Chapter 13: The Temporomandibular Joint

STRUCTURE AND FUNCTION

Temporomandibular Joint

Anatomy

The temporomandibular joint (TMJ) is the articulation between the mandible, the articular disc, and the temporal bone of the skull (Fig. 13.1A, B). The disc divides the joint into two distinct parts, which are referred to as the upper and lower joints. The larger upper joint is formed by the convex articular eminence, concave mandibular fossa of the temporal bone, and the superior surface of the disc. The lower joint consists of the convex surface of the mandibular condyle and the concave inferior surface of the disc.^1,2,3 The articular disc helps the convex mandible conform to the convex articular surface of the temporal bone.²

The TMJ capsule is described as being thin and loose above the disc but taut below the disc in the lower joint. Short capsular fibers surround the joint and extend between the mandibular condyle and the articular disc and between the disc and the temporal eminence.³ Longer capsular fibers extend from the temporal bone to the mandible.

The primary ligament associated with the TMJ is the temporomandibular ligament. The stylomandibular and the sphenomandibular ligaments (Fig. 13.2) are considered to be accessory ligaments.^4,5 The muscles associated with the TMJ are the medial and lateral pterygoids, temporalis, masseter, digastric, stylohyoid, mylohyoid, and geniohyoid.

Osteokinematics

The upper joint is an amphiarthrodial gliding joint, and the lower joint is a hinge joint. The TMJ as a whole allows motions in three planes around three axes. All of the motions except mouth closing begin from the resting position of the joint in which the teeth are slightly separated (freeway space).^3,6 The amount of freeway space, which usually varies from 2 mm to 4 mm, allows free anterior, posterior, and lateral movement of the mandible.

The functional motions permitted are mandibular depression (mouth opening), mandibular elevation (mouth closing), protrusion (anterior translation; Fig. 13.3) and retrusion (posterior translation; Fig. 13.4), and right and left lateral excursion or laterotrusion (lateral deviation; Fig. 13.5). Maximal contact of the teeth in mouth closing is called centric occlusion.