Bone Allograft Pedicle Screw Augmentation: A Biomechanical Study.

ABSTRACT

STUDY DESIGN: We performed a comprehensive cadaveric biomechanical study to compare the fixation strength of primary screws, screws augmented with bone allograft, and screws augmented with polymethylmethacrylate cement. OBJECTIVE: To evaluate a novel technique for screw augmentation using morselized cortico-cancellous bone allograft to fill the widened screw track of failed pedicle screws. BACKGROUND: To date, there are no known biological methods available for failed pedicle screw augmentation or fixation. MATERIALS AND METHODS: Biomechanical tests were performed using 2 different testing modalities to quantify fixation strength including axial screw pullout and progressive cyclic displacement tests. RESULTS: Fifty vertebrae were instrumented with pedicle screws. Our study showed that bone allograft augmentation using the same diameter screw was noninferior to the fixation strength of the initial screw. In the axial pullout test, screws undergoing bone allograft repair failed at 25% lower loads compared with native screws, and screws augmented with cement showed approximately twice as much strength compared with native screws. In the cyclic displacement test, screws fixed with cement resisted loosening the best of all the groups tested. However, screws augmented with bone graft were found to have an equal strength to native screw purchase. our study did not find a correlation with bone mineral density as a predictor for failure in axial pullout or cyclic displacement tests. CONCLUSION: Bone allograft augmentation for pedicle screw fixation was noninferior to the initial screw purchase in this biomechanical study. This bone allograft technique is a viable option for screw fixation in the revision setting when there is significant bone loss in the screw track.