Temporomandibular Joint (TMJ)

• The temporomandibular joint (TMJ) is a modified hinge-type synovial joint that allows gliding (translation) and a small degree of rotation (pivoting).
• It also allows for flexion (elevation) and extension (depression) movements typical of hinge joints.
• The bony articular surfaces are the mandibular fossa and articular tubercle of the temporal bone superiorly, and the head of the mandible (condylar process) inferiorly.
• The articular surfaces are covered with fibrous cartilage.
• The joint cavity is divided into upper and lower cavities by a fibrocartilaginous articular disc.

Articular Disk

• The articular disk is composed of dense fibrous connective tissue.
• It is non-vascularized and non-innervated, allowing it to resist pressure.
• Anatomically, the disk has three regions: the anterior band, the central intermediate zone, and the posterior band.
• The intermediate zone is the thinnest and is where function occurs, between the mandibular condyle and the temporal bone.

Retrodiscal Tissue

• The articular disk blends posteriorly with a highly vascular, highly innervated structure called the bilaminar zone.
• The bilaminar zone is involved in the production of synovial fluid.
• The superior aspect of the retrodiskal tissue, or the superior retrodiskal lamina, contains elastic fibers attached to the tympanic plate.
• These elastic fibers act as a restraint to disk movement in extreme translator movements.
• The inferior aspect of the retrodiskal tissue, or the inferior retrodiskal lamina, consists of collagen fibers connected to the posterior margin of the condyle.
• These collagen fibers prevents extreme rotation of the disk on the condyle in rotational movements.

Capsule

• The capsule surrounds the joint and attaches above to the articular tubercle and the margins of the mandibular fossa.
• It attaches below to the neck of the mandible and permits side-to-side motion, protrusion, and retrusion.
• The two bony articular surfaces are separated by fibrocartilage: the articular disc of the TMJ (Meniscus), attached at its periphery to the internal aspect of the fibrous capsule.
• This creates superior and inferior articular cavities, or compartments, lined by superior and inferior synovial membranes.

Synovial Membrane

• The synovial membrane is thin, smooth, and richly innervated vascular tissue (without an epithelium) that lines the capsule.
• Lining the inner aspect of all synovial joints, including the TMJ, are articular cartilage and synovium.
• The space bound by these two structures is the synovial cavity, filled with synovial fluid containing hyaluronic acid.
• Hyaluronic acid is though to be responsible for the fluid's high viscosity.
• The synovium can rapidly and completely regenerate following injury.
• Synovial fluid functions include lubrication of the joint, phagocytosis of particulate debris, and nourishment of the articular cartilage.
• The concentration of hyaluronic acid, and, therefore, the viscosity of the synovial fluid, is greater at the point of load, thus protecting the articular surfaces.

Ligaments

• The lateral temporomandibular ligament strengthens the lateral aspect of the capsule.
• Its fibers run downward and backward from the tubercle (in the root of zygoma) to the lateral surface of the neck of the mandible, limiting the movement of the mandible in a posterior direction.
• It thus protects the external auditory meatus.
• The articular disc divides the joint into upper and lower cavities.
• It is an oval plate of fibrocartilage attached circumferentially to the capsule.
• The disc is attached in front to the tendon of the lateral pterygoid muscle and by fibrous bands to the head of the mandible.
• These bands cause the disc move forward and backward with the head of the mandible during protraction and retraction.
• The upper surface of the disc is concavo-convex from before backward to fit the shape of the articular tubercle and the mandibular fossa.
• The lower surface is concave to fit the head of the mandible.
• The stylomandibular ligament lies behind and medial to the joint.
• It extends from the apex of the styloid process to the angle of the mandible, limiting anterior protrusion of the mandible.
• The sphenomandibular ligament lies on the medial side of the joint.
• It is a thin band that attaches above to the spine of the sphenoid bone and below to the lingula of the mandible.
• This ligament may act as a pivot by providing tension during opening and closing.

Nerve Supply

• The nerve supply to the TMJ is predominantly from branches of the auriculotemporal nerve.
• It also get anterior contributions from the masseteric nerve and the posterior deep temporal nerve.

Vascular Supply

• The vascular supply of the TMJ comes from branches of the superficial temporal and maxillary arteries posteriorly and the masseteric artery anteriorly.
• There is a rich plexus of veins in the posterior aspect of the joint, which fills and empties with protrusive and retrusive movements respectively.
• Veins also function in the production of synovial fluid.

Movements

• TMJ movements are produced chiefly by the muscles of mastication.
• The mandible can be depressed or elevated, protruded or retracted, and rotation can occur as in chewing.
• In the position of rest, the teeth of the upper and lower jaws are slightly apart.
• On closure of the jaws, the teeth come into contact.
• Depression of the mandible is generally produced by gravity.
• The suprahyoid and infrahyoid muscles raise and depress the hyoid bone and larynx.
• They can indirectly help depress the mandible, especially when opening the mouth suddenly, resisting or inverted.
• The platysma can be similarly used.

Important Relations of the Temporomandibular Joint

• Anteriorly: The mandibular notch and the masseteric nerve and artery.
• Posteriorly: The tympanic plate of the external auditory meatus and the glenoid process of the parotid gland
• Laterally: The parotid gland, fascia, and skin.
• Medially: The maxillary artery and vein and the auriculotemporal nerve.

Clinical Notes

• The temporomandibular joint lies immediately in front of the external auditory meatus.
• The lateral temporomandibular ligament prevents the head of the mandible from passing backward and fracturing the tympanic plate when a severe blow falls on the chin.
• The articular disc of the temporomandibular joint may become partially detached from the capsule, which results in a noisy movement and an audible click during movements at the joint.

Dislocation of TMJ

• Excessive contraction of the lateral pterygoids may cause the heads of the mandible to dislocate anteriorly (pass anterior to the articular tubercles).
• The mandible remains depressed, and the person cannot close his or her mouth.
• Posterior dislocation is uncommon, being resisted by the postglenoid tubercle and the strong intrinsic lateral ligament.
• Reduction of the dislocation is achieved by pressing gloved thumbs downward on the lower molar teeth and pushing the jaw backward.
• The downward pressure overcomes the tension of the temporalis and masseter muscles.
• The backward pressure overcomes the spasm of the lateral pterygoid muscles.

Surgery of TMJ

• Because of the close relationship of the facial and auriculotemporal nerves to the TMJ, care must be taken during surgical procedures to preserve both of them.
• Injury to these nerves usually leads to laxity and instability of the TMJ.

Arthritis of TMJ

• The TMJ may become inflamed from degenerative arthritis.
• Abnormal function of the TMJ may result in structural problems such as dental occlusion and joint clicking (crepitus).
• The clicking results from delayed anterior disc movements during mandibular depression and elevation.