Temporomandibular joint

Articular surfaces

The two main bones involved in the formation of the temporomandibular joint are the mandible and the temporal bone. The temporal bone forms the superior part of the joint with two components: mandibular fossa and articular tubercle. The inferior part of the joint is formed mainly by the head of the mandible. The head of the mandible, also known as the mandibular condyle, is the posterior end of the ramus of the mandible. 

Between these two articular surfaces, there is an interposed structure composed entirely of fibrocartilage, known as the articular disc of the temporomandibular joint. This disc attaches to the internal aspects of the fibrous capsule and completely separates the articular surfaces of the TMJ. The disc stabilizes the condyle of the mandible within the joint, reduces frictional forces between the articular surfaces and may aid in lubrication of the joint.

Test your knowledge on the temporomandibular joint in this quiz.

Compartments (cavities)

The articular disc divides the joint into two anatomically and functionally separate compartments: the superior compartment and the inferior compartment. Each compartment is lined by its own corresponding synovial membrane. 

• The superior compartment is bordered superiorly by the mandibular fossa of the temporal bone and inferiorly by the articular disc itself. It is lined by the superior synovial membrane. It contains 1.2 mL of synovial fluid and is responsible for the translational movement of the joint.
• The inferior compartment is bounded superiorly by the articular disc and inferiorly by the condyle of the mandible. It is lined by the inferior synovial membrane. It is slightly smaller with an average synovial fluid volume of 0.9 mL and allows rotational movements of the joint.

Ligaments and joint capsule

The joint capsule originates from the borders of the mandibular fossa, encloses the articular tubercle of temporal bone and inserts at the neck of mandible above the pterygoid fovea, which is a depression on the anterior surface of the neck. The joint capsule is so loose that the mandible can naturally dislocate anteriorly without damaging any fibers of the capsule.

The ligaments involved in the function of the temporomandibular joint can be divided into intrinsic and extrinsic ligaments. 

• The lateral ligament is an intrinsic ligament of the TMJ. This ligament represents the thickened part of the joint capsule and strengthens the joint laterally. Together with the postglenoid tubercle, the intrinsic lateral ligament acts to prevent posterior dislocation of the joint.

• The two extrinsic ligaments of the TMJ are the stylomandibular and sphenomandibular ligaments. The stylomandibular ligament runs from the styloid process to the angle of the mandible, while the sphenomandibular ligament runs from the spine of the sphenoid to the lingula on the medial aspect of the ramus of the mandible. These ligaments do not provide significant support for the joint, however, the sphenomandibular ligament serves as the “swinging hinge” for the movements of the mandible at the TMJs.

Blood supply and innervation

The TMJ is supplied mainly by three arteries. The main supply comes from the deep auricular artery (from the maxillary artery) and the superficial temporal artery (a terminal branch of the external carotid artery). In addition, the joint is supplied by the anterior tympanic artery (also a branch of the maxillary artery). 

The venous drainage of the TMJ is via the superficial temporal vein and the maxillary vein.

The innervation to the temporomandibular joint is by branches from the mandibular division of the trigeminal nerve (CN V3), mostly through the auriculotemporal branch, along with branches from the masseteric and deep temporal nerves. The articular tissues and the dense part of the articular disc have no nerve supply.

Movements

The temporomandibular joint is a hinge type of synovial joint that allows for a wide range of movements to occur. The two compartments of the TMJ have different functions in terms of movements. 

The superior compartment allows translational movements to occur at the joint. Translation (sliding) can be described as a movement in which every point of the moving object simultaneously has the same direction and velocity. An example of translation at the TMJ  occurs when the mandible moves forward causing the teeth, condyles, and rami to all move in the same direction and to the same degree. This movement occurs between the superior surface of the articular disc and the inferior surface of the articular fossa. These movements include the: 

• Protrusion - the anterior movement of the mandible. 
• Retraction - the posterior movement of the mandible. 
• Left and right lateral deviation of the mandible - this usually refers to the acts of grinding and chewing. 

The inferior compartment allows rotational movements. More specifically, rotation occurs between the superior surface of the condyle and the inferior surface of the articular disc. Rotational movement of the mandible can occur in all three planes: horizontal, frontal (vertical), and sagittal. These movements include the:

• Depression - a movement of opening the mouth. This movement is mostly caused by gravity, however, if there is resistance, then muscles facilitate this movement.
• Elevation - a strong movement of closing the mouth. 

In most cases, both rotation and translation occur simultaneously. This results in complex movements that allow people to chew and talk.

Muscles acting on the temporomandibular joint

There are four main muscles of mastication including the temporalis, masseter, lateral pterygoid and medial pterygoid muscles. Mastication is also facilitated  by the infrahyoid and suprahyoid muscles. Each movement in the TMJ activates a certain group of muscles:

• Elevation: Temporalis, masseter and medial pterygoid muscles;
• Depression: Lateral pterygoid, digastric, geniohyoid and mylohyoid muscles;
• Protrusion: Lateral pterygoid and medial pterygoid muscles;
• Retraction: Posterior fibers of temporalis, deep part of masseter, geniohyoid and digastric muscles;
• Lateral deviation: Posterior fibers of temporalis, digastric, mylohyoid and geniohyoid muscles (ipsilateral movement); lateral and medial pterygoid muscles (contralateral movement).