 Osteoarthritis OA is the most common kind of arthritis and primarily affects the large, weight bearing joints including the knees, lower back, hips, neck and the feet and the joints in the hands. OA results from the breakdown of cartilage in joints, leading to joint pain and damage. It affects about 8 million people in the United Kingdom and nearly 27 million people in the United States, most of whom are over the age of 45. 