Comprehending the three-dimensional mandibular morphology of facial asymmetry patients with mandibular prognathism
Kamata et al. Progress in Orthodontics
Comprehending the three-dimensional mandibular morphology of facial asymmetry patients with mandibular prognathism
Hideki Kamata 0
Norihisa Higashihori 0
Hiroki Fukuoka 0
Momotoshi Shiga 0 1
Tatsuo Kawamoto 0 1
Keiji Moriyama 0
0 Section of Maxillofacial Orthognathics, Department of Maxillofacial/Neck Reconstruction, Graduate School, Tokyo Medical and Dental University , Tokyo , Japan
1 Division of Orofacial Functions and Orthodontics, Department of Health Improvement, Faculty of Dentistry, Kyushu Dental University , Kitakyushu , Japan
Background: The purpose of this study was to elucidate the factors that cause facial asymmetry by comparing the characteristics of the mandibular morphology in patients with mandibular prognathism with or without facial asymmetry using three-dimensional computed tomography (3D-CT). Methods: We studied 28 mandibular prognathism patients whose menton deviated by ≥ 4 mm from the midline (FA group, n = 14) and those with a < 4-mm deviation (NA group, n = 14). DICOM data from multislice CT images were reconstructed and analysed using 3D image analysing software. Mandibular structures were assessed via linear, angular, or volumetric measurements and analysed statistically. Results: The lengths of the ramal and body components and condylar volume in the FA group were significantly greater on the nondeviated side than those on the deviated side. The mandibular body length of the nondeviated side in the FA group was significantly longer than that of the NA group. Other components of the FA group did not significantly differ from those of the NA group. Conclusions: Imbalances in the sizes of the ramal and body components as well as the increased body length of the nondeviated side in the FA group compared with that of the NA group may contribute to facial asymmetry in patients with mandibular prognathism.
3D; Mandibular prognathism; Facial asymmetry; Mandibular morphology
Completely symmetrical faces are almost nonexistent. Most
people have some degree of asymmetry in their facial
]. Using triangulation, Sharad and Joshi assessed
the asymmetry of the component areas of the facial
complex in college students with normal occlusion and no
history of orthodontic therapy, and found that one side of the
facial surface area was significantly larger than that of the
other . Furthermore, a study on craniofacial morphology
using cone-beam CT showed minor asymmetry in 30
orthodontic patients [
]. Therefore, it can be concluded
that facial asymmetry is a common phenomenon in the
normal human craniofacial complex. However, there are
wide variations in the facial asymmetry of patients who
need orthodontic treatment—from cases in which
occlusion can be improved by orthodontics alone to cases of
severe asymmetry where orthognathic surgery is needed to
improve occlusion and/or facial features. Regardless of the
issue resulting in facial asymmetry, investigating the
characteristics and causes of asymmetry is important in
orthodontic diagnosis and treatment.
Many years of study have been dedicated to elucidating
the factors that cause facial asymmetry. These studies have
been based on the use of conventional two-dimensional
(2D) images such as frontal facial photographs or
cephalometric radiographs. However, it is difficult to
conduct evaluations using conventional 2D analysis because
of distortion and magnification problems, especially in
cases of asymmetry [
]. Recent developments in 3D
measuring devices and image analysis software have resolved
these problems. Among these measuring devices, 3D
computed tomography (3D-CT) has enabled us to obtain
3D hard- and soft-tissue data at the same time with fewer
magnification and distortion problems [
]. The 3D
measurements of the length and angle of the midfacial,
mandibular, and cranial bases by Baek et al. showed that
menton deviation was significantly correlated with ramal
height differences between the two sides, leading the
authors to conclude that greater growth of the ramus was
one of the causes of asymmetry in skeletal class III
patients . Furthermore, You et al. showed that ramal
height (condylion superior–gonion midpoint) and menton
deviation were strongly correlated, implying differences in
deviated and nondeviated ramal heights. However, they
did not mention whether asymmetry was caused by
onesided hyper- or hypo-active mandibular growth [
The present study was conducted to elucidate the
factors that cause facial asymmetry by comparing the
characteristics of mandibular morphology in patients with
mandibular prognathism with or without facial
asymmetry using 3D-CT.
Twenty-eight patients with mandibular prognathism
(ANB < 0°) who had undergone orthognathic surgical
procedures at the Tokyo Medical and Dental University
Dental Hospital were included in this study. On
posteroanterior cephalometric radiographs taken at the
initial examination, the midline was defined as the line
that passed through the crista galli of the ethmoid bone,
perpendicular to the Lo-Lo′ line. Patients whose menton
deviated 4 mm or more (mean 8.9 ± 4.3 mm) from the
midline were assigned to the group considered to have
skeletal mandibular prognathism with facial asymmetry
(FA group: 14 cases). Patients who had less than 4 mm
deviation (mean 1.6 ± 0.8 mm) were assigned to the group
with skeletal mandibular prognathism without facial
asymmetry (NA group: 14 cases). In both groups, we
defined the deviated side as the one with menton deviation.
