Chin Morphology in Relation to the Skeletal Pattern, Age, Gender, and Ethnicity

Abstract

Chin morphology is visually impactful and significantly influences harmonious facial appearance. Therefore, it is important to know the morphological characteristics of the chin in relation to gender and skeletal pattern when performing an orthodontic and surgical orthognathic treatment. This study aimed to examine the relationship between chin size, skeletal pattern, age, gender, and ethnicity. In this study, cone-beam computed tomography images of 208 participants (males: 90, females: 118; 63 Koreans and 145 Egyptians), aged 18 years and older, were used to evaluate the size of the chin in linear dimension, volume, and skeletal pattern. The larger the vertical skeletal pattern, the larger the chin volume and the smaller the width (p < 0.01). In the anteroposterior skeletal pattern, Class III showed a larger volume than that of Class II and Class I (p < 0.01). There was no significant association between age and any of the chin-related measurements. Regarding gender, all measurements of chins were larger for men than for women (p < 0.01). In terms of ethnicity, Koreans had larger chin volumes than Egyptians p < 0.01). Chin volume was related to population, sex, anteroposterior skeletal pattern, and vertical skeletal pattern, indicating the combined effects of various factors.

1. Introduction

The unique anatomical morphology of the chin is considered an autapomorphy of Homo sapiens and is used as a measure of the degree of evolution from the earliest H. sapiens to modern humans [1]. The evolutionary and functional importance of the chin has long been explored in terms of several factors, including masticatory biomechanics, speech, and anterior tooth size; however, the effect of population differences, such as ethnicity, on chin morphology remains unclear [2,3,4]. On comparing the growth changes of the mandible between H. sapiens and other primates, it was found that the size of the mandible increases with bone addition to the labial aspect in primates, while in H. sapiens, the size of the mandibular base increases with bone addition with growth; however, the alveolar portion is characterized by a shift to the lingual side owing to bone resorption on the labial side [5]. In H. sapiens, the change in the head position associated with the acquisition of bipedal locomotion is believed to have caused a retroversion of the maxilla and mandible relative to the cranium [1]. Recent anthropological studies have hypothesized that the chin was formed due to the retraction of the anterior portion of the mandibular submarginal plane, which was left behind to spatially compensate for the narrowing of the laryngopharynx associated with the retraction of the maxilla and mandible [1]. Medically, the horizontal and vertical skeletal patterns of the maxillary bones have been shown to be related to the laryngeal volume. In fact, a posteriorly positioned mandible or backward rotation of the mandible [6] is known to be an important risk factor for obstructive sleep apnea (OSA), as it reduces the laryngeal area and compresses the airway [7]. An oral appliance that allows the mandible to assume an anterior position during sleep [8], and a surgical forward shift of the chin are effective treatment options to reduce OSA [9]. In contrast, it is well known that males have larger chins than females in terms of height, width, and depth [10]. The feminine chin also frequently appears tapered or oval. As a result, it has been hypothesized repeatedly that sexual selection and sexual dimorphism are variables that influence the development of the chin [11]. Differences in the size of the chin also affect the evaluation and classification of facial contours, which are important for aesthetically harmonious facial features [12]. Therefore, the relationship between gender differences, skeletal patterns, and the morphological characteristic of the chin is an important topic in orthodontics [13,14]. Patients with mandibular prognathism frequently choose to have chin retraction added to mandibular setback during orthognathic surgery [15]. Additionally, morphological corrections of the chin are part of the feminization operations used on transgender patients to create feminine effects [10]. Since the cross-sectional morphology of the symphysis, where the right and left mandibular bodies meet, can be well evaluated on lateral head radiographs, the relationship between the skeletal pattern, mandibular growth direction, and the chin has long been investigated [16]. It is generally believed that the thicker the symphysis, the more the mandible grows anteriorly; and the longer the symphysis, the more the mandible grows vertically [17,18]. Previous studies have pointed out the problem of confusing the two, owing to their anatomical proximity [19]. The chin is located anteriorly and inferiorly to the symphysis; therefore, it is necessary to distinguish between the symphysis and the chin to properly evaluate the chin. Recently, the relationship between symphysis, chin, and skeletal pattern has been reported using cone-beam computed tomography (CBCT) [19,20,21], not only in terms of linear dimensions of the cross-section of the chin region as in the past but also in terms of the relationship between surface area and skeletal pattern. CBCT is an appropriate imaging method to evaluate the three-dimensional volume of the chin [22,23], which may provide useful information for the diagnostic process in orthodontic treatment. This study aims to investigate the relationship between chin size and volume and vertical and anteroposterior skeletal patterns, gender, ethnicity, and age.

