Osteoarthritis of the knee. Coloured x-rays of the right knee of a 55-year-old patient with advanced osteoarthritis (OA) of the knee. The patient's right knee is shown in 2 different positions - straight (left) and bent (right). Osteoarthritis is a disease resulting from the breakdown of joint cartilage (connective tissue) and the underlying bone. It often affects the large weight-bearing joints, such as the hips and knees, and causes joint pain and stiffness. Here, the patient's knee osteoarthritis has caused severe narrowing of the patellofemoral joint space, which causes bone on bone contact when the knee is bent (centre right).