Piovesan Agnese, Berti Francesca, Villa Tomaso, Pennati Giancarlo, La Barbera Luigi
Laboratory of Biological Structure Mechanics,Department of Chemistry, Materials andChemical Engineering "Giulio Natta",Politecnico di Milano,Piazza Leonardo da Vinci 32,Milan 20133, Italy.
J Biomech Eng. 2019 Apr 1;141(4). doi: 10.1115/1.4042767.
Posterior fixation with contoured rods is an established methodology for the treatment of spinal deformities. Both uniform industrial preforming and intraoperative contouring introduce tensile and compressive plastic deformations, respectively, at the concave and at the convex sides of the rod. The purpose of this study is to develop a validated numerical framework capable of predicting how the fatigue behavior of contoured spinal rods is affected by residual stresses when loaded in lordotic and kyphotic configurations. Established finite element models (FEM) describing static contouring were implemented as a preliminary simulation step and were followed by subsequent cyclical loading steps. The equivalent Sines stress distribution predicted in each configuration was compared to that in straight rods (SR) and related to the corresponding experimental number of cycles to failure. In the straight configuration, the maximum equivalent stress (441 MPa) exceeds the limit curve, as confirmed by experimental rod breakage after around 1.9 × 105 loading cycles. The stresses further increased in the lordotic configuration, where failure was reached within 2.4 × 104 cycles. The maximum equivalent stress was below the limit curve for the kyphotic configuration (640 MPa), for which a run-out of 106 cycles was reached. Microscopy inspection confirmed agreement between numerical predictions and experimental fatigue crack location. The contouring technique (uniform contouring (UC) or French bender (FB)) was not related to any statistically significant difference. Our study demonstrates the key role of residual stresses in altering the mean stress component, superposing to the tensile cyclic load, potentially explaining the higher failure rate of lordotic rods compared to kyphotic ones.
使用塑形棒进行后路固定是治疗脊柱畸形的一种既定方法。均匀的工业预塑形和术中塑形分别在棒的凹侧和凸侧引入拉伸和压缩塑性变形。本研究的目的是建立一个经过验证的数值框架，能够预测在脊柱前凸和后凸构型加载时，塑形脊柱棒的疲劳行为如何受到残余应力的影响。将描述静态塑形的既定有限元模型（FEM）作为初步模拟步骤，随后进行后续的循环加载步骤。将每种构型中预测的等效正弦应力分布与直棒（SR）中的进行比较，并与相应的实验失效循环次数相关联。在直棒构型中，最大等效应力（441MPa）超过了极限曲线，约1.9×10⁵次加载循环后实验棒断裂证实了这一点。在脊柱前凸构型中应力进一步增加，在2.4×10⁴次循环内达到失效。对于脊柱后凸构型（640MPa），最大等效应力低于极限曲线，达到了10⁶次循环的无失效运行。显微镜检查证实了数值预测与实验疲劳裂纹位置之间的一致性。塑形技术（均匀塑形（UC）或法式弯曲器（FB））与任何统计学上的显著差异均无关。我们的研究表明残余应力在改变平均应力分量方面的关键作用，叠加在拉伸循环载荷上，这可能解释了脊柱前凸棒比脊柱后凸棒更高的失效率。
