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The temporomandibular joint (TMJ) is a modified synovial joint where the mandible articulates (forms a joint) with the temporal bone. It has three articulating surfaces (the parts of bone that form the joint): the mandibular head (the rounded top of the jaw bone), the mandibular fossa (the hollow in the temporal bone that the head sits in, also called the glenoid fossa) and the articular tubercle (the bony bump just in front of the hollow).
The TMJ is special in that the bony articular surfaces do not actually come into contact with each other. Instead, they are separated by a disc of fibrocartilage (tough, flexible cartilage that absorbs shock). This disc separates the joint into two cavities filled with synovial fluid.
Diagram - Articular surfaces of the temporomandibular joint
Public Domain Source by Henry Gray: Gray's Anatomy (20th edition) (NIH) [Public domain]
The TMJ is stabilised by a fibrous joint capsule and three ligaments:
- Lateral ligament - runs from the articular tubercle to the mandibular neck (the narrow part just below the head). It is a thickening of the joint capsule and helps to prevent the jaw dislocating backwards.
- Sphenomandibular Ligament - runs from the spine of the sphenoid (a small pointed projection from the sphenoid bone at the base of the skull) to the lingula of the mandible (a small tongue-shaped bony projection on the inner surface of the ramus).
- Stylomandibular Ligament - runs from the styloid process (a thin, pointed projection of the temporal bone below the ear) to the angle of the mandible (the corner of the jaw).
The mandible has a horizontal body anteriorly and two vertical rami posteriorly.
The body is U-shaped and has two borders: the alveolar border (superior) and base (inferior). The alveolar border contains the sockets for the lower teeth. The mental protuberance is the bony prominence of the mandible in the anterior midline. Lateral to the mental protuberance, on the outer surface of the body below the second premolar tooth, is the mental foramen (a hole through the bone; here "mental" means relating to the chin). It transmits (lets pass through) the mental nerve and mental artery. The mental nerve is the continuation of the inferior alveolar nerve and supplies the skin of the lower lip and chin.
The body joins to the mandibular rami at the angle of the mandible.
The mandibular rami have a number of bony landmarks:
- Head - articulates at the TMJ.
- Neck - connects the head to the rest of the ramus.
- Coronoid Process - anterosuperior projection (pointing up and forwards) to which the temporalis muscle (the fan-shaped muscle over the temple) attaches. The masseter muscle (the thick muscle of the cheek) attaches to the ramus.
Diagram - Osteology of the mandible
(Note: The mandibular condyle is synonymous with the mandibular head)
Creative commons source by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
The TMJ is capable of a range of movements:
- Elevation - closing the mouth, a very strong action created by contraction of the masseter, temporalis and medial pterygoid muscles (the medial pterygoid is a deep muscle on the inner surface of the ramus).
- Depression - opening the mouth, assisted by gravity, with active contribution from the lateral pterygoid (a deep muscle attached to the neck of the mandible) and the suprahyoid muscles (muscles above the hyoid bone, the small U-shaped bone in the neck), particularly when opening against resistance.
- Protrusion - forward movement of the jaw, created by the lateral pterygoid (assisted by the medial pterygoid).
- Retraction - backward movement of the jaw, created by the posterior fibres of temporalis.
- Lateral Deviation - sideways movement of the jaw, produced by unilateral (one-sided) contraction of the lateral pterygoid, which swings the jaw towards the opposite side.
In the initial opening (depression) of the jaw, the mandibular head rotates against the disc in a hinge-type fashion (in the lower joint cavity), within the mandibular fossa. When opening the jaw more widely, the mandibular head and disc must glide forwards over the articular tubercle (in the upper joint cavity).
The muscles of mastication (the chewing muscles: masseter, temporalis, medial pterygoid and lateral pterygoid) are innervated by the mandibular division of the trigeminal nerve (CN V3, the third division of cranial nerve V).
Diagram - The muscles of mastication
Creative commons source by OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
TMJ dislocation (the head of the mandible slipping out of its fossa) can occur due to a lateral blow to the jaw, yawning or taking a very large bite. The mandibular head dislocates anteriorly over the articular tubercle and becomes locked in position. This prevents the patient from being able to close their mouth. The locking is worsened by a reactive contraction (spasm) of the muscles of mastication. Reduction (putting the joint back in its normal position) requires a specific clinical technique and should be performed by a trained healthcare professional.
TMJ disorder (temporomandibular disorder, or TMD) is a condition affecting how the jaw moves. It refers to pain and/or stiffness of the jaw. Causes include overuse (for example teeth grinding), trauma (such as a blow to the head or face), arthritis (wear and tear of the joint), stress and an uneven bite. Pain is commonly felt around the jaw, ear and temple, and may cause headaches. Patients may also experience mechanical symptoms (problems with how the joint moves) such as clicking, locking and grinding noises.
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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