Biomechanical properties of pedicle screws in human vertebrae of reduced bone density- Development of a new cadaver corpectomy model and comparison to pull-out only test.

Until today, the stability of dorsal implant systems of the spine has often been evaluated by means of simple pull-out or cyclic loading tests. However, the existing loading scenarios do not adequately represent the physiological loading and the clinical situation. The aim of the present study was to develop an alternative setup and parameters to compare the static and dynamic properties of pedicle screws at the bone-implant interface in lumbar vertebrae of reduced bone quality. The corpectomy model is based on the ASTM-1717 standard. 12 human osteoporotic vertebrae from L1 to L4 were examined by computed tomography and divided into four groups. In groups A and C the screws were inserted natively, in B and D augmented with bone cement. In groups A and B, a postoperative loading was performed during walking simulating two months, followed by pull-out. In groups C and D, only pull-out was performed. Screw loosening was recorded by optical measurement and analyzed for clinical failure patterns. In addition, correlations between CT data, T-score, and in vitro parameters were determined. In groups A and B, the subsidence of the screws relative to the vertebral endplates was measured, which was reflected in a visible permanent deflection. The increase in subsidence was greatest within the first and last cycles just before failure. The early primary stability may already be an indication of the overall stability. Augmentation of the screws reduced this significantly. Failure between the left and right sides does not occur simultaneously at the same extent, but gradually and subsequently. However, there was no significant difference in the pull-out. With the new model, typical clinical failure mechanisms, such as windshield wiper-like loosening and loss of correction, could be simulated. The described parameters also allow an analysis of the complex changes in load distributions and the interaction between the left and right side of the bone-implant interface.