QUANTIFYING THE EXTENT OF OSTEONECROSIS OF THE FEMORAL-HEAD - A NEW METHOD USING MRI

In a randomised trial comparing core decompression with conservative t reatment we tested the hypothesis that the extent of necrosis at the i nitial MRI predicts the subsequent risk of collapse of the femoral hea d. After the initial clinical evaluation, including plain radiography and MRI, 37 hips with early-stage osteonecrosis (ON) in 33 patients we re randomly assigned to a core-decompression group or conservatively-t reated group. All were followed regularly by clinical evaluation, plai n radiography and MRI at intervals of three months. The extent of ON w as estimated on the basis of abnormal signal intensity in the weight-b earing portion of the femoral head as determined from a combination of coronal and sagittal MRIs. The are of the necrotic portion in the mid -coronal image (A) and that in the mid-sagittal image (B) were used to quantify the extent of necrosis by the formula: (A/180) X (B/180) X 1 00. There was a strong correlation between this index and the risk of collapse before and after adjustment for age, gender, stage and treatm ent group. We conclude that the extent of the necrotic portion ascerta ined by this method is a major predictor of future collapse. We propos e a systematic method of determining the index of the necrotic portion which may be clinically useful in the management of early-stage ON of the femoral head.