The temporomandibular joint (TMJ) makes possible the various movements of the mandible (see figure 2-12). It allows for the up, down, forward, backward, and side to side movements. All movements of the mandible and the functioning of the teeth are closely associated with the TMJ. The structures of the TMJ and some information about how they function follow.
a. Bones. The temporomandibular joint derives its name from the two bones that form the joint, the temporal bone and the mandible. The condyloid process of the mandible and the mandibular fossa of the temporal bone form the joint. The condyle moves (articulates) inside the fossa and makes the movements of the mandible possible. It provides for up and down movements (elevation and depression), forward and backward movements (protrusive and retrusive), and side-to-side movements (lateral and rotational).
b. Articular Disc. The articular disc is a thin, biconcave, oval plate made of fibrous tissue, located between the mandibular fossa and the condyle of the mandible and the articular tubercle (posterior tubercle) of the temporal bone. The disc divides the TMJ into upper and lower cavities, each having synovial membranes which secrete synovial fluid to lubricate the joint. The edges of the disc are attached to the capsular ligament and, in front, it is attached to the lateral pterygoid muscle.
c. Ligaments. A ligament is a tough, fibrous band that connects bones. There are four ligaments that limit the extreme movement of the mandible. These are the capsular ligament, the lateral (temporomandibular) ligament, the sphenomandibular ligament, and the stylomandibular ligament.
d. Movements of the Mandible. The articulating joint of the mandible allows for a combination of movements, such as hinge, lateral, and protrusive movements. Different types of teeth perform various functions (incisors and cuspids for cutting, bicuspids and molars for crushing) The articular disc is situated between the condyle and the fossa to allow for the many different movements and functions required by different teeth. To contain the TMJ and to seal in the synovial fluid, the TMJ is encapsulated by the capsular ligament with a synovial lining. Thus, the TMJ is protected from wear and can absorb minor blows to the jaw.
e. Use of Articulators. Articulators, metal instruments used in the making of dentures, are designed to reproduce the movements of the mandible. The temporomandibular joint and jaw movements vary widely from person to person. Nonadjustable articulators are designed to reproduce the average articulation measurement for most individuals. The vast majority of prosthetic restorations can be accomplished using average jaw movements. For those situations where more precise reproduction of jaw movements is required, adjustable articulators are available.
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