<0.001
<0.001
NS
<0.001
Mean
6.27
7.09
10.68
15.07
15.46
28.46
±SD
0.71
1.27
1.95
1.88
1.33
2.72
NS
<0.001
<0.001
<0.001
NS
<0.001
T2
T3
W1
W2
W3
W4
W5
Height
p<0.05
The “curvature” of the symphysis was assessed by
measuring the coordinates of the anterior and posterior
position at each level and comparing their horizontal
differences.
Table 3.8 describes the horizontal
differences between the two groups in terms of the labial
and lingual position of the symphysis.
With regard to the
lingual contour, the less divergent group (SN-MP≤30°)
exhibits a symphysis that is positioned significantly more
36
anterior at its superior region (P1 and P2).
With regard
to the labial contour, the symphysis is positioned
significantly more posterior in the more divergent group
(SN-MP≥35°) at levels A1, A2, A3, and A4.
This can be
appreciated in Figure 3.5.
Figure 3.5. Comparison of symphysis thickness and horizontal landmark
position between groups based on pre-treatment SN-MP. Height has been
standardized for better visualization.
37
Table 3.9.
Horizontal symphysis landmarks
SN-MP≥35°
SN-MP≤30°
Group Differences
T1
P1
P2
P3
P4
P5
A1
A2
A3
A4
A5
Menton
Mean
1.79
0.93
0.59
0.75
1.51
7.99
7.62
9.26
13.01
15.70
12.55
±SD
1.70
1.00
0.55
0.70
0.96
2.13
1.66
1.44
1.56
1.76
1.75
P1
P2
P3
P4
P5
A1
A2
A3
A4
A5
Menton
Mean
1.24
0.87
0.87
1.16
1.86
6.85
6.69
8.59
13.07
16.33
13.57
±SD
1.47
0.98
0.95
1.29
1.46
2.15
1.98
1.89
1.90
2.24
2.05
Mean
1.20
0.86
0.93
1.39
2.43
7.16
6.69
9.06
13.99
17.20
14.17
±SD
1.64
1.01
1.00
1.44
1.68
2.04
1.71
1.82
1.95
2.17
2.41
Mean
3.22
1.54
0.72
0.43
1.30
9.83
9.40
11.37
14.60
16.17
12.63
±SD
1.51
1.19
0.65
0.58
1.16
1.85
1.59
1.67
1.81
1.75
1.71
0.002
0.048
NS
NS
NS
<0.001
<0.001
<0.001
<0.001
NS
NS
Mean
3.17
1.47
0.63
0.35
1.36
9.46
8.89
11.09
14.80
16.33
12.69
±SD
1.45
1.01
0.61
0.35
1.01
1.87
1.67
1.80
1.81
1.75
1.91
<0.001
0.033
NS
0.004
NS
<0.001
<0.001
<0.001
<0.001
NS
NS
Mean
3.18
1.67
0.76
0.30
1.59
9.46
8.76
11.44
15.37
17.05
13.43
±SD
1.48
0.98
0.62
0.41
1.10
1.82
1.74
1.87
1.83
1.82
1.93
<0.001
0.005
NS
<0.001
0.038
<0.001
<0.001
<0.001
<0.001
NS
NS
T2
T3
P1
P2
P3
P4
P5
A1
A2
A3
A4
A5
Menton
p<0.05
38
Figure 3.6. Comparison of overall symphysis shape between groups based
on SN-MP. True height and width comparisons are shown.
Discussion
In this retrospective study, longitudinal
cephalometric data was collected to assess a potential
relationship between the mandibular symphysis and long-term
incisor irregularity.
Several significant linear
correlations between symphysis dimensions and irregularity
were found ranging from -0.210 to -0.382.
The negative
relationship between symphysis width and irregularity
suggests that a narrower symphysis may predispose patients
39
to more incisor irregularity over time.
However, these
correlation coefficients are small suggesting a weak
relationship exists between symphysis width and long-term
crowding.
It appears just as was found in other recent
studies,19,20 the causes of incisor irregularity are complex
with few individual factors accounting for the wide
variability of orthodontic relapse.
Although weak relationships were found, the symphysis
characteristics that are significantly correlated with
long-term crowding are the same defining features that are
exemplified in a symphysis of an individual with an above
average facial divergence.
The average facial divergence
(SN-MP) in this study was 32.33°.
When the sample was subdivided based upon pretreatment facial divergence, symphysis measurements were
compared, and several significant differences were found.
