Elements of a general theory of joints. 6. General kinematical structure of mandibular movements

Movements of the mandible are recorded in vivo by a measuring system (MT 1602) that takes all 6 degrees of freedom of a rigid body into account. Class-I-patients were asked to move their mandible in the sagittal-vertical plane. The evaluation of the measurements yields an almost plane mandibular movement that only uses 2 degrees of freedom although a general plane movement normally possesses 3 degrees and although the human temporomandibular joint (TMJ) has a certain space of motion. This quantitative reduction of the degrees of freedom by one is produced by a neuro-muscularly guided dimeric link chain that cannot directly be related to anatomical landmarks. The diverse types of mandibular motion of a sound patient differ in the constant ratio of the angular velocities around the 2 axes of the dimeric link chain. Therefore, the paths of the individual mandibular points are epicycloids or hypocycloids. Patients with disorders of the TMJ and the neuromuscular feedback system do no longer show this constancy of the angular velocities' ratio. Besides that, we theoretically derive and empirically prove the fact that common axiographs do not record the "path of the hinge axis" of the TMJ, on principle. In this context we discuss some--in dentistry and anatomy widespread--fundamental misconceptions of the rigid body's kinematics.