- vol.10, pp.71-81, 1990
Numerous studies on the neurophysiology and anatomy of the control system of mastication have been done, but most of them are only qualitative, not quantitative and dynamic. The authors believe that the mechanism of mastication can be analyzed quantitatively using a mathematical model and that an artificial mastication system can be developed on the basis of the results. The purpose of this study was to develop a 3 DOF (degrees of freedom) artificial mandible model WJ (Waseda Jaw)-2 and to simulate mandible motion in mastication. We hypothesized that a mandible modeled as a rigid body has 4DOF because the left and right condylar paths are constrained by the kinematic axis. The WJ-2 has 3DOF since the thrust movement along the kinematic axis is so small that it can be negligible. Artificial muscle actuators (AMAs) using DC servo motors drive the WJ-2. The WJ-2 has four kinds of AMAs: the m-colliant. (to open), the m-masseter (to close), the m-pterygoideus lateralis (to move forward), and the m-temporalis post. (to move backward). They are assigned around the jaw with reference to the anatomic data and the authors' computer simulation. We decided that the incisal point tracing shown by Gysi is the target motion of the WJ-2. It consists of 4-phases. Phase 1 is the opening phase, 2 is the turning phase (opening to closing), 3 is the closing phase, and 4 is the occlusal phase. The authors considered that the incisal point tracing can be drawn by fixing the working side condyle, moving the balancing side condyle, and rotating the jaw around the kinematic axis. The authors have determined that there are 2 levels of control systems in mastication. One is the muscle control system of each masseter muscle, which is the lower-level system. The other is the mastication control system, the upper-level system, which supervises and controls the muscle control systems. The muscle control system controls the muscle by position and velocity feedback of the muscle spindle. Feedback gain is variable. The mastication control system creates movement patterns of the mandible, sends it to the muscle control system, and supervises jaw reflexes. The control sequence of the WJ-2 is also divided into 4 phases according to Gysi's theory. The change from the occlusal to the opening is under the control of a normal position pattern. The jaw opening reflex and unloading reflex are used when accidents occur in mastication. And the WJ-2 has the function that the maximum mandible opening position changes adaptively as food is deformed. The authors conducted a mastication experiment with food. As a result, the WJ-2 realized mandible movement that suits Gysi's 4-phase theory and realized mastication movement that matches deformation of food.