Table 2.

Classification systems used in the studies

System	Definition
ARCO	Stage 0—bone biopsy positive, all imaging normal Stage I—normal findings on radiographs and abnormal MRI findings  IA, < 15%; IB, 15–30%; IC, > 30% Stage II—abnormal X-ray findings, no femoral head collapse on X-ray and CT, and lesions were subdivided into medial, central, and lateral depending on the location of femoral head involvement  IA, < 15%; IB, 15–30%; IC, > 30% Stage III—crescent sign, the lesion can be subdivided into medial, central, and lateral according to the position of femoral head  Involvement   IIIA: crescent sign, < 15% or collapse, < 2 mm   IIIB: crescent sign, 15–30% or collapse, 2–4 mm   IIIC: crescent sign, > 30% or collapse, > 4 mm Stage IV—osteoarthritic acetabular changes and cartilage changes	Wei et al. [19], Zeng et al. [20], Wu et al. [21], Feng et al. [23]
Steinberg	Stage 0—normal or nondiagnostic X-ray, bone scan, and MRI Stage I—normal X-ray, abnormal bone scan, and/or MRI Stage II—lucent and sclerotic changes in femoral head Stage III—subchondral collapse (crescent sign) without flattening Stage IV—flattening of femoral head Stage V—joint narrowing and/or acetabular changes Stage VI—advanced degenerative change	Cao et al. [22]