Surgical management of osteonecrosis of the femoral head in patients with sickle cell disease

Table 1 University of Pennsylvania System for staging avascular necrosis[38]

Stage	Criteria
0	Normal or non-diagnostic radiograph, bone scan, MRI
I	Normal radiographs, abnormal bone scan and/or MRI
	A: Mild (< 15% of femoral head affected)
	B: Moderate (15%-30%)
	C: Severe (> 30%)
II	Cystic and sclerotic changes in femoral head
	A: Mild (< 15% of femoral head affected)
	B: Moderate (15%-30%)
	C: Severe (> 30%)
III	Subchondral collapse (crescent sign) without flattening
	A: Mild (< 15% of articular surface)
	B: Moderate (15%-30%)
	C: Severe (> 30%)
IV	Flattening of femoral head
	A: Mild (< 15% of surface and < 2 mm depression)
	B: Moderate (15%-30% of surface and 2-4 mm depression)
	C: Severe (> 30% of surface and 4 mm depression)
V	Joint narrowing or acetabular changes
	A: Mild1
	B: Moderate1
	C: Severe1
VI	Advanced degenerative changes

¹Average of femoral head involvement; as determined in stage IV and estimated acetabular involvement. MRI: Magnetic resonance imaging.