A model to describe how ligaments may control symmetrical jaw opening movements in man

A mechanical model of the temporomandibular (TM) joint was converted into a computer program, with a graphic output, which tested the simultaneous effects on symmetrical jaw opening of the following three constraints: 1) the condyle could not move above the articular surface of the temporal bone; 2) the TM ligament and 3) the sphenomandibular (SN) ligament could not be stretched. Cartesian coordinates describing the ligaments and bones of six different skulls were measured and entered into the model. Although the constraints in the model allowed five different types of jaw opening movement, only one of these was physically possible for any given position of the mandible during opening. Each movement changed the geometry of the constraints. The opening movements of the condyle followed those of the hinge and kinematic axes which have been observed in studies of symmetrical jaw opening. Together with the constraint of the articular eminence, the early phase of opening was controlled by the backwardly inclined TM ligament. The late phase of opening was controlled by the forwardly inclined SN ligament. This mechanical explanation for the observed movements of the condyle is consistent with the principle that accessory ligaments have evolved around joints to reduce the need for some neuromuscular controls by replacing them with mechanical controls.