TABLE II.

The 2019 Revised ARCO Classification System for Osteonecrosis of the Femoral Head^45*

Stage	Description
I	Normal radiograph and abnormal MRI findings
II	No crescent sign, radiographic evidence of sclerosis, osteolysis, or focal osteoporosis
III	Subchondral fracture, fracture in the necrotic portion, and/or flattening of the femoral head on radiograph or CT scan
IIIA	Femoral head depression of ≤2 mm
IIIB	Femoral head depression of >2 mm
IV	Evidence of osteoarthritis, joint space narrowing, and degenerative acetabular change

^*ARCO = Association Research Circulation Osseous, MRI = magnetic resonance imaging, and CT = computed tomography.