Abstract
During mastication of hard and solid food, the occlusal vertical dimension is raised by the food to be crushed between upper and lower molar teeth on the working side. Hence, the upper and lower molar teeth are widely apart on the balancing side, resulting in such unbalanced occlusion that there is no effective vertical stop on the balancing side. This study examined the mechanism of stabilization of the working-side temporomandibular joint during the mastication of hard and solid food. EMG activities of the MSS and lateral pterygoid muscle (LP) were simultaneously recorded with movements of the condyle as well as the incisor point during masticatory-like jaw movement induced by electrical stimulation of the cortical masticatory area of anesthetized rabbits. Movement of the condyle in the sagittal plane was directly video-recorded with a high-speed CCD camera. The functional role of the MSS and LP in the condyle movement was examined by taking account of the time lag that exists between EMG and actual mechanical effects such as bite force or jaw movement. Abnormal condyle movement on the working side was observed in 4 of 8 rabbits by applying an occlusal bite-raising splint on the upper molar on the working side. It was revealed that the unusual condyle movements appear in the period between the maximum mechanical effects of the MS and the LP and that the interval between these maximum mechanical effects was significantly prolonged. In addition, the working-side MSS activity showed little changes in magnitude, while the muscle activity was markedly increased in the other 4 rabbits that showed no abnormal movement of the condyle even when the splint was applied. These findings suggest that unusual condyle movements are closely related to the muscle activities of the MS and LP.
