椎弓根螺钉长度变化对螺钉-骨复合体模型应力影响的三维有限元分析研究

Three dimensional finite element analysis of stress distribution on continuously varying of length of pedicle screw

目的利用三维有限元模型研究椎弓根螺钉长度变化对生理载荷下螺钉-骨复合体模型的应用影响。方法建立椎弓根螺钉和L1椎体的三维模型,并对其进行网格划分,设置椎弓根钉长度尺寸的变化范围。模拟生理载荷条件下,对不同长度尺寸的椎弓根钉有限元模型进行应力分析。结果椎弓根螺钉长度在30～50mm范围内变化时,随着螺钉长度的增大,螺钉-骨复合体模型的骨质部分承担的应力均减小,而螺钉承担的应力则增加。螺钉最大平均主应变出现在螺钉的尾端,皮质骨发生的最大平均主应力位置出现在螺钉与皮质骨接触面两侧,松质骨发生的最大平均主应力位置出现在螺钉头部与松质骨接触面两侧。当螺钉长度达到50mm时,载荷力传递到皮质骨和松质骨分别减小了43.1%和42.3%,而螺钉上出现的则增加了38%。当椎弓根螺钉长度大于45mm时,螺钉-骨复合体模型各部分应力变化不明显。结论椎弓根螺钉长度在30～50mm范围变化时,在生理载荷下,椎弓根螺钉长度的增大有利于改善螺钉、皮质骨及松质骨上轴向应力的力学分布;只要骨量允许,临床选择椎弓根螺钉的长度应不小于45mm。

Objective To study the stress distribution on different length of pedicle screws under the physiological load by using the three dimensional finite element analysis.Method A three-dimension model of the pedicle screw and L1 vertebral body were constructed with the model meshed on the basis of the finite element method.The feature dimension of pedicle screw in the model was set in a specified varied range.Under the physiological load,the stress on every model with different diameter sizes of pedicle screw was analyzed.Results The stress on every bone model decreased with the increase of stress on screw under the axially pullout force as the length of screw ranged from 30mm to 50mm.And the maximum Equivalent Stress（EQV Stress） in the pedicle screw appeared in the central area of the pedicle screw,the maximum Equivalent Stress（EQV Stress） in the cortical bone appeared in both sides of contact surface,the maximum Equivalent Stress（EQV Stress） of the cancellous bone appeared in both sides of contact surface of the top of pedicle screw and cancellous bone.In a certain external load,with the 50mm length of screw,the load that transfers to the cortical bone and cancellous bone is reduced by 43.1% and 42.3%,respectively,while the maximum Equivalent Stress（EQV Stress） of screw was increased 38%.When L≥45mm,the variable stress on all models become stable.Conclusions While the length of screw is in range of 4.0 mm to 6.5 mm,30～50 mm,the increase of pedicle screw length could improve the distribution of axial pullout stress on the screws,cortical bone and cancellous bone.As long as the bone mass allowed,the length of pedicle screws should be not less than 45mm in clinical choice.