2. Materials and Methods

2.1. Participants

A total of 208 patients (male: 90, female: 118; 63 Koreans and 145 Egyptians), aged 18 years and older, were included in the study. The Korean participants consisted of 63 orthodontic patients (male: 28, female: 35) from the Pusan National University Dental Hospital. The study was approved by the Institutional Review Board of Pusan National University Dental Hospital (IRB PNUDH-2019-025). The Egyptian participants consisted of 145 orthodontic patients (male: 62, female: 83) from Suez Canal University (Ismailia, Egypt). The study was approved by the Ethics Committee of Suez Canal University (IRB 8). We note that the CBCT images were taken for therapeutic purposes, and not specifically for this study. This research was performed as an exploratory study without setting the sample size. This study was conducted according to the current standards recommended for reporting observational studies in epidemiology (STROBE) and is in accordance with the Declaration of Helsinki. Written consent was obtained from all the patients before the start of the treatment. Only patients aged 18 years or older were included in the study, and the exclusion criteria were as follows:

(1)

Patients with significant mandibular deviation (menton deviation of 4 mm or more from the midface reference plane)

(2)

Patients with congenital diseases such as cleft lip or palate

(3)

Patients with a history of orthognathic treatment or orthognathic surgery

(4)

Patients with a history of maxillofacial trauma

(5)

CBCT images of inadequate quality due to artifacts

(6)

CBCT images with inadequate coverage

2.2. Measurements

CBCT images of Korean patients were acquired using a CBCT system (Zenith3D, Vatech Co., Seoul, Korea). CBCT images of Egyptian patients were acquired using a CBCT system (Soredex SCANORA 3D, Nahkeatine 16, Tuusula, Finland). CBCT data were stored in Digital Imaging and Communications in Medicine (DICOM) format. Data analysis was performed by a single operator. All DICOM data were imported into Invivo™ 6 (Anatomage, San Jose, CA, USA) for further processing and analysis. Landmarks and reference planes for analysis were defined after consulting previous studies [24]. The Frankfort horizontal plane was defined as the plane that intersects both porions and the left orbitale, and the mandibular plane was defined as the plane formed by connecting the gonion and menton on both sides (Figure 1). The chin was defined as a plane descending through point B, parallel to the left and right gonions, perpendicular to the mandibular submarginal plane, and anterior to that plane (Figure 2). The mandible segmentation with facial midsagittal plane was defined as a plane perpendicular to the Frankfort horizontal plane through the nasion and sera. The longest vertical distance that the excised chin faced in the midsagittal plane was defined as the chin height, the longest horizontal distance was defined as the chin width, and the difference between the distance from the gonion to pogonion and the distance from the gonion to point B (average of the left and right sides) was defined as the chin depth. Chin volume was measured using the Invivo™ 6 imaging software. Using the pre-defined values in the software, the threshold values for “bone” were set. The chin region was then removed from the 3D model of the mandible from the DICOM picture series using the cropping tool, and the volume was determined using the automatic measurement function (Figure 3). Aluminum bars were used to confirm that the difference in CBCT systems did not affect the measured values since CBCT image data obtained from two different CBCT systems were used in this study, as shown in a previous study [25]. Thirty CBCT pictures were randomly selected and measured using Dahlberg’s formula at intervals of two weeks under identical circumstances [26] to analyze the potential operator error. Anteroposterior skeletal patterns were classified into three groups based on the ANB angle: Class I (−1° < ANB < 4°), Class II (ANB ≥ 4°), and Class III (ANB ≤ −1°). Vertical skeletal patterns were classified into three groups based on the mandibular plane angle (MP): hypodivergent (MP ≤ 23°), normodivergent (23° < MP < 30°), and hyperdivergent (MP ≥ 30°). The demographics of the patient’s skeletal pattern are shown in

Table 1. The demographics of skeletal pattern of the participants.

