Temporomandibular Joint (TMJ): Anatomy, Movements, and Function

How does the TMJ move?

The temporomandibular joint (TMJ) is a complex synovial joint that facilitates the intricate movements of the jaw, combining both rotational and translational actions to enable essential functions like speaking, chewing, and swallowing.

Anatomy of the Temporomandibular Joint

The TMJ is a bilateral joint, meaning there are two joints, one on each side of the head, that must function synchronously. It connects the mandible (lower jawbone) to the temporal bone of the skull. Key anatomical components include:

• Mandibular Condyle: The rounded end of the mandible that articulates with the temporal bone.
• Mandibular Fossa (Gelnoid Fossa): A depression in the temporal bone where the mandibular condyle rests.
• Articular Eminence: A bony projection anterior to the mandibular fossa, over which the condyle slides during jaw movement.
• Articular Disc (Meniscus): A biconcave, fibrous disc located between the condyle and the fossa, dividing the joint into two compartments: an upper (superior) compartment and a lower (inferior) compartment. This disc is crucial for smooth, frictionless movement and shock absorption.

Types of TMJ Movement

The TMJ is unique in its ability to perform a combination of hinge-like (rotational) and gliding (translational) movements. The primary movements are:

Depression (Opening of the Mouth)

This is the action of lowering the mandible. It involves two distinct phases:

• Initial Rotation: In the early phase of opening (approximately the first 20-25mm), the mandibular condyle primarily rotates within the lower compartment of the joint, around a horizontal axis. This is a pure hinge movement.
• Anterior Translation (Gliding): For wider opening, the condyle and the articular disc simultaneously slide forward and downward along the articular eminence in the upper compartment. This combined rotation and translation allows for the full range of motion required for biting large foods or yawning.

Primary Muscles Involved:

• Suprahyoid Muscles: Digastric, geniohyoid, mylohyoid (pull the mandible down and back).
• Lateral Pterygoid: Primarily involved in the forward translation of the condyle and disc.

Elevation (Closing of the Mouth)

This is the action of raising the mandible. It essentially reverses the movements of depression:

• Posterior Translation (Gliding): The condyle and disc slide backward and upward along the articular eminence.
• Rotation: The condyle then rotates back into the mandibular fossa.

Primary Muscles Involved:

• Masseter: Powerful muscle for biting and chewing, elevates the mandible.
• Temporalis: Elevates the mandible and also retracts it.
• Medial Pterygoid: Elevates the mandible and aids in protrusion.

Protrusion (Forward Movement)

This involves moving the mandible straight forward, bringing the lower incisors anterior to the upper incisors.

• Both mandibular condyles and the articular discs slide forward and downward along the articular eminence.

Primary Muscles Involved:

• Lateral Pterygoid (Bilateral contraction): Main protruder.
• Medial Pterygoid (Bilateral contraction): Assists in protrusion.
• Masseter (Superficial fibers): Assists in protrusion.

Retrusion (Backward Movement)

This involves moving the mandible straight backward from a protruded position.

• Both mandibular condyles and the articular discs slide backward into the mandibular fossa.

Primary Muscles Involved:

• Temporalis (Posterior fibers): Main retruder.
• Digastric: Assists in retrusion.
• Suprahyoid Muscles: Can assist.

Lateral Excursion (Side-to-Side Movement)

This is the grinding or chewing motion, where the jaw moves from side to side. It is a complex, asynchronous movement of the two TMJs:

• Working Side: On the side towards which the jaw moves, the condyle primarily rotates.
• Balancing Side (Non-working Side): On the opposite side, the condyle translates forward, downward, and medially along the articular eminence. This coordinated action allows for the efficient grinding of food.

Primary Muscles Involved:

• Lateral Pterygoid (Unilateral contraction): Contraction of the lateral pterygoid on one side causes the opposite condyle to move forward and medially, resulting in lateral excursion towards the side of the contracting muscle.
• Medial Pterygoid (Unilateral contraction): Assists the lateral pterygoid.
• Temporalis (Unilateral contraction): Can stabilize the working side.

The Role of the Articular Disc

The articular disc is paramount to the TMJ's unique movement capabilities. It acts as a shock absorber and helps to distribute forces across the joint surfaces. Its biconcave shape allows the condyle to smoothly glide along the articular eminence during translational movements, while also providing a stable surface for rotational movements. The disc is firmly attached to the mandibular condyle, ensuring that it moves in coordination with the condyle, particularly during the initial phase of opening.

Clinical Significance

Understanding the intricate biomechanics of TMJ movement is crucial for recognizing and addressing temporomandibular disorders (TMDs). Dysfunction can arise from issues with the muscles, ligaments, or the articular disc itself, leading to pain, clicking, limited range of motion, or locking of the jaw. As fitness educators and kinesiologists, appreciating the normal kinematics of the TMJ allows for a more holistic understanding of head and neck posture, masticatory function, and the potential impact of musculoskeletal imbalances on this vital joint.