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Multiple classification systems have been developed for ONFH. The Ficat and Arlet classification system was first developed in the 1960s and is based on clinical symptoms, radiographic findings, and uptake on bone scan. It is the most commonly used classification system as well as the most straightforward but does not account for size or location of the osteonecrotic lesion, which are now considered important factors predicting treatment outcomes.Stage 0 is preclinical, with no findings on imaging expect for reduced uptake on bone scan. Stage I is preradiographic, with clinical symptoms and increased uptake on bone scan. Stage II involves sclerotic or cystic changes on radiographs without femoral head flattening or crescent sign. Stage III involves the radiographic crescent sign with preserved joint space, whereas stage IV is defined as femoral head flattening and arthritic changes of the hip joint.