应用有限元模型分析椎弓根拉力螺钉固定枢椎环骨折的稳定性

Biomechanical study of different kinds of Pedicle lag screw in posterior fixation for the fracture of the ring of axis:a finite-element analysis

目的:应用有限元模型分析半螺纹椎弓根拉力螺钉和全螺纹椎弓根拉力螺钉后路固定枢椎环骨折的生物力学稳定性,为临床椎弓根拉力螺钉的选择提供理论参考。方法:选取1例28岁健康成年男性志愿者,采用256排螺旋CT对其从C0~3节段进行薄层扫描,扫描层厚0.625mm。利用Mimics 20、3-matic 13.0及Ansys16.0,建立正常颈椎C0~3节段三维六面体网格有限元模型(FE/Intact)并与体外实验及其他模型文献数据进行有效性对比验证;在已验证的模型上通过弱化网格单元强度的方法模拟建立三种枢椎环骨折模型(FE/Fracture A、B、C);分别建立半螺纹椎弓根拉力螺钉及全螺纹拉力内固定模型,用两种不同螺钉分别固定FE/Fracture A、B、C骨折模型,进行边界约束后分别施加前屈、后伸、侧屈、旋转四种生理载荷,比较各模型在不同工况下三维活动的角位移(range of motion,ROM)及骨折端节点位移变化。结果:建立的有限元模型外观逼真,几何相似性好,经验证有效。在相同条件下FE/Fracture A、B、C模型三维活动度较FE/Intact模型明显增大。半螺纹椎弓根拉力螺钉模拟固定FE/Fracture A后模型前屈、后伸、侧屈及旋转方向上C2~3 ROM分别为2.9°、2.2°、2.0°、1.8°,较固定前的7.8°、5.8°、4.8°、3.6°相比,为固定前的37.18%、37.93%、41.67%、50.00%;而相同情况下,全螺纹椎弓根螺钉C2~3 ROM分别为3.0°、2.3°、2.1°、1.9°,为固定前的38.46%、39.66%、43.75%、52.78%。半螺纹椎弓根拉力螺钉模拟固定FE/Fracture B后模型各工况下C2~3 ROM分别为3.2°、2.4°、2.3°、2.5°,较固定前的8.1°、6.2°、5.0°、4.0°相比,为固定前的39.51%、38.71%、46.00%、62.50%;而相同情况下,全螺纹椎弓根螺钉C2~3 ROM分别为3.3°、2.4°、2.5°、2.6°,分别为固定前的40.74%、40.32%、48.00%、65.00%。半螺纹椎弓根拉力螺钉模拟固定FE/Fracture C后模型各工况下C2~3 ROM分别为3.6°、3.0°、2.8°、2.5°,较固定前的8.9°、6.3°、5.7°、4.6°相比,分别为固定前的40.45%、47.62%、49.12%、54.35%;而相同情况下,全螺纹椎弓根螺钉C2~3 ROM分别为3.7°、3.3°、2.9°、2.7°,分别为固定前的41.57%、52.38%、50.88%、58.70%。同一种骨折模型中,将两种螺钉固定后所得节点位移采用Wilcoxon符号秩检验,两组差异具有统计学意义(P<0.05),半螺纹椎弓根拉力螺钉内固定系统在固定枢椎环骨折端和限制骨折端位移方面效果优于全螺纹螺钉。结论:半螺纹椎弓根拉力螺钉固定枢椎环骨折较全螺纹椎弓根拉力螺钉提供较强的生物力学稳定性,能够满足C2~3节段正常生理活动,达到稳定骨折断端以重建上颈椎稳定性的目的。

Objectives:To evaluate the biomechanical stability of posterior fixation of fracture of the ring of axis(FRA)with semi-threaded and full-threaded pedicle lag screw using finite element analysis,and to provide theoretical reference for clinical selection of pedicle lag screw.Methods:A 28-year-old healthy male volunteer was scanned by 256-slice spiral CT from C0 to C3 with a slice thickness of 0.625mm.Using Mimics 20.0,3-matic 13.0 and Ansys 16.0,a three-dimensional hexahedral mesh finite element model(FE/Intact)of C0-3 segments of normal cervical spine was established and validated.Three models of axis ring fracture(FE/Fracture A,B and C)were simulated and validated by weakening the strength of mesh element on the validated model.A semi-threaded screw and a full-threaded screw internal fixation model were established,and used to fix the FE/Fracture A,B,and C fracture models respectively.After boundary restraint,four kinds of physiological loads were applied:forward flexion,backward extension,lateral flexion and axial rotation.The three-dimensional angular range of motion(ROM)and the displacement of fracture end nodes under different working conditions were compared among these models.Results:The ROM of FE/Fracture A,B and C models increased significantly compared with the FE/Intact model under the same loading conditions.After the FRA was fixed by the semi-threaded screw,the C2-3 ROM of the FE/Fracture A model in the flexion,extension,lateral flexion and axial rotation were 2.9°,2.2°,2.0°,and 1.8°,which were 37.18%,37.93%,41.67%,and 50.00%prior to fixation;As for the full-thread screws,C2-3 ROM were 3.0°,2.3°,2.1°,and 1.9°,which were 38.46%,39.66%,43.75%,and 52.78%of the ROM prior to fixation.After the FRA was fixed by the semi-threaded screw,the C2-3 ROM of the FE/Fracture B model in the flexion,extension,lateral flexion and axial rotation were 2.9°,2.2°,2.0°,and 1.8°,which were 37.18%,37.93%,41.67%,and 50.00%prior to fixation;the full-thread screws C2-3 ROM were 3.0°,2.3°,2.1°,and 1.9°,being 38.46%,39.66%,43.75%,and 52.78%prior to fixation.The C2-3 ROM was 3.2°,2.4°,2.3°,2.5°in each working condition of the model after the FE/Fracture B was fixed by semi-threaded screw.Compared with 8.1°,6.2°,5.0°,and 4.0°before fixation,they were respectively 39.51%,38.71%,46.00%,and 62.50%;the full-thread screws C2-3 ROM were 3.3°,2.4°,2.5°and 2.6°,being 40.74%,40.32%,48.00%,and 65.00%prior to fixation.The C2-3 ROM was 3.6°,3.0°,2.8°,2.5°in each working condition of the model after in each working condition of the model after the FE/Fracture C was fixed by semi-threaded screw.Compared with 8.9°,6.3°,5.7°,and 4.6°before fixation,they were respectively 40.45%,47.62%,49.12%,and 54.35%;the full-thread screws C2-3 ROM were 3.7°,3.3°,2.9°and 2.7°,being 41.57%,52.38%,50.88%,and 58.70%prior to fixation.In the same fracture model,Wilcoxon′s signed-rank test was used to analyze and compare the node displacement after two kinds of screws were fixed.The difference between the two groups was statistically significant(P<0.05).The semi-threaded pedicle lag screw internal fixation system was superior to the full-screw screw in fixing the fracture of FRA and limiting the displacement of the fracture end node.Conclusions:Semi-threaded pedicle lag screw fixation for FRA provides greater biomechanical stability than full-thread screw.It can meet the normal physiological activities of the C2-3 segment and achieve the purpose of stabilizing the fracture end to reconstruct the stability of the upper cervical vertebra.