November 28, 2014
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Temporomandibular joint (TMJ): What is it?

TMJ is the abbreviation used to represent the jaw joint.It stands for temporomandibular joint. TMJ is an anatomical term but is often used to refer to any problem with this joint or the associated jaw muscles. Dentists will generally use the term temporomandibular disorders (TMD) to refer to abnormalities that affect the TMJ or the associated jaw muscles.

The upper part of the mandibular joint is a hollow (the mandibular fossa) formed by the temporal bone of the skull. The lower part is formed by the mandibular condyle (end of the lower jaw), hence the term temporomandibular joint (see Figure 1). The right and left lower joint bones are joined together by the body of the mandible, and are able to rotate and also move in and out of the upper part of the fossa. This makes the mechanics of jaw movement complex. When one joint is not working well the other is often affected.

There are 3 paired and powerful muscles that close the jaw and bring the teeth together for the biting and grinding of food (see Figure 1): the masseter, temporalis, and medial pterygoid muscles. The paired lateral pterygoids protrude the lower jaw and produces jaw opening.

The Temporomandibular Joint

Figure 1

The temporomandibular joint.

Legend

Mandibular fossa - the hollow formed from the temporal bone of the skull where the mandibular condyle (lower joint bone) sits when the mouth is closed.

Mandibular condyle - the lower joint bone that is rounded and moves in and out of the fossa during mouth opening and closing. The right and left condyles are joined together by the mandible (lower jaw).

Articular disc - a firm pad of tissue occupying the space between the upper and lower joint bones. The disc helps to maintain smooth movement and position between the 2 joint bones. Changes in disc position are often the cause of noises occuring in the joint during mouth movements. The disc itself does not have sensation but the surrounding ligaments such as the posterior attachment are sensitive and may become painful due to a disc disorder. The posterior attachment connects the disc to the mandibular fossa.

Temporalis muscle - one of the large jaw-closing muscles that when strained can casue headache in and around the temples.

Masseter muscle - one of the powerful jaw-closing muscles that is attached on the outside of the lower jaw.

Mandible (lower jaw) - ends on both sides of the face to form the mandibular condyle, the lower joint bones.

Lateral pterygoid muscle - when this muscle contracts the condyle is pulled forward and down producing mouth opening.

TMJ anatomy details

A firm pad of tissue (the articular disc) occupies the space between the upper and lower joint bones (see Figure 1). Ligaments attach the disc to the lower bone and the upper fossa. Changes in disc position are common and can cause jaw clicking and locking. A ligament attached to the upper and lower joint bones surrounds the joint parts. Ligaments help to provide stability to the disc and condyle during movements.

 
Bruce Blasberg, DMD

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