Spiegel D A, Cunningham B W, Oda I, Dormans J P, McAfee P C, Drummond D S
Division of Orthopaedic Surgery, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104, USA.
Spine (Phila Pa 1976). 2000 Nov 1;25(21):2755-61. doi: 10.1097/00007632-200011010-00007.
This in vitro biomechanical study examines segmental anterior vertebral screw strain and solid rod construct stiffness with and without the addition of multilevel, threaded cortical bone dowels in a bovine model.
To determine whether strain at the bone-screw interface is higher at the end levels during physiologic range loading, and whether solid interspace support decreases segmental strain on the implant.
Anterior instrumentation provides greater correction and preserves distal motion segments. However, nonunion and implant failure are observed more frequently than with posterior segmental instrumentation, and when observed, loss of fixation occurs at the end levels.
Eight calf spines underwent mechanical testing in the following sequence: 1) intact condition, 2) anterior release with anterior solid rod and bicortical rib grafts, and 3) anterior release with anterior solid rod and threaded cortical bone dowels (L2-L5). Instrumented vertebral screws were used to assess strain within the vertebral body by the near cortex, whereas an anterior extensometer spanning the instrumented segments was used to measure segmental displacements to calculate construct stiffness. The protocol included axial compression (-400 N), right lateral bending (4 Nm (Newton-meter), away from the implant), and left lateral bending (4 Nm, toward the implant). Statistical analysis included a one-way analysis of variance and a Student-Newman-Keuls post hoc test. A pilot study was performed using four additional specimens loaded for 4000 cycles to investigate macroscopic loosening after fatigue loading.
In lateral bending toward the implant, the strain was higher at both end levels, with no differences between the rib and dowel reconstructions. The stiffness values were greater than the intact values for both groups. In lateral bending away from the implant, the strain also was higher at both end screws, and the dowel group had less strain at these levels than the rib group. Both groups were stiffer than the intact condition, and the dowel group was stiffer than the rib group. Axial compressive strain also was higher at the end levels, but this difference did not reach statistical significance. The rib group did not reach intact stiffness values, whereas the dowel group was stiffer than the intact condition. The fatigue study showed gross loosening at one or both end levels in all cases.
Higher strain was observed at the bone-screw interface in both end screws of an anterior solid rod construct during lateral bending, which correlates with the clinically observed failure location. This suggests that physiologic range loading may predispose to failure at the end levels. Disc space augmentation with solid implants increased construct stiffness in all three load paths and decreased strain at the end levels in lateral bending away from the implant. Future implant modifications should achieve better fixation at the end screws, and the current model provides a means to compare different strategies to decrease strain at these levels.
这项体外生物力学研究在牛模型中，研究了添加和不添加多级带螺纹皮质骨销时节段性前路椎体螺钉应变及实心棒结构刚度。
确定在生理范围负荷期间，终末节段骨 - 螺钉界面处的应变是否更高，以及实心间隙支撑是否会降低植入物上的节段应变。
前路器械固定提供了更大的矫正效果并保留了远端运动节段。然而，与后路节段性器械固定相比，不愈合和植入物失败更频繁地被观察到，并且当出现这种情况时，固定丢失发生在终末节段。
八具小牛脊柱按以下顺序进行力学测试：1）完整状态；2）前路松解并植入前路实心棒和双皮质肋骨移植；3）前路松解并植入前路实心棒和带螺纹皮质骨销（L2 - L5）。使用仪器化的椎体螺钉通过近皮质评估椎体内的应变，而跨越仪器化节段的前路引伸计用于测量节段位移以计算结构刚度。实验方案包括轴向压缩（-400 N）、右侧弯曲（4 Nm（牛顿 - 米），远离植入物）和左侧弯曲（4 Nm，朝向植入物）。统计分析包括单因素方差分析和Student - Newman - Keuls事后检验。使用另外四个标本进行4000次循环加载的预试验，以研究疲劳加载后的宏观松动情况。
在朝向植入物的侧弯中，两个终末节段的应变均较高，肋骨和骨销重建之间无差异。两组的刚度值均大于完整状态的值。在远离植入物的侧弯中，两个终末螺钉处的应变也较高，并且骨销组在这些节段的应变低于肋骨组。两组均比完整状态更硬，并且骨销组比肋骨组更硬。轴向压缩应变在终末节段也较高，但这种差异未达到统计学意义。肋骨组未达到完整的刚度值，而骨销组比完整状态更硬。疲劳研究显示所有病例中一个或两个终末节段均出现明显松动。
在前路实心棒结构的两个终末螺钉的骨 - 螺钉界面处，侧弯时观察到更高的应变，这与临床观察到的失败位置相关。这表明生理范围负荷可能使终末节段易于发生失败。使用实心植入物增加椎间盘间隙可提高所有三种负荷路径中的结构刚度，并降低远离植入物侧弯时终末节段的应变。未来的植入物改进应在终末螺钉处实现更好的固定，并且当前模型提供了一种比较不同策略以降低这些节段应变的方法。
