Virtual stress testing of fracture stability in soldiers with severely comminuted tibial fractures.

Virtual stress testing (VST) provides a non-invasive estimate of the strength of a healing bone through a biomechanical analysis of a patient's computed tomography (CT) scan. We asked whether VST could improve management of patients who had a tibia fracture treated with external fixation. In a retrospective case-control study of 65 soldier-patients who had tibia fractures treated with an external fixator, we performed VST utilizing CT scans acquired prior to fixator removal. The strength of the healing bone and the amount of tissue damage after application of an overload were computed for various virtual loading cases. Logistic regression identified computed outcomes with the strongest association to clinical events related to nonunion within 2 months after fixator removal. Clinical events (n = 9) were associated with a low tibial strength for compression loading (p < 0.05, AUC = 0.74) or a low proportion of failed cortical bone tissue for torsional loading (p < 0.005, AUC = 0.84). Using post-hoc thresholds of a compressive strength of four times body-weight and a proportional of failed cortical bone tissue of 5%, the test identified all nine patients who failed clinically (100% sensitivity; 40.9% positive predictive value) and over three fourths of those (43 of 56) who progressed to successful healing (76.8% specificity; 100% negative predictive value). In this study, VST identified all patients who progressed to full, uneventful union after fixator removal; thus, we conclude that this new test has the potential to provide a quantitative, objective means of identifying tibia-fracture patients who can safely resume weight bearing. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:805-811, 2017.