Articular Disc – Function and Disorders

What Is the Articular Disc?

The articular disc (Latin: discus articularis) is a fibrocartilaginous structure located between the bony surfaces of certain joints. It is attached to the joint capsule and divides the joint space into two separate compartments. Its primary role is to distribute mechanical forces evenly across the joint surfaces, absorb shock, and improve the congruence (fit) between opposing joint surfaces. As a result, it is a key structural element for joint stability and function.

Location in the Human Body

Not all joints contain an articular disc. It is typically found in joints where the articulating surfaces are not ideally matched in shape, or where particularly high mechanical loads are encountered. The most clinically relevant joints with an articular disc include:

• Temporomandibular joint (TMJ): The disc here enables the complex movements of the lower jaw during chewing and speaking, while protecting the joint from excessive loading.
• Sternoclavicular joint: Located between the breastbone and the collarbone, the disc improves joint fit and absorbs forces transmitted through the shoulder girdle.
• Distal radioulnar joint: In the wrist region, the disc supports force transmission and shock absorption between the radius and ulna.
• Knee joint: The equivalent structures in the knee are called menisci, which are functionally analogous to the articular disc and play a critical role in load distribution and joint stabilization.

Structure and Composition

The articular disc is composed primarily of fibrocartilage, a tissue that combines dense collagen fibers with a proteoglycan-rich ground substance. This composition gives the disc high resistance to both compressive and tensile forces. The disc is typically attached to the fibrous joint capsule and divides the joint cavity into two chambers -- a structure known as a bicameral joint. This arrangement allows different types of movement to occur in each compartment, increasing the functional versatility of the joint.

Functions of the Articular Disc

The key functions of the articular disc can be summarized as follows:

• Pressure distribution: The disc spreads mechanical loads evenly over the joint surfaces, preventing localized overloading and cartilage damage.
• Shock absorption: It absorbs impact forces and protects the subchondral bone (the bone beneath the joint cartilage) from repeated stress.
• Improved joint congruence: By filling gaps between mismatched joint surfaces, the disc enhances the fit and stability of the joint.
• Increased range of motion: The bicameral joint division allows more complex, multi-directional movements than would otherwise be possible.
• Joint stabilization: The disc contributes to mechanical stability, particularly under load-bearing conditions.

Common Disorders and Injuries

Because the articular disc is subject to repeated mechanical stress, it can be damaged by wear, injury, or dysfunction. The most common conditions include:

• Disc displacement (disc dislocation): Particularly frequent in the temporomandibular joint. The disc shifts out of its normal position, causing clicking or popping sounds, pain, and limited jaw opening.
• Disc perforation: A tear or breakthrough of the disc, often resulting from chronic overloading or inflammation.
• Osteoarthritis: Degenerative wear of the disc and adjacent cartilage surfaces, most commonly affecting the jaw or wrist joint.
• Traumatic lesions: Injuries from accidents or sudden mechanical impact can cause partial or complete tears of the disc.

Diagnosis

Several diagnostic approaches are used to evaluate the articular disc:

• Clinical examination: Palpation of the joint, assessment of range of motion, pain provocation tests, and detection of joint sounds (clicking, crepitation).
• Magnetic resonance imaging (MRI): MRI is considered the gold standard for soft tissue evaluation and provides detailed information about the position, shape, and integrity of the articular disc.
• Arthroscopy: A minimally invasive procedure that allows direct visualization of the joint interior and can be used for both diagnosis and treatment.
• Ultrasound: In some joints, such as the temporomandibular joint or shoulder, ultrasound can provide supplementary information about disc position and mobility.

Treatment

Treatment of articular disc disorders depends on the underlying cause, severity of the damage, and the joint involved. Options include:

• Conservative management: Physiotherapy, splint therapy (e.g., occlusal splints for TMJ disorders), physical therapy modalities, anti-inflammatory medications (e.g., NSAIDs), and behavioral modifications to reduce joint loading.
• Minimally invasive procedures: Arthroscopic surgery to reposition the disc or release adhesions within the joint space.
• Surgical treatment: In cases of severe damage, open surgery may be required for disc reconstruction or replacement.

References

1. Standring, S. (ed.) (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd edition. Elsevier, Edinburgh.
2. Okeson, J.P. (2013). Management of Temporomandibular Disorders and Occlusion. 7th edition. Elsevier Mosby, St. Louis.
3. Netter, F.H. (2019). Atlas of Human Anatomy. 7th edition. Elsevier, Philadelphia.