Table 1.
Association Research Circulation Osseous (ARCO) international classification of osteonecrosis [60].
| ARCO stage | Image findings | Anteroposterior images | Description |
|---|---|---|---|
| 1 | X-ray normal MRI abnormal |
 |
A band lesion of low signal intensity around the necrotic area is seen on MRI. A cold spot is seen on bone scan. No changes are seen on plain radiographs. |
| 2 | X-ray abnormal MRI abnormal |
 |
Osteosclerosis, focal osteoporosis or cystic changes are seen in the femoral head on plain radiographs or CT scan. Still there is no evidence of subchondral fracture, fracture in the necrotic portion or flattening of the femoral head. |
| 33A(early)3B(late) | Subchondral fracture on X-ray or CT |
 
|
Subchondral fracture, fracture in the necrotic portion and/or flattening of the femoral head is seen on plain radiography or CT scan. →3A: Femoral head depression ≤ 2 mm →3B: Femoral head depression >2 mm |
| 4 | X-ray osteoarthritis |  |
Osteoarthritis of the hip joint with joint space narrowing, acetabular changes and destruction are seen on plain radiographs. |