Table 2.

Location analysis of necrosis in classification and correlation with collapse.

Author	Year	Country	Study point	Classification	Relations with collapse
Ohzono et al [17]	1991 1992	Japan	Osteonecrosis features (X-ray)	Type I: sclerosis band formation (IA, IB, IC) Type II: weight-bearing area deformation Type III: cystic change	IC, IIB, IIIB are more likely to collapse.
Sugano et al [33]	1994	Japan	Necrosis portion and range (MRI)	A type: <1/3 of the weight-bearing area; B type: <2/3; C type: >2/3	C type, with above 50% of necrotic area in both the coronal and the sagittal position, is more likely to collapse.
Takatori et al [34]	1996	Japan	Necrosis portion	A type: anterior medial of the head C type: half of the head A type < B type < C type D type > C type	Collapse rate increases in the upper type. Collapse is correlated with necrosis portion and range.
Sakamoto et al [35]	1997	Japan	Necrosis portion and range	Add a D type based on Sugano et al’s [33] study: C type does not expand outside the acetabular, but D type does.	D type seldom leads to a collapse.
Ito et al [36]	1999	Japan	Necrosis portion and range	Sakamoto et al’s [35]	Relatively stable microstructure of the femoral head can maintain an asymptomatic stage.