Wagnac Eric, Michardière Denis, Garo Anaïs, Arnoux Pierre-Jean, Mac-Thiong Jean-Marc, Aubin Carl-Eric
Ecole Polytechnique de Montréal, Mechanical Engineering Dept, Montreal, Canada.
Stud Health Technol Inform. 2010;158:167-71.
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.
在脊柱内固定手术中，尽管已经进行了多项体外实验，但考虑到皮质/松质骨质量、几何形状和特性分布以及螺钉设计的椎弓根螺钉最佳置入位置仍未确定。本研究的目的是评估使用植入器械的椎骨详细有限元模型（FEM）模拟螺钉轴向拔出并分析骨-螺钉力学相互作用的可行性。使用一名第50百分位男性志愿者L3椎骨的CT扫描图像（0.6毫米厚连续切片）构建有限元模型，以便在直接位置虚拟植入完全可定制的椎弓根螺钉。这个拥有753,000个单元的模型考虑了局部皮质骨厚度，并整合了模拟骨破坏的先进材料行为（弹塑性）定律。模拟了螺钉轴向拔出并与体外实验数据进行比较，并分析了螺钉螺纹-骨界面处的应力分布。模拟的螺钉拔出力（非线性响应，在640N时失效）在实验数据范围（500 - 660N）内。骨结构中的冯·米塞斯应力集中在每个内螺纹根部周围，最大应力位于近侧第一螺纹附近，在椎弓根后壁的皮质骨中。本研究表明使用详细的有限元模型模拟螺钉拔出并分析骨-螺钉力学相互作用的可行性和相关性。
