Temporomandibular joint Commonly known as TMJ, is the articulation of the mandible with the opposing surface of the temporal bone and because of the arched shape of mandible tow articulating units are necessary and so the term “ bilateral diarthrosis ” is used. The function of TMJ is the movement of mandible in various planes and direction so the activities involved in ingestion, mastication and speech can be executed.

Type of joint TMJ is a synovial joint, this joint permits significant movement and forms between glenoid fossa, articular eminence (parts of temporal bone) and condyle process (part of mandibular bone) which are united and surrounded by a capsule that creats a joint cavity. This cavityis lined by synovial membrane and filled by synovial fluid and separated by articular disk into upper and lower synovial cavity.

Development of TMJ At 3 months of gestation the first evidence of TMJ begins to form by appearance of tow distinct regions of mesenchymal condensation temporal and condylar blastema between developing ramus of mandible and developing squamus tympanic bone. Ossification begins first in temporal blastema while condylar blastema is still condensed mesenchyme, a cleft appears immediately above it that becomes the inferior joint cavity. The condylar blastema differentiates into (condylar cartilage), and second cleft appears in relation to temporal ossification that becomes the upper joint cavity, with the appearance of this cleft, the primitive articular disk is formed

The condyle: The condyle is mainly cancellous bone so its central mass is comprised of plates, tubules and mostly trabeculae seperated by marrow.The articulating surface is composed of dense fibrous connective tissue or fibrocartilage, beneath it found very thin layer of hyaline cartilage.

Articular fossa and eminence: Articular fossa is composed of compact bone that is continuous with that of the articular eminence. Athin fibrous connective tissue layer covers the fossa and eminence uniformly, except on the posterior aspect of eminence tissue forms much thicker band this is believed to be a product of increased stress because in this area the condyle and articular disk slide across the eminence. The cellular componets of the fibrous bundles are few, consisting mainly of fibroblasts, chondrocytes and mesenchymal cells.

Articular disk: Articular disk is composed of fibrocartilage, as maturation progresses the fibrocartilage is replaced by dense fibrous connective tissue. The cells of the disk are mostly fibroblasts. The principal fibers is collagen; elastic fibers occur only in small amounts.

Synovial membrane and synovial fluid: Synovial membrane lines synovial cavity consists of intimal layer of synoviocytes that overlie subintimal layer which is highly vascular, loose, areolar connective tissue. The deeper connective tissue is densely fibrous with fibrocytes that maintain collagen bundles.

Synoviocytes produce synovial fluid. It is a viscous, transparent, yellowish liquid consisting of a protein-mucopolysaccharide complex. This fluid provides nutrition to disk and articular surfaces.

The capsule and ligaments: The capsule consists of a vascular dense collagenous membrane that seals the joint space and provides passive stability. The lateral aspect of capsule is thickened to form the temporomandibular ligament, this ligament with stylomandibular ligament and sphenomandibular ligament are associated with synovial joint to strengthen the articulation and check excess movement

blood and nerve supply The vascular supply of TMJ is via the superficial temporal and maxillary branches of the external carotid artery. Venous drainage by retromandibular vein. Nerve supply is contributed by branches of auriclotemporal and masseteric nerves of the mandibular division of the trigeminal nerve. Vasomotor and sensory endings are present in the connective tissue of synovial membrane.

Movement of the mandible Three movements characterize TMJ articulation: hinge and slide (opening and closing); retrusive and protrusive (liding back and forth); and lateral (sideward) movements. These mivement are result of the action of the jaw and cervical muscles.

This figure shows the muscles that associated in movement of mandible:

The defects of TMJ 1.Myofascial pain dysfunction (MPD) syndrome, this is related masticatory muscle spasm. 2.Osteoarthritis, this is related to trauma or excessive stress on articular system. 3.TMJ may also affected by congenital and developmental anomalies, traumatic injuries and various forms of benign and malignant neoplasia. TMJ disorders and MPD syndrome, both can cause difficulty in opening the mouth and in eating; and both can lead to clicking or popping sounds in the joint.

Oral histology the temporomandibular joint