Articular Cartilage: Functions, Composition, and Clinical Relevance

What is the function of the articular cartilage?

Articular cartilage serves as a specialized connective tissue covering the ends of bones within synovial joints, primarily functioning to reduce friction and absorb shock, thereby facilitating smooth, pain-free movement and protecting the underlying bone from damaging forces.

Introduction to Articular Cartilage

Articular cartilage, specifically hyaline cartilage, is a remarkable tissue found at the articulating surfaces of bones within synovial joints, such as the knee, hip, shoulder, and elbow. Unlike most tissues, it lacks blood vessels, nerves, and lymphatic vessels, making its unique composition and properties crucial for its function. Its integrity is paramount for joint health and the ability to move efficiently and without discomfort.

Primary Functions of Articular Cartilage

The specialized structure of articular cartilage enables it to perform several critical functions essential for joint mechanics and longevity:

Shock Absorption and Load Distribution

Articular cartilage acts as a highly effective shock absorber. Its viscoelastic properties allow it to deform under compressive loads, dissipating forces across the joint surface and protecting the underlying subchondral bone from peak stresses. When a joint is loaded (e.g., during walking, running, or lifting), the cartilage compresses, and the fluid within its matrix is momentarily displaced. This hydraulic dampening mechanism helps to distribute the load over a wider area, preventing concentrated stress points that could damage the bone.

Friction Reduction and Smooth Gliding

Perhaps its most celebrated function is to provide an exceptionally low-friction surface, allowing for effortless and smooth movement between bones. The coefficient of friction of healthy articular cartilage is remarkably low, often cited as being lower than that of ice on ice. This incredible slipperiness is attributed to:

• Smooth Surface: The microscopic smoothness of the cartilage surface.
• Boundary Lubrication: The presence of lubricating molecules (e.g., lubricin) adsorbed onto the cartilage surface, preventing direct contact between surfaces.
• Fluid-Film Lubrication: The synovial fluid, a viscous fluid within the joint capsule, forms a thin film between the cartilage surfaces, further reducing friction during movement. This mechanism is particularly effective under dynamic loading conditions.

Composition and Structure of Articular Cartilage

The unique functions of articular cartilage are directly attributable to its specific composition and structural organization:

Chondrocytes

These are the sole cells found within articular cartilage. Chondrocytes are responsible for synthesizing and maintaining the cartilage's extracellular matrix (ECM). They are relatively sparse and metabolically less active compared to cells in other tissues, which contributes to cartilage's limited capacity for self-repair.

Extracellular Matrix (ECM)

The ECM constitutes the bulk of articular cartilage and is primarily composed of:

• Water (65-80%): Water is the largest component by weight, held within the matrix by proteoglycans. Its presence is vital for the cartilage's ability to withstand compressive forces and for fluid-film lubrication.
• Collagen Fibers (10-30%): Primarily Type II collagen, these fibers provide the cartilage with its tensile strength and structural integrity. They are arranged in a complex, arcading pattern that resists tension and helps contain the swelling pressure generated by proteoglycans.
• Proteoglycans (5-10%): These large molecules, particularly aggrecan, are highly negatively charged and attract large amounts of water, giving cartilage its stiffness, resilience, and ability to resist compression. They create a swelling pressure that keeps the collagen network taut.

Avascular, Aneural, and Alyphatic Nature

The absence of blood vessels (avascular), nerves (aneural), and lymphatic vessels (alymphatic) has profound implications for articular cartilage. It means that cartilage relies on diffusion from the synovial fluid for nutrient supply and waste removal. This also explains why cartilage injuries often do not cause immediate pain (no nerve endings) and why cartilage has a very limited capacity for self-repair (no direct blood supply to bring repair cells or nutrients).

Clinical Relevance and Injury

Understanding the function and unique properties of articular cartilage is critical for appreciating the impact of its degeneration or injury.

Osteoarthritis (OA)

This is the most common joint disease, characterized by the progressive breakdown and loss of articular cartilage. As the cartilage erodes, the joint's ability to absorb shock and provide a smooth gliding surface diminishes, leading to increased friction, bone-on-bone rubbing, pain, stiffness, and reduced mobility.

Acute Injuries

Traumatic injuries, such as direct impact or sudden twisting motions, can cause acute damage to articular cartilage, leading to cracks, tears, or delamination. Due to its avascular nature, these injuries often heal poorly or not at all, potentially progressing to early-onset osteoarthritis.

Limited Healing Capacity

The low metabolic activity of chondrocytes and the lack of direct blood supply mean that once damaged, articular cartilage has a very limited capacity for intrinsic repair. Small defects may not heal, and larger defects often fill with fibrocartilage, a mechanically inferior tissue that does not possess the same functional properties as hyaline cartilage.

Maintaining Cartilage Health

While articular cartilage has limited repair capabilities, certain strategies can help preserve its health and function:

• Controlled Movement: Regular, moderate joint movement is crucial. It helps circulate synovial fluid, which is vital for delivering nutrients to the chondrocytes and removing waste products.
• Appropriate Loading: Cartilage requires intermittent, moderate loading to maintain its health. Overloading (e.g., excessive impact, heavy weights without proper form) or underloading (sedentary lifestyle) can both be detrimental.
• Weight Management: Maintaining a healthy body weight significantly reduces the mechanical stress on weight-bearing joints (hips, knees, ankles), thereby slowing down the rate of cartilage wear.
• Balanced Nutrition: A diet rich in anti-inflammatory foods, antioxidants, and adequate hydration can support overall joint health. While specific supplements are often marketed for joint health, their efficacy in regenerating cartilage is not conclusively proven.
• Proper Biomechanics: Correct movement patterns and muscular balance around a joint can optimize load distribution and reduce abnormal stresses on the cartilage.

Conclusion

Articular cartilage is a marvel of biological engineering, providing the essential functions of shock absorption, load distribution, and friction reduction within our joints. Its unique composition and avascular nature, while enabling these critical roles, also render it highly vulnerable to damage and limit its capacity for self-repair. Preserving the health of this vital tissue through mindful movement, appropriate loading, and a healthy lifestyle is fundamental to maintaining lifelong joint function and mobility.