[Avascular necrosis of the femoral head: staging by MR imaging].

Magnetic resonance (MR) images and conventional radiographs were compared in 142 hips with avascular necrosis, and a staging system for the disease based on MR imaging was developed. MR images were classified into three patterns: class I: a band of low signal intensity, class II: an area of low signal intensity with internal spot (s) of high signal, and class III: an area of low signal intensity without internal spots of high signal. Most MR class I lesions were in radiographic stage I (normal) or II (sclerotic or cystic changes without collapse). Most MR class II lesions were in radiographic stage III (segmental collapse), and most MR class III lesions were in stage III or IV (secondary degenerative changes). The MR image classification was closely correlated with radiographic staging (p < 0.01, using chi square test). We considered that this classification closely reflected the different stages of the disease according to the histopathology of the bone marrow.