Joints Containing an Articular Disc
An articular disc is described in the following joints:
- Temporomandibular joint (TMJ) — the disc divides the cavity into two compartments where different types of movement occur.
- Sternoclavicular joint (articulatio sternoclavicularis) — the disc divides the cavity into two chambers, making it a complex joint. It represents the only true bony attachment between the upper limb and the axial skeleton.
- Acromioclavicular joint (articulatio acromioclavicularis) — an articular disc is sometimes present.
Structure of the Articular Disc (TMJ Example)
Histologically, the disc is composed of dense fibrous connective tissue that resembles cartilage and contains chondrocyte-like cells.
In the TMJ, the disc forms a biconcave plate. Its inferior surface mirrors the shape of the head of the mandible, while its superior surface mirrors the mandibular fossa of the temporal bone. The disc features thickenings—anterior and posterior borders—and is topographically divided into:
- Base (root) of the articular disc.
- Thin avascular intermediate zone.
- Posterior thickened band.
Throughout its periphery, the disc fuses with the joint capsule (capsula articularis), dividing the joint cavity into two non-communicating compartments containing synovial fluid:
- Superior compartment: adult volume of 1.0–1.5 mL.
- Incompatible inferior compartment: adult volume of 0.5–0.8 mL.
Blood Supply and Nutrition
Blood vessels are present only in the anterior and posterior regions of the articular disc. The thin intermediate zone is avascular, receiving its nutrients via diffusion from tissue fluid and lymph.
Fixation and Ligamentous Support
The disc is maintained between the mandibular condyle and the articular surface of the temporal bone via three main structures:
- Muscular attachment: The superior head of the lateral pterygoid muscle inserts into the anterior margin of the disc.
- Bilaminar zone (retrodiscal tissue): A loose connective tissue structure connecting the posterior wall of the fossa to the posterior aspect of the condylar head.
- Intra-articular ligaments of the disc:
- Superior retrodiscal lamina: Extends from the upper surface of the disc to the tympanic plate; contains elastic fibers that help pull the disc back during anterior translation.
- Inferior retrodiscal lamina: Extends from the lower surface of the disc to the posterior margin of the articular surface of the condyle; composed of dense, coarse fibrous connective tissue that prevents excessive anterior displacement of the disc.
Biomechanics of Movement
Dividing the joint into two compartments dictates distinct movements within each space:
- Rotational movements occur in the inferior compartment.
- Translational movements (gliding of the disc along with the condyle down the articular eminence) occur in the superior compartment.
During mouth opening, the condyle carries the disc forward. In the first phase, the disc remains largely within the mandibular fossa; in the second phase, it slides anteriorly and inferiorly down the posterior slope of the articular eminence; at maximum opening in the third phase, it rests near the crest of the articular eminence.
Clinical Significance and Pathology
The presence of two isolated compartments provides high compensatory capacity for the TMJ: if one compartment is impaired, movement in the other may be preserved. Severe movement restriction occurs only during major structural damage involving destruction of the disc itself.
Key pathologies associated with the disc include:
- Disc displacement with reduction: Anterior displacement of the disc that reduces (snaps back into place) upon mouth opening, accompanied by a click. Range of motion is generally preserved.
- Disc displacement without reduction (closed lock): An anterior non-reducing displacement during an episode of locking. The displaced disc acts as a mechanical barrier blocking translation of the mandibular condyle. Locking may last from minutes to hours.
- Chronic disc displacement: Permanent anteromedial disc position and anterior non-reducing displacement visible on MRI. Characterized by pain, a sensation of mechanical obstruction, joint sounds, limited mouth opening (less than 2.5 cm), and deviation of the mandible toward the affected side.
- Chronic posterior disc displacement: An extremely rare condition where the disc is displaced posterior to the condyle and becomes trapped between the condyle and the posterior wall of the fossa upon movement.
- Secondary osteoarthritis: The end-stage of internal TMJ derangements seen in long-standing cases (longer than 6 months of locking). Characterized by disc deformation and adhesion, intra-articular adhesions, cartilage defects, bony remodeling, and osteophyte formation.