Influence of the dynamical properties of the human masticatory muscles on jaw closing movements.

A dynamic six degrees-of-freedom mathematical model of the human masticatory system has been developed in order to study the contribution of the different masticatory muscles and their dynamical properties to the closing movement of the jaw. Muscles were included as forces acting according to their lines of action and the temporomandibular joints were modelled by linear elastic surfaces. Ligaments were not included. The geometry of the model was derived from a human cadaver. With this model symmetrical jaw closing movements were simulated. It was found that the normally observed jaw closing movement which includes a swing-slide movement of the condyle along the articular eminence, can be generated by various separate pairs of masticatory muscles. Among the masticatory muscles the different parts of the masseter as well as the medial pterygoid muscle appeared to be the most suitable to complete this action. The dynamical muscle properties appeared to provide a mechanism causing the movements to pass off more smoothly. The force/length relationship of muscle fibres, which also introduced a limit for protrusive excursions of the lower jaw, turned out to play a predominant role in this mechanism.