Osteoarthritis, also known as degenerative joint disease, is a type of arthritis that is caused by breakdown of cartilage with eventual loss of the cartilage of the joints. Cartilage is a protein substance that serves as a "cushion" between the bones of the joints. Osteoarthritis is also known as degenerative arthritis. Among the over 100 different types of arthritis conditions, osteoarthritis is the most common, affecting over 15 million people in the United States. Before age 45, osteoarthritis occurs more frequently in males. After age 55 years, it occurs more frequently in females. In the United States, all races appear equally affected. A higher incidence of osteoarthritis exists in the Japanese population, while South African blacks, East Indians and southern Chinese have lower rates.
Osteoarthritis usually affects the hands, feet, spine, and large weight-bearing joints, such as the hips and knees. Most cases of osteoarthritis have no known cause, and are called primary osteoarthritis. When the cause of the osteoarthritis is known, the condition is called secondary osteoarthritis.