Physiology of the jaw-opening movement
Jaw opening is accomplished through the action of the suprahyoid muscles (rotation) and the lateral pterygoid muscles (translation). In centric condylar position the elastic fibers in the temporomandibular joint are in equilibrium.
Initial phase :
Illustration of the structural loading during the initial opening movement. The condyle makes a rotational movement with a small translational component, changing its position relative to the fossa only slightly. Because of the condylar rotation, the disk moves posteriorly relative to the condyle. The only part of the lateral pterygoid muscle that is active is its lower head (1). The elastic fibers are brought out of equilibrium only minimally.
Intermediate phase :
In this phase the condyle executes a definite translation. The disk moves anteriorly relative to the fossa, but posteriorly in relation to the condyle. Tension becomes steadily increased in the superior stratum of the bilaminarzone and in the lower anterior wall of the joint capsule. The inferior stratum relaxes to the same extent. The venous plexus of the genu vasculosum expands, creating a negative pressure, and fills with blood.
Terminal phase :
Left : In this phase the condyle reaches the maximum extent of its rotation and translation. The translational component passively moves the disk farther forward, while the rotation makes it lie farther posteriorly on the condyle. The superior stratum and the lower anterior capsule wall are now stretched to their maximum. The retrocondylar space is filled by the blood flowing into the genu vasculosum. The inferior stratum is completely relaxed.
Right: Degree of opening in Pos-selt’s envelope of motion.