Is a “movable hinge axis” used by the human stomatognathic system?

This treatise deals with sagittal in vivo motions of the human mandible. The concept of a “movable hinge axis”, which is commonly used in dentistry, was scrutinised theoretically and empirically. We wondered whether a “movable hinge axis”‐ or better a mandibularly fixed hinge axis (MFHA) ‐ was actually used by subjects with sound temporomandibular joints. To answer this question we first showed that the assumptions of a MFHA would comprise that of the neuromuscular apparatus of the stomatognathic system piloting the mandible by solely two kinematical degrees of freedom (DOF). We spatially recorded in vivo motions of mandibles with high-precision ultrasonic devices. The subjects were asked to guide their mandibles in sagittal movements so that the lower incisal edges ran along the Posselt diagrams. The mathematical procedure is described in detail, hence a possible use of two DOF by a subject could quickly be puzzled out from a set of motions. These analyses revealed that the quasi-plane mandibular movements were approximately piloted by two kinematical DOF in subjects with sound temporomandibular joints. The grade of approximation was measured. Thus, the ensemble of possible positions of the moved body (mandible) can be described by a coordinate system, which is inherent in the stomatognathic system. Lacking precision and poor reproducibility in using only two variables for mandibular position control yield hints that the subject has clinical problems in his stomatognathic system.