Julius Wolff Institute, Charité-Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany.
Eur Spine J. 2012 Apr;21(4):666-73. doi: 10.1007/s00586-011-2047-4. Epub 2011 Oct 20.
Pedicle-screw-based dynamic implants are intended to preserve intervertebral mobility while releasing certain spinal structures. The aim of the study was to determine the as yet unknown optimal stiffness value of the longitudinal rods that fulfils best these opposing tasks.
A finite element model of the lumbar spine was used which includes the posterior implant at level L4/5. More than 250 variations of this model were generated by varying the diameter of the longitudinal rods between 6 and 12 mm and their elastic modulus between 10 MPa and 200 MPa. The loading cases flexion, extension, lateral bending and axial rotation were simulated. Evaluated optimization criteria were the ranges of motion, forces in the facet joints, posterior bulgings of the intervertebral disc and the intradiscal pressures. Various objective functions were evaluated.
The results show that the objective values depend more on the axial stiffness of the rods than on bending and torsional stiffness, rod diameter and elastic modulus. The optimal stiffness value for most of the investigated objective functions is approximately 50 N/mm and is achieved, e.g. using a rod diameter of 6 mm and an elastic modulus of 50 MPa. The design with the least axial stiffness was the best one with regard to the mobility. The forces in the facet joints and the intradiscal pressures were reduced mostly by an implant with the highest axial stiffness. When minimal posterior disc bulging was the criterion, the optimal axial stiffness was also approximately 50 N/mm.
The optimal axial stiffness of a pedicle-screw-based motion preservation implant for the lumbar spine is approximately 50 N/mm.
基于椎弓根螺钉的动态植入物旨在保持椎间活动度，同时释放某些脊柱结构。本研究的目的是确定满足这些相反任务的纵向杆的未知最佳刚度值。
使用包括 L4/5 水平后路植入物的腰椎有限元模型。通过改变纵向杆的直径在 6 到 12 毫米之间，弹性模量在 10 到 200 兆帕之间，生成了超过 250 种该模型的变体。模拟了屈曲、伸展、侧屈和轴向旋转等加载情况。评估的优化标准是运动范围、关节突关节力、椎间盘后膨出和椎间盘内压力。评估了各种目标函数。
结果表明，目标值更多地取决于杆的轴向刚度，而不是弯曲和扭转刚度、杆直径和弹性模量。对于大多数研究的目标函数，最佳刚度值约为 50 N/mm，例如使用直径为 6 毫米、弹性模量为 50 MPa 的杆即可实现。在大多数情况下，与活动度有关的最佳设计是轴向刚度最小的设计。关节突关节力和椎间盘内压力主要通过轴向刚度最高的植入物降低。当最小的椎间盘后膨出是标准时，最佳轴向刚度也约为 50 N/mm。
基于椎弓根螺钉的腰椎运动保留植入物的最佳轴向刚度约为 50 N/mm。
