Are chin and symphysis morphology facial type–dependent? A computed tomography-based study

Introduction

The chin is a major determinant of the facial profile; hence, it plays a major role in orthodontics and orthognathic surgery. It is thus essential to follow and better understand its expression in different facial types. The major objectives of the current study were to characterize morphometrically the chin and symphysis and reveal their association with different facial types.

Methods

Computed tomography scans of the head and neck of 311 adults (163 males, 148 females; age range, 18-95 years) were classified into 3 facial types: short, average, and long. Height, width, projection, inclination, thickness, and area were measured on the chin and symphysis.

Results

The majority of the population (70%) manifested an average facial type; the other 30% were almost equally distributed between short and long facial types. The long facial type was more common among females and the short facial type among males. Chin projection, area, and size were significantly greater in short-faced patients. Chin width in males was similar for all facial types, whereas, in females, chin width was the widest in the short facial type and the narrowest in the long facial type. Symphysis height was significantly greater in long-faced patients in both sexes. The mandibular incisors’ inclination relative to the mandibular plane was not significantly associated with the chin or symphysis morphology.

Conclusions

Chin and symphysis morphology is facial type–dependent. Orthodontists and maxillofacial surgeons should be aware of the complex relationship between facial types and chin/symphysis size and shape when planning treatment.

Highlights

  • Chin and symphysis size and shape are facial type–dependent.

  • A short face is associated with a square-shaped protruding chin.

  • Patients with a long face present long symphysis.

  • Chin width is facial type–dependent in females but not in males.

  • Mandibular incisor inclination is independent of chin and symphysis morphology.

The chin is a major component of the lower third of the face. Its size is one of the important facial characteristics that determine a balanced facial profile . Variability in chin dimensions contributes to changes in the facial curve from convex to concave; consequently, it affects facial profile classification. It also plays a significant role in planning treatment for orthodontic patients; that is, the degree of chin prominence helps determine the mandibular incisors’ placement during the treatment. This relationship is referred to as the Holdaway ratio. Thus, when planning orthodontic treatment, one should consider chin size in terms of the stability of the outcomes and the esthetic benefits for the patient.

Although the functional significance of the chin shape is obscure, its association with facial types is of significant clinical importance because it helps determine the direction of mandibular growth. Björk, for example, used symphysis inclination to predict the directionality of mandibular growth. In addition, Sassouni associated skeletal deep bite with short (vertically) and broad (antero-posteriorly) symphysis as well as large chin button , and skeletal open bite with narrow (antero-posteriorly) and long (vertically) symphysis and lack of chin. However, neither Sassouni nor Björk provided quantitative data to support their assertions. Aki et al, in a quantitative-based analysis, confirmed that males’ symphysis morphology is associated with mandibular growth, namely, that short and wide symphysis is associated with the anterior growth of the mandible, whereas long and narrow symphysis is associated with the posterior growth direction. Females exhibited a similar tendency, although it was not statistically significant. The measuring method used by Aki et al differed from that used in other studies, because they did not include in their calculations the alveolar part of the symphysis when determining its shape. Khan et al investigated chin dimensions among patients with different divergent patterns. No statistically significant differences were found between hyper-, normo-, and hypodivergent groups in any of the symphysis dimensions: vertical, sagittal, or transverse. However, this study was carried out using a very small number of samples that combined males and females. Arruda et al examined the association between symphysis size and facial types. Their study was also based on small samples, and although a significant difference was noted in symphysis height between the sexes, they did not control for this parameter in their final test. Their findings confirm that dolichofacial types have narrower and higher symphyses, whereas brachyfacial types manifest shorter symphyses. A recent study by Gómez et al sought to find a relationship between mandibular symphysis characteristics and craniofacial structures on the basis of a large number of 3-dimensional cone-beam computed tomographic images. However, many of the measurements of the symphysis were either linear or angular (ie, 2-dimensional) and were taken at the sagittal plane between the mandibular central incisors, which does not always coincide with the midsagittal plane. Using only linear measurements might be insufficient to explore the associations between symphysis and facial types. In a previous study, we demonstrated that the association between masticatory forces and mandibular size and shape is better expressed by the shape measures than linear measures. The main findings of Gómez et al suggest differences in symphysis vertical dimensions between sexes and facial types. In addition, the inclination of the mandibular incisors was positively correlated with symphysis concavity and inclination. Molina-Berlanga et al also observed an association between mandibular incisor inclination and symphysis morphology, although their method of measuring the symphysis was different from the method used by Gómez et al.

