Morphologic variations in the craniofacial structures in Japanese adults and their relationship with sex differences

Abstract

The morphology of the human face varies broadly, with genetic and environmental factors determining these variations. Examining variations in the 3-dimensional (3D) craniomandibular morphology and identifying related factors (eg, sex differences) are important in orthodontic clinics. This study observed shape variations in the 3D facial morphology of Japanese adults showing skeletal Class 1 malocclusion and examined the association of sexual dimorphism with shape variations. Sixty cone-beam computed tomography images of Japanese adults (30 males and 30 females) with skeletal Class I malocclusion were employed. In each cone-beam computed tomography image, wire mesh fitting was conducted as previously described. A principal component (PC) analysis after Procrustes registration and the PC clustering method was conducted to observe the shape variations. A PC regression analysis was conducted to determine the sexual morphologic characteristics. Nine PCs depicting 62% of the morphology were determined. Four typical phenotypes were found, mainly related to mandibular protrusion (PC1) and the vertical divergence of the face (PC2). PCs related to sex determination were PC3 (robustness of the mandibular angle in males), PC5 (greater size and shape of the coronoid and mastoid processes in males), and PC7 (greater maxillary width in males), accounting for 16% of total variations. The major shape variations in skeletal Class 1 subjects were related to nonsexual dimorphic characteristics (ie, mandibular protrusion and facial divergence). Sexual dimorphic characteristics were evaluated in detail and accounted for 16% of total morphologic variations.