Gayet L E, Hamcha H, Charbonneau A, Texereau J, Bertheau D, Bellicaud D, Pries P
Service d'Orthopédie-Traumatologie, CHRU de la Milétrie, Hôpital Jean-Bernard, BP 577, 86021 Poitiers.
Rev Chir Orthop Reparatrice Appar Mot. 2001 Sep;87(5):459-68.
The purpose of this experimental study was to compare posterior fixation systems using hooks and screws implanted in the thoracic spine. This study was completed by a digital analysis using the finished element method.
For the experimental study, we used 7 human thoracic spines. Forty-nine groups of 2 vertebrae were individualized. Traction was applied to maximum breaking force measured on an Instron. We used two types of instrumentations, alternating 4 pedicle screws and 2 pedicle-lamina hooks. For the digital study, we used a vertebral model composed of nearly 63 000 nodes and 14 000 elements. Elastic field calculations were carried out with a finished element abacus.
The base of the pedicles broke when traction was applied to a pedicle-lamina hook assembly. The medial part of the pedicle broke when traction was applied to a pedicle screw assembly. Maximul break strength for hooks was 1 108 +/- 510 N. It was 820 +/- 418 N for 4 mm diameter screws and 1 395 +/- 435 for 5 mm screws. The most fragile vertebrae were T5-T6 and T7-T8. the screw-instrumented model showed that stress concentrated on the medial aspect of the pedicle, inside the medullary canal. Using a long screw did not reduce the stress force significantly. The hook-instrumented model showed that stress was greatest on the lower part of the pedicle.
From a mechanical point of view, screw instrumentation is the more appropriate type of fixation. Screw fixation did not however demonstrate its superiority during the traction tests. For 4 mm screws, resistance was 23% weaker than with a hook assembly and for 5 mm screws, it was only 12% stronger. Pullout may be attributed to two principal causes, either fracture of the bony anchoring of the screw system or breakage of the pedicle. Bone thread pullout occurs when the screw threads do not penetrate sufficiently deep into the cortical bone due to the small diameter of the screw shaft. Using a larger diameter screw raises however the problem of damaging the pedicle. Pedicle breakage is seen with stronger stress forces and constitutes the upper limit of maximum break force. This leads us to formulate the hypothesis that in most cases, screw pullout occurs by breakage of the bony threading. Screws are less effective if they are not properly anchored in the pedicles, probably the reason for their relative weakness. Screw diameter should be adapted to the size of each pedicle. This would allow better transmission of stress from the screw to the pedicle. Hooks apply further stress to the vertebrae. The digital study showed that using a long screw crossing through the vertebra does not reduce the stress applied to the pedicles enough to justify its use.
本实验研究的目的是比较用于胸椎的使用钩和螺钉的后路固定系统。本研究通过使用有限元法的数字分析完成。
对于实验研究，我们使用了7具人类胸椎。将49组2个椎体个体化。在英斯特朗万能材料试验机上施加牵引力直至测量到最大断裂力。我们使用了两种类型的内固定器械，交替使用4枚椎弓根螺钉和2枚椎弓根-椎板钩。对于数字研究，我们使用了一个由近63000个节点和14000个单元组成的椎体模型。使用有限元计算软件进行弹性场计算。
当对椎弓根-椎板钩组件施加牵引力时，椎弓根基部断裂。当对椎弓根螺钉组件施加牵引力时，椎弓根内侧部分断裂。钩的最大断裂强度为1108±510N。直径4mm的螺钉为820±418N，直径5mm的螺钉为1395±435N。最脆弱的椎体是T5-T6和T7-T8。螺钉内固定模型显示应力集中在椎弓根内侧，髓腔内。使用长螺钉并不能显著降低应力。钩内固定模型显示应力在椎弓根下部最大。
从力学角度来看，螺钉内固定是更合适的固定类型。然而，在牵引试验中，螺钉固定并未显示出其优越性。对于4mm的螺钉而言，其抗拉力比钩组件弱23%，对于5mm的螺钉，仅强12%。拔出可能归因于两个主要原因，要么是螺钉系统的骨锚固断裂，要么是椎弓根断裂。当螺钉螺纹由于螺钉杆直径小而没有足够深地穿透皮质骨时，就会发生骨螺纹拔出。然而，使用较大直径的螺钉会带来损伤椎弓根的问题。在更强的应力作用下会出现椎弓根断裂，这构成了最大断裂力的上限。这使我们提出一个假设，即在大多数情况下，螺钉拔出是由骨螺纹断裂引起的。如果螺钉没有正确锚固在椎弓根内，其效果就会较差，这可能是它们相对较弱的原因。螺钉直径应与每个椎弓根的大小相适应。这将使应力从螺钉更好地传递到椎弓根。钩会对椎体施加额外的应力。数字研究表明，使用穿过椎体的长螺钉并不能充分降低施加在椎弓根上的应力，从而证明其使用的合理性。
