Morphologic and biomechanical correlates with maximum bite forces in orthognathic surgery patients

Purpose: The purpose of this study was to determine which factors of craniofacial morphology best predict maximum bite forces and jaw muscle strength (based on [electromyogram] EMG/force slopes) in patients selected for various orthognathic surgical procedures. These factors were then compared for their ability to separate orthognathic surgery patients by their clinical diagnosis. Patients and Methods: Standard lateral cephalograms were taken of 121 orthognathic surgery patients (before treatment) and 80 control subjects to establish multivariate sagittal and biomechanical factors of craniofacial form. Maximum and submaximal bite forces were recorded at 8 tooth positions for each subject. EMG activity was recorded for 3 pairs of muscles (anterior temporalis, posterior temporalis, and superficial masseter) during the isometric bites. The EMG and bite force measurements were used to calculate EMG/force slopes as a measure of jaw muscle strength. The study looked for significant correlations between the morphologic factors and maximum bite force or jaw muscle strength. Results: Factor analysis determined 12 sagittal and 6 biomechanical factors. However, only 3 of the sagittal and 4 of the biomechanical factors were significantly correlated with maximum bite force or jaw muscle strength. Factors reflecting jaw size were correlated with maximum bite forces and jaw muscle strength but generally did not separate patient groups. The factor most strongly correlated with maximum bite forces separated patients by their relative difference between anterior and posterior facial height. The factor for anteroposterior maxillomandibular discrepancies was not correlated with maximum bite force or jaw muscle strength. Conclusions: Many cephalometric measurements used to diagnose craniofacial deformities and to assign patients to particular orthognathic surgical procedures are not correlated with maximum bite forces or jaw muscle strength. Only measurements reflecting relative differences between anterior and posterior facial height are both strongly correlated with maximum bite force and reflect assignment of surgical procedures. (C) 2000 American Association of Oral and Maxillofacial Surgeons.