The Temporomandibular Joint (TMJ) is a complex structure connecting the lower jaw (mandible) to the temporal bone of the skull, located directly in front of each ear. The TMJ is unique because it functions bilaterally, meaning the joints on both sides of the head must work together for movement to occur. It is often cited as the most frequently used joint in the body, constantly engaged in activities like speaking, chewing, and swallowing.
The Osseous Components
The temporomandibular joint is formed by the articulation of the mandible and the temporal bone. The lower jaw contributes the Mandibular Condyle, a rounded, oblong bony prominence at the top end of the mandible. This condyle acts like the ball in a ball-and-socket joint, though the TMJ is structurally more complex.
The temporal bone forms the socket component, which is comprised of two primary structures. The Mandibular Fossa is a shallow, concave depression that serves as the resting place for the condyle when the jaw is closed. Anterior to this fossa is the Articular Eminence, a convex, downward-sloping bony ridge.
For the jaw to open widely, the mandibular condyle must slide down and forward along the slope of the Articular Eminence. This sliding movement, called translation, combined with the initial rotating motion, allows for the full range of mouth opening. The distinct shapes of the condyle, fossa, and eminence permit the joint’s dual hinge and glide functionality.
The Articular Disc
Positioned between the mandibular condyle and the temporal bone surfaces is the Articular Disc (meniscus). This disc is composed of dense fibrous connective tissue. Its shape is biconcave, meaning it is thinner in the center and thicker at the edges.
The disc’s primary function is to act as a smooth, non-bony articulating surface and a shock absorber, helping to evenly distribute forces across the joint during chewing and clenching. It effectively divides the temporomandibular joint space into two separate compartments, a superior and an inferior cavity. Each of these cavities contains synovial fluid, which lubricates the joint surfaces for smooth movement.
The inferior cavity, between the disc and the condyle, is where the initial rotational movement occurs when the mouth begins to open. The superior cavity, between the disc and the temporal bone, facilitates the gliding or translational movement as the disc and condyle complex move forward together. The disc is firmly attached to the condyle’s medial and lateral poles by collateral ligaments, ensuring that they move in a coordinated manner. Disruption of this synchronized movement, such as the disc slipping out of place, is often the cause of clicking or popping sounds during jaw function.
Movers and Stabilizers
The complex movements of the TMJ are orchestrated by a group of muscles collectively known as the muscles of mastication. These four primary muscles on each side control the elevation, depression, protrusion, and lateral movements of the mandible. The Masseter muscle, a thick, powerful, rectangular muscle located on the side of the jaw, is one of the main elevators responsible for closing the mouth.
The Temporalis muscle, a large, fan-shaped muscle covering the side of the head, also powerfully elevates the jaw to close it and uses its posterior fibers to retract, or pull the jaw backward. Deep within the skull is the Medial Pterygoid muscle, which works in conjunction with the masseter to elevate the jaw. The lateral movement necessary for grinding food, known as lateral excursion, is achieved by the medial pterygoid on one side working with the lateral pterygoid on the opposite side.
The fourth muscle, the Lateral Pterygoid, is the primary mover for opening the mouth by pulling the condyle and disc forward (protrusion). Jaw opening is also assisted by gravity and other muscles below the jaw, such as the digastric and mylohyoid.
Movement is supported and limited by the Temporomandibular Ligament, a thickening of the joint capsule. This ligament prevents the mandibular condyle from moving too far backward, protecting the delicate structures behind the joint. Other ligaments, like the sphenomandibular and stylomandibular ligaments, provide additional support by restricting excessive movements.