The temporomandibular joint, or TMJ, is the joint that connects the mandible, or jawbone, to the skull’s temporal bone. It is located on the side of the face, just in front of the ear. This bilateral synovial joint facilitates the movements necessary for chewing, speaking, and yawning. The TMJ is one of the most frequently moved joints in the human body, capable of both hinge-like rotation and gliding motions.
Key Bony Components of the TMJ
The primary osseous structures that form the temporomandibular joint are the mandibular condyle and specific parts of the temporal bone. The mandibular condyle is the rounded, elliptical knob at the top of the mandible that serves as the articulating surface. This convex structure fits into a corresponding concave area on the temporal bone known as the mandibular fossa or glenoid fossa. The bony surfaces of the joint are covered with fibrocartilage, a dense, durable tissue, rather than the more common hyaline cartilage found in many other joints.
Anterior to the mandibular fossa is a convex bony prominence called the articular eminence. During the act of opening the mouth widely, the mandibular condyle glides forward out of the fossa and onto this eminence. This translational movement, combined with the initial rotational or hinge movement, allows for the full range of jaw motion. The specific shapes and relationship between the condyle, fossa, and eminence dictate the mechanical limits and possibilities of jaw movement.
The Articular Disc and Joint Capsule
A feature of the temporomandibular joint is the articular disc, a piece of dense fibrocartilage situated between the mandibular condyle and the temporal bone. This disc has a biconcave shape, being thinner in the center and thicker at the edges, and divides the joint into two separate synovial cavities: an upper and a lower joint space. The lower compartment is primarily responsible for the initial rotational (hinge) movement, while the upper facilitates the secondary translational (gliding) motion. Each compartment is lined by a synovial membrane that produces lubricating synovial fluid.
The primary function of the articular disc is to absorb shock, distribute loads evenly across the joint, and facilitate smooth, gliding movements between the bony surfaces. It is attached to the mandibular condyle, moving with it during jaw actions, and also connects to the surrounding joint capsule. The entire TMJ is enclosed by this fibrous joint capsule, which attaches to the temporal bone and the neck of the mandibular condyle. The capsule helps to contain the synovial fluid, provides stability, and contains nerve endings that contribute to proprioception, or the sense of joint position.
Ligaments and Muscles of the TMJ
The stability and movement of the TMJ are governed by ligaments and several muscles of mastication. The main ligament is the temporomandibular ligament, a thickened part of the joint capsule located on the lateral side. It works to prevent the condyle from being displaced too far backward or downward. Two other accessory ligaments, the sphenomandibular and stylomandibular ligaments, provide additional support, acting as passive restraints to limit excessive jaw protrusion.
Movement of the jaw is actively produced by the four primary muscles of mastication. The masseter and temporalis muscles are elevators of the mandible, responsible for closing the jaw. The temporalis also helps in retracting the jaw, pulling it backward. The medial pterygoid muscle assists in elevating the jaw and also works with the lateral pterygoid to produce side-to-side grinding motions.
The lateral pterygoid muscle has a complex role in jaw function. Its superior head attaches to the articular disc and joint capsule, helping to stabilize these structures during movement. The inferior head attaches to the mandibular condyle and is responsible for pulling the condyle and disc forward during jaw opening and protrusion.
Innervation and Vascularization of the TMJ
The sensory information and blood supply to the temporomandibular joint are provided by specific nerves and arteries. The primary nerve supplying the TMJ is the auriculotemporal nerve, a branch of the mandibular nerve that carries most sensory signals like pain and position. Additional innervation is provided by the masseteric and deep temporal nerves.
The blood supply to the TMJ originates from branches of the external carotid artery. The most significant contributions come from the superficial temporal and maxillary arteries, which give rise to smaller arteries that nourish the joint capsule. The central, weight-bearing part of the articular disc itself is avascular, meaning it has no direct blood supply. It receives its nutrients from the surrounding synovial fluid.