BIOMECHANICAL ANALYSIS OF JAW-CLOSING MOVEMENTS

This study concerns the complex interaction between active muscle forc es and passive guiding structures during jaw-closing movements. It is generally accepted that the Ligaments of the joint play a major role i n condylar guidance during these movements. While these ligaments perm it a wide range of motions, it was assumed that they are not primarily involved in force transmission in the joints. Therefore, it was hypot hesized that muscle forces and movement constraints caused by the arti cular surfaces imply a necessary and sufficient condition to generate ordinary jaw-closing movements. This hypothesis was tested by biomecha nical analysis. A dynamic six-degrees-of-freedom mathematical model of the human masticatory system has been developed for qualitative analy sis of the contributions of the different masticatory muscles to jaw-c losing movement. In simulated symmetrical jaw-closing movements, it wa s found that the normally observed movement, which includes a swing-sl ide condylar movement along the articular eminence, can be generated b y various separate pairs of masticatory muscles, among which the diffe rent parts of the masseter as well as the medial pterygoid muscle appe ared to be the most suitable to complete this action. The results seem to be in contrast to the general opinion that a muscle with a forward -directed force component may not be suitable for generating jaw movem ents in which the condyle moves backward. The results can be explained , however, by biomechanical analysis which includes not only muscle an d joint forces as used in standard textbooks of anatomy, but also the torques generated by these forces.