CONTROLLABILITY OF TEMPOROMANDIBULAR JOINT LOAD BY COORDINATED ACTIVITIES OF MASTICATORY MUSCLES : A Static Analysis Using a Two-Dimensional Jaw Model

摘要

Masticatory force and biting force applied to the teeth are shown experimentally to generate compressive force to the temporo-mandibular joint (TMJ). This force is referred to as "temporomandibular joint load". Normal TMJs are both tolerable and adaptable to a certain amount of load, although excessive loads are believed to be a cause of TMJ dysfunctions. Thus, TMJ load can be considered to be controlled to a certain extent by the stomatognathic system, so as not to exceed a certain limit. In order to clarify the amount of load applied to the TMJ, the magnitude and direction of TMJ load have been estimated intensively under various biting conditions, employing a static equilibrium analysis. The controllability of TMJ load, however, has yet to be investigated sufficiently, due to the difficulty of manipulating various model parameters. To simplify this analysis, we employed a two-dimensional jaw model incorporating three masticatory muscles, masseter (assumed to include internal pterygoid), the anterior portion of temporalis, and lateral pterygoid, which function dominantly during biting. We determined the location, the orientation and the magnitude of muscle forces of our model, employing morphological and electro myographic data reported previously. Biting force is assumed to be applied solely to the first molar. We carried out two distinct experiments to obtain TMJ load: 1) under various magnitudes of force on each muscle, and 2) under various directions of TMJ force, while other model parameters are all fixed. Experimental results indicated that the anterior portion of the temporalis can generate biting force without amplifying the magnitude of TMJ load, and additionally can control the direction of TMJ load. This load was clarified, for the first time, to be minimized when it points in a certain direction, under the condition of a fixed biting force. This direction corresponded to that anatomically optimum to support compressive forces, and was independent of the direction of biting force. It was also verified to be reduced by applying biting force somewhat antero-inferiorly. These results suggest that TMJ load can be considered to be controllable by coordinated activities of masticatory muscles.