新型变径全皮质骨螺纹螺钉设计以及在腰椎改良皮质骨轨迹的应用

Design of a Novel Variable-Diameter Cortical Threaded Screw and its Application in Improving Cortical Bone Trajectory in Lumbar Spine

目的 设计一款用于改良皮质骨轨迹(modified cortical bone trajectory,MCBT)置钉技术的新型变径全皮质骨螺纹螺钉,验证新型变径全皮质骨螺纹螺钉在MCBT技术中的力学特性。方法 根据MCBT技术,设计螺钉的螺距为2 mm,全长45 mm,粗杆部分直径恒定5.5 mm,细杆部分直径为4.0~4.5 mm变径,粗杆和细杆连接的变径位置长度为2 mm。从变径位置、螺纹深度、螺纹类型3个方面设置参数,开展三因素三水平L9正交试验,并建立螺钉模型。基于有限元方法对设计的螺钉进行扭转、弯曲和拔出力计算,对结果进行极差分析并确定螺钉模型。建立骨质疏松标本L4椎体三维模型,并按照MCBT技术置钉。比较新型变径全皮质骨螺纹螺钉和常规非变径全皮质骨螺纹螺钉抗拔出力。结果 通过极差分析,得出螺钉6(变径位置距离螺钉头部24 mm,螺纹深度0.7 mm,螺纹类型为45°对称螺纹)为最优螺钉。在抗拔出力方面,第6款变径全皮质骨螺纹螺钉比4.5 mm常规非变径全皮质骨螺纹螺钉提高13.1%,与5.5 mm常规非变径全皮质骨螺纹螺钉相比不具有统计学差异。结论 螺钉变径位置对螺钉拔出力影响最小,螺纹类型对拔出力影响最大,螺纹深度对扭转、弯曲影响最大。与常规非变径全皮质骨螺纹螺钉相比,变径全皮质骨螺纹螺钉前端较细,可避免进钉点发生劈裂;螺钉后端直径较粗,具有较好的抗拔出力性能。研究结果为MCBT技术的临床应用提供新的理论基础。

Objective A novel variable-diameter cortical threaded screw used in a modified cortical bone trajectory(MCBT) was designed to verify its mechanical properties using the MCBT technique.Methods According to MCBT technology,the screw pitch was fixed at 2 mm,the total length was 45 mm,the diameter of the thick rod was 5.5 mm,the diameter of the thin rod was 4.0-4.5 mm,and the length of variable-diameter position connecting the thick rod and the thin rod was 2 mm.The parameters were set based on three aspects:variable-diameter position,thread depth,and thread type.Three-factor and three-level L9 tests were conducted and screw models were established.The torsion and the bending and pull-out force of the designed screws were calculated based on the finite element method,the results were analyzed using range analysis,and then the screw models were determined.The three-dimensional(3D) model of L4 vertebral body in osteoporosis specimens was established and screws were placed according to the MCBT technique.The pull-out force of the novel variable-diameter cortical threaded screw was compared with that of a conventional non-variable-diameter cortical threaded screw.Results Range analysis showed that screw No.6(variable-diameter position:24 mm from the screw head,thread depth:0.7 mm,45° symmetrical thread) was the optimal screw.The anti-pull-out force of the No.6 variable-diameter cortical threaded screw was 13.1% higher than that of the 4.5 mm conventional non-variable-diameter cortical threaded screw,and no statistical difference in anti-pull-out force was found between the No.6 variable-diameter cortical threaded screw and the 5.5 mm conventional non-variable-diameter cortical threaded screw.Conclusions The variable-diameter position has the smallest influence on pull-out force of the screw,the thread type has the largest influence on pull-out force,and the thread depth has the largest influence on torsion and bending.Compared with that of the conventional non-variable-diameter cortical threaded screw,the variable-diameter cortical threaded screw had a smaller front end,which prevented splitting at the entrance point of the screw.The screw has a large diameter at rear end,thereby showing improved pull-out performance.The results provide a new theoretical basis for the clinical application of MCBT technology.