This classification was based on the report of Baek et al.
]. Patients with congenital disorders, such as cleft lip
and/or cleft palate, were not included. The institutional
ethics committee of Tokyo Medical and Dental University
approved the research protocol (approval #731).
3D-CT data were obtained from multislice CT images
(SOMATOM Plus-S®; Siemens Japan, Tokyo, Japan)
acquired immediately before surgery. The CT imaging
conditions were as follows: 120 kV, 200 A, 15–20-s exposure
time, and 4 mm/s table speed. Digital Imaging and
Communications in Medicine (DICOM) data from
multislice CT images were reconstructed and analysed using
SimPlant OMS® software (Materialise Dental Japan,
Tokyo, Japan). The data were created with 0.5-mm slice
thicknesses. The segmentation level and segmentation
width were 1285.5 and 1785.5 HU, respectively.
Landmarks were inscribed on the 3D model as
described in Table 1 and Fig. 1.
Definition of the mandibular condyle
The most inferior point of the mandibular foramen (F) is
recognised as a good reference point among the
landmarks in the area surrounding the mandibular foramen
]. Connecting F and the most superior point of the
condyloid process (Cd) creates a reproducible axis for the
mandibular ramus. We hypothesised that using the plane
passing through the most recessed area of the lateral
mandibular neck (Cdneck) orthogonal to the mandibular ramal
axis (Cd–F) would be as reproducible as the plane
partitioning the mandibular condyle from the rest of the
ramus. Therefore, a plane was defined orthogonal to the
line connecting Cd and F that passed through Cdneck
(hereafter referred to as the mandibular condylar base).
The area superior to this plane was defined as the
mandibular condyle (Fig. 2). The software measured the
volume of the mandibular condyle in an automated manner.
Body length was measured as the distance from Gomid
to Me. Ramal height was measured as the distance from
Cd to Gomid (Fig. 3a). A point was designated where the
Cd–Gomid line intersected with the mandibular condylar
base. Condylar height was measured as the distance
superior to this point along the length of the ramus.
Inferior to this point was the inferior ramal height.
The gonial (Go) angle was created by the Cdpost–Gomid
line and the Gomid–Me line (Fig. 3b).
Midpoint between Gopost and Goinf on the mandibular angle
Condylar volume measurements and method accuracy
To examine the inter- and intra-examiner reliability
of the measurements, mandibular condyles were
extracted from the CT data of randomly selected cases
and their volumes were measured according to the
method of Kwon et al. [
]. Three orthodontists
performed the volume measurements, with each one
separately conducting two measurements.
Measurement errors were evaluated with Dahlberg’s formula:
Se = √ΣD2/2n, where D is the difference between the
two measurements, and n is the number of times
the measurements were repeated. No significant
inter- or intra-observer differences were observed.
The Mann-Whitney U test was used to compare
differences in the nondeviated and deviated sides
between the two groups. The measurements on both
sides of the FA group were compared with those of
the NA group using the Mann-Whitney U test. The
Wilcoxon signed-rank test was used to compare the
mandibular measurements of the deviated and
nondeviated sides in the FA and NA groups. Spearman’s
rank correlation coefficient was used to examine
correlations between the mandibular measurements.
Comparison of the bilateral differences in the mandibular components between the FA and NA groups
To identify the mandibular components that create
facial asymmetry, we compared the differences in the
nondeviated and deviated sides between the FA and
NA groups. Compared with the NA group, significant
differences were observed in body length (P = 0.001),
ramal height (P = 0.027), condylar height (P < 0.001),
and condylar volume (P < 0.001) in the FA
group—indicating that asymmetry was caused by the
mandibular morphology (Table 2).
Bilateral comparison of the mandibular components in the FA and NA groups
In the FA group, linear measurements of body length,
ramal height, and condylar height were significantly
smaller on the deviated side. Condylar volume and the Go
angle were also smaller on the deviated side in the FA
group. There were no significant differences between the
measurements on both sides in the NA group (Table 3).
Comparison of the mandibular components of each side in the FA group and of those in the NA group
To identify whether the linear measurements and the
volume of each side of the FA group were similar to
or different from those of the NA group, we
compared the mandibular components of each side in the
FA group and the corresponding mandibular
components in the NA group. No difference was observed
in the linear measurement of body length between
the deviated side of the FA group and that of the NA
group. In the nondeviated side of the FA group,
however, body length was significantly longer than that in
the NA group (P = 0.011) (Table 4).
Correlations between the measurements of mandibular morphology in the FA group
Many previous reports have used differences in the
deviated–nondeviated sides of actual measurements
to compare groups or investigate correlations [
In this study, we used the ratios of the deviated–
nondeviated sides to eliminate the effect of size
differences among the mandibles and to make the
evaluations more objective. Menton deviation was
negatively correlated with the body length ratio (P =
0.044), ramal height ratio (P = 0.016), condylar
height ratio (P = 0.023), inferior ramal height ratio
(P = 0.016), and condylar volume ratio (P = 0.010).
