一种基于影像特征经皮椎体成形术生物力学稳定性评估的有限元模型

背景: 生物力学研究表明椎体成形术在注入骨水泥后单个骨质疏松椎体的硬度和强度有很大的增长, 但对于保持椎体力学稳定性所需骨水泥的体积和其注入位置还有待进一步研究。目的: 通过参数有限元模型比较椎体成形手术腰椎 L1- L2 的生物力学稳定性评估椎体成形术的有效性。设计: 非随机对照观察。单位: 北京神经外科研究所。材料: 选择 1 例 54 岁女性骨质疏松性导致腰椎 L1- L2 压缩性骨折患者, 经 CT 确诊, 该患者对实验项目知情同意。通过椎体成形手术前后CT 扫描, 分别得到了各 90 张连续切片, 层厚 1 mm, 每个像素的尺寸是 0.33 mm。方法: 实验于 2005- 10/2006- 06 在北京神经外科研究所完成。①三维有限元模型的创建: 通过整合 CT 和 MRI 图像的解剖结构, 建立了一种腰椎功能性脊椎单元的三维几何模型, 基于该几何模型对搜集到 90 张连续 CT 切片数据进行转换并对图像进行分割, 创建两种分别表示患者通过椎体成形手术前后的 L1～2 段椎体三维有限元模型。②模型评估: 在模型 L1 椎体的上表面分别施加 500,1 000,1 500,2 000,2 500 N 的轴向负载,观察有限元模型腰椎 L1～2 段的位移、应力、应变的分布变化情况; 观察骨水泥体积的增加对有限元椎体模型位移、应力、应变的分布的影响。主要观察指标: ①不同负载下有限元模型腰椎 L1～2 段的位移、应力、应变的分布变化情况。②骨水泥体积的增加对有限元椎体模型位移、应力、应变的分布的影响。结果: ①在轴向负载下 L1～2 段的应力应变随着负载的增大而增大, 在椎间盘的右侧部有很大的应力应变集中。而这一区域也是在负载情况下经常发生损伤的部位。在椎间盘上应力的大小取决于应用于椎体单元负载的大小。负载越大, 应力越大。②通过椎间盘上接触应力的增加, 随着骨水泥体积的增加导致了临近椎体的负载转移。结论: 建立了椎体成形手术前后 L1～2 段椎体三维有限元模型, 在有限元模型上计算腰椎 L1～2 相对位移可以说明椎体三维的生理状态。这一仿真计算也能清楚地显示椎体应力应变的分布和术前术后的变形。

A finite element model based on medical image for evaluating biomechanical stability of percutaneous vertebroplasty

BACKGROUND: Researches on vitodynamics present that percutaneous vertebroplasty (PVP) can strengthen hardness and intension of single osteoporosis vertebral body after injection of bone cement; however, the infused volume and site of bone cement for maintaining mechanical stability of vertebral body should be further studied.OBJECTIVE: A numerical calculation method on finite element models (FEM) for biomechanical analysis has been developed, while a boundary condition describing the relative L1 -L2 displacement is imposed on the FEM to account for three-dimensional physiological states.DESIGN: Non-randomized control study.SEITING: Beijing Neurosurgical Institute.MATERIALS: One female patient aged 54 years with compressibility fracture at lumbar vertebrae L1-L2 induced by osteoporosis was diagnosed with CT examination, and the patient was told the fact. Based on CT scanning before and after PVP, 90 serial sections with the thickness of 1 mm were obtained and the size of each pixei was 0.33 mm.METHODS: The experiment was carried out Beijing Neurosurgery Institute from October 2005 to June 2006. ①Establishment of three-dimensional FEM: Integrating the anatomical structure from the spine CT and MRI image of a patient, a novel three-dimensional geometric model of lumbar functional spinal units (FSUs) has been built. Meanwhile, 90 serial sections were obtained to exchange data of CT sections and divide imagings. Based on the geometric model, two kinds of three-dimensional FEM of L1-L2 segments for preoperative and postoperative vertebrae were created. ② Model evaluation: The lcad of (500 N, 1 000 N, 1 500 N, 2 000 N, 2 500 N) axial compression were applied to the superior surface of the model in the form of a uniformly concentrated lcad over all L1 superior surface nodes respectively. We could observe the stress distribution of L1-L2 segment by applying the load and clue on the high stress concentration region as the most likely areas fracture.MAIN OUTCOME MEASURES: ① Changes of displacement, stress and strain distributions of FEM at L1-L2 segment of lumbar vertebrae under various loads; ② Effect of increase of bone cement volume on displacement, stress and strain distribution.RESULTS: ① The increase in displacement, stress and strain of FE model with the increase of loading in the postoperarive cases. The tendency was approximately linear which also illustrates the spine have flexible biomechanical characteristics. The region was a common place for injures due to loading. The magnitude of stress in the intervertebral disc depended on the proportion of load applied to the superior surface of the motion segment. The heavier the lcad was, the stronger the stress was. ② With the increase of stress on intervertebral disc, the increasing volume of bone cement could induce transfusion of load of nearby vertebral body.CONCLUSION: Building three-dimensional FEM of L1-L2 segments for preoperative and postoperative PVP can explain three-dimensional physiological status of vertebral body based on calculating relative shift of L1-L2 segment of lumbar vertebrae. Meanwhile, simulative calculation can clearly express distribution of stain and stress and preoperative and postoperative deformity of vertebral body.