Table 2. ARCO classification^*.

Stage	Clinical findings, pathology, imaging
0	Normal findings on radiography, magnetic resonance imaging, and scintigraphy Signs of necrosis on histology
I	Normal radiography/computed tomography Pathological findings on magnetic resonance imaging or scintigraphy Subclassification according to part of femoral head affected (lateral, medial, or ventral) and proportion of femoral head circum ference involved (<15%, 15–30%, >30%)
II	Changes of bone structure without alteration of femoral head contour on radiography Normal joint space Magnetic resonance imaging findings specific for femoral head necrosis Subclassification according to part of femoral head affected (lateral, medial, or ventral) and proportion of femoral head circum ference involved (<15%, 15–30%, >30%)
III	Changes of bone structure with subchondral fracture showing up as a brighter sickle-shaped zone (crescent sign) on radiography Flattening of femoral head contour Normal width of joint space Subclassification according to part of femoral head affected (lateral, medial, or ventral), proportion of femoral head circumference involved (<15%, 15–30%, >30%), and magnitude of flattening (<2 mm, 2–4 mm, >4 mm)
IV	Development of rheumatoid arthritis Flattening of femoral head Narrowing of joint space

^*From (20)