Knee Cartilage: Structure, Function, and Maintaining Joint Health

How does knee cartilage work?

Knee cartilage, primarily articular (hyaline) cartilage and meniscal (fibrocartilage), functions as a critical component of joint health by providing incredibly smooth, low-friction surfaces for movement, absorbing shock, distributing mechanical loads across the joint, and enhancing stability.

The Knee Joint: An Overview

The knee is one of the body's largest and most complex joints, crucial for mobility and bearing the body's weight. It is a modified hinge joint formed by the articulation of three bones: the femur (thigh bone), tibia (shin bone), and patella (kneecap). To enable the smooth, pain-free movement essential for activities ranging from walking to jumping, the surfaces of these bones are covered and cushioned by specialized connective tissues known collectively as cartilage. Understanding how these cartilaginous structures function is fundamental to appreciating knee mechanics and overall joint health.

What is Cartilage?

Cartilage is a resilient and flexible type of connective tissue found throughout the body. Unlike bone, it is avascular (lacks blood vessels) and aneural (lacks nerves), meaning it does not have a direct blood supply or nerve innervation. This unique composition significantly impacts its ability to heal and receive nutrients. The primary cells within cartilage are chondrocytes, which are embedded within an extracellular matrix composed of collagen fibers, proteoglycans, and a high percentage of water. The specific composition of this matrix determines the type and mechanical properties of the cartilage.

Types of Cartilage in the Knee

The knee joint contains two distinct types of cartilage, each with a specialized structure and function:

Articular (Hyaline) Cartilage

• Location: This smooth, glistening tissue covers the ends of the femur, the top of the tibia, and the posterior surface of the patella. It forms the actual joint surfaces where bones meet and move against each other.
• Structure and Composition: Articular cartilage is primarily composed of Type II collagen fibers, which provide tensile strength, and proteoglycans, which attract and retain water, giving the cartilage its compressive stiffness and elasticity. Approximately 65-80% of its wet weight is water. Its surface is incredibly smooth, often described as having a lower coefficient of friction than ice on ice.
• Function: Its primary roles are to reduce friction between articulating bones, allowing for effortless gliding, and to distribute compressive loads evenly across the joint surface, preventing stress concentrations that could damage the underlying bone.

Meniscal (Fibrocartilage)

• Location: Within the knee joint, between the femur and tibia, lie two C-shaped wedges of fibrocartilage: the medial meniscus (on the inner side of the knee) and the lateral meniscus (on the outer side).
• Structure and Composition: Menisci are tougher and more fibrous than articular cartilage. They are primarily composed of Type I collagen fibers, which provide greater tensile strength and resistance to tearing. Their wedge shape creates a deeper socket for the femoral condyles to sit in. Unlike articular cartilage, the outer edges of the menisci have a limited blood supply, which allows for some healing, but the inner portions are avascular.
• Function: The menisci serve multiple critical functions:
  • Shock absorption: They act as cushions, absorbing up to 20% of the load during walking and up to 80% during deep knee bends.
  • Load transmission and distribution: They help to transmit the load from the femur to the tibia, increasing the contact area between the two bones by up to three times, thereby reducing stress on the articular cartilage.
  • Joint stability: Their wedge shape helps to deepen the shallow tibial plateau, enhancing the congruity and stability of the knee joint.
  • Lubrication and nutrition: They assist in the circulation of synovial fluid, aiding in the nutrition of articular cartilage.
  • Proprioception: Recent research suggests they also contain mechanoreceptors, contributing to the knee's sense of position and movement.

The Mechanics of Articular Cartilage Function

The exceptional function of articular cartilage is due to its unique biomechanical properties and interaction with synovial fluid:

• Shock Absorption: When a load is applied to the joint (e.g., during walking or jumping), the water within the cartilage is momentarily squeezed out, causing the tissue to deform. This elastic deformation dissipates energy, effectively cushioning the impact. As the load is removed, the water is reabsorbed, and the cartilage returns to its original shape.
• Friction Reduction (Weeping Lubrication): The high water content and smooth surface of articular cartilage, combined with synovial fluid, create an incredibly low-friction environment. As pressure is applied, synovial fluid is "wept" out from the cartilage, forming a thin, lubricating film between the joint surfaces. This mechanism, known as weeping lubrication, minimizes friction and wear during movement.
• Load Distribution: By deforming under pressure, articular cartilage increases the contact area between the bones, effectively distributing the applied force over a larger surface. This prevents excessive stress from being concentrated in small areas, protecting the underlying bone from damage.

The Role of Meniscal Cartilage

The menisci, with their distinct fibrocartilaginous structure, contribute to knee function in a complementary manner:

• Increased Contact Area: By conforming to the shapes of both the femoral condyles and the tibial plateau, the menisci significantly increase the surface area over which forces are transmitted. This reduces the peak stress on the articular cartilage.
• Stability Enhancement: Their wedge shape acts as a dynamic stabilizer, particularly against rotational and anterior-posterior forces, preventing excessive movement between the femur and tibia.
• Load Transmission: Approximately 50% of the compressive load through the knee is transmitted through the menisci in extension, increasing to 85% at 90 degrees of flexion. This highlights their critical role in bearing weight.
• Circulation of Synovial Fluid: The movement of the menisci during joint motion helps to pump synovial fluid throughout the joint, aiding in the nutrition of the avascular articular cartilage.

Cartilage Nutrition and Healing

Due to its avascular and aneural nature, cartilage has a limited capacity for self-repair. Chondrocytes, the cells responsible for maintaining the cartilage matrix, receive nutrients primarily through the diffusion of synovial fluid. This fluid, produced by the synovial membrane lining the joint capsule, lubricates the joint and carries essential nutrients. Movement and weight-bearing are crucial for this process, as they create a "pumping" action that facilitates fluid exchange within the cartilage. Damage to cartilage, especially articular cartilage, often leads to chronic conditions like osteoarthritis because the body struggles to repair the tissue effectively.

Maintaining Healthy Knee Cartilage

Preserving the integrity of knee cartilage is vital for long-term joint health. Strategies include:

• Regular, Moderate Exercise: Activities that involve controlled, non-impact loading (e.g., cycling, swimming, elliptical) promote synovial fluid circulation and nutrient delivery without excessive stress.
• Maintaining a Healthy Weight: Reducing excess body weight significantly lowers the compressive forces on the knee joints, thereby reducing wear and tear on cartilage.
• Proper Biomechanics and Form: Using correct technique during exercise and daily activities minimizes abnormal loading patterns that can accelerate cartilage degeneration.
• Balanced Nutrition: A diet rich in anti-inflammatory foods, antioxidants, and adequate protein supports overall joint health.
• Listen to Your Body: Avoiding activities that cause pain and seeking professional advice for persistent knee discomfort can prevent minor issues from escalating into significant cartilage damage.

Conclusion

The intricate interplay between articular and meniscal cartilage, supported by the lubricating properties of synovial fluid, forms the cornerstone of knee joint function. These specialized tissues enable smooth, low-friction movement, absorb considerable shock, and distribute forces efficiently, allowing for the wide range of motion and weight-bearing capabilities of the knee. Understanding their structure and mechanical workings underscores the importance of protective measures and proper care to maintain this vital component of our musculoskeletal system throughout life.