Biomechanical analysis of pedicle screw placement: a feasibility study

In spinal instrumentation surgery, the optimal placement of pedicle screws that takes into account the cortical/cancellous bone quality, geometry and property distribution, and screw design is still undetermined despite several in vitro experiments. The objective of this study was to evaluate the feasibility of using a detailed finite element model (FEM) of an instrumented vertebra to simulate screw axial pull-out and to analyze the bone-screw mechanical interaction. The FEM was built using CT-scan images of the L3 vertebra (0.6mm thick contiguous slices) of a 50th percentile human male volunteer, in order to virtually implant a fully customizable pedicle screw in a straight-forward position. The 753,000 elements model takes into account local cortical bone thickness and integrates advanced material behavior (elasto-plastic) laws that simulate bone failure. Screw axial pull-out was simulated and compared to in vitro experimental data, and the stress distribution at the screw thread-bone interface was analyzed. The simulated screw pull-out force (non-linear response with a failure at 640N) was within the range of experimental data (500-660N). Von Mises stresses in the bony structures were concentrated around the root of each internal thread, with the maximum stress located near the first proximal thread, in the cortical bone of the posterior wall of the pars. This study shows the feasibility and relevance of using a detailed FEM to simulate screw pull-out and to analyze the bone-screw mechanical interaction.