Knee diseases. X-ray of an eighteen year old patient with osteochondritis dissecans (OCD) of the knee. The patella (kneecap) is at left, the femur (thigh bone) at top, the tibia (shin bone) at bottom centre and the fibula at bottom right. OCD occurs when a small piece of bone and cartilage (shown by arrows) separates from a joint. The separated pieces may stay in place or fall into the joint. It is most prevalent in boys aged between 10 and 20. OCD causes pain and swelling and, if the bone has moved, locking of the joint. If the fragment is still in place treatment is with rest, but if it has moved surgery may be needed.