FUNCTIONAL MOVEMENTS OF PUTATIVE JAW MUSCLE INSERTIONS

Background: The craniomandibular muscles control jaw position and forc es at the teeth and temporomandibular joints, but little is known rega rding their biomechanical behaviour during dynamic function. The objec tive of this study was to determine how jaw muscle insertions alter po sition during different jaw movements in living subjects. Methods: Com puter 3D reconstruction of MR images and jaw-tracking were combined to permit the examination of movement with six degrees of freedom. Maxim um mandibular opening, protrusive and laterotrusive positions were rec orded in four subjects, and the translation and rotation of the putati ve insertions of masseter, temporal, medial, and lateral pterygoid mus cles were measured. Results: The sizes and shapes of regional attachme nts varied markedly among subjects, and their displacement patterns we re different in specific muscles. For instance, when the jaw closed to the dental intercuspal position from maximum gape, the region near th e superior insertion site of the masseter moved backward and upward, w hereas the region near the inferior insertion site displaced mainly fo rward. In three subjects, the jaw's rotational center during this act was similar to 26-34 mm below the mandibular condyles. Conclusions: Si nce the movements of each muscle part differ according to variations i n the size and shape of insertion areas, individual musculoskeletal fo rm, and patterns of jaw motion during function, the prediction of moti on-related muscle mechanics in any one subject is unlikely to be possi ble without direct measurement of the motion of visualized muscle part s. The present study shows that this information can be obtained. (C) 1995 Wiley-Liss, Inc.