BACKGROUND CONTEXT

Pedicle screw-rod assembly procedures following pedicle screw insertion include contouring and placing rods into screw tulips, introducing set screws into the tulip along the screw thread, applying a counter-torque holder and tightening all the set screws clockwise. Even if an appropriate pedicle screw is implanted, screw dislodgement after tightening of the tulip and set screw is not uncommon. Pedicle wall violation resulting from excessive rotational force due to inadequate use of a counter-torque holder might be the reason. However, the strain change in the pedicle during tulip-set screw tightening and the role of counter-torque have never been investigated.

PURPOSE

This study determined differences in the strain change in the outer and inner pedicle walls during tulip-set screw tightening; additionally, the influence of counter-torque on pedicle wall violation was elucidated.

STUDY DESIGN

A controlled biomechanical study; the strain values of outer and inner pedicle walls in cadaveric porcine L4-L5 vertebrae during tulip-set screw tightening with or without a counter-torque holder were measured.

METHODS

Twelve L4-L5 fresh-frozen porcine lumbar vertebrae were implanted with screw-rod constructs; the set screw was randomly locked into the tulip in the right L5, right L4, left L5 and left L4 testing groups. The maximal values from eight strain gauges (P-R-O: outer cortex of right pedicle in proximal vertebra; P-R-I: inner cortex of right pedicle in proximal vertebra; D-R-O: outer cortex of right pedicle in distal vertebra; D-R-I: inner cortex of right pedicle in distal vertebra; P-L-O: outer cortex of left pedicle in proximal vertebra; P-L-I: inner cortex of left pedicle in proximal vertebra; D-L-O: outer cortex of left pedicle in distal vertebra; D-L-I: outer cortex of left pedicle in proximal vertebra) for each specimen during tightening to 12 Nm were measured.

RESULTS

The maximal strain values of the ipsilateral strain gauges in all testing groups were almost significantly higher when a counter-torque holder was not used than when one was used. The strain values in the adjacent pedicle of specimens without a counter-torque holder were significantly increased: P-R-O and P-R-I in the right L5 group; D-R-I in the right L4 group; P-L-I and P-L-O in the left L5 group; D-L-O and D-L-I in the left L4 group.

CONCLUSIONS

The constraint effect of counter-torque during tulip-set screw tightening is necessary. Clockwise rotational force with a fragile lateral pedicle wall suggests that caution is required when using a counter-torque holder to tighten the right L5 and left L4 constructs.

CLINICAL SIGNIFICANCE

A counter-torque holder is important during tulip-set screw tightening; improper use may lead to adjacent pedicle wall violation, sequentially resulting in pedicle screw loosening.

中文翻译：

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反扭矩支架在拧紧椎弓根螺钉杆结构中的作用：猪模型的生物力学研究

背景语境

椎弓根螺钉插入后的椎弓根螺钉杆组装程序包括修整轮廓并将杆放入螺钉郁金香中，将固定螺钉沿螺纹引入郁金香，应用反扭矩支架并顺时针拧紧所有固定螺钉。即使植入合适的椎弓根螺钉，在拧紧郁金香和固定螺钉后螺钉移位也并不少见。由于反扭矩支架使用不当而导致过度旋转力导致的椎弓根壁侵犯可能是原因。然而，从未研究过郁金香固定螺钉拧紧过程中椎弓根的应变变化和反扭矩的作用。

目的

本研究确定了郁金香固定螺钉拧紧过程中外、内椎弓根壁应变变化的差异；此外，还阐明了反扭矩对椎弓根壁侵犯的影响。

学习规划

受控生物力学研究；测量了在使用或不使用反扭矩支架的情况下，尸体猪 L4-L5 椎骨外椎弓根和内椎弓根壁的应变值。

方法

12 块新鲜冷冻的 L4-L5 猪腰椎植入螺钉杆结构；右L5、右L4、左L5、左L4测试组将固定螺钉随机锁入郁金香内。八个应变片的最大值（PRO：近端椎体右侧椎弓根外皮质；PRI：近端椎体右侧椎弓根内侧皮质；DRO：远端椎体右侧椎弓根外侧皮质；DRI：远端椎体右侧椎弓根内侧皮质椎体；PLO：近端椎体左侧椎弓根外皮质；PLI：近端椎体左侧椎弓根内侧皮质；DLO：远端椎体左侧椎弓根外侧皮质；DLI：近端椎体左侧椎弓根外侧皮质）测量到 12 Nm 的拧紧力。

结果

当不使用反扭矩支架时，所有测试组中同侧应变仪的最大应变值几乎显着高于使用反扭矩支架时的值。没有反扭矩支架的标本的相邻椎弓根的应变值显着增加：右侧 L5 组的 PRO 和 PRI；右 L4 组 DRI；左侧 L5 组中的 PLI 和 PLO；左 L4 组中的 DLO 和 DLI。

结论

梅花紧定螺钉拧紧过程中反扭矩的约束作用是必要的。顺时针旋转力与脆弱的外侧椎弓根壁表明，在使用反扭矩支架收紧右侧 L5 和左侧 L4 结构时需要谨慎。

临床意义

在郁金香紧定螺钉拧紧过程中，反扭矩支架很重要；使用不当可能会侵犯相邻椎弓根壁，继而导致椎弓根螺钉松动。