Articular Disc

The articular disc (discus articularis), also known as the joint disc, is made up of tight connective tissue fibers in which fibrocartilage is embedded. It is a curved disc that is thickened at the edges; it is 1 to 1.5 mm thick in the middle and 3 to 4 mm thick at the edges. The articular disc is fused throughout to the joint capsule, so that it divides the joint into two compartments and makes it into a double joint.

There is a capillary gap between the articular disc and the condyle, which is known as the inferior joint space (spatium articulare inferius). As the articular disc rests on the condyle like a cap, rotary movement can take place in the inferior joint space. Sliding movement is permitted by the superior joint space (spatium articulare superius) between the disc and the mandibular fossa. Viewed as a double joint, the superior joint space can be seen as a sliding joint in which the disc is able to slide forward and backward with the condyle on the articular surface sloping forward and downward. In the inferior joint space, which can be seen as a hinge joint, the condyle rotates in the concave surface of the disc.This means that if one part of the joint is unable to function, the functioning of the other part is still guaranteed.

The lateral pterygoid muscle with a few tendinous fibers radiates into the joint capsule and into the disc, which is then pulled forward during forward and lateral movements.

The functions of the articular disc are as follows:

• It divides the joint cavity into two functional compartments, which makes the special functional movement of the mandible possible.
• Compressive forces are cushioned by the disc or distributed to a larger area to protect the mandibular fossa and condyle.
• The separation into a pivot (rotary) joint and a sliding joint means that friction is distributed over twice the area.
• The incongruence is balanced out by the articular disc.

The TMJ permits all movements that enable humans as omnivores to process both plant and animal food. Furthermore, the TMJ enforces a separation of the dentition into a working side and a nonworking side. Lateral jaw movements are necessary to prepare the food, and the bites of food are always processed on one side of the dentition only. This makes it necessary to separate the nonworking side so that there is no interference with gliding movement on this no-load side. This separation is achieved by:

• The slope of the condylar path in two directions in space, namely in the horizontal and vertical planes
• The inclination of the teeth within the occlusal curves
• The particular direction of pull of the muscles, especially of the lateral pterygoid muscle, which enforces Bennett movement
• The articular tubercle, on which the condyle glides during lateral and forward movement

The structure of the human TMJ permits complex movements of the mandible, in contrast to the limited movement possibilities in animal jaws. The jaw joints in pure carnivores will only allow its cylindric joints, which are firmly held by the close-fitting joint fossa, to perform simple hinge movements; these allow very simple vertical opening and closing movements. The pointed, sharp-edged teeth of these animals slide past each other when the mandible is closed, and they interlock closely with each other. Carnivores only need to tear their food or crush bones; they do not require any lateral or forward movement.

In herbivores, the jaw joints have very flat, convex joint cavities, in which flat condyles sloping in a transverse direction come to rest. This joint structure permits flat, lateral grinding movements. The noncanine chewing surfaces are designed for crushing raw, solid plants.

Jaw joints of rodents have narrow, grooveshaped joint cavities in which narrow cylindric joints are set in a sagittal direction; these permit forward and backward movement. The rodents' teeth are very flat and inclined in accordance with the type of movement performed: the maxillary teeth inclined outward and the mandibular teeth inward.