		Number of Participants	Age (y)	Gender (m: Male, f: Female)	Ethnicity (k: Korean, e: Egyptian)
Anteroposterior skeletal pattern	Class I	105 n	24.7 ± 5.1	m: 47 n	k: 31 n
				f: 58 n	e: 74 n
	Class II	93 n	24.7 ± 5.3	m: 37 n	k: 23 n
				f: 56 n	e: 70 n
	Class III	10 n	20.0 ± 2.3	m: 7 n	k: 9 n
				f: 3 n	e: 1 n
Vertical skeletal pattern	Hypodivergent	74 n	24.3 ± 4.4	m: 38 n	k: 14 n
				f: 36 n	e: 60 n
	Normodivergent	92 n	24.9 ± 6.0	m: 37 n	k: 27 n
				f: 55 n	e: 65 n
	Hyperdivergent	42 n	23.8 ± 4.3	m: 16 n	k: 22 n
				f: 26 n	e: 20 n

.

2.3. Statistical Analysis

First, we evaluated whether there were differences in chin size (chin width, chin height, chin depth, and chin volume) in terms of anteroposterior skeletal pattern, vertical skeletal pattern, ethnicity (Korean/Egyptian), sex (male/female), and age (year) using univariate analysis. Comparison of two groups was done using the t-test, and the comparison of three or more groups was done using one-way analysis of variance followed by Bonferroni’s multiple comparisons. Age was evaluated using the Pearson’s correlation coefficient. Subsequently, ordinal logistic regression analysis was performed with the skeletal pattern as the objective variable and chin volume, chin width, chin height, and chin depth as explanatory variables. SPSS Statistics 26 (IBM Corporation, Armonk, NY, USA) software was used to carry out the statistical analyses, and the statistical significance was set at p < 0.05.

3. Results

Differences in chin width, height, depth, and volume in terms of anteroposterior and vertical skeletal pattern, ethnicity, and gender are shown in

Table 2. Chin morphology in relation to skeletal pattern, gender, and ethnicity.

		Volume (mm3)	Width (mm)	Hight (mm)	Depth (mm)
Ethnicity	Korean	3668.19 ± 1297.25 *	31.94 ± 5.27	19.68 ± 2.23	2.95 ± 1.58
	Egyptian	2838.46 ± 856.26	32.39 ± 4.85	19.27 ± 2.54	2.61 ± 1.18
Gender	Male	3648.51 ± 1106.16 §	34.36 ± 4.61 §	20.42 ± 2.71 §	2.99 ± 1.45 §
	Female	2655.20 ± 827.98	30.61 ± 4.63	18.60 ± 1.89	2.50 ± 1.18
Anteroposterior skeletal pattern	Class I	3071.95 ± 1142.58	33.05 ± 4.91	19.36 ± 2.87	2.88 ± 1.26 ‡
	Class II	2994.61 ± 908.68	31.37 ± 4.92	19.44 ± 1.97	2.32 ± 1.09
	Class III	4161.80 ± 1335.24 †‡	32.14 ± 5.10	19.36 ± 1.88	4.57 ± 2.00 †‡
Vertical skeletal pattern	Hypodivergent	2900.99 ± 970.53	33.19 ± 5.33	19.05 ± 2.06	2.87 ± 1.38
	Normodivergent	3029.86 ± 1026.29	31.43 ± 4.53	19.42 ± 2.90	2.56 ± 1.27
	Hyperdivergent	3553.62 ± 1244.88 ⁑⁂	32.39 ± 5.04	19.94 ± 1.91	2.78 ± 1.34

* Significant difference vs. Egyptians. ^§ Significant difference vs. Female. ^† Significant difference vs. I. ^‡ Significant difference vs. II. ^⁑ Significant difference vs. Hypodivergent. ^⁂ Significant difference vs. Normodivergent. ^{§, †, ‡, *, ⁑, ⁂}: p < 0.01, t-test or multiple comparison after one-way analysis of variance.

.

In terms of the anteroposterior skeletal pattern, differences were observed in chin volume and chin depth. Class III showed significantly larger chin volumes than those of Class I and Class II (Class I: 3071.95 ± 1142.58 and Class II: 2994.61 ± 908.68, both p < 0.01 compared to the Class III: 4161.80 ± 1335.24). Significant differences in chin depth were seen among all groups, with the largest values seen in Class III, followed by Class I and Class II (Class I: 2.88 ± 1.26, Class II: 2.32 ± 1.09, and Class III: 4.57 ± 2.00, each other p < 0.01). Ordinal logistic regression analysis (

Table 3. Ordinal logistic regression analysis of anteroposterior skeletal patterns.

