Articular Cartilage: Structure, Function, and Biomechanics

Biomechanical Properties: Load Distribution and Friction

The primary "job" of articular cartilage is to manage two physical challenges: compression and friction.

1. Load Distribution (The Biphasic Nature)

Articular cartilage behaves as a "biphasic" material, meaning it consists of a solid phase (the collagen/proteoglycan matrix) and a fluid phase (interstitial water).

When you jump or walk, the weight of your body applies a compressive force to the joint. Because the proteoglycans are negatively charged, they repel each other and pull water into the matrix. When the load is applied, the water is squeezed through the tiny pores of the collagen mesh. This movement of fluid creates "interstitial fluid pressure," which actually carries the majority of the load, protecting the solid matrix from being crushed.

2. Low Friction

The surface of hyaline cartilage is incredibly smooth. When combined with synovial fluid—the lubricant found within the joint capsule—the coefficient of friction is even lower than that of ice sliding on ice. This ensures that the bones glide past each other with minimal wear and tear.

The Importance of Synovial Fluid

Because articular cartilage lacks its own blood supply, it cannot receive nutrients through traditional circulation. Instead, it relies on a process called diffusion and the mechanical action of movement.

The synovial fluid, contained within the joint cavity, acts as both a lubricant and a nutrient delivery system. As the joint moves, the "pumping" action of compression and decompression helps circulate the synovial fluid, bringing glucose and oxygen to the chondrocytes and carrying metabolic waste products away.

Worked Example: The "Pumping" Mechanism

Consider a person sitting still for several hours versus a person walking.

• Static Loading: When sitting, the lack of movement means minimal fluid exchange occurs. The chondrocytes may experience a slight buildup of metabolic waste.
• Dynamic Loading: During walking, the rhythmic compression of the cartilage acts like a sponge being squeezed and released. This maximizes the flow of synovial fluid, ensuring the cartilage stays "fed" and lubricated.

Clinical Significance: When Properties Fail

When the structural integrity of the hyaline cartilage is compromised—either through acute trauma or chronic overuse—the mechanical properties change. If the collagen network breaks down, the proteoglycans can no longer hold water effectively.

As the water content decreases and the surface becomes rough, the friction increases. This leads to:

1. Increased Wear: The "smooth glide" is lost.
2. Bone-on-Bone Contact: As the cartilage thins, the underlying subchondral bone is exposed to direct pressure, leading to pain and inflammation.

In summary, the ability of a joint to function smoothly depends on the delicate balance of the hyaline cartilage's biochemical makeup and its ability to manage fluid pressure through movement.