The temporomandibular joint, commonly known as the TMJ, is the intricate connection where your lower jawbone (mandible) meets the temporal bone of your skull. This pair of joints, one on each side of the head, facilitates a wide range of movements necessary for daily life, such as speaking, chewing, and yawning. Understanding its structure helps explain its ability to perform smooth and efficient jaw motion.
Bones of the Jaw Joint
The temporomandibular joint is formed by two bony structures. The first is the condyle, the rounded upper end of the mandible. This condylar process fits precisely into a depression on the skull, allowing for articulation.
The second bony component is the temporal bone, located on the side of the skull. Within the temporal bone, two specific areas contribute to the joint. The mandibular fossa, a shallow, concave depression, serves as the resting place for the mandibular condyle.
Anterior to the mandibular fossa lies the articular tubercle. This raised, convex bone provides a guiding surface for the condyle during jaw movements, particularly when the mouth opens wide. The interplay between the condyle, the mandibular fossa, and the articular tubercle forms the bony framework of this joint.
Supporting Structures
Within the temporomandibular joint, several non-bony structures are important for its function and stability. The articular disc, a fibrocartilage disc, sits between the mandibular condyle and the temporal bone. This disc acts as a shock absorber, distributing forces during chewing and other jaw movements.
The articular disc divides the joint into two separate compartments: an upper compartment above the disc and a lower compartment below it. These compartments facilitate different types of movement. The disc moves in coordination with the mandibular condyle, ensuring smooth gliding and rotational actions.
Encasing the joint is the joint capsule, a fibrous sac that encloses the articular surfaces and the disc. This capsule holds components together. Ligaments reinforce the joint capsule and limit excessive movement.
The temporomandibular ligament, on the lateral side, helps prevent posterior displacement of the condyle. Two other ligaments, the sphenomandibular ligament and the stylomandibular ligament, provide additional support. These ligaments ensure the joint’s stability during its movements.
Muscles Controlling Movement
Four primary muscles, known as the muscles of mastication, control the movements of the temporomandibular joint. Each muscle has specific actions, allowing for precise jaw movements.
Masseter
The masseter muscle, located on the side of the face, elevates the mandible, closing the mouth and clenching teeth.
Temporalis
The temporalis muscle, a large muscle covering much of the side of the skull, also elevates the mandible, contributing to biting force. Its posterior fibers retract the jaw.
Medial Pterygoid
The medial pterygoid muscle is situated internally. It works with the masseter to elevate the mandible and aids in jaw closure. It also contributes to the side-to-side grinding movements of the jaw during chewing.
Lateral Pterygoid
The lateral pterygoid muscle is responsible for depressing the mandible, opening the mouth. It also moves the jaw forward and, when acting on one side, moves the jaw to the opposite side, aiding chewing. The coordinated action of these four muscle pairs allows for the complex movements of the TMJ.
How the Jaw Joint Works
The temporomandibular joint performs two types of movement: rotation and translation. During the initial phase of mouth opening, the mandibular condyle performs a rotational movement within the lower compartment of the joint. This hinge-like action occurs between the condyle and the articular disc.
As the mouth opens wider, a translational movement begins. The mandibular condyle and the articular disc glide forward and downward along the articular tubercle, moving into the upper compartment of the joint. This gliding action allows for full jaw opening, such as during yawning or taking a large bite of food.
When closing the mouth, these movements are reversed. The condyle and disc first glide backward and upward, followed by a rotational movement as the condyle returns to its resting position in the mandibular fossa. The articular disc moves with the condyle to reduce friction.
Side-to-side movements, important for grinding food, involve combined actions. On one side, the lateral pterygoid muscle contracts, causing the condyle and disc to glide forward and medially. The opposing condyle remains stable, rotating within its fossa.
Forward protrusion of the jaw involves bilateral contraction of the lateral pterygoid muscles, while retraction is by the posterior fibers of the temporalis muscle. The coordination of the bony components, the articular disc, and the muscles of mastication enables the TMJ to perform complex movements.