		B	SE	Wald	df	Sig	EXP(B) (95% Cl)
							Min	Max
Threshold	[Anteroposterior skeletal patterns = Class II vs. Class I]	−0.22	1.41	0.03	1	0.88	−2.99	2.55
	[Anteroposterior skeletal patterns = Class II vs. Class III]	3.37	1.45	5.37	1	0.02	0.52	6.22
Position	Volume (mm3)	0.00	0.00	2.20	1	0.14	0.00	0.00
	Width (mm)	0.01	0.03	0.17	1	0.68	−0.05	0.08
	Height (mm)	−0.12	0.07	3.45	1	0.06	−0.25	0.01
	Depth (mm)	0.52	0.13	15.99	1	0.00	0.26	0.77

B, regression coefficient; Cl, confidence interval, minimum and maximum; df, degrees of freedom; Exp(B), exponential B; SE, standard error; and Sig, significance.

) indicated that the anteroposterior skeletal pattern was positively associated with chin depth (B = 0.52, p = 0.00).

In terms of vertical skeletal pattern, differences were observed only in chin volume, with the chin volume in the hyperdivergent pattern being significantly greater than that of the hypodivergent and normodivergent patterns in the univariate analysis (Hypodivergent: 2900.99 ± 970.53 and Normodivergent: 3029.86 ± 1026.29, both p < 0.01 compared to the hyperdivergent: 3553.62 ± 1244.88). Ordinal logistic regression analysis (

Table 4. Ordinal logistic regression analysis of vertical skeletal patterns.

			B	SE	Wald	df	Sig	EXP(B) (95% Cl)
								Min	Max
Threshold	[Vertical skeletal patterns = Hypodivergent vs. Normodivergent]		−1.81	1.31	1.90	1	0.17	−4.39	0.77
	[Vertical skeletal patterns = Hypodivergent vs. Hyperdivergent]		0.37	1.31	0.08	1	0.78	−2.19	2.94
Position	Volume (mm3)		0.00	0.00	19.70	1	0.00	0.00	0.00
	Width (mm)		−0.09	0.03	8.74	1	0.00	−0.15	−0.03
	Height (mm)		0.04	0.06	0.47	1	0.49	−0.08	0.16
	Depth (mm)		−0.26	0.12	5.11	1	0.02	−0.48	−0.03

B, regression coefficient; Cl, confidence interval, minimum and maximum; df, degrees of freedom; Exp(B), exponential B; SE, standard error; and Sig, significance.

) revealed that the vertical skeletal pattern was positively correlated with chin volume (B = 0.00, p = 0.00), and negatively correlated with chin width and chin depth (B = −0.26, p = 0.02).

In terms of ethnicity, the Korean group showed significantly larger values for chin volume than the Egyptian group (3668.19 ± 1297.25 vs. 2838.46 ± 856.26, respectively, p < 0.01), but there were no significant differences in chin width, height, and depth (Table 2).

In terms of gender, males showed significantly larger values than females for all parameters including chin width (34.36 ± 4.61 vs. 30.61 ± 4.63, respectively, p < 0.01), chin height (20.42 ± 2.71 vs. 18.60 ± 1.89, respectively, p < 0.01), chin depth (2.99 ± 1.45 vs. 2.50 ± 1.18, respectively, p < 0.01), and chin volume (3648.51 ± 1106.16 vs. 2655.20 ± 827.98, respectively, p < 0.01).

In terms of age, we found no association exceeding |r| > 0.2, and no significant correlations (

Table 5. Pearson correlation analysis of Chin morphology and age.

	Age (y)	Volume (mm3)	Height (mm)	Width (mm)	Depth (mm)
Correlation coefficient	1	0.10	0.08	0.09	0.02
p		0.17	0.23	0.21	0.78

).

4. Discussion

In this study, we investigated the relationship between chin size and vertical and anteroposterior skeletal patterns, gender, ethnicity, and age. Furthermore, the relationship between the three-dimensional volume of chin and the parameters of maxillofacial morphology have never been investigated before.

More anterior growth of the mandible corresponds to a thicker symphysis, which has been reported in many studies [27]. Kale et al. found that the chin surface area in Class III patients was the greatest and could be used as a parameter for differential diagnosis of Class III malocclusion [28]. The current study showed similar results, with chin volume and chin depth showing the greatest values in Class III patients. On the other hand, in the multivariate analysis, the only parameter related to anteroposterior skeletal pattern was chin depth. The greater the chin depth, the greater the likelihood that the mandible was anterior to the maxilla, as in all Classes (Table 3).

