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

What type of joint between jaw and skull?

The joint connecting the jaw (mandible) to the skull (temporal bone) is known as the Temporomandibular Joint (TMJ), a highly specialized synovial joint classified as a modified hinge joint or ginglymoarthrodial joint.

Understanding the Temporomandibular Joint (TMJ)

The temporomandibular joint (TMJ) is a bilateral articulation, meaning there are two such joints, one on each side of the head, working in concert. These joints are crucial for a wide range of oral functions, including chewing, speaking, yawning, and swallowing. Its unique structure allows for complex, multi-directional movements essential for these daily activities. The TMJ is formed by the articulation of the mandibular condyle (the rounded end of the lower jawbone) with the mandibular fossa and articular tubercle (parts of the temporal bone of the skull).

Classification of the TMJ

To fully understand the TMJ, it's helpful to categorize it based on both its function and structure:

• Functional Classification: Diarthrosis
  • The TMJ is classified as a diarthrosis, indicating it is a freely movable joint. This contrasts with synarthroses (immovable joints) or amphiarthroses (slightly movable joints). The extensive range of motion at the TMJ is vital for its diverse functions.
• Structural Classification: Synovial Joint
  • Structurally, the TMJ is a synovial joint. This means it possesses key characteristics that facilitate smooth, low-friction movement:
    • Articular Cartilage: Covers the articulating bone surfaces.
    • Joint Capsule: Encloses the joint, forming a joint cavity.
    • Synovial Fluid: Lubricates the joint and nourishes the cartilage.
    • Synovial Membrane: Lines the capsule, producing synovial fluid.
• Specific Type: Modified Hinge Joint (Ginglymoarthrodial Joint)
  • The TMJ is often specifically termed a modified hinge joint or ginglymoarthrodial joint due to its unique combination of movements:
    • Ginglymus (Hinge) Component: Allows for rotational movement, primarily responsible for the opening and closing (depression and elevation) of the mouth, similar to a door hinge. This occurs in the lower joint compartment.
    • Arthrodial (Gliding/Sliding) Component: Permits translational movement, allowing the jaw to slide forward (protrusion), backward (retrusion), and side-to-side (lateral excursion). This occurs predominantly in the upper joint compartment.

Key Anatomical Components of the TMJ

The intricate function of the TMJ relies on the precise interaction of several anatomical structures:

• Articular Surfaces:
  • Mandibular Condyle: The convex, articulating surface of the mandible.
  • Mandibular Fossa (Glenoid Fossa): The concave depression in the temporal bone where the condyle rests.
  • Articular Tubercle (Articular Eminence): A convex projection anterior to the mandibular fossa, over which the condyle slides during jaw opening.
• Articular Disc (Meniscus):
  • A biconcave, oval-shaped fibrous cartilage disc located between the mandibular condyle and the temporal bone.
  • It divides the joint into two separate compartments: an upper (superior) compartment and a lower (inferior) compartment, each with its own synovial fluid.
  • The disc improves the congruence of the joint surfaces, absorbs shock, and facilitates the complex hinge and gliding movements.
• Joint Capsule:
  • A fibrous sac that encloses the entire joint, attaching superiorly to the temporal bone and inferiorly to the neck of the mandibular condyle.
  • It helps to contain the synovial fluid and provides passive stability to the joint.
• Ligaments:
  • Capsular Ligament: Surrounds the joint capsule, strengthening it.
  • Lateral Ligament (Temporomandibular Ligament): The primary ligament supporting the joint laterally. It prevents excessive posterior movement of the condyle and helps to limit the range of opening.
  • Accessory Ligaments: Two additional ligaments provide further support but are not directly part of the joint capsule:
    • Sphenomandibular Ligament: Runs from the sphenoid bone to the medial aspect of the mandibular ramus.
    • Stylomandibular Ligament: Extends from the styloid process of the temporal bone to the angle of the mandible. Both help to limit excessive jaw movements.

Movements Permitted by the TMJ

The modified hinge nature of the TMJ allows for a complex array of movements:

• Depression (Opening the Mouth): Involves both rotation (hinge action) and anterior translation (gliding) of the condyle and disc.
• Elevation (Closing the Mouth): The reverse of depression, involving posterior translation and rotation.
• Protrusion (Moving Jaw Forward): Primarily involves anterior translation (gliding) of both condyles and discs.
• Retrusion (Moving Jaw Backward): Posterior translation (gliding) of the condyles and discs.
• Lateral Excursion (Side-to-Side Movement): Occurs when one condyle glides anteriorly and medially (protrudes and rotates slightly), while the other condyle largely remains in the fossa and rotates. This alternating action allows for grinding movements during chewing.

Clinical Significance and Common Issues

Given its constant use and complex biomechanics, the TMJ is susceptible to various conditions collectively known as Temporomandibular Disorders (TMDs). These can include:

• Muscle pain: Often due to overuse or tension in the masticatory muscles.
• Joint derangement: Problems with the articular disc, such as displacement or perforation.
• Arthritis: Degenerative changes within the joint.
• Trauma: Injuries to the jaw or TMJ.

Understanding the precise anatomical classification and biomechanics of the TMJ is fundamental for healthcare professionals, including physical therapists, dentists, and kinesiologists, to accurately diagnose and treat TMDs, and to provide effective rehabilitation strategies for jaw function.

Conclusion

The joint between the jaw and the skull, the Temporomandibular Joint (TMJ), is a highly specialized and fascinating modified hinge (ginglymoarthrodial) synovial joint. Its unique structure, featuring an articular disc and a combination of rotational and translational movements, enables the complex and essential functions of the jaw. This intricate design underscores its importance in human anatomy and its susceptibility to dysfunction, making its study crucial for anyone interested in musculoskeletal health and biomechanics.