Coloured X-ray of the right knee of an 18 year old male patient with osteochondritis dissecans (OCD) and aseptic necrosis. A small piece of bone and cartilage has separated from the rear of the patella (knee cap). The separated piece may stay in place or fall into the joint. OCD causes pain and swelling and, if the bone has moved, locking of the joint. Aseptic necrosis, also known as avascular necrosis or osteonecrosis is a condition where bone dies due to poor blood supply to the affected area and the bone marrow.