BEYOND WEAR AND TEAR

To understand how arthritis develops, consider how limb joints function. The ends of long bones are covered by articular cartilage, which is made up of 80 percent water. The remainder is collagen and long chains of proteins called proteoglycans, which give cartilage stiffness to withstand shearing forces as the joint moves. Areas of higher motion have higher levels of proteoglycans. Articular cartilage compresses and expands under pressure from the bones each time the joint flexes, forcing water and other fluids in and out, keeping the tissues hydrated and healthy.

Even routine activity might cause minute damage and small amounts of inflammation within a joint. Usually, the body makes repairs quickly and effectively. If the insults are too severe or repetitive, however, healing processes may be overwhelmed, starting the cascade of events that leads to arthritis:

• Inflammatory enzymes break down the synovial fluid that lubricates the joint space, making it less protective.

• Meanwhile, collagen and proteoglycans are lost, reducing the cartilage’s ability to retain water.

• Without rest and/or medical intervention, damage mounts and the joint fills with inflammatory fluids, leading to pressure, pain and stiffness.

Left unchecked, these processes can erode cartilage to the extent that bone ends rub painfully against each other. This is an advanced case of osteoarthritis, also called degenerative joint disease.