Scheme of ARCO classification system (1992)

Stage

Radiological findings

Subclassification

0

Positive: histology. Negative/normal: Radiograph/CT/MRI/Scintigraphy

-

I

Positive: MRI and/or bone scintigraphy. Negative/normal: radiograph/CT

+’(a)

II

Radiograph: sclerotic, cystic or osteoporotic changes of femoral head

+’(a)

III

Radiograph: subchondral fracture (“crescent sign”)

+’(a)

IV

Radiograph: flattening of femoral head

++’(b)

V

Radiograph: flattening of femoral head and osteoarthrotic changes: decreased joint space and acetabular changes

++’(b)

VI

Complete joint destruction

-

(a) Location of femoral head necrosis: 1) medial third, 2) median third, 3) lateral third. Size of femoral head necrosis: A) <15%, B) 15% - 30%, C) >30%. (b) Intrusion degree of femoral head contour: A) <2 mm, B) 2 - 4 mm, C) >4 mm.