Joints with Discs: Intervertebral, TMJ, and Other Key Articular Structures

What joints have discs?

Joints that contain discs are primarily found where significant shock absorption, load distribution, and enhanced joint congruence are required, allowing for smooth, stable, and often complex movements.

Understanding Articular Discs

Articular discs, also known as menisci (though menisci are specifically C-shaped cartilages in the knee, often confused with full discs), are specialized pads of fibrocartilage located within certain synovial joints. Unlike hyaline cartilage, which covers the ends of bones, these discs are distinct structures that serve several critical biomechanical functions:

• Shock Absorption: They cushion the impact between bones, protecting articular cartilage.
• Load Distribution: They spread forces more evenly across the joint surfaces, reducing peak pressures.
• Improved Congruence: They enhance the fit between incongruent joint surfaces, leading to greater stability.
• Facilitating Movement: They can guide movement and, in some cases, divide the joint cavity into two compartments, allowing for different types of motion within a single joint.

While the term "disc" is often colloquially used, anatomically, true articular discs are typically circular or oval, completely or nearly completely dividing the joint space.

The Vertebral Column: Intervertebral Discs

The most well-known and functionally critical discs in the human body are the intervertebral discs. These 23 discs are found between adjacent vertebrae, from the second cervical vertebra (C2) down to the sacrum, forming the cartilaginous joints of the vertebral column.

• Anatomy: Each intervertebral disc consists of two main parts:
  • Annulus Fibrosus: The tough, outer fibrous ring composed of concentric lamellae (layers) of fibrocartilage. It provides structural integrity and contains the inner nucleus.
  • Nucleus Pulposus: The gelatinous, highly elastic inner core, rich in water, which acts like a ball bearing, allowing movement and distributing pressure.
• Function: Intervertebral discs are vital for:
  • Shock absorption: Protecting the spinal cord and brain from impact forces during movement.
  • Flexibility: Allowing for a wide range of motion in the spine (flexion, extension, lateral flexion, rotation).
  • Load bearing: Distributing compressive forces evenly across the vertebral bodies.
• Clinical Relevance: Due to their constant load-bearing role, intervertebral discs are susceptible to degeneration, herniation (slipped disc), and bulging, which can lead to nerve compression and pain.

The Temporomandibular Joint (TMJ)

The temporomandibular joint (TMJ), connecting the mandible (jawbone) to the temporal bone of the skull, contains a crucial articular disc.

• Anatomy: This biconcave (oval-shaped) disc is positioned between the condyle of the mandible and the mandibular fossa of the temporal bone. It divides the joint into two separate synovial cavities.
• Function: The TMJ disc enables the complex movements of the jaw, including:
  • Hinge-like rotation: For opening and closing the mouth.
  • Gliding translation: For protrusion, retrusion, and side-to-side movements. The disc's ability to move with the condyle facilitates smooth, coordinated jaw function, crucial for speaking, chewing, and swallowing.
• Clinical Relevance: Dysfunction of the TMJ disc can lead to pain, clicking, popping, and limited jaw movement, collectively known as TMJ disorders.

The Knee Joint: Menisci (Not True Discs, But Similar Function)

While often colloquially referred to as "discs," the structures in the knee joint are specifically called menisci (plural of meniscus). These are crescent-shaped (C-shaped) pads of fibrocartilage, rather than fully circular discs, but they perform similar vital functions.

• Anatomy: The knee has two menisci: the medial meniscus (larger, C-shaped) and the lateral meniscus (more O-shaped). They sit on the tibial plateau, between the femur and tibia.
• Function: Menisci are essential for:
  • Load transmission: Distributing forces across the knee joint, reducing stress on the articular cartilage.
  • Shock absorption: Cushioning impacts.
  • Stability: Deepening the tibial plateau to better articulate with the femoral condyles, enhancing joint stability.
  • Lubrication and nutrition: Aiding in the distribution of synovial fluid.
• Clinical Relevance: Meniscal tears are common injuries, especially in sports, leading to pain, swelling, and locking of the knee.

The Sternoclavicular Joint

The sternoclavicular (SC) joint, which connects the sternum (breastbone) to the clavicle (collarbone), also possesses an articular disc.

• Anatomy: This small, flat disc is located between the medial end of the clavicle and the manubrium (upper part of the sternum). It completely divides the joint cavity.
• Function: The SC joint disc is crucial for the wide range of motion of the shoulder girdle, acting as a pivot point and shock absorber for forces transmitted from the upper limb to the axial skeleton. It enhances the congruence between the relatively incongruent joint surfaces.

The Distal Radioulnar Joint: Triangular Fibrocartilage Complex (TFCC)

At the wrist, the distal radioulnar joint (DRUJ), where the ulna and radius meet near the hand, features a disc-like structure that is part of the Triangular Fibrocartilage Complex (TFCC).

• Anatomy: The TFCC is a complex of ligaments and fibrocartilage, with its central component being a triangular-shaped articular disc. This disc effectively separates the ulna from the carpal bones.
• Function: The TFCC, including its disc, is vital for:
  • Stabilizing the DRUJ: Especially during pronation (palm down) and supination (palm up) of the forearm.
  • Transmitting axial loads: From the hand to the forearm, particularly through the ulna.
  • Cushioning: Absorbing compressive forces at the wrist.
• Clinical Relevance: Tears or degeneration of the TFCC can lead to chronic wrist pain, instability, and limited range of motion.

The Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint, connecting the acromion of the scapula (shoulder blade) to the lateral end of the clavicle, sometimes contains a small, incomplete articular disc.

• Anatomy: When present, this disc is typically small and wedge-shaped, partially separating the joint surfaces. Its presence can vary between individuals.
• Function: If present, the AC joint disc contributes to improving joint congruence and acting as a minor shock absorber for forces transmitted through the shoulder.
• Clinical Relevance: AC joint injuries (separations) are common, regardless of the presence or absence of a disc.

Why Discs Matter: Functional Implications

The presence of articular discs in these specific joints underscores their critical role in the biomechanics of the human body. They are not merely passive pads but active contributors to joint health, stability, and optimal movement. Understanding their structure and function is fundamental for appreciating how our bodies manage forces, maintain flexibility, and perform complex motor tasks. Their health directly impacts our ability to move freely, without pain, and to engage in physical activity.

Conclusion

In summary, the primary joints in the human body that contain true fibrocartilaginous discs are the vertebral column (intervertebral discs), the temporomandibular joint (TMJ), the sternoclavicular joint, and the distal radioulnar joint (as part of the TFCC). While the knee contains menisci, these are distinct C-shaped cartilages with similar functions, rather than full discs. The occasional presence of a disc in the acromioclavicular joint is also noted. These specialized structures are indispensable for shock absorption, load distribution, and enhancing the congruence and stability of their respective joints, enabling the complex and resilient movements of the human body.