In Avascular Necrosis (AVN) of the femoral head parts of the thigh bone loose their blood supply.
AVN starts as a demarcation of dead bone with the cartilage still intact (upper picture). Later the bone disappears and leaves a defect in the hip joint (lower picture). In childhood the body can partially repair this deficiency (Perthes). In adults joint replacement is often the best option. Avascular necrosis and femoral head deformity can also occur after trauma.
Northern Hip & Groin Institute