Biomechanical considerations in the surgical correction of mandibular deficiency.

A two-dimensional model is presented that allows calculation of mechanical advantage for the human masseter and temporalis muscles. Ten patients with high-angle mandibular deficiency and vertical maxillary excess had greatly reduced mechanical advantages with an anticipated elevated muscle activity. Superior repositioning of the maxilla with mandibular advancement, however, increased the overall mechanical efficiency of the masticatory system. When compared with mandibular advancement alone, the bimaxillary procedure resulted in a 13% increase in the mechanical advantage of the temporalis muscle and a 21% increase for the masseter muscle. This model suggests that various surgical procedures designed to correct facial disharmonies may have either desirable or undesirable effects on jaw biomechanics and long-term stability. Biomechanical influences are important considerations for the clinician in planning surgical procedures to correct dentofacial deformities.