胸腰椎椎体骨-螺钉界面剪切载荷的生物力学研究

Shear load on bone-screw interface of thoracolumbar vertebrae：a biomechanical study

目的：在胸腰椎脊柱前路手术中，术中术后常见椎体钉切割椎体而松动或拔出，尤其是骨质疏松的病人。通过生物力学实验，探讨胸腰椎椎体所能耐受前路椎体钉的最大剪切力、相关螺钉位移值及其与椎体骨密度的关系。方法取8具新鲜成人尸体胸腰段脊柱标本获取32个单个椎体标本并测量骨密度值，椎体钉置于椎体中心点并穿透对侧皮质固定，随机分成实验组和对照组，实验组先行剪切载荷，再行拔出载荷，对照组只行拔出载荷，记录最大剪切力和最大轴向拔出力，实验后观察椎体破坏和钉道情况。结果实验组钉道较对照组宽，以螺钉尖为顶点成扇形扩大，螺纹内只有少量骨屑。椎体能承受的最大剪切力为（536.63±82.99）N，相对应的螺钉位移为（8.79±1.08）mm。线性回归分析显示最大剪切力、螺钉位移值与椎体骨密度值之间具有相关性（P〈0.01），相关系数（r）分别是0.958、-0.933。实验组与对照组的骨密度值统计学上差异无统计学意义（P〉0.05）。实验组与对照组的最大轴向拔出力，两者差异有统计学意义（P〈0.01）。结论胸腰椎椎体能承受椎体钉的最大剪切力与椎体骨密度呈正相关，相应螺钉位移值与椎体骨密度呈负相关。胸腰椎前路手术行脊柱畸形的矫正力量也是影响骨—螺钉界面固定强度的重要因素。

Objective Vertebral screw may frequently become loosened or easy to extract due to incision of the vertebrae during or after surgery of thoracolumbar spine via the anterior approach ,especially in osteoporotic patients. This study aims to investigate the correlation between the maximum tolerable shear force of anterior vertebral screw on thoracolumbar vertebrae,displacement value of related screw and bone mineral density（BMD）of vertebra. Methods Thirty？two single vertebral specimens were obtained from 8 fresh adult cadaveric thoracolumbar spine specimens,and measure for BMD. The vertebral screws were placed in the center point of vertebrae and penetrated the contralateral cortex for fixation. All the specimens were randomly divided into the experimental group and control group. The experimental group experienced shear load first,followed by pull？out force,whereas the control group experienced pull？out force alone. The maximum shear force and maximum axial pull？out force were recorded. The condition of vertebral destruction and screw channel were observed after the experiment. Results The screw channel in the experiment group was wider than that in the control group,expanding into a sector from the screw tip,with a little bone debris in the thread. The maximum tolerable shear force on vertebrae was 536.63 ± 82.99 N,with related＆amp;nbsp;screw displacement of 8.79 ± 1.08mm. Linear regression analysis showed significant correlations between the maximum shear force,screw displacement and vertebral BMD（P〈0.01）,and the correlation coefficient（r）was 0.958 and ？0.933,respectively. There was no statistically significant difference in BMD between the experimental group and control group（P〉0.05）. However,statistically significant difference was found in the maximum axial pull？out force between the experimental group and control group （P〈0.01）. Conclusion Vertebral BMD is positively correlated with the maximum tolerable shear force of anterior vertebral screw on thoracolumbar vertebrae,and negatively correlated with the screw displacement. Orthopedic force on spinal deformity is an important factor in anterior approach of thoracolumbar spine,which may affect fixation strength of bone？screw interface.