Internal derangement, temporomandibular joint

Discal dislocation and associated signs of internal derangement can be visualized using MRI. In an early stage, there is simple displacement of the disc in the closed mouth position, usually anteriorly, due to weakness of the discal ligaments. When the patient opens the mouth, the disc takes again his normal position relative to the condyle: there is early clicking and the disc is said to be reduced early, due to the remaining elasticity of the bilaminar zone (the retrodiscal elastic ligament). The mouth opening will continue normally (Fig.1). This weakness of the ligaments may become worse; the disc will then glide anteriorly to a lower rest position. Initially, when opening the mouth, the disc will be further displaced anteriorly, but eventually will be recuperated by the remaining elasticity of the bilaminar zone. In such a case there is late reduction of the disc.

In other cases, there will be no reduction of the disc: it will remain stuck in the anterior joint recess: this is called anterior disc displacement without reduction. Such a dislocated disc limits the mouth opening, as the condyle can not pass beneath it ('closed lock') (Fig.2). The joint blocking is initially probably only intermittent. The disc may becomed deformed, taking a biconvex, stretched or thinned appearance instead of the normal biconcave configuration.

The evolution of internal derangement may go on after the stade of irreductible disc luxation. Once the disc is blocked in the anterior joint recess, the condyle continously hurts the junction of the bilaminar zone with the disc, producing microtraumas. Synovial modifications, joint effusions and adherences are the consequence. The bilaminar zone becomes thinner, eventually fissurating and perforating. These perforations cannot be seen directly on MRI, except in the case of complete rupture of the disc, where the anterior and posterior band will be seen to move separately. Nevertheless, the presence of a perforation can be suspected when internal derangement is associated with signs of osteoarthrosis (Fig.3). The presence of a perforation can be demonstrated directly by arthrography, where filling of both joint compartments occurs when only one of them is injected.

Discal displacements are not only seen to the anterior side; discs may also be displaced medially or laterally, and this is usually associated with an anterior dislocation.

Sometimes, in patients with pain and severe reduction of the mouth opening, a normal disc position is found; this exclude the possibility of disc dislocation, and suggests a myogeneous origin of the problem.

It should be noted that MRI has a very high sensitivity (ability to detect abnormal joints) but a lower specificity. This problem is related to frequent finding of anterior disc dislocation in asymptomatic persons with a history of orthodontic intervention, mouth trauma or multiple teeth extractions.

RH