The cause for osteoarthritis is unknown. However, aging, obesity, joint injury, joint stress, heredity, and muscle weakness all seem to play a role.

Osteoarthritis occurs when the cartilage, which cushions the joints, breaks down over time. Normally, the surface of the articular cartilage is smooth. In osteoarthritis, the surface of the joint cartilage becomes rough, and the joint becomes inflamed during use. When the cartilage breaks down completely, the surfaces of the bone rub against one another. This causes damage to the bone, and over time, the joint can become deformed and painful.