Table 1.

University of Pennsylvania Classification of OST

Stage	Criteria
0	Normal or nondiagnostic radiograph, bone scan, and MRI
I	Normal radiograph: abnormal bone scan and/or MRI
	A ‒ Mild	(<15% of head affected)
	B ‒ Moderate	(15%-30%)
	C ‒ Severe	(>30%)
II	Lucent and sclerotic changes in femoral head
	A ‒ Mild	(<15%)
	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 or 2- to 4-mm depression)
	C ‒ Severe	(>30% of surface or >4-mm depression)
V	Joint narrowing and/or acetabular changes
	A ‒ Mild
	B ‒ Moderate
	C ‒ Severe
VI	Advanced degenerative changes

Reprinted from Lee et al¹⁷ with permission.