IV. Joint erosion in rheumatoid arthritis

Abstract In rheumatoid arthritis, destruction of the collagenous structures that comprise the joint is associated with proliferation of synovial lining cells. Erosion of cartilage, bone, and soft connective tissue occurs predominantly in areas adjacent to these cells and the collagen of these tissues is susceptible to degradation at neutral pH by collagenase from the proliferating cells. The collagen fragments thus solubilized are further broken down by other proteases. Cartilage collagen is attacked more slowly by synovial collagenase than are soft tissue collagens, presumably due to its particular primary structure and post‐translational modifications of this primary structure. Proteoglycans are degraded probably by proteolytic attack on the protein backbone. The inorganic mineral phase of bone prevents collagenolytic attack on bone matrix; mechanisms of mineral dissolution may include a local decrease in pH and release of substances such as prostaglandins from the inflammatory cells. The degradation of each structural component results in specific alterations in form and function of the joint.