In a randomised trial comparing core decompression with conservative treatment we tested the hypothesis that the extent of necrosis at the initial MRI predicts the subsequent risk of collapse of the femoral head. After the initial clinical evaluation, including plain radiography and MRI, 37 hips with early-stage osteonecrosis (ON) in 33 patients were randomly assigned to a core-decompression group or a conservatively-treated group. All were followed regularly by clinical evaluation, plain radiography and MRI at intervals of three months. The extent of ON was estimated on the basis of abnormal signal intensity in the weight-bearing portion of the femoral head as determined from a combination of coronal and sagittal MRIs. The arc 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 100. There was a strong correlation between this index and the risk of collapse before and after adjustment for age, gender, stage and treatment group. We conclude that the extent of the necrotic portion ascertain by this method is a major predictor of future collapse. We propose 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.

We performed a randomised trial on 37 hips (33 patients) with early-stage osteonecrosis (ON). After the initial clinical evaluation, including plain radiography and MRI, 18 hips were randomly assigned to a core-decompression group and 19 to a conservatively-treated group. All the patients were regularly followed up by clinical evaluation, plain radiography and MRI at intervals of three months. Hip pain was relieved in nine out of ten initially symptomatic hips in the core-decompression group but persisted in three out of four initially painful hips in the conservatively-treated group at the second assessment (p < 0.05). At a minimum follow-up of 24 months, 14 of the 18 core-decompressed hips (78%) and 15 of the 19 non-operated hips (79%) developed collapse of the femoral head. By survival analysis, there was no significant difference in the time to collapse between the two groups (log-rank test p = 0.79). Core decompression may be effective tin symptomatic relief, but is of no greater value than conservative management in preventing collapse in early osteonecrosis of the femoral head.