STATIC ANALYSIS OF CONTROL MECHANISM OF TEMPOROMANDIBULAR JOINT LOADING : MODIFICATION OF COORDINATED ACTIVITIES OF MASTICATORY MUSCLES BY CHANGING BITE POINT

Masticatory or bite forces applied to the teeth are known to generate compressive forces in the temporomandibular joint (TMJ), which are referred to as "TMJ loading." Normal TMJs are both tolerant and adaptable to a certain amount of loading, but excessive loading is believed to be one of the causes of TMJ dysfunctions. Thus TMJ loading can be considered to be controlled to a certain extent by the stomatognathic system, so as not to exceed a certain limit. In an attempt to clarify that control mechanism during biting, we previously analyzed TMJ loading using a two-dimensional static jaw model and then proved, by computer simulation, that the TMJ loading is minimized when the load vector points in a certain direction. This minimization can be achieved merely by coordinating the activities of the masseter and the temporalis. We then analyzed the modification of such coordinated activities of masticatory muscles by changing the bite point and validated the jaw model by comparing our simulation results with somatometric data. Our jaw model comprises two rigid bodies, the upper and lower jaws, and a spring-element model of the articular disk, including masticatory muscles as follows: 1) the masseter, including the internal pterygoid; 2) the temporalis; and 3) the lateral pterygoid, all of which function dominantly during biting. Bite force is assumed to be applied to a single point on the occlusal plane. Muscle forces applied to our model were determined by referring to morphological and electromyographic data reported previously. First, we carried out experiments in order to obtain masseter and temporalis forces minimizing TMJ loading under various locations of bite point on the occlusal plane. Simulation results indicated that 1) the location of the bite point solely affects the activity of the masseter under the condition that the magnitude and direction of bite force are both constant and TMJ loading is minimized; and 2) the activity balance between the masseter and the temporalis minimizing TMJ loading is sensitive to the direction of bite force. Comparison of our simulation results with EMG data for the masseter and temporalis and biteforce data demonstrated that our results almost coincided with somatometric data for biting at the canine or the premolar, and for biting weakly at the first molar. This strongly suggests that minimization of TMJ loading is actually realized by the stomatognathic system under certain bite conditions