Relationships between mandibular symphysis morphology and lower incisor inclination in skeletal class III malocclusion requiring orthognathic surgery.

The purpose of this study is to clarify symphysis morphological characteristics in skeletal class III malocclusion requiring orthognathic surgery and their relationships with symphysis morphology and inclination of the long axis of the lower incisor. The materials were pretreatment lateral cephalometric radiographs from 50 subjects diagnosed as requiring orthognathic surgery. Controls were 30 adults with normal occlusion and well-balanced faces. Detailed cephalometric measurements on the mandibular symphysis (angular, linear and proportional measurements) were recorded and analyzed statistically. In the surgical group, the long axis of the lower incisor demonstrated a significant lingual inclination in relation to the mandibular plane and symphysis. Moreover, because the symphysis demonstrated lingual inclination everywhere except in the basal bone region in relation to the mandibular plane, the curvature between the alveolar and the basal bone region was significantly smaller than that of normal occlusion group. The symphysis labial external surface in the alveolar and basal bone regions demonstrated lingual inclination in relation to the mandibular plane, however, there was no difference in curvature. Small mean values were obtained for Pt.B width, Pog width, and Sym height. Significant differences were observed only in Pt.B width. In both the surgical and normal occlusion groups, a significant correlation was observed between the inclination of the long axis of the lower incisors and symphysis inclination. In the basal bone region, however, there was no significant correlation.