Wu Shing-Sheng, Edwards W. Thomas, Yuan Hansen A.
Department of Orthopaedic Surgery, Tri-Service General Hospital, 8. Section 3, Ting-Chow Road, National Defence Medical Center, Taipei, Taiwan, ROC.
Clin Biomech (Bristol). 1998;13(1 Suppl 1):S1-S8. doi: 10.1016/s0268-0033(97)00069-7.
This study was undertaken to evaluate the variation in bone density within the vertebral body and to determine the biomechanical stiffness of the screw-bone interface for different superior-inferior transpedicular screw orientations in the vertebral body. DESIGN: The stiffness of three directions of screw placement (upper, middle, and lower) were measured in two modes of loading (flexion and torsion). All screws were inserted to 80% of the distance along the path of the screw from the point of insertion to the anterior vertebral cortex. BACKGROUND: The placement of transpedicular screws within the vertebral body is a fundamental determinant the stiffness and strength of the bone-screw interface and consequently the stability of an implant construct. The bone stiffness within the pedicle and vertebra must be sufficient to resist spine forces and moments. METHODS: The stiffness of pedicle-screw fixation was tested for three different screw orientations based on the position of the screw tip in the vertebral body (upper, middle, and lower). Fixation rigidity was measured for two modes of loading (flexion and torsion). In all, eighteen individual vertebrae (L(2)-L(4)) from 6 cadaveric spine segments were examined. All the screws were inserted to 80% of the depth of the posterior to anterior vertebral cortex dimension. Quantitative bone density measurements were obtained from vertebral mid-sagittal cross sections of the additional vertebral bodies using an image analysis system to determine the distribution of bone density in the mid-sagittal plane. RESULTS: Based on area fraction measurements of cancellous bone in seventeen L1 mid-sagittal regions, highest densities were observed adjacent to the endplates (area fractions > 0.29). Regions of lowest density were found in the central portions of the vertebral bodies, above the mid-line of the body (area fractions < 0.20). In flexion, the stiffness of all three groups of screws were similar, ranging from 4.0-4.2 Nm deg(-1). In torsion the upper and middle directions were significantly more rigid than lower position (p < 0.04). CONCLUSION: Pedicle screw insertion aimed toward the superior-anterior aspect of the vertebral body, adjacent to the superior endplate provides the best overall rigidity considering both directions of loading. Screws placed in the mid-portion of the vertebral body may provide good lateral torsion rigidity, but would not be optimal for flexion stability if bone density is reduced in that region.
本研究旨在评估椎体骨密度的变化，并确定椎体中不同上下方向椎弓根螺钉置入时螺钉与骨界面的生物力学刚度。
在两种加载模式（前屈和扭转）下测量三种螺钉置入方向（上、中、下）的刚度。所有螺钉均沿螺钉路径从置入点插入至椎体前皮质距离的80%。
椎体椎弓根螺钉的置入是骨-螺钉界面刚度和强度以及植入物结构稳定性的基本决定因素。椎弓根和椎体内的骨刚度必须足以抵抗脊柱的力和力矩。
根据螺钉尖端在椎体中的位置（上、中、下），测试三种不同螺钉置入方向的椎弓根螺钉固定刚度。测量两种加载模式（前屈和扭转）下的固定刚度。总共检查了来自6个尸体脊柱节段的18个单个椎体（L2-L4）。所有螺钉均插入至椎体后皮质到前皮质深度的80%。使用图像分析系统从额外椎体的椎体矢状中截面获得定量骨密度测量值，以确定矢状中平面内的骨密度分布。
基于17个L1矢状中区域的松质骨面积分数测量，在终板附近观察到最高密度（面积分数>0.29）。最低密度区域位于椎体中央部分，在椎体中线以上（面积分数<0.20）。在前屈时，三组螺钉的刚度相似，范围为4.0-4.2 Nm deg-1。在扭转时，上方向和中方向比下位置明显更刚硬（p<0.04）。
考虑到两种加载方向，朝向椎体上前方、靠近上终板置入椎弓根螺钉可提供最佳的整体刚度。置于椎体中部的螺钉可能提供良好的侧向扭转刚度，但如果该区域骨密度降低，则对前屈稳定性而言并非最佳。