One of the drawbacks of many previous studies is the confusion between the chin and the symphysis, which is partially responsible for the inconsistent results. Although the chin is positioned at the anterior-inferior part of the mandibular symphysis, it evolved relatively recently (late Pleistocene) and is considered a unique trait of our species. Therefore, separating the measurements of the symphysis from those of the chin is critical to evaluate how these 2 structures are associated with facial type.

This study aimed to develop a series of measurements for the chin (midsagittal and frontal aspects) and symphysis separately and to assess their association with facial types. The results of such a study should be of interest to both clinicians and basic science researchers.

Material and methods

This study was carried out on computed tomographic (CT) scans of the head and neck of 311 adults of Caucasian origin: 163 males and 148 females; the ages ranged between 18 and 95 years. All scans were taken at Carmel Medical Center, Haifa, Israel (Brilliance 64; Philips Medical System, Cleveland, Ohio), using the following parameters: slice thickness of 0.9-3.0 mm, pixel spacing 0.3-0.5 mm, 120 kV, 250-500 mAs, number of slices 150-950, and Matrix 512 × 512. The CT scans were carried out for diagnostic purposes unrelated to the present study between 2000 and 2013. The research was approved by the ethical board of the Carmel Medical Center (CMC 11-0066). Inclusion criteria were as follows: aged ≥18 years, intact mandibular incisors, and teeth at centric occlusion (maximum intercuspation). Exclusion criteria included the following: absence of mandibular incisors; presence of dental implants or metal restorations that could interfere with the measurements; evidence of orthodontic treatment (brackets, appliances, lingual fixed retainers); previous surgery in the head and neck region (medical files or signs on the skull); prominent facial and mandibular asymmetry; craniofacial, temporomandibular joint, and muscular disorders; trauma; and technically aberrant CT scans. The records of all those who met the inclusion criteria, who were not excluded, and who fit into a single category of facial type were selected. Of the more than 2000 subjects initially enrolled in this study, only 311 met the inclusion criteria, and therefore, they were selected for further analysis.

Patients were classified into 3 groups of facial types: the short facial type (SFT), the average facial type (AFT), and the long facial type (LFT) ( Fig 1 ). The classification was based on 3 parameters: (1) the facial height index (FHI); (2) the steepness of the mandibular plane (MP), following the methods described by Bishara and Jakobsen and Swasty et al ; and (3) the lower anterior facial height (LAFH). The latter was defined as the distance between the anterior nasal spine and the Menton (Me), measured perpendicular to the Frankfort horizontal (FH) plane. Shortface group was characterized by low MP angle, high FHI, and short LAFH; Average-face group by average MP angle, FHI, and LAFH; and Longface group by high MP angle, small FHI, and long LAFH. Only patients that manifested at least 2 characteristics of a given category were included in the study. Six patients were excluded from the study because they manifested 3 of the characteristics that fell into different facial categories.

Fig 1
Examples of facial types and their respective chin and symphysis morphologies. The SFT is presented on the left, the AFT in the middle, and the LFT on the right (images taken from computed tomography).

All measurements were taken directly from the CT scans, using a multiplanner reformatting technique ( Extended Brilliance Workspace portal, version 2.6.0.27; Philips Medical Systems, Cleveland, Ohio). Previous studies found that skull and facial bones measured from 3-dimensional CT are quantitatively accurate and valid. To obtain comparable measurements, all skulls were positioned parallel to the FH plane. Landmarks were identified following Swennen et al and Jacobson and Jacobson. Most of the chin and symphysis measurements were carried out on the midsagittal section of the mandible. The location of this plane was determined regardless of the mandibular incisors’ position. First, we performed a transverse section parallel to the FH plane through the pogonion ( red line ). Then, we performed a second section that passed through the most protruding anterior and posterior points at the symphysis region ( blue line ) ( Fig 2 ). Chin and symphysis were considered as 2 separate structural entities. Linear, angular, and area measurements were carried out to evaluate chin and symphysis size, shape, and position from the CT scans.

