Modeling rabbit temporomandibular joint torques during a power stroke

Little information exists regarding the effects of changes in mandibular form as a result of orthognathic surgery on torques produced about the temporomandibular joint (TMJ). In this study, we have modeled torques produced about the working side TMJ by selected compartments of the rabbit masseter muscle based on published electromyographic activity. The masseter muscle is composed of multiple subregions or compartments that have unique mechanical actions. In a previous study, forces were elicited by electrical stimulation of each compartment and were recorded by a multiaxis force transducer attached to the anterior mandible. Torques were calculated using mandibular lever arms measured from the center of the TMJ. We have extended this modeling to include variations in mandibular width, length, or height to determine the torques that would be generated with variations in mandibular form. Three superficial masseter compartments on the working side and one posterior deep compartment from the balancing side masseter were examined using data collected from a companion study. It was found that the working and balancing side compartments were synergists for pitch torque components but were antagonists for roll and yaw. In modeling an increase of each mandibular dimension by 20%, nonuniform changes in compartment-generated torques were found. The largest increase was found for the posterior superficial masseter yaw torque component. The effects of changing mandibular form on torques produced about the TMJ may be greater than predicted by previous models that assumed a single line of force produced by each jaw muscle.