Table 1.

Classification Systems of Osteonecrosis of the Femoral Head

	Hip
	Stage	ARCO	UPenn
0			No radiographic evidence, but suspicion or clinical symptoms
I	Pain but no radiographic anomalies	Normal radiograph, abnormal MRI, or bone scan	Suggestive MRI, but a normal plain radiograph
II	Increased density, cystic changes, or porosity	Abnormal radiograph without fracture or flattening	Lucency, sclerosis, or plain radiograph and MRI
III	Flattening of the femoral head and crescent sign	Subchondral or necrotic zone fracture. IIIA: ≤2 mm flattening IIIB: >2 mm flattening	Crescent sign without flattening
IV	Full collapse of the femoral head with decrease in joint space	Radiographic evidence of arthritis with joint space narrowing, acetabular changes, and/or joint destruction	Flattening of the femoral head without joint space narrowing
V			Flattening of the femoral head with joint space narrowing
VI			Joint deformity and joint space obliteration

ARCO = Association Research Circulation Osseous