2007 Volume 115 Issue 1 Pages 55-62
Investigations of nonhuman primate mandibles have demonstrated that they are bent, twisted, and sheared during the power stroke of mastication. Inferences have been made regarding potential relationships between local stress patterns and the external morphology of the mandibular symphysis. This study reports the quantitative assessment of cross-sectional bone distribution patterns in the modern human symphysis by use of high-resolution microfocal X-ray computed tomography. Parameters that were examined include (1) bone substance area, (2) the ratio of bone substance to total cross-sectional area, and (3) cortical thicknesses along the perimeters of the symphyseal cross-section. The observed bone distribution was then compared with the hypothetical patterns of mechanical stress during mastication. Results showed that cortical bone was significantly thicker on the lingual than on the labial aspect of the symphysis at all superoinferior levels. The thickest cortical bone was observed on the lingual aspect of the symphysis immediately inferior to the mental spine, and labially at the mental protuberance. Bone area measurements were largest and second largest in the inferolingual and inferolabial quadrants of the symphyseal cross-section. These results show that bone is concentrated particularly at the lower lingual aspect of the symphysis, which is thought to experience high concentrations of tensile stress during mastication. Such a bone distribution pattern contributes to decreasing stress gradients in the mandibular symphysis, and therefore provides some support to the idea that bone distribution of the mandibular symphysis is in part determined by function.