Table 1.
The ARCO international classification of osteonecrosis of the femoral head
| Classification | ||
|---|---|---|
| 0 | biopsy results consistent with osteonecrosis; normal findings on all other tests | |
| I | Positive scintiscan or magnetic resonance image, or both | |
| I-A | < 15% involvement of femoral head | |
| I-B | 15 – 30% involvement of femoral head | |
| I-C | > 30% involvement of femoral head | |
| II | Radiographic abnormalities (mottled appearance of femoral head, osteosclerosis, cyst formation, and osteopenia); no crescent sign or evidence of collapse of femoral head on radiographs | |
| II-A | < 15% involvement of femoral head | |
| II-B | 15 – 30% involvement of femoral head | |
| II-C | > 30% involvement of femoral head | |
| III | Crescent sign | |
| III-A | < 15% involvement of femoral head or <2mm depression of femoral head | |
| III-B | 15 – 30% involvement of femoral head or 2 – 4 mm depression of femoral head | |
| III-C | > 30% involvement of femoral head or > 4 mm depression of femoral head | |
| IV | Osteoarthritis: Articular surface flattened radiographically and joint space shows narrowing; changes in acetabulum with evidence of osteosclerosis, cyst formation, and marginal osteophytes |