Muscle activity patterns and control of temporomandibular joint loads

The strategy used by the neuromuscular system to distribute reaction forces to the temporomandibular joints on the working and balancing sides has not been fully defined. Theoretical studies and experimental measurements suggest that the two joints are unevenly loaded during unilateral biting or closures for posterior teeth. However, previous electromyographic studies suggest that muscle activity patterns may attempt to balance the distribution of the two joint forces. This study measured bilateral activity in the anterior temporal, posterior temporal, and superficial masseter muscles during isometric bites or closures and chewing at five different positions along the teeth. The resulting ratios of muscle activity on the working/balancing sides were compared with ratios required to maintain equal joint forces. The values of the muscle activity were also used to estimate the ratio of joint forces on the working/balancing side at each tooth position. Results indicate that the muscle activity patterns do not maintain equal joint forces, nor are the muscles responding to joint forces exceeding critical limits. These results suggest that patterns of muscle activity are designed to control the position and magnitude of occlusal forces rather than temporomandibular joint forces. If these same patterns of activity are maintained following repositioning of dental and skeletal elements, adverse temporomandibular joint forces could result.