Fig. 3. Japanese Investigation Committee classification system of femoral head osteonecrosis. Four types: type A lesion < medial 1/3 of the weight-bearing portion; type B lesion < medial 2/3 of the weight-bearing portion; type C1 lesion > medial 2/3 of the weight-bearing portion but not extending laterally to the acetabular edge; and type C2 lesion extending laterally to the acetabular edge.