The group with greater divergence (SN-MP≥35°) was found to
have significantly more crowding at the start of
orthodontic treatment and more incisor irregularity change
over the long-term post-retention interval.
This suggests
that individuals with greater facial divergence may be more
susceptible to the development of crowding over time.
The greater prevalence of irregularity among the
individuals with greater facial divergence is noteworthy
40
because this group also exhibited a much more narrow and
taller symphysis.
The characteristic tall and thin
symphysis of a high-angle or hyperdivergent individual is
well-reported in current literature.11,21–23 Although a
narrow symphysis cannot be considered the direct cause of
incisor irregularity increases over time, crowding in this
study is shown to be significantly greater among
individuals who possess a more narrow and tall symphysis.
Goldberg amalgamated data from several studies that
investigated long-term rates for incisor irregularity.24
He
found that irregularity increased at a rate of +0.12mm/yr
in treated individuals.
Rates observed in the present
study were measured similarly using mean increases in
irregularity and the mean time interval between the end of
orthodontic treatment and the long-term follow up.
This
overall sample exhibited an average rate of +0.11mm/year.
Separating the sample based on facial divergence revealed
an increase in irregularity at a rate of +0.13mm/yr (SNMP≥35°) and +0.09mm/yr (SN-MP≤30°).
Although similar,
there is an apparently larger yearly increase in
irregularity among subjects with greater facial divergence.
Long-term changes in IMPA were not significantly
different between the two groups, but the group with
greater facial divergence exhibited significantly more
41
upright mandibular incisors at all time points.
In
accordance with upright incisors, the symphysis of these
subjects was considerably much more upright in its most
superior region.
This suggests that some alveolar
remodeling occurs allowing the symphysis to “bend” in the
direction of the mandibular incisor inclination.
The
entire region of the symphysis remodels during the
adolescent growth spurt,16 and the most coronal portion of
the symphysis can be affected to some extent by incisor
tooth movement.17,18
However, reports of fenestrations,
dehiscences, and bone loss are well reported in the
literature when incisors are excessively retracted or
proclined.11,12,14
Furthermore, the labial contour of the symphysis was
positioned significantly more posterior in the group with
the greater facial divergence.
In these individuals, the
average symphysis appears to demonstrate a much more
accentuated labial curvature.
A narrower and more
lingually positioned symphysis in combination with the
natural lingual drift16 that occurs during the adolescent
growth spurt suggests that mandibular incisor proclination
be performed with caution in patients with greater than
average facial divergence.
42
The bony resorption that occurs with growth in the
tooth-bearing region of the symphysis implies that
practitioners should avoid excessive proclination of the
mandibular incisors into naturally resorptive labial bone.
Long-term consequences of mandibular incisor tooth movement
in the presence of a narrow symphysis may include bone
loss, root resorption, or perhaps, as suggested by this
study, crowding.
Conclusions
1. A significant yet weak relationship exists between the
width of the mandibular symphysis and long-term incisor
irregularity.
2. Individuals with greater facial divergence exhibited a
significantly more narrow and taller symphysis.
3. Individuals with a more narrow and taller symphysis
exhibited significantly more incisor irregularity prior
to orthodontic treatment and during the post-retention
interval.
4. Subjects in this study with SN-MP≥35° and SN-MP≤30°
exhibited annual increases of incisor irregularity of
+0.13mm/yr and +0.09mm/yr, respectively.
5. The labial cortical plate of the symphysis is positioned
significantly more posterior in individuals with greater
43
facial divergence.
Excessive incisor proclination among
these individuals may increase the potential for crowding
over time.
6. The narrowness of the symphysis provides reduced bony
support for the mandibular incisors, and this may be a
contributing factor for the development of crowding over
time.
44
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47
VITA AUCTORIS
Joseph Mess was born in Columbus, Ohio on May 16,
1984.
He is the youngest of three children and has lived
the majority of his life in Ohio.
He graduated from St.
Charles Preparatory High School in 2002.
After high
school, he attended The Ohio State University in Columbus,
Ohio where he earned his Bachelor of Science degree in
Microbiology in 2006.
Joseph continued his education at The Ohio State
University College of Dentistry in Columbus, Ohio and
received his Doctor of Dental Surgery degree in June of
2010.
He began his orthodontic training at Saint Louis
University later that month in 2010.
Joseph married his wife, Nicole, during his final year
at Saint Louis University.
He plans to complete his
Masters of Science in Dentistry degree in December 2012.
Upon graduation, Dr. Mess and his wife are planning to move
to Columbus, Ohio where Dr. Mess will enter private
practice.
48