Fig 2
Determining the mandible’s midsagittal plane: first, we performed a transverse section parallel to FH plane through the pogonion ( red line ); second, we performed a section that passes through the most protruding anterior and posterior points ( blue line ).

The following chin measurements were used ( Fig 3 , A and B ):

  • 1.

    Height (mm): the distance between the B point and the Me.

  • 2.

    Projection (mm): the maximum thickness of the chin, measured as the shortest distance between the pogonion and the chin height line.

  • 3.

    Area (mm 2 ): the portion of the symphysis area that is located anterior to the chin height line.

  • 4.

    Width (mm): the distance between the right and left mental tubercles.

Fig 3
Measurements of the chin: (A) height, projection, and area ( in pink ), and (B) chin width; measurements of the symphysis: (C) height, thickness, area ( in light blue ), and (D) symphysis orientation (β angle) and inclination (α angle).

The following symphysis measurements were used ( Fig 3 , C and D ):

  • 1.

    Height (mm): the distance between the most superior point on the alveolar bone and the Me.

  • 2.

    Thickness (mm): the distance between the pogonion and the most posterior point on the symphysis.

  • 3.

    Area (mm 2 ): the total area of the symphysis in the midsagittal plane.

  • 4.

    Inclination (°): the inclination of the symphysis relative to the MP, which is the angle (α) created between the line passing from the Infradentale to the Gnathion (Id-Gn line), and the line passing from Gonion to Gnathion.

  • 5.

    Orientation (°): the inclination of the symphysis relative to the FH, which is the angle (β) measured at the cross-point between the Id-Gn line and the FH plane.

Three indexes relating to the size and shape of the chin and symphysis were calculated: (1) the chin size index (%) = the ratio between the chin area and the symphysis area, multiplied by 100; (2) the chin shape index (%) = the ratio between the chin projection and the chin height, multiplied by 100; and (3) the symphysis shape index (%) = the ratio between the symphysis thickness and the symphysis height, multiplied by 100.

All linear and area measurements were controlled for mandible size (relative measures): linear measurements were divided by mandible body length, and area measurements were divided by body length squared. In addition, the inclination of the mandibular incisors was measured relative to MP (IMPA). Following Downs, patients were classified into 3 groups: retroinclination, proclination, and normal inclination.

To determine the ability to accurately replicate the CT measurements, the intratester and intertester reliabilities for each measurement were calculated on 15 different patients. To check the intratester reliability, measurements were carried out twice with a 2-week interval by an independent researcher (T.S.T). For intertester reliability, the measurements were taken by an additional independent researcher (H.M). Intraclass correlation coefficient (ICC) analysis was carried out to examine the reproducibility of the measurements and was interpreted according to the categorization method of Cicchetti.

Statistical analysis

The data were recorded and analyzed using SPSS (version 20.0; IBM, Armonk, NY). All measurements in the study were distributed normally. Assessment of normal distribution was based on a 1-sample Kolmogorov-Smirnov test, a Q-Q plot linear distribution, and a histogram with a normal curve. An independent-samples t test was carried out to identify significant differences in age between sexes. A chi-square test was carried out to detect any association between facial types and sex. A one-way analysis of variance (ANOVA) test was carried out to detect significant differences in age, chin, and symphysis characteristics between the facial types. Post-hoc multiple comparisons were carried out to detect significant differences between the groups. Two-way ANOVA was carried out to detect significant interactions between IMPA and facial type. The level of statistical significance was set at P <0.05.

The datasets analyzed during the current study are available from the corresponding author on request.

Results

ICC results showed high reproducibility for chin measurements, excellent agreement (0.838 ≤ ICC ≤ 0.907) for intratester variation, and good agreement (0.715 ≤ ICC ≤ 0.785) for intertester variation. All symphysis measurements showed excellent agreement (0.903 ≤ ICC ≤ 0.986) for intratester variation and excellent agreement (0.852 ≤ ICC ≤ 0.980) for intertester variation. All measurements used for facial-type classification showed excellent agreement for both intratester and intertester variation (0.895 ≤ ICC ≤ 0.991) ( P <0.001).

The study sample included 311 patients: 163 males (52.4%) and 148 females (47.6%). The mean age was 49 ± 20.3 years (range, 18-95 years). The mean age did not significantly differ between sexes and was 47.5 ± 19.5 years for males and 50.8 ± 21.1 years for females ( P = 0.153). In addition, there was no statistical difference between the mean age of the different facial types in both males and females ( P >0.188).

