BACKGROUND: Atlantoaxial posterior approach is currently the main surgical treatment for atlantoaxial instability and/or dislocation, but the shape of rod in normal screw-rod device system is cylindrical. To obtain satisfactory reduction of atlantoaxis, the rod will be pre-bent obviously before fixation; however, the cylindrical rod will be rotated when tighten the nuts. Extra devices will be required to adjust and maintain the direction of the rod, thereafter, the surgical field will be blocked by the device, and spinal injury will occur once the devices are not held tightly. While the novel automatic anti-rotation rod has the function of anti-rotation during nut-tightening process, and also holds all the advantages of normal rod. Further research should be performed for the differences in biomechanical characteristics between two methods.OBJECTIVE: To investigate the biomechanical properties of the novel automatic anti-rotation rod for internal fixation system of atlantoaxial posterior approach based on three-dimensional finite-analysis model of upper cervical spine.METHODS: The three-dimensional finite element model of upper cervical spine with internal rod fixation system was developed. The biomechanical characteristics of the internal fixation system were analyzed. RESULTS AND CONCLUSION: The traditional and novel three-dimensional finite element model with realistic and geometric similarity contained 198330 elements, 964747 nodes and 246788 elements, 996069 nodes,correspondingly. There was no obvious stress concentration in both two systems, stress was concentrated mainly in the screw-bone and screw-rod interfaces. The stress values of the novel system were higher than those of the traditional system, but the maximum Von Misses Stress of two systems was lower than the yield (795-827 MPa ) and ultimate (860-896 MPa) strength of titanium alloys. These results show that the design of the novel automatic anti-rotation rod-screw fixation system has matched the biomechanical requirements for new internal fixation instruments, and is one safe, effective and practical device for atlantoaxial posterior procedure showing promising application prospect.%背景:寰枢椎后路内固定是目前治疗寰枢椎失稳和或脱位的主要术式,但是常规后路钉棒内固定系统中的连接棒是圆柱形,为增强复位功能,连接棒预弯的曲度一般较大,在拧螺母固定时连接棒容易发生旋转,需要额外的辅助器械把持和固定连接棒的方向,但辅助器械存在手术术野遮挡,且当辅助器械把持不稳时可能造成脊髓的过度骚扰和意外损伤.自行防旋转寰枢椎钉棒系统是一种既保持了常规钉棒系统的优点,又具有自行防旋转的功能,但其生物力学性能与常规钉棒系统之间的差异需进一步研究.目的:通过建立寰枢椎后路钉棒内固定系统的三维有限元模型,评估新型自行防旋转寰枢椎后路钉棒内固定系统的生物力学特性.方法:应用有限元分析软件创建上颈椎后路钉棒内固定系统的有限元模型,分析得出钉棒内固定系统的应力分布云图.结果与结论:建立的有限元模型几何相似性好,其中常规及新型钉棒系统模型分别包含198330个节点和964747个单元,246788个节点和996069个单元.两种内固定系统均无明显应力集中现象,且主要应力分布区域基本一致,主要集中螺钉-骨质界面、螺钉-连接棒界面,其中新型自行防旋转寰枢椎后路钉棒内固定系统的主要应力分布区域的应力均值高于常规钉棒系统,但2种钉棒内固定系统连接棒的最大应力值均小于钛合金棒的屈服强度(795-827 MPa)和极限强度(860-896 MPa).提示新型自行防旋转寰枢椎后路钉棒内固定系统连接棒的设计符合生物力学的要求,新型连接棒是一种有效、安全的脊柱内固定装置,可在临床推广应用.
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