Characteristics and Composition of Articular Cartilage

The Extracellular Matrix (ECM)

The bulk of articular cartilage consists of an extracellular matrix (ECM). This matrix is primarily composed of water, which is interspersed with inorganic salts, proteins, glycoproteins, and lipids. Within this matrix, two main components work in tandem to provide mechanical integrity:

1. Collagen Fibers: These fibers are made of elastin and collagen. They provide the tissue with stability, strength, and tensile stiffness. Crucially, collagen fibers are designed to tolerate tension (pulling forces) but are not the primary responders to compression.
2. Proteoglycans: These are molecules encased in a solution within the network of collagen fibrils.

Chondrocytes

While the matrix provides the structure, the living component of the cartilage is the chondrocyte. Chondrocytes are the specialized cells responsible for manufacturing and maintaining the organic components of the extracellular matrix, including the collagen and proteoglycans.

Proteoglycan Aggregation and Functional Properties

The interaction between these components defines the functional properties of the joint. While collagen handles tension, proteoglycan aggregation is responsible for providing structural rigidity and superior tolerance to compression. This allows the cartilage to act like a pressurized sponge, absorbing the shock of impact and spreading the load applied to the bone ends over a large area, which decreases contact stress.

Wear, Injury, and Degeneration

Despite its efficiency, articular cartilage is susceptible to various forms of mechanical and biological breakdown.

Mechanisms of Wear

There are two primary ways the cartilage surface can degrade:

• Interfacial Wear: This occurs when there is insufficient lubrication between the moving surfaces. In a healthy joint, synovial fluid (produced by the synovial membrane) coats the surfaces to reduce friction.
• Fatigue Joint Wear: This results from repetitive stress injuries over time.

Degenerative Joint Disease

When the cartilage is subjected to sudden, excessive stress, the joint matrix can suffer acute trauma. As the tissue wears down over time, microcracks may form. These cracks can eventually lead to the creation of "loose bodies"—small fragments of tissue that float within the joint cavity.

As the surface becomes rough and irregular, the joint becomes even more susceptible to further wear and accelerated degeneration. This progressive breakdown is the hallmark of osteoarthritis and other forms of degenerative joint disease.

Summary Table: Mechanical Roles of Matrix Components