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