Knee Cartilage: Types, Functions, and Importance for Joint Health

What is the function of cartilage in the knee?

Cartilage in the knee serves as a vital component for smooth, pain-free movement, primarily by reducing friction between bones, absorbing shock, and distributing mechanical loads across the joint surfaces.

Introduction to the Knee Joint and Cartilage

The knee is a complex hinge joint, crucial for locomotion, weight-bearing, and athletic performance. It is formed by the articulation of three bones: the femur (thigh bone), tibia (shin bone), and patella (kneecap). To facilitate movement and withstand the significant forces placed upon it, the knee relies heavily on specialized connective tissues, among the most critical being cartilage. Far from a simple cushion, cartilage in the knee performs distinct and indispensable biomechanical roles.

Types of Cartilage in the Knee

The knee joint houses two primary types of cartilage, each with a unique structure and specific functions:

• Articular Cartilage (Hyaline Cartilage): This is a smooth, slippery tissue that covers the ends of the bones within the joint (the femoral condyles, tibial plateau, and the posterior surface of the patella). It is remarkably resilient, with a low coefficient of friction.
• Menisci (Fibrocartilage): These are two C-shaped pads (medial and lateral menisci) located between the femoral condyles and the tibial plateau. Unlike articular cartilage, menisci are tougher and more fibrous, designed to absorb and distribute forces.

Primary Functions of Articular Cartilage

Articular cartilage is fundamental to the knee's ability to move efficiently and without pain. Its key functions include:

• Friction Reduction: Articular cartilage provides an incredibly smooth, low-friction surface that allows the bones to glide effortlessly against each other during movement. This minimizes wear and tear on the joint, preventing damage to the underlying bone. The synovial fluid, a viscous liquid within the joint capsule, works in conjunction with articular cartilage to provide superior lubrication.
• Shock Absorption: While not its primary role, articular cartilage does contribute to absorbing compressive forces. Its elastic properties allow it to deform under load and then return to its original shape, helping to dissipate stress.
• Load Distribution: By covering the ends of the bones, articular cartilage increases the contact area between them. This larger surface area helps to distribute the forces passing through the joint over a wider region, effectively reducing the peak stresses on any single point of the subchondral bone. This protective mechanism is crucial for the long-term health of the joint.

Primary Functions of the Menisci (Fibrocartilage)

The menisci, composed of fibrocartilage, are uniquely adapted to handle the high stresses within the knee. Their functions are distinct from, yet complementary to, articular cartilage:

• Enhanced Shock Absorption: The menisci are the knee's primary shock absorbers. Their wedge shape and pliable nature allow them to absorb up to 50% of the compressive load in extension and up to 85% in 90 degrees of flexion. This cushioning effect protects the articular cartilage and underlying bone from excessive impact forces.
• Superior Load Distribution: By conforming to the shape of both the femur and tibia, the menisci significantly increase the contact area between these bones. This broadens the distribution of forces, reducing the stress per unit area on the articular cartilage and subchondral bone, thereby delaying degenerative changes.
• Joint Stability: The C-shaped menisci act as "spacers" or "wedges" that deepen the articular surface of the tibial plateau, improving the congruence between the rounded femoral condyles and the relatively flat tibial plateau. This enhanced fit contributes significantly to the knee's stability, preventing excessive anterior-posterior and rotational movements.
• Lubrication and Nutrition Assistance: While not directly producing synovial fluid, the menisci facilitate its distribution across the articular surfaces during joint movement, aiding in lubrication and nutrient delivery to the avascular articular cartilage.

The Importance of Cartilage Health

The integrity of both articular cartilage and the menisci is paramount for optimal knee function and longevity. Unlike many other tissues, articular cartilage has a limited capacity for self-repair due to its avascular (no blood supply) and aneural (no nerve supply) nature. Damage to either type of cartilage, whether through acute injury (e.g., meniscal tear) or chronic degeneration (e.g., osteoarthritis), can lead to pain, swelling, reduced range of motion, and a significant decline in joint function. Understanding the critical roles of these cartilaginous structures underscores the importance of proper biomechanics, injury prevention, and appropriate rehabilitation strategies for maintaining knee health throughout the lifespan.

Conclusion

In summary, cartilage in the knee is not a monolithic entity but rather a system of specialized tissues working in concert. Articular cartilage ensures smooth, low-friction movement and broad load distribution, while the menisci provide critical shock absorption, further load distribution, and enhanced joint stability. Together, these cartilaginous structures are indispensable for the knee's ability to withstand immense forces, perform complex movements, and maintain long-term health, enabling us to walk, run, jump, and live active lives.