116 Terminal phase
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 Posselt’s envelope of motion.
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. As described previously, the temporal and masseter muscles also insert on the anterior part of the joint capsule. This maintains the basic level of tension necessary for the receptors in the joint capsule to function. It is a fundamental fact that during closing movements the disk executes an anterior movement relative to the condyle. As the condyle is being drawn posteriorly by the muscles, other structures are guiding the disk posteriorly and preventing its anterior displacement at the end of the closing movement. The elastic superior stratum is responsible for moving the articular disk back during the initial phase of closure (Ries 1954, Dauber 1987). In the intermediate phase the disk, because of the convexity of its pars posterior, is carried along passively with the condyle in a posterior direction (Carpentier et al. 1988). During the final closing rotation the taut inferior stratum holds the disk on the condyle (Carpentier et al. 1988, Luder and Bobst 1991).
Jaw-closing movement
117 Initial phase
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 vasoilosum (Finlay 1964, Ward et al. 1990).
118 Intermediate phase
In this phase the upper head further stabilizers 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 nonphysiological 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.
119 Terminal phase
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 Posselt diagram.
Physiology of Movements in the Horizontal Plane
During lateral movements of the mandible, the condyle on the working side moves in laterotrusion and the condyle on the nonworking side in mediotrusion. In the centric condylar position all the structural components of the temporomandibular joint are in equilibrium and are not subjected to any nonphysiological loads. In the ideal situation, the working condyle rotates around a vertical axis during laterotrusion. The condylar position can then be stabilized by either muscles or ligaments. If the laterotrusion is stabilized by the lateral pterygoid muscle, the center of rotation will lie more medial within the condyle. If, on the other hand, stabilization is ligamentary, the center of rotation will lie more in the lateral portion because of the insertion of the lateral ligament. A lateroretrusion of the condyle is possible only if the lateral ligament is overstretched. In this case the posterolateral part of the condyle would compromise the bilaminar zone but the joint surfaces would be relieved of pressure.
During mediotrusion the corresponding condyle moves anteriorly, inferiorly, and medially. This causes loading of the joint surfaces and the capsule and unloading of the bilaminar zone.
120 Centric condylar position
Schematic drawing of the position of a right condyle in relation to the posterior and medial borders of the bony fossa in the horizontal plane. At this level the lower head (1) of the lateral pterygoid muscle inserts on the anteromedial surface of the condyle. In centric condylar position the bilaminar zone and the genu vasculosum (lilac colored) are not overloaded. Transverse movements can amount to 0.9 mm in the working condyle and 0.4 mm in the balancing condyle (Lückerath and Helfgen 1991).
121 Laterotrusion in the horizontal plane
Position of a right condyle relative to the posterior and medial borders of the bony fossa during laterotrusion. Ideally, the condyle would turn around a vertical axis running through the center of the condyle. But if the lateral ligament is overstretched, a lateroretrusion can also occur. In this case the lateral portion of the bilaminar zone and the genu vasculosum would become overloaded (Coffey et al. 1989).
122 Mediotrusion in the horizontal plane
Illustration of the position of a right condyle during a mediotrusive movement. The condyle moves forward, medially, and downward. The entire genu vasculosum is relieved of pressure and the lateral part of the superior stratum becomes stretched more than the medial part. Correspondingly, most of the elastic fibers seen histologically are in its posterolateral region. The result of overloading of the medial portion of the bilaminar zone will always be traumatic rather than functional.
The Teeth and Periodontal Receptors
The periodontium is innervated by both myelinated and unmyelinated nerve fibers. The receptors can be divided into type I (mechanoreceptors) and type II (nociceptors) (Griffin and Harris 1974) and are analogous to the “Ruffini type” (Linden and Millar 1988a, Sato et al. 1992), although other forms also appear (Lambrichts et al. 1992, Fukuda and Tazaki 1994). The cell nuclei of the mechanoreceptors are found either in the trigeminal ganglion or in the mesencephalic nucleus of the trigeminal nerve. Receptor cells with their nuclei in the mesencephalic nucleus (high threshold, rapid adaptation) are located predominantly at the root apices, while receptors with their nuclei in the ganglion (low threshold, slow adaptation) are more numerous around the root, especially in the middle third (Byers and Dong 1989). The afferent nerves are made up of Aβ fibers for mechanoreception and Aδ fibers and C fibers for pain perception (Mengel et al. 1993, Linden et al. 1994).
Approximately 50% of the afferent nerves also react to stimuli acting on adjoining teeth (Trulsson 1993, Tabata et al. 1995). Therefore, there is no neurophysiological difference between canine guidance and group function.