Liu Jia-Ming, Zhang Yu, Zhou Yang, Chen Xuan-Yin, Huang Shan-Hu, Hua Zi-Kai, Liu Zhi-Li
Department of Orthopedic Surgery, The First Affiliated Hospital of Nanchang University, No. 17 Yongwaizheng Street, Donghu District, Nanchang, 330006, Jiangxi Province, People's Republic of China.
Department of Orthopedic Surgery, Jiujiang No. 1 People's Hospital, Jiujiang, 332000, People's Republic of China.
Int Orthop. 2017 Jun;41(6):1183-1187. doi: 10.1007/s00264-017-3453-y. Epub 2017 Mar 28.
Posterior reduction and pedicle screw fixation is a widely used procedure for thoracic and lumbar vertebrae fractures. Usually, the pedicle screws would be removed after the fracture healing and screw tunnels would be left. The aim of this study is to evaluate the effect of screw tunnels on the biomechanical stability of the lumbar vertebral body after pedicle screws removal by finite element analysis (FEA).
First, the CT values of the screw tunnels wall in the fractured vertebral bodies were measured in patients whose pedicle screws were removed, and they were then compared with the values of vertebral cortical bone. Second, an adult patient was included and the CT images of the lumbar spine were harvested. Three dimensional finite element models of the L1 vertebra with unilateral or bilateral screw tunnels were created based on the CT images. Different compressive loads were vertically acted on the models. The maximum loads which the models sustained and the distribution of the force in the different parts of the models were recorded and compared with each other.
The CT values of the tunnels wall and vertebral cortical bone were 387.126±62.342 and 399.204±53.612, which were not statistically different (P=0.149). The models of three dimensional tetrahedral mesh finite element of normal lumbar 1 vertebra were established with good geometric similarity and realistic appearance. After given the compressive loads, the cortical bone was the first one to reach its ultimate stress. The maximum loads which the bilateral screw tunnels model, unilateral screw tunnel model, and normal vertebral model can sustain were 3.97 Mpa, 3.83 Mpa, and 3.78 Mpa, respectively. For the diameter of the screw tunnels, the model with a diameter of 6.5 mm could sustain the largest load. In addition, the stress distributing on the outside of the cortical bone gradually decreased as the thickness of the tunnel wall increased.
Based on the FEA, pedicle screw tunnels would not decrease the biomechanical stability and strength of the vertebral body. A large diameter of screw tunnel and thick tunnel wall were helpful for the biomechanical stability of the vertebral body.
后路复位及椎弓根螺钉固定术是治疗胸腰椎骨折广泛应用的一种术式。通常，骨折愈合后会取出椎弓根螺钉并留下螺钉通道。本研究旨在通过有限元分析（FEA）评估椎弓根螺钉取出后螺钉通道对腰椎椎体生物力学稳定性的影响。
首先，测量接受椎弓根螺钉取出术患者骨折椎体中螺钉通道壁的CT值，并与椎体皮质骨的值进行比较。其次，纳入一名成年患者并采集其腰椎CT图像。基于CT图像创建具有单侧或双侧螺钉通道的L1椎体三维有限元模型。不同的压缩载荷垂直作用于模型。记录模型承受的最大载荷以及模型不同部位的力分布，并相互比较。
通道壁和椎体皮质骨的CT值分别为387.126±62.342和399.204±53.612，差异无统计学意义（P = 0.149）。建立了正常L1腰椎三维四面体网格有限元模型，具有良好的几何相似性和逼真外观。施加压缩载荷后，皮质骨最先达到其极限应力。双侧螺钉通道模型、单侧螺钉通道模型和正常椎体模型能够承受的最大载荷分别为3.97 Mpa、3.83 Mpa和3.78 Mpa。对于螺钉通道直径，直径为6.5 mm的模型能够承受最大载荷。此外，随着通道壁厚度增加，皮质骨外侧的应力分布逐渐减小。
基于有限元分析，椎弓根螺钉通道不会降低椎体的生物力学稳定性和强度。较大直径的螺钉通道和较厚的通道壁有助于椎体的生物力学稳定性。
