Validated numerical modeling of the effects of combined orthodontic and orthognathic surgical treatment on TMJ loads and muscle forces

Investigations of the changes in the mechanics of the craniomandibular system as a result of treatment have been limited by the lack of validated models of this system. The aims of this, project were to (1) validate numerical model predictions of temporomandibular joint (TMJ) eminence morphology and muscle forces produced during molar biting and (2) use the validated models to calculate the changes in TMJ and muscle forces as a consequence of treatment involving orthognathic surgery. Ten volunteers participated; their combined orthodontic and orthognathic surgical treatments were completed. Three-dimensional anatomical data from each subject were used in computer models to predict the sagittal TMJ eminence morphology and joint and muscle forces for each subject, consistent with the neuromuscular objectives of minimizing joint loads and muscle effort. The actual shape of the eminence in each subject was measured with jaw tracking. Surface electromyographic recordings were a measure of the muscle forces involved in static molar biting. Model predictions were compared with measured data from the subjects for eminence shape (R-2 = 0.96) and for muscle activity ratios (R-2 = 0.98). The strength of these relationships validated the models for use in calculating changes in joint loads and muscle forces after treatment. The results suggested that the mechanics of the masticatory system are affected by the combined treatments. The TMJ loads increased in 8 subjects. The average increases in condylar and muscle forces were 4% relative to the applied bite force, but in 1 case the increases were up to 20%. Therefore, although average increases in the forces were small, some persons may experience biologically significant increases in joint and muscle forces as a result of treatment.