Force ratio in masticatory muscles after total replacement of the temporomandibular joint.

The temporomandibular (TM) joint is one of the most active joints in the human body, and any defect in this joint has a significant impact on the quality of life. The objective of this study was to analyze changes in the force ratio after TM joint replacement on contralateral TM joint loading. Implantation of an artificial TM joint often requires removal of 3 of the 4 masticatory muscles (activators). In order to perform true loading of the TM joint, loading during mastication was investigated. Input kinematic variables and mastication force were experimentally examined. The inverse dynamics approach and static optimization technique were used for solution of the redundant mechanism. Muscle forces, and reactions in the TM joint were calculated. We modified the model for several different tasks. The m. temporalis and m. masseter were removed individually and together and the forces of mastication on the TM joint were calculated for each variation. To evaluate the results, a parametric numerical FE analysis was created to compare the magnitude of the TM joint loading during the bite process for four different muscle resections. The results show an influence relative to the extent of muscle resection on contralateral TM joint loading in a total TM joint replacement. The biggest increase in the loading magnitude on the contralateral TM joint is most evident after m. masseter and m. temporalis resection. The results from all simulations support our hypothesis that the greater the extent of muscle resection the greater the magnitude of contralateral TM joint overloading.