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| ARCO stage | Image findings | ||
|---|---|---|---|
| I | X-ray: normal | ||
| MRI: low-signal band on T1-weighted MRI | |||
| II | X-ray: abnormal | ||
| MRI: abnormal | |||
| III | Subchondral fracture on X-ray or CT | ||
| IIIA (early) | Femoral head depression ≤ 2 mm | ||
| IIIB (late) | Femoral head depression > 2 mm | ||
| IV | X-ray: osteoarthritis | ||
ARCO = Association Research Circulation Osseous, MRI = magnetic resonance imaging, CT = computed tomography.
