Study design

Cadaveric biomechanical with imaging analysis.

Objective

This study aims to compare the fixation failure between pedicel screws (PS) and cortical screws (CS), thus to investigate their failure mechanisms under vertical migration.

Summary of background data

Due to their minimal invasive nature, CS are gaining popularity. However, contradictions exist in the literature regarding whether CS may have superior fixation failure resistance compared to PS under vertical migration.

Methods

Human vertebral specimens were examined under Dual-energy X-ray. For each specimen, PS were inserted on the left and CS on the right with rods secured. Vertical force-displacement tests were applied to rods. MicroCT images were taken pre and post-MTS® for microstructural analysis.

Results

The average T-scores of the specimens were −4±0.25. Three phases of force-displacement behaviour featuring different PS and CS failure-resistance were discovered. For phase I, the force required to migrate PS tended to be slightly higher than CS. However, during phase II, a fixation instability occurred for PS and the CS fixation strength was superior. For phase III under large displacement, CS did not require increased force to displace, whereas PS re-stabilised and revealed improved displacement resistance. Both force analysis and microstructural analysis indicated that PS migrated along the direction of the vertical loading, whereas CS had a force component in the longitudinal axis of the screw.

Conclusions

Different failure mechanisms underlay PS and CS under large vertical displacement. PS fail with trabecular bone compaction possibly altering the initial material property surround the screw. CS fail with screw cut-out due to the force component along the screw axis.

中文翻译：

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大位移下椎弓根螺钉和皮质螺钉固定的失效机理：基于临床病例的生物力学和微观结构研究。

学习规划

尸体生物力学与影像分析。

目的

本研究旨在比较椎弓根螺钉（PS）和皮质螺钉（CS）的固定失效，从而研究其在垂直移动下的失效机理。

背景数据汇总

由于其极小的侵入性，CS越来越受欢迎。但是，在垂直移动下，与PS相比CS是否具有更好的固定失败抵抗力，文献中存在矛盾。

方法

在双能X射线下检查人的椎骨标本。对于每个标本，在固定杆的情况下将PS插入左侧，将CS插入右侧。垂直力位移测试应用于杆。在MTS®之前和之后拍摄MicroCT图像以进行微结构分析。

结果

样品的平均T分数为-4±0.25。发现了具有不同抗压强度和抗压强度的三个阶段的力-位移行为。对于第一阶段，迁移PS所需的力往往比CS高。然而，在第二阶段，PS发生了固定不稳定性，CS的固定强度更高。对于大位移下的第三阶段，CS不需要增加位移力，而PS重新稳定并显示出更高的位移阻力。力分析和微观结构分析均表明PS沿垂直载荷方向迁移，而CS在螺杆的纵轴上有力分量。

结论

在大的垂直位移下，不同的破坏机制会为PS和CS奠定基础。PS失败，导致小梁骨压紧，可能会改变螺钉周围的初始材料属性。由于沿螺钉轴的力分量，CS不能通过螺钉切除而失效。