Osteoarthritis (OA) is a degenerative disease of the joints. In normal joints, cartilage surrounds the ends of bones to provide cushioning and to help them glide smoothly. In people with OA the cartilage becomes worn and thin, therefore the joint can’t move as smoothly and can become painful and stiff. The most common joint affected by OA is the knee and the second most commonly affected joint is at the base of the thumb.