The study sample included 216 average-faced patients (69.45%), 49 long-faced patients (15.75%), and 46 short-faced patients (14.8%). The AFT group comprised 115 males (53.2%) and 101 females (46.8%). The LFT group comprised 18 males (36.7%) and 31 females (63.3%). The SFT group included 30 males (65.2%) and 16 females (34.8%). A significant association was found between the facial types and sex ( P = 0.019): LFT was more common among females and SFT among males.

Chin absolute and relative measures were compared between the facial types. In males, the chin was found to be significantly thicker and greater (projection, area, shape, and size index) in the SFT group, compared with the AFT and LFT groups ( Table I ). No significant differences were found in these parameters between the LFT and AFT groups ( P >0.489). Similar results were obtained when measurements were controlled for mandible size. Of particular interest is the fact that no significant differences in chin height and width were found between the 3 facial groups. In females, all chin parameters differed statistically between the facial types ( Table II ). Chin was significantly greater, wider, and thicker (projection, area, width, shape, and size indexes) in females with SFT ( P <0.041). The LFT group manifested the greatest chin height. Unlike males, chin width in females was significantly different between all the facial types ( P <0.003). SFT was characterized by having the widest chin, whereas LFT by the narrowest one.

Table I
Morphometric characteristics of the chin and symphysis in different facial types in males
Measurement Facial type Mean SD Minimum Maximum P values Post-hoc
Absolute measures Relative measures Multiple comparison
Chin
Height (mm) SFT 22.36 3.119 15.78 28.90 0.366 0.653
AFT 21.49 3.013 13.30 27.80
LFT 21.84 2.915 17.00 28.80
Projection (mm) SFT 4.60 0.890 3.10 7.10 0.001 0.001 SFT > AFT
AFT 3.86 1.023 1.40 7.10
LFT 3.94 0.738 2.10 4.90
Area (mm 2 ) SFT 60.62 19.547 27.10 113.40 0.020 0.035 SFT > AFT
AFT 50.43 17.812 13.80 113.30
LFT 55.95 18.097 24.00 107.70
Width (mm) SFT 30.08 5.756 19.60 40.40 0.065 0.109
AFT 27.53 5.322 16.30 40.50
LFT 27.32 5.535 19.50 40.40
Shape index (%) SFT 20.61 2.966 14.55 26.73 0.005 SFT > AFT
AFT 18.03 4.176 5.32 27.84
LFT 18.07 2.833 11.80 21.46
Size index (%) SFT 18.43 5.335 9.46 31.09 0.024 SFT > AFT
AFT 15.44 5.274 5.29 28.89
LFT 16.27 5.321 7.69 28.02
Symphysis
Height (mm) SFT 32.04 3.065 26.00 38.10 <0.001 0.009 LFT > AFT > SFT
AFT 33.70 2.884 24.60 40.30
LFT 35.99 3.572 29.00 42.60
Thickness (mm) SFT 16.55 1.846 13.60 20.30 0.013 0.019 SFT > AFT
AFT 15.30 2.165 11.20 23.20
LFT 15.46 1.324 13.10 18.20
Area (mm 2 ) SFT 327.80 48.003 230.50 428.50 0.353 0.403
AFT 329.82 56.319 212.90 481.10
LFT 348.98 53.052 275.70 437.90
Shape index (%) SFT 52.04 7.197 37.81 68.85 <0.001 SFT > AFT; SFT > LFT
AFT 45.53 6.074 32.37 70.73
LFT 43.15 3.586 37.56 50.70
Orientation (°) SFT 85.23 7.154 67.40 102.00 <0.001 SFT > AFT > LFT
AFT 79.61 7.043 57.10 100.30
LFT 74.67 6.769 64.40 86.70
Inclination (°) SFT 78.43 4.531 66.00 86.00 <0.001 SFT > AFT; AFT > LFT
AFT 76.10 5.321 61.00 89.00
LFT 71.24 5.178 64.00 81.00
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Jul 16, 2021 | Posted by in Orthodontics | Comments Off on Are chin and symphysis morphology facial type–dependent? A computed tomography-based study
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