Physiology of the jaw-closing movement :
The jaw is closed by the temporal, masseter, and medial pterygoid muscles and the upper heads of the lateral pterygoid muscles. The lower head of the lateral pterygoid muscle is inactive during jaw closure.
Schematic representation of the structural loading during the initial jaw-closing movement. The upper head (1) of the lateral pterygoid muscle retards distal movement of the condyle through eccentric muscle activity. The disk can only be passively guided posteriorly. In the initial phase this is brought about by the tension in the elastic superior stratum. A physiological positive pressure arises in the genu vasculo-sum (Finlay 1964, Ward et al. 1990).
In this phase the upper head further stabilizes the condyle on the articular protuberance. Tension in the superior stratum steadily diminishes, and the disk, because of the bulge of its pars posterior, is passively carried farther distally. A nonphysio-logical increase of pressure in the genu vasculosum due to sympathetic or hormonal influences would exert an anteriorly directed force on the disk (Ward et al. 1990). This can lead to increased tension in the inferior stratum and flattening of the disk.
Right: Once the jaws are closed the elastic structures are again relaxed. The inferior stratum becomes increasingly tense and finally prevents anterior disk displacement in case the condyle moves too far distally. Anterior disk displacement can occur only in the presence of an overstretched inferior stratum, with or without flattening of the pars posterior (Eriksson et al. 1992).
Left: Degree of closure in the Pos-selt diagram.