Articular disc

In which joints is it present?

The articular disc is described in the following joints:

• Temporomandibular joint (TMJ) — the disc divides the cavity into two compartments; movements in them differ.
• Sternoclavicular joint (articulatio sternoclavicularis) — the disc divides the cavity into two chambers, making the joint complex. The joint provides the sole connection between the upper limb and the trunk.
• Acromioclavicular joint (articulatio acromioclavicularis) — an articular disc is sometimes present.

Structure of the articular disc (using the TMJ as an example)

Histologically, the disc is formed by dense fibrous connective tissue that externally resembles cartilage and contains cartilage cells.

The shape of the TMJ disc is a biconcave plate. Its lower surface matches the shape of the mandibular head, while the upper surface matches the shape of the mandibular fossa of the temporal bone. The disc has thickenings—anterior and posterior poles—and is topographically divided into parts:

• Base (root) of the articular disc.
• Thin non-vascularized part.
• Posterior thickened part.

Throughout its extent, the disc is fused with the joint capsule (capsula articularis) and divides its cavity into two non-communicating compartments containing synovial fluid:

• Upper compartment: volume in adults is 1.0–1.5 mL.
• Lower compartment: volume in adults is 0.5–0.8 mL.

Blood supply and nutrition

Blood vessels are present only in the anterior and posterior parts of the articular disc. The thin intermediate zone is non-vascularized. Its nutrition is provided diffusely by tissue fluid and lymph.

Fixation and ligamentous apparatus

The disc is held between the condylar process and the articular surface of the temporal bone by three structures:

• Muscular attachment: the superior head of the lateral (external) pterygoid muscle attaches to the anterior margin of the disc.
• Bilaminar zone (retrodiscal pad): a connective tissue structure connecting the posterior part of the fossa and the posterior part of the articular surface of the mandibular head.
• Intra-articular ligaments of the disc:
• Superior ligaments: run from the upper surface of the disc to the mandibular fossa. They consist of elastic bundles and pull the disc back when it moves anteriorly.
• Inferior ligaments: run from the lower surface of the disc. They are formed by thick, coarse fibrous connective tissue and help prevent anterior displacement of the disc.

Biomechanics of movements

The division of the joint into two compartments by the disc determines the different character of movements within them:

• Rotational movements occur in the lower compartment.
• Translational movements—gliding of the disc together with the head along the slope of the articular tubercle—occur in the upper compartment.

When opening the mouth, the head drags the disc along with it. In the first phase, the disc remains in the mandibular fossa; in the second, it glides forward and downward along the posterior slope of the articular tubercle; and at maximum mouth opening, in the third phase, it rests upon the articular tubercle.

Clinical significance and pathologies

The presence of two isolated compartments ensures high compensatory capabilities of the TMJ: if one compartment is damaged, movement in the other is preserved. Significant restriction of movement occurs only in severe lesions accompanied by destruction of the disc itself.

Main pathologies associated with the disc:

• Articular disc subluxation: anterior displacement with reduction. It is accompanied by a click when opening and closing the mouth. Mandibular movements are preserved in full range, and at maximum opening, the head does not leave the mandibular fossa.
• Recurrent articular disc dislocation: anterior unreduced displacement during a locking episode. The displaced disc acts as a mechanical obstacle blocking the movement of the mandibular head. Locking can last from several minutes to several hours.
• Chronic articular disc dislocation: persistent anteromedial position of the disc and anterior unreduced displacement seen on MRI. Characterized by pain, a sense of mechanical obstruction, joint sounds, restricted mouth opening of less than 2.5 cm, and deviation of the mandible toward the affected joint.
• Chronic posterior articular disc dislocation: an extremely rare pathology in which the disc is displaced posteriorly relative to the condylar process and, upon initiation of movement, becomes trapped between the condylar process and the posterior surface of the articular fossa.
• Secondary osteoarthritis: the final stage of TMJ internal derangements, observed in late stages of locking, after more than 6 months. Characterized by disc deformation and adhesion, intra-articular adhesions, defects of the cartilage coating, deformation of bony structures, and osteophyte formation.

Class and exam questions

What else to study on this topic

• Temporomandibular joint (TMJ)
• Sternoclavicular joint
• Bilaminar zone
• Synovial fluid and membrane

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.