Mechanical Evaluation of Collapse Risk for Osteonecrosis of Femoral Head Post-Medical Treatment

Background: : Early-stage osteonecrosis of the femoral head can progressively induce the bone collapse and the accuracy of the collapse risk evaluation is limited. This study aims to propose a potential collapse indicator by biomechanical evaluating the treatment effectiveness. Methods: : Six patient-specific models pre- (initial presentation) and post- medical treatment were established and elastic properties of the bone were assigned with a function of spatial-variant Hounsfield unit values. Morphological and mechanical analyses were compared between pre- and post-treatment models. Results: : In the morphological study, four cases show the reduced ratio of low-density necrotic volume, but not consistent with one-year follow-up results. In the mechanical analysis, the equivalent stress variation near the Ward triangle shows consistency in the one-year outcome. Moreover, a parameter - relative stress index, the ratio between the mean volume stress index and the mean volume equivalent stress, is proposed. A smaller value of the relative stress index indicates stronger bone compressive strength and its variation is consistent with the follow-up results. Conclusions: : Results implicate that to evaluate the effectiveness of medical treatments, the morphologic analysis should be considered but the mechanical capability and the loading transfer path through the necrotic and viable bone play a more important role.