For the ramal height ratio, positive correlations
were found with the condylar height ratio (P =
0.001), inferior ramal height ratio (P < 0.001), and
condylar volume ratio (P = 0.044). For the condylar
height ratio, a positive correlation was found with
the inferior ramal height ratio (P = 0.003). For the
inferior ramal height ratio, a positive correlation
was found with the condylar volume ratio (P =
0.037) (Table 5).
The present study focused on the 3D mandibular
morphology to elucidate the characteristics of facial
asymmetry in Japanese jaw deformity patients with
mandibular prognathism. We found that most of the
components of the mandible significantly differed
between the deviated and nondeviated sides in the
FA group (Table 3). Furthermore, the linear
measurement of the mandibular body in the nondeviated
side of the FA group was larger than that of the NA
group (Table 4).
Posteroanterior cephalometric analysis has been
used to identify the mandibular components that
cause the facial asymmetry. A 2D study by Fong et al.
reported that asymmetry was found in the mandibular
body in patients with menton deviation, although
ramal heights were not significantly different [
The results were evaluated using posterior-anterior
cephalometric analysis, which has limitations because
of the magnification and distortion errors inherent in
the projection techniques [
]. To overcome these
problems, 3D-CT analysis has been frequently used to
evaluate factors that may induce facial asymmetry
]. Our findings and those of others showed
that the ramal component of the deviated side was
shorter or smaller than that of the nondeviated side
in patients with facial asymmetry.
The mandible is composed of a body, condyles, a
chin, and several processes, including the alveolar
process, coronoid process, and angular process. To
identify the mandibular components that contribute
to facial asymmetry, we evaluated the correlations of
menton deviation with several measurements. The
results showed that the ramal height ratio, condylar
height ratio, condylar volume ratio, inferior ramal
height ratio, and body length ratio were correlated
with menton deviation. You et al. reported a similar
conclusion that the ramal component and the body
component both correlated with menton deviation,
although they did not analyse the ramal components
]. Additionally, correlations were found
between the condylar component ratio (condylar
volume and height ratio) and the ramal component
ratio (ramal height ratio or inferior ramal height
ratio), suggesting that the size of the condyle may
control the ramus component.
Whether the body component is asymmetrical in
terms of length remains controversial. Baek et al. showed
no significant differences in body size between the two
sides in patients with facial asymmetry [
]. In contrast,
our results showed that body length was significantly
different on the nondeviated and deviated sides (Table 3),
and these findings are consistent with those of You et al.
and Kwon et al. [
The pathogenesis of facial asymmetry reportedly
involves two factors: congenital and acquired factors
]. Genetic factors and syndromic disorders are
included among the congenital factors, with hemifacial
*The nondeviated side of the FA group was significantly larger (P < 0.05) than
the NA group
microsomia as one example. Hemifacial microsomia
is a disorder in which the lower half of one side of
the face is underdeveloped [
]. Acquired factors,
such as a functional shift during the growth period
of the mandible, are known to be causes of facial
asymmetry. When a functional shift is maintained
for a long time, especially during the growth period,
the sustained functional load on the mandible
transforms the functional shift into structural asymmetry
]. However, although imbalanced growth of the
condyle induces facial asymmetry, it is difficult to
distinguish whether the asymmetry has occurred
through hyperactivity of the nondeviated side of the
condyle or through hypoactivity of the shifted side
of the condyle, or both, during the growth period.
Therefore, we compared the deviated side and the
nondeviated side of the FA group with those of the
NA group. Our data showed significant left–right
differences in the FA group (Table 3). However, we
did not observe any differences between the two
groups in the deviated and nondeviated sides, except
for body length in the nondeviated side of the FA
group, which was longer (Table 4). Based on our
results, we could not conclude whether this was a
result of hyperactivity in the nondeviated side or
hypoactivity in the shifted side. The phenomenon of
the body length of the nondeviated side of the FA
group, which was larger than that of the NA group,
causing the menton to move distally to the shifted
side may be one of the mechanisms by which facial
We assessed mandibular morphology using 3D-CT in
patients with skeletal mandibular prognathism and facial
asymmetry. Imbalances in the sizes of the ramal and
body components and the longer body length of the
nondeviated side of the FA group compared with that of
the NA group may possibly contribute to facial
asymmetry in patients with mandibular prognathism.
Go angle ratio
− 0.501 (0.068)
We thank all the member of Maxillofacial Orthognathics, Tokyo Medical and
Dental University, for the helpful comments.
Availability of data and materials
The datasets supporting the conclusions of this article are included within
HK contributed to data acquisition and analysis and drafted the manuscript.
NH coordinated the research project, participated in the interpretation of the
results, and drafted and critically revised the manuscript. HF, MS, and TK
participated in the interpretation of the results and critically revised the
manuscript. KM coordinated the research project and critically revised the
manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
This study was approved by the Institutional Ethical Committee of Tokyo
Medical and Dental University (no. 731) and conducted in accordance with
the Declaration of Helsinki.
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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