Clinical meaning
Osteoarthritis (OA) is the most common form of arthritis and the leading cause of chronic disability in adults, affecting over 300 million people worldwide. Once considered a simple 'wear and tear' disease, OA is now understood as a complex, multifactorial process involving active biochemical degradation of articular cartilage, subchondral bone remodeling, synovial inflammation, and ultimately, whole-joint failure.
Normal Articular Cartilage Biology: Articular (hyaline) cartilage is a specialized connective tissue that covers the ends of bones in synovial joints, providing a near-frictionless surface for joint movement and distributing mechanical loads across the subchondral bone. The cartilage matrix is composed of: (1) type II collagen fibers (60% of dry weight) providing tensile strength; (2) aggrecan proteoglycans (30% of dry weight) — large molecules with glycosaminoglycan side chains (chondroitin sulfate and keratan sulfate) that attract and bind water through osmotic effects, giving cartilage its compressive resilience and ability to withstand loading forces; (3) water (65-80% of wet weight) — the high water content is critical for nutrient diffusion (cartilage is avascular) and load distribution. Chondrocytes are the sole cellular component of cartilage, comprising only 1-5% of tissue volume, but are responsible for synthesizing and maintaining the entire extracellular matrix.
Cartilage Degradation Mechanisms: In OA, the balance between matrix synthesis and degradation shifts toward net degradation through several interconnected mechanisms. Chondrocyte dysfunction is central: aging, mechanical overload, and inflammatory cytokines cause chondrocytes to shift from an anabolic (matrix-producing) to a catabolic (matrix-degrading) phenotype. Stressed chondrocytes produce matrix metalloproteinases (MMPs) — particularly MMP-1, MMP-3, and MMP-13 — that cleave type II collagen fibers, and aggrecanases (ADAMTS-4 and ADAMTS-5) that degrade aggrecan proteoglycans. As aggrecan is lost, the cartilage loses its water-binding capacity and compressive resilience. Collagen fiber disruption exposes the cartilage surface to further mechanical damage. Once type II collagen is cleaved, it cannot be repaired — the damage is irreversible. Chondrocytes also produce pro-inflammatory cytokines (IL-1beta, TNF-alpha) and reactive oxygen species (ROS) that create a self-amplifying degradation loop. Eventually, chondrocyte apoptosis occurs, leaving acellular regions of cartilage that cannot be maintained.