Structure

The main components are the joint capsule, articular disc, mandibular condyles, articular surface of the temporal bone, temporomandibular ligament, stylomandibular ligament, sphenomandibular ligament, and lateral pterygoid muscle.

Capsuleedit

The articular capsule (capsular ligament) is a thin, loose envelope, attached above to the circumference of the mandibular fossa and the articular tubercle immediately in front; below, to the neck of the condyle of the mandible.

Articular discedit

The unique feature of the temporomandibular joint is the articular disc. The disc is composed of dense fibrocartilagenous tissue that is positioned between the head of the mandibular condyle and the mandibula fossa of the temporal bone. The temporomandibular joints are one of the few synovial joints in the human body with an articular disc, another being the sternoclavicular joint. The disc divides each joint into two compartments, the lower and upper compartments. These two compartments are synovial cavities, which consists of an upper and a lower synovial cavity. The synovial membrane lining the joint capsule produces the synovial fluid that fills these cavities.

The central area of the disc is avascular and lacks innervation, thus getting its nutrients from the surrounding synovial fluid. In contrast, the posterior ligament and the surrounding capsules along has both blood vessels and nerves. Few cells are present, but fibroblasts and white blood cells are among these. The central area is also thinner but of denser consistency than the peripheral region, which is thicker but has a more cushioned consistency. The synovial fluid in the synovial cavities provides the nutrition for the avascular central area of the disc. With age, the entire disc thins and may undergo addition of cartilage in the central part, changes that may lead to impaired movement of the joint. The synovial membrane covers the inner surface of the articular capsule in the TMJ, except for the surface of the articular disc and condylar cartilage.

The lower joint compartment formed by the mandible and the articular disc is involved in rotational movement—this is the initial movement of the jaw when the mouth opens. The upper joint compartment formed by the articular disc and the temporal bone is involved in translational movement—this is the secondary gliding motion of the jaw as it is opened widely. The part of the mandible which mates to the under-surface of the disc is the condyle and the part of the temporal bone which mates to the upper surface of the disk is the articular fossa or glenoid fossa or mandibular fossa.

The articular disc is a fibrous extension of the capsule in between the two bones of the joint. The disc functions as articular surfaces against both the temporal bone and the condyles and divides the joint into two sections, as already described. It is biconcave in structure and attaches to the condyle medially and laterally. The anterior portion of the disc splits in the vertical dimension, coincident with the insertion of the superior head of the lateral pterygoid. The posterior portion also splits in the vertical dimension, and the area between the split continues posteriorly and is referred to as the retrodiscal tissue. Unlike the disc itself, this piece of connective tissue is vascular and innervated, and in some cases of anterior disc displacement, the pain felt during movement of the mandible is due to the condyle compressing this area against the articular surface of the temporal bone.

Ligamentsedit

There are three ligaments associated with the temporomandibular joints: one major and two minor ligaments. These ligaments are important in that they define the border movements, or in other words, the farthest extents of movements, of the mandible. Movements of the mandible made past the extents functionally allowed by the muscular attachments will result in painful stimuli, and thus, movements past these more limited borders are rarely achieved in normal function.

• The major ligament, the temporomandibular ligament, is actually the thickened lateral portion of the capsule, and it has two parts: an outer oblique portion (OOP) and an inner horizontal portion (IHP). The base of this triangular ligament is attached to the zygomatic process of the temporal bone and the articular tubercle; its apex is fixed to the lateral side of the neck of the mandible. This ligament prevents the excessive retraction or moving backward of the mandible, a situation that might lead to problems with the joint.
• The two minor ligaments, the stylomandibular and sphenomandibular ligaments are accessory and are not directly attached to any part of the joint.
  • The stylomandibular ligament separates the infratemporal region (anterior) from the parotid region (posterior), and runs from the styloid process to the angle of the mandible; it separates the parotid and submandibular salivary glands. It also becomes taut when the mandible is protruded.
  • The sphenomandibular ligament runs from the spine of the sphenoid bone to the lingula of mandible. The inferior alveolar nerve descends between the sphenomandibular ligament and the ramus of the mandible to gain access to the mandibular foramen. The sphenomandibular ligament, because of its attachment to the lingula, overlaps the opening of the foramen. It is a vestige of the embryonic lower jaw, Meckel cartilage. The ligament becomes accentuated and taut when the mandible is protruded.

Other ligaments, called "oto-mandibular ligaments", connect the middle ear (malleus) with the temporomandibular joint:

• discomallear (or disco-malleolar) ligament,
• malleomandibular (or malleolar-mandibular) ligament.

Nerve supplyedit

Sensory innervation of the temporomandibular joint is derived from the auriculotemporal and masseteric branches of V₃ or mandibular branch of the trigeminal nerve. These are only sensory innervation. Recall that motor is to the muscles.

The specific mechanics of proprioception in the temporomandibular joint involve four receptors. Ruffini endings function as static mechanoreceptors which position the mandible. Pacinian corpuscles are dynamic mechanoreceptors which accelerate movement during reflexes. Golgi tendon organs function as static mechanoreceptors for protection of ligaments around the temporomandibular joint. Free nerve endings are the pain receptors for protection of the temporomandibular joint itself.

Free nerve endings, many of which act as nociceptors, innervate the bones, ligaments, and muscles of the TMJ. The fibrocartilage that overlays the TMJ condyle is not innervated and is avascular in healthy TMJs. When bone tissue, ligaments, or muscles become inflamed or injured, sensory signals are relayed along small-diameter primary afferent nerve fibers that form the trigeminal nerve. Signals are directed through the trigeminal nerve and modulated by neuronal cell bodies in the trigeminal ganglion. Nociceptive signals are subsequently routed to the spinal trigeminal nucleus, which contains second-order sensory neurons. From the trigeminal nucleus, sensory signals are relayed to higher-order brain regions, including the somatosensory cortex and thalamus.

Blood supplyedit

Its arterial blood supply is provided by branches of the external carotid artery, predominately the superficial temporal branch. Other branches of the external carotid artery, namely the deep auricular artery, anterior tympanic artery, ascending pharyngeal artery, and maxillary artery, may also contribute to the arterial blood supply of the joint.

Developmentedit

Formation of the temporomandibular joints occurs at around 12 weeks in utero when the joint spaces and the articular disc develop. At approximately 10 weeks the component of the fetus future joint becomes evident in the mesenchyme between condylar cartilage of the mandible and the developing temporal bone. Two slits like joint cavities and intervening disk make their appearance in this region by 12 weeks. The mesenchyme around the joint begins to form the fibrous joint capsule. Very little is known about the significance of newly forming muscles in joint formation. The developing superior head of the lateral pterygoid muscle attaches to the anterior portion of the fetal disk. The disk also continues posterior through the petrotympanic fissure and attaches to the malleus of middle ear.

A growth center is located in the head of each mandibular condyle before an individual reaches maturity. This growth center consists of hyaline cartilage underneath the periosteum on the articulating surface of the condyle. This is the last growth center of bone in the body and is multidirectional in its growth capacity, unlike a typical long bone. This area of cartilage within the bone grows in length by appositional growth as the individual grows to maturity. Over time, the cartilage is replaced by bone, using endochondral ossification. This mandibular growth center in the condyle allows the increased length of the mandible needed for the larger permanent teeth, as well as for the larger brain capacity of the adult. This growth of the mandible also influences the overall shape of the face, and thus is charted and referred to during orthodontic therapy. When an individual reaches full maturity, the growth center of bone within the condyle has disappeared.