Mandibular joint (Temporomandibular joint)

The bony parts entering into the formation of the mandibular joint are above, the articular eminence and the anterior portion of the articular fossa of the temporal bone; below, the head of the mandible. The articular surfaces are covered with a variety of white fibrocartilage in which the fibers predominate and the cartilage cells are few in number. An articular disc divides the joint into an upper and a lower cavity. The ligaments of the joint are the following; capsular, sphenomandibular, temporomandibular, stylomandibular.

The capsular ligament is a thin, loose envelope, attached, above, to the circumference of the articular fossa and the articular eminence; below, to the neck of the mandible. The synovial membrane of the joint lines the capsular ligament and is continued over the upper and lower surfaces of the articular disc.

The temporomandibular ligament (fig. 508) consists of two short, narrow fasciculi, one in front of the other, attached, above, to the lateral surface of the zygomatic process of the temporal bone and to the tubercle on its root; below, to the lateral surface and posterior border of the neck of the mandible. It is broader above than below, and its fibers are directed obliquely downwards and backwards. The ligament is covered superficially by the parotid gland.

Figure 508

The sphenomandibular ligament (fig. 509) is a flat, thin band, which is attached above to the spine of the sphenoid bone, and, becoming broader as it descends, is fixed to the lingula, of the mandibular foramen. Its lateral surface is in relation, above, with the lateral pterygoid muscle and the auriculotemporal nerve; lower down, it is separated from the neck of the mandible by the maxillary (internal maxillary) vessels; still lower, the inferior dental (inferior alveolar) vessels and nerve and a lobule of the parotid gland lie between it and the ramus of the mandible. Its medial surface is in relation with the medial pterygoid muscle. The upper attachment of the ligament is a secondary attachment, and some, of its fibers can be traced through the medial end of the petrotympanic fissure to the primary attachment, viz. the anterior process of the malleus. It represents a portion of the cephalic extremity of Meckel's cartilage.

Figure 509

The articular disc (fig. 510) is a thin, oval plate consisting mainly of fibrous tissue; it is placed between the condyle of the mandible and the articular fossa, and divides the joint into two cavities. Its upper surface is concavoconvex from before backwards, to accommodate itself to the form of the articular fossa and the articular eminence. Its under surface, in contact with the head of the mandible, is concave. Its circumference is connected to the capsular ligament, and in front to the tendon of the lateral pterygoid muscle. It is usually thickest a little behind its center, where it occupies the deepest part of the articular fossa.

The stylomandibular ligament (fig. 509) is a specialized band of the deep cervical fascia, which stretches from the apical part of the styloid process of the temporal bone to the angle and posterior border of the ramus of the mandible between the masseter and medial pterygoid muscles. It separates the parotid from the submandibular (submaxillary) gland, and from its deep surface some fibers of the styloglossus take origin. Although classed among the ligaments of the mandibular joint, it can only be considered as accessory to it. The nerves of the mandibular joint are derived from the auriculotemporal and masseteric branches of the mandibular nerve, the arteries from the superficial temporal branch of the external carotid artery, and from the maxillary artery (internal maxillary artery).

Figure 510

Movements.-The mandible may be depressed and elevated, or carried forwards and backwards; a slight amount of side-to-side movement is also permitted. When the mouth is opened the body of the mandible is depressed, and the head and articular disc are pulled forwards and downwards on to the articular eminence on each side; on closure of the mouth the reverse action takes place. When the mandible is carried horizontally forwards, as in protruding the lower incisor teeth in front of the upper, the disc and the head of the mandible glide forwards and downwards on the articular fossa and articular eminence on each side. The grinding or chewing movement is produced by the head, with its disc, gliding alternately forwards and backwards, on one side, while on the other they move simultaneously in the opposite directions -, at the same time the head undergoes a vertical rotation on the disc. On one side the heard advances and rotates, while on the other it recedes and rotates.

Muscles producing the movements;

Depression.- Digastric, Mylohyoid, Geniohyoid and Lateral Pterygoid (of both sides).
Elevation.- Masseter, Temporal and Medial Pterygoid (of both sides).
Protrusion.- Medial and Lateral Pterygoid (of both sides).
Retraction.- Temporal (posterior fibers-both sides).
Lateral movement.- Medial and Lateral Pterygoid (of one side).

Applied Anatomy.-The mandible can be dislocated in one direction only-viz. forwards. When the mouth is open, the head of the mandible is situated on the articular eminence, and any sudden violence, or even a sudden muscular spasm, as during a convulsive yawn, may displace it forwards into the infratemporal fossa. The displacement may be unilateral or bilateral. Reduction is accomplished by depressing the jaw with the thumbs placed on the last molar teeth, and at the same time elevating the chin. The downward pressure overcomes the spasm of the masseter, temporal, and pterygoid muscles, and elevation of the chin throws the head of the mandible backwards; the above-mentioned muscles then draw the head back into its normal position.

In close relation to the head of the mandible are the external auditory meatus and the tympanic cavity; any force, therefore, applied to the bone is liable to be attended with damage to these parts, or inflammation in the joint may extend to them. On the other hand inflammation of the tympanic cavity may involve the articulation and cause its destruction, thus leading to ankylosis of the joint.