Temporomandibular joint

Embryology

The TMJ develops in membrane as two separate condensations of neural crest mesenchyme between the eighth and twelfth weeks in utero ; a recognizable joint is present at 12 weeks. The articular disc is initially cellular, vascular and rich in elastic fibres. The growth centre, located just inferior to the condylar head, may be active until the third decade. Prolonged growth activity can be seen in the form of condylar hyperplasia and usually manifests in the second to third decade of life. Lack of growth will be evident in the first decade of life, e.g. in hemi­facial microsomia. The growth centre can be damaged by trauma; arrest may also be induced with a high condylar shave procedure following excess growth.

Clinical anatomy

The condylar head is the most proximal aspect of the mandible and forms the lower part of the TMJ. The head and part of the neck (approximately 1 cm) are contained within the capsule. On mouth opening there is an initial hinge-type movement due to rotation of the condylar head against the articular disc in the lower joint space; this relates clinically to mouth opening of approximately 26 mm. The disc–condyle complex is then able to translate anteriorly on to the articular eminence, permitting mouth opening of over 35 mm; this translation is facilitated within the upper joint space, which also allows for protrusion and lateral movements. Limited mouth opening can be due to an extra-articular issue, such as spasm in the muscles of mastication, or an intra-articular problem usually involving the upper joint space or disc. Degenerative changes of the TMJ manifest mainly on the condylar head with the presence of osteophytes, subchondral cysts and abnormal bony sclerosis. Intracapsular fractures of the condylar head can occasionally result in avascular necrosis from disrupted blood supply.

The glenoid fossa (part of the temporal bone) is the thin roof of the TMJ and has a sigmoidal shape ( Ch. 14 ). The articular eminence is anterior and the retro-articular prominence, which is in continuity with the bony external auditory canal (EAC, external acoustic meatus), is posterior. An anterior dislocation results when the condylar head rests superior and anterior to the articular eminence, as is seen in hypermobility of the TMJ.

The joint capsule is formed of fibroelastic tissue that surrounds the condylar head; it is attached to the glenoid fossa superiorly and the articular eminence anteriorly. The lateral ligament is a thickening of the capsule that is attached to the condylar neck up to 1 cm from the joint ( Fig. 10.1 ). The TMJ is innervated by branches from the mandibular division of the trigeminal nerve, mostly through the auriculo­temporal branch, together with branches from the masseteric and deep temporal nerves. Postganglionic sympathetic nerves supply the tissues associated with the capsular ligament and the looser posterior bilaminar extension of the disc. The capsule of the TMJ, lateral ligament and retroarticular tissue contain mechanoreceptors and nociceptors: proprioceptive input from mechanoreceptors helps to control mandibular posture and movement.

The articular disc divides the joint space into a superior (discotemporal) space and an inferior (discomandibular) space, both filled with synovial fluid. It is composed of multidirectional type I fibrocartilage and is smooth, biconcave and avascular. The disc is attached medially and laterally to the condylar head, anteriorly to the lateral pterygoid muscle and posteriorly to the retrodiscal tissues. The latter can be stretched in cases of internal derangement, resulting in a loss of disc mobility and (potentially) surface damage. In sagittal section, the disc has three distinct parts: an anterior band, a thinner intermediate zone and a posterior band. The disc becomes avascular soon after birth and is then incapable of repair.

The synovial membrane lines the inside of the capsule: it does not cover the disc or the articular surfaces. The membrane contains free nerve endings, which may explain why synovitis manifests with pain. Synovial fluid produced by the synovial membrane provides protection and nutrition to the articular surfaces and is the sole supplier of nutrients to the avascular disc; it contains surface-active phospholipids and hyaluronic acid that lubricate the joint. Repeated bruxism can cause extrusion of joint fluid and adhesions of the joint surfaces. Anchored disc phenomena can result from loss of lubrication, leading to an acute loss of joint mobility. Relubrication can be achieved by washing out the inflammatory mediators and hydrodissection with an arthrocentesis procedure. The cells of the synovial membrane can then recover, secreting lubricating fluid that facilitates joint movement.