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Disc

The articular disc starts out as a biconcave structure composed primarily of dense fibrous connective tissue. The disc generally is divided into thicker posterior and anterior bands with a thinner mid-portion that will take the greatest compressive force from the adjacent osseous surfaces. At the periphery, the disc has some vascular networking. This anatomic arrangement allows the disc to lock onto the underlying condyle during compressive loading. Furthermore, as the condyle translates anteriorly during normal function, the perimeter of the disc can make immediate adjustments in relative thickness by minute changes in vascular shunting. This will allow the disc to remain in a stable relationship over the mandibular condyle while simultaneously changing its relative thickness on the anterior and posterior bands to match the space available for it between the condyle and fossa or between the condyle and eminence. Thus, as the relative space between the bony structures changes, the disc can make appropriate adjustments in its thickness, at least at the perimeter.

The consistency of the disc is such that it is somewhat pliable, yet still firm enough to compress against adjacent articulating structures for participation in normal synovial nutrition. Furthermore, the disc will be dependent on synovial fluid compression for its nourishment in the mid-portion. Because of the relative limitation of the depth that synovial fluid will penetrate, this mid-portion must remain between one and three millimeters in thickness. At the periphery, the disc can receive nutrients both directly and indirectly from the synovial fluid and directly from its own blood supply.

Discal positioning over the superiorly seated condyle should have the posterior band just proximal to the mid-fossa. In this relation, any compressive forces from the mandibular condyle will tend to lock the disc into this posture. Furthermore, the disc must retain this relative position just proximal to mid-fossa on both the medial and lateral poles. As long as this relation is maintained, a superiorly compressed condyle cannot slip posterior to the disc, because of the locking forces generated by the posterior band over the top of the condyle. During forward translation of the mandibular condyle, the disc will rotate more posteriorly in relation to the condyle, and the condyle will slide closer to the anterior band of the disc as these structures reach the level of eminence. At this forward posture, the shunting of blood out of the anterior band of the disc will allow this area to compress, and the shunting of blood flow into the posterior band will cause the area to thicken in order to fill the gap intervening between the posterior slope of the condyle and the proximal slope of the eminence. Finally, the proper positioning of the disc within the articular space will allow the adjacent intrarticular hard and soft tissues to perform their individual physiologic functions, as will be explained subsequently.

 

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