Sometimes called degenerative joint disease, osteoarthritis occurs when the cartilage breaks down and gradually becomes rougher and thinner. Swelling can occur if the synovial membrane becomes irritated and produces excess fluid that collects inside the joint. As the cartilage wears away, growths of bone (called bone spurs) may form around the edges of the joint, making it look knobby and swollen. As the process continues, a substantial amount of cartilage wears away, causing the bones that meet at the joint to rub against each other. Because bone is very sensitive, this can be extremely painful and can severely reduce movement in the joint. Osteoarthritis usually results from injury to a joint or from wear and tear on the joints over time.
More than 420,000 Americans had total hip or knee replacement surgery in 1998 at an average age of 68. Deterioration from osteoarthritis is the most common reason for replacing joints with artificial ones, and the numbers are projected to rise as the population ages.