Knee Cartilage: Functions, Types, Properties, and Maintaining Health

What is the Function of the Cartilage in the Knee Joint?

The cartilage within the knee joint serves crucial functions: primarily reducing friction between articulating bones, absorbing shock from impacts, and distributing mechanical loads across the joint surfaces, all of which facilitate smooth, stable, and pain-free movement.

An Overview of the Knee Joint and Its Cartilage

The knee is one of the body's largest and most complex joints, primarily functioning as a hinge joint while also allowing for some rotational movement. It connects the femur (thigh bone) to the tibia (shin bone) and includes the patella (kneecap). To enable the remarkable range of motion and withstand the immense forces it endures daily, the knee relies heavily on specialized cartilaginous tissues: articular (hyaline) cartilage and the menisci (fibrocartilage). While both are types of cartilage, their distinct structures and locations equip them for unique, yet complementary, roles in joint mechanics.

The Primary Functions of Articular (Hyaline) Cartilage

Articular cartilage is a smooth, glistening, white tissue covering the ends of the femur, tibia, and the posterior surface of the patella where they meet within the joint. Its primary functions are:

• Friction Reduction: Articular cartilage boasts one of the lowest coefficients of friction of any biological material, significantly lower than ice on ice. This incredibly smooth surface allows the bones to glide effortlessly over each other during movement, minimizing wear and tear and preventing heat buildup. Without this, even simple movements would cause significant pain and damage.
• Shock Absorption: Composed of a matrix rich in water, collagen, and proteoglycans, articular cartilage acts as a visco-elastic cushion. When the joint is loaded (e.g., during walking, running, or jumping), the cartilage deforms, compressing and expelling water. Upon load removal, it reabsorbs water and returns to its original shape, effectively dampening impact forces and protecting the underlying subchondral bone from excessive stress.
• Load Distribution: Instead of concentrating forces on small points, articular cartilage spreads the compressive loads over a wider surface area of the bone. This distribution is critical in preventing localized stress concentrations that could otherwise lead to bone damage or fracture.
• Joint Lubrication: While synovial fluid is the primary lubricant, the porous nature of articular cartilage contributes to the lubrication process. As the joint moves and loads shift, fluid is squeezed out and reabsorbed, creating a self-lubricating mechanism that further enhances the smooth gliding of joint surfaces.

The Specialized Role of the Menisci (Fibrocartilage)

Unlike the articular cartilage that covers bone ends, the menisci are two C-shaped wedges of fibrocartilage located between the femoral condyles and the tibial plateau. The medial meniscus is on the inner side of the knee, and the lateral meniscus is on the outer side. Their unique structure and placement contribute to several vital functions:

• Deepening the Tibial Socket and Enhancing Congruity: The tibial plateau is relatively flat, while the femoral condyles are rounded. The menisci effectively deepen the shallow tibial surface, creating a more congruent fit with the femoral condyles. This improves the contact area between the bones, enhancing joint stability and mechanical efficiency.
• Secondary Shock Absorption: While articular cartilage handles the primary shock absorption, the menisci provide an additional layer of cushioning, particularly during high-impact activities and rotational movements. They can absorb up to 20% of the compressive load on the knee.
• Load Transmission: The menisci play a critical role in transmitting axial loads across the knee joint. They effectively increase the contact area between the femur and tibia by up to three times, significantly reducing stress on the articular cartilage and subchondral bone. This function is particularly crucial in the weight-bearing positions.
• Joint Stability: By improving the congruity between the bones and acting as dynamic stabilizers, the menisci contribute to the overall stability of the knee joint, preventing excessive anterior-posterior and rotational movements.
• Proprioception: The menisci contain nerve endings that provide sensory feedback to the brain regarding knee position and movement. This proprioceptive input is vital for coordinated movement and balance.

The Unique Properties of Knee Cartilage

The effectiveness of knee cartilage stems from its unique biological properties:

• Avascularity: Cartilage does not have a direct blood supply. It receives nutrients primarily through diffusion from the synovial fluid, which limits its ability to repair itself after injury.
• Aneurality: It lacks nerve endings, meaning cartilage itself does not feel pain. Pain associated with cartilage damage usually comes from inflammation of the surrounding joint structures or exposed subchondral bone.
• Almyphaticity: It also lacks lymphatic vessels, which further contributes to its limited healing capacity.
• Composition: Both types of cartilage are composed of specialized cells called chondrocytes embedded in an extracellular matrix. For articular cartilage, this matrix is rich in Type II collagen and proteoglycans (which attract water), giving it its resilient, shock-absorbing qualities. Menisci contain Type I collagen, making them tougher and more fibrous.

Consequences of Cartilage Damage

Given its critical functions and limited capacity for self-repair, damage to knee cartilage (e.g., tears to the menisci or erosion of articular cartilage) can have severe consequences:

• Pain and Swelling: Resulting from increased friction, inflammation, and potential exposure of underlying bone.
• Reduced Mobility: Impaired smooth gliding surfaces lead to stiffness and a decreased range of motion.
• Joint Instability: Particularly with meniscal tears, the joint may "give way" or lock.
• Accelerated Osteoarthritis: Cartilage damage significantly increases the risk of developing osteoarthritis, a progressive degenerative joint disease characterized by further cartilage loss, bone spur formation, and chronic pain. Without the protective layer of cartilage, bone-on-bone friction occurs, leading to irreversible joint destruction.

Maintaining Cartilage Health

While cartilage has limited healing potential, proactive measures can help preserve its health:

• Balanced Exercise: Regular, moderate-impact activities promote nutrient exchange within the cartilage. Avoid excessive, repetitive high-impact activities or prolonged inactivity.
• Proper Nutrition: A diet rich in anti-inflammatory foods, vitamins, and minerals supports overall joint health.
• Weight Management: Reducing excess body weight significantly decreases the load on the knee joints, slowing down wear and tear.
• Prompt Injury Treatment: Addressing knee injuries promptly can prevent further damage to cartilage and other joint structures.

Conclusion: The Indispensable Role of Knee Cartilage

The cartilage in the knee joint, comprising both the smooth articular cartilage and the resilient menisci, is fundamental to the joint's ability to function. It allows for effortless movement by minimizing friction, protects the underlying bones by absorbing shock and distributing loads, and contributes significantly to joint stability. Understanding these vital roles underscores the importance of protecting this remarkable tissue, as its limited capacity for self-repair means that damage can have profound and lasting implications for mobility and quality of life.