[Diagnostic criteria, staging system and classification of idiopathic osteonecrosis of the femoral head].

Five criteria which showed high specificity were selected as follows. 1) collapse of the femoral head or crescent sign, 2) demarcating sclelosis in the femoral head, 3) cold in hot in bone scan, 4) band -like pattern on T1 weighted image of magnetic resonance imaging (MRI) , 5) trabecular and marrow necrosis on histology. Definite idiopathic osteonecrosis of the femoral head (ION) is diagnosed by fulfilling any two of the five criteria. Staging is judged on both AP and lateral X-ray. Stage 1 is defined as a period of no abnormality on X-ray. Stage 2is a period of demarcating sclelosis seen in X-ray. Stage 3 is a period of collapse of the head. Stage 3 is subclassified into Stage 3A when collapse is less than 3mm and 3B when collapse is 3mm or more. Stage 4 is a period of osteoarthritic changes. According to location of necrotic lesion on T1 weighted images or on x-ray, the lesions are classified into into four types. Type A lesion is defined to occupy medial one third or less of weight bearing portion. Type B lesion is defined to occupy medial two third or less of weight bearing surface. Type C1 lesion is defined to occupy more than medial two third, but not beyond acetabular edge while Type C2 lesion is defined to extend lateral to the acetabular edge.