TMJ located between three articulating surfaces (Fig. 7):

-the mandibular fossa of the temporal bone;

-the articular eminence of the temporal bone;

-the condylar process of the mandible.

The articular disc (Fig. 8) is comprised of fibrocartilage and has a biconcave shape with a thin center and thickened periphery. The thickened anterior and posterior parts are called the anterior and posterior bands respectively. The thinner central part is the intermediate zone.

The retrodiscal (bilaminar) zone (Fig. 9)  is located between the posterior band of the TMJ disc and the posterior portion of the TMJ capsule. The 'bilaminar' refers to the two posterior attachments of the disc: 

-superior layer: elastic to allow anterior translation of the disc over the articular eminence;

-inferior layer: inelastic collagen to maintain relationship with the mandibular condyle.

It also contains a small neurovascular bundle.

Temporomandibular joint (TMJ) effusion (Fig. 10, 11) are unusual in asymptomatic patients, and thus should trigger a careful search for underlying pathology. It usually precedes osteoarthritis of the TMJ. Effusions are seen in:

-TMJ dysfunction;

-septic arthritis;

-rheumatoid arthritis (RA).

The disc divides the joint into two compartments (Fig. 10): the superior and inferior compartments (cavities).

Normally, the posterior band of the disc is located above the apex of the condylar head at the 12 o'clock position ±10° (the posterior band of the articular disc should “sit” on top of the condyle). Some authors suggest using 30° as the cutoff to improve specificity (Fig. 12).

In the open mouth position, the condyle moves anteriorly under the centre of the articular eminence, with the articular disc located between them. When a disc is displacement the mouth is closed; on mouth opening, there may be reduction of the disc to a normal position, or the disc becomes distorted without reduction on mouth opening (Fig. 13). Coronal view of TMJ disc position (Fig. 14).

Dynamic images:

-normal disc position (Fig. 15);  

-anterior disc displacement with reduction (Fig. 16);

-anterior disc displacement without reduction (Fig. 17);

Bifid mandibular condyle (duplication of the head of the mandibular condyle) is a rare condition, more frequently unilateral and typically discovered incidentally on imaging (Fig. 18).

A stuck disc (Fig. 19, 20), also called anchored disc phenomenon, refers to a temporomandibular joint disc which does not translate anteriorly out of the mandibular fossa onto the articular eminence but rather remains (thus "stuck") in the fossa. It is a form of TMJ dysfunction.

Decentralization (asymmetrical position) of the mandibular condyle head without spilt (Fig. 21): anterior disc displacement with reduction due to decentralization of the mandibular condyle head in the articular fossa of the temporal bone (arrows).  After three months with split (Fig. 21), the decentralization of the mandibular condyle head is eliminated (arrows) and the normal disc positioning is restored.