It has been reported that the chin depth is greater in Class III patients [29], which is consistent with the present results. Based on the hypothesis that the origin of the chin is due to the spatial compensation of the laryngeal area, the need for spatial compensation is rather reduced in Class III, where the mandible is located anteriorly, and the laryngeal volume is large. The results of this study are consistent with this hypothesis.

In the univariate analysis, vertical skeletal pattern was significantly associated with chin volume with the chin volume being larger in the hyperdivergent pattern. Multivariate analysis showed a positive correlation with chin volume, but negative correlations with chin width and chin depth and no association with chin height. In other words, as chin volume increased, it was more likely to be hypodivergent, normodivergent, and hyperdivergent; similarly, as chin width increased, it was more likely to be hyperdivergent, normodivergent, and hypodivergent (Table 4). Regarding the relationship between vertical skeletal pattern and symphysis, an increase in height and decrease in width has been reported with increasing hypertrophy, and chin size decreases with increasing hypertrophy in vertical skeletal pattern [17,18,29]. Contrastingly, Tunis et al. reported that the width becomes smaller with hypertrophy, but chin height is not affected in the chin evaluation based on point B [19]. The results of this study based on point B were the same. The results of the present study are consistent with the finding that the vertical skeletal pattern has more influence on the shape of the symphysis than the anteroposterior skeletal pattern [30]. The vertical skeletal pattern is associated with mandibular rotation relative to the base of the cranium, with an anterior rotation in patients with hypodivergent pattern and a posterior rotation in hyperdivergent pattern [31,32]. Patients with hyperdivergent pattern with a posterior mandible are known to have a smaller laryngeal volume [6] and based on the hypothesis that the origin of the chin is due to the spatial compensation for the laryngeal volume; the results of this study are consistent with this hypothesis.

The addition of genioplasty, a surgical vertical or horizontal shift of the chin, to orthognathic surgery of the mandible leads to a more harmonious facial soft tissue morphology and a higher degree of satisfaction with the result than mandibular advancement alone [33]. However, changes in the position of the chin may result in bad joints and unexpected soft tissue changes [34]. This study demonstrates the connection between chin shape and skeletal pattern. When altering the position and morphology of the chin in orthognathic surgery, three-dimensional evaluations of the chin, such as the direction of the slide, the osteotomy line, and the degree of bone grinding, may be helpful in treatment planning.

Among the ethnic groups, chin volume was greater in Koreans than in Egyptians. Based on molecular, biological, and forensic evidence, Egyptians are Caucasian in origin [35]. The mandible is known to vary by ethnicity, with patients belonging to the Asian ethnicity showing larger mandibles than that of the patients belonging to the Caucasian ethnicity [36,37]. The results of this study are consistent with these findings. However, there are differences in the Korean sample size used in this study. It is crucial to take into account the possibility that the sample size had an impact on the outcomes.

Age revealed no significant relationship or a low correlation coefficient with any of the chin-related measurements (Table 5). It is known that the morphology of the mandible, such as the mandibular angle, continues to change throughout life, even after the growth period has passed, because of changes in masticatory pressure [38]. Evangelista et al. have shown that the height of symphysis changes with age even after the growth spurt [39]. However, the results of the present study did not show any effect of age-related changes of the mandible on the chin, which is a small proportion of the mandible.

Chin size was significantly larger in males than in females, showing an association not only with the chin volume but also with the chin width, height, and depth. The chin has long been considered a sexual symbol. It is well known that a cleft chin and a large chin width are symbols of masculinity, while the female chin has a smaller width [10,11,40,41,42], and the results of this study support these findings.

This study had a limitation. Without determining the sample size, this study was carried out as an exploratory study. It is necessary to do more research utilizing a sizable sample after running a power analysis. Furthermore, all participants in this study were patients with malocclusions who underwent orthodontic treatment. Since the evaluation of dentition, occlusal status, or occlusal forces could not be obtained in this study, the evaluation of healthy participants needs to be conducted in future studies.

5. Conclusions

With the exception of age, all other variables, including ethnicity, sex, anteroposterior skeletal pattern, and vertical skeletal pattern, were strongly correlated with chin morphology. For the purposes of orthognathic surgery, evaluations of chin morphological parameters that use both linear analysis and volume analysis may be helpful.