ABSTRACT
We tested the hypothesis that the extent of necrosis at the initial MRI predicts the subsequent risk of collapse of the femoral head in a randomized clinical trial conducted to compare the core decompression to the conservative treatment. After the initial clinical evaluation including plain roentgenography and MRI, 37 hips of early-stage osteonecrosis (ON) in 33 patients were randomly assigned to core decompression group or conservative treatment group. All the patients were regularly followed by clinical evaluation including plain roentgenography and MRI at three-month intervals. The extent of ON was estimated on the basis of the percentage of abnormal signal intensity in the weight-bearing portion of the femoral head as determined on a combination in coronal aod sagittal MR images. The angle of necrotic portion in mid-coronal image (A) and that in mid-sagittal image (B) were used to quantify the extent of necrotic portion by the formula; (A/180) x (B/180) x 100%. A strong association was observed between the percentage of necrotic portion and the development of collapse. We concluded that the extent of necrotic portion is a major risk factor of the collapse and proposed a systematic method of classifying the percentage of necrotic portion, which might be useful as a predictive index for the fate of early-stage ON.