Ficat Classification of Avascular Necrosis (AVN) of the Femoral Head

Stage	Name	Clinical features	Radiography
Early
0	Preclinical
I	Preradiographic	+
II		+	Osteoporosis, sclerosis, cysts
Transition			Flattening and crescent sign
Late
III	Collapse	++	Irregular contour of head, sequestrum, normal joint space
IV	Osteoarthritis	+++	Flattened femoral head, narrowed joint space; collapse of head (see image)

References

Ficat RP. Idiopathic bone necrosis of the femoral head. Early diagnosis and treatment. J Bone Joint Surg Br. 1985 Jan;67(1):3-9.