Causes

Osteoarthritis, which causes swelling and reduced motion in joints, results from the breakdown of cartilage. Healthy cartilage, which is the slippery tissue that covers the ends of bones in a joint, absorbs shock resulting from movement. When the top layer of cartilage breaks down and wears away, rubbing between bones and joints occurs, resulting in joint damage. With reduced cartilage, rubbing causes pain, swelling, and loss of motion of the joint, which may eventually lose its normal shape. Bone spurs may also grow on the edges of the joint. Bits of bone or cartilage may break off and float inside the joint space, causing more pain and damage.

Cartilage breakdown may occur gradually, through mechanical wear and tear, such as over a lifetime of running on hard surfaces. A single injury to a joint, such as a sports injury or a car accident, may be enough to trigger ongoing cartilage damage.

Rheumatoid arthritis, the second most common form of the disease, may affect joints, but also attacks tissue in the skin, lungs, eyes, and blood vessels. Symptoms of rheumatoid arthritis that can accompany pain may include feeling sick, tired, and sometimes feverish. The cause of rheumatoid arthritis is unknown.

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This article was reviewed and updated June 2007.