MRI study on predicting the collapse of avascular necrosis of the femoral head

Objective: To study the risk factors of MRI for the prediction of collapse in patients with avascular necrosis of the femoral head. Methods: Twenty-two patients (39 hips) diagnosed avascular necrosis of femoral head by MR were enrolled in our study. The following MR appearances were evaluated: bone marrow edema,joint fluids, signal intensity and location of the lesion. The volume and surface area of the necrosis zone were calculated. The time of follow-up was 18-84 months (median, 25 months). Logistic regression analysis was used to predict the risk factors by SPSS 13.0. The maximum value of Youden index was selected as the critical point to predict the collapse of femoral head and to define the sensitivity, specificity and accuracy. Results: In the 39 hips with femoral head necrosis, 21 hips had collapse. Bilateral collapse occurred in 5 cases. In 25 hips with the necrosis surface larger than 25%, collapse occurred in 21 (84%); In 8 hips with the volume of femoral head necrosis larger than 30%, collapse occurred in all cases; In 33 hips with the necrosis locating at the superolateral quadrant, collapse occurred in 21 (63.6%); In 22 hips with necrotic areas showing heterogeneous signal intensity, collapse occurred in 18(81.8%); In 25 hips with large amount of joint effusion, collapse occurred in 16 (64%); in 18 hips with bone marrow edema, collapse occurred in 13 (65%). Joint fluid, heterogeneous signal intensity and lesions in the superolateral quadrant, volume ratio, and area ratio were the high risk factors, while bone marrow edema was a relatively low risk factor. The area under ROC curves for area ratio of NASA was greater than that for volume ratio (0.987 vs 0.902). When the critical value for area ratio was 26.7%, the true positive rate was 95.2%, true negative rate was 94.4%, and Youden's index was 0.896. Conclusions: The collapse of necrosis of femoral head may result from many factors. The femoral head was easy to collapse when it had large enough area of necrosis and mixed signal intensity, a large amount of joint effusion, bone marrow edema, and superolateral quadrant location. The critical value for area ratio to predict the collapse of femoral head was about 26.1%. The area ratio is more accurate than volume ratio in predicting the collapse of necrosis of femoral head. (authors)