Table 2: Association Research Circulation Osseous (ARCO) Classification of Femoral Head Necrosis

Stage
0	All diagnostic studies normal, diagnosis by histology only
1	Plain radiographs and computed tomography normal, magnetic resonance imaging positive and biopsy positive, extent of involvement A, B, or C (less than 15 percent, 15 to 30 percent, and greater than 30 percent, respectively).
2	Radiographs positive but no collapse, extent of involvement A, B, or C
3	Early flattening of dome, crescent sign, computed tomography or tomograms may be needed, extent of involvement A, B, or C, further characterization by amount of depression (in millimeter)
4	Flattening of femoral head with joint space narrowing, possible other signs of early osteoarthritis