Osteonecrosis is a disease of unknown origin which degenerates the structure of the joint and causes progressive loss of function. While osteonecrosis mostly affects hip joint, it is a pathologic condition which is rarely seen in knee joint. Osteonecrosis in the knee includes three different clinical conditions: spontaneous knee osteonecrosis, secondary knee osteonecrosis and post-arthroscopic osteonecrosis. Spontaneous knee osteonecrosis is a disease which occurs without a certain underlying cause with single joint involvement and single focus. In secondary osteonecrosis there is generally involvement in both knees with multiple joint involvement which has an underlying cause. Post-arthroscopic osteonecrosis is characterised by the involvement of single or both femoral condyles with sudden onset of pain after the arthroscopic surgery.