Fig. 4. Modified Kerboul classification system of femoral head osteonecrosis. A: necrotic angle in mid-coronal MR image. B: necrotic angle in mid-sagittal MR image. Combined necrotic angle: A + B. Three categories: small lesion (combined necrotic angle ≤ 190°), medium-sized lesion (combined necrotic angle between 190° and 240°), and large lesion (combined necrotic angle ≥ 240°).

MR = magnetic resonance.