Sengupta Dilip K, Mulholland Robert C
Department of Orthopaedics, Spine Center, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire 03756-0001, USA.
Spine (Phila Pa 1976). 2005 May 1;30(9):1019-29; discussion 1030. doi: 10.1097/01.brs.0000160986.39171.4d.
An experimental study on cadaver spine and spine model for biomechanical evaluation of a novel dynamic stabilization device.
First, to test the hypothesis that in dynamic stabilization of a lumbar spine using pedicle screws and ligament, addition of a fulcrum in front of the ligament can unload the disc. Second, to determine the relationship between the length and stiffness of the fulcrum and the ligament on disc unloading, lordosis and motion preservation.
Activity related low back pain may be attributable to abnormal disc loading or abnormal movement. Spinal fusion addresses both the mechanisms, but it has limitations. Soft stabilization with Graf ligament restricts abnormal movement but increases the disc pressure. The Dynesys system uses a plastic cylinder around the ligament to prevent overloading the disc, but it restricts extension and loses lordosis.
A novel dynamic stabilization system (fulcrum assisted soft stabilization or FASS) was developed in which a flexible fulcrum was placed in front of a ligament between the pedicle screws. It was hypothesized that the fulcrum should transform the compressive force of a ligament behind into a distraction force in front and unload the disc. Three spine models were developed using wooden blocks for vertebral bodies and neoprene rubber of different hardness for disc. Their load-deformation character was tested and compared with that of the cadaver spine in a spine tester. The spine model with the closest load-deformation property to cadaver spine was then tested for the effect of a FASS system, consisting of high density polythene rod as fulcrums and rubber "O" rings as ligaments. The disc pressure in the spine models were recorded with strain gauge in the center.
Application of ligaments alone across the pedicle screws increased the disc pressure, produced a lordosis, and reduced the range of motion. Application of fulcrums reduced the disc pressure and maintained the lordosis. Increasing the fulcrum length resulted in progressive unloading of the disc but increased stiffness of the motion segment. As the fulcrum length approximated the height of the motion segment, the lordosis was lost, and the disc was completely unloaded. Decreasing the lateral bending stiffness of the fulcrum had minimal effect on disc unloading and motion-segment stiffness.
The novel FASS system can unload the disc, control the range of motion, and maintain lordosis. These parameters may be controlled with a suitable combination of ligament and fulcrum system. The study provides an indication toward the desirable biomechanical properties of the fulcrum and ligament for future development of a clinically applicable prototype.
一项针对尸体脊柱和脊柱模型的实验研究，用于对一种新型动态稳定装置进行生物力学评估。
首先，检验以下假设：在使用椎弓根螺钉和韧带对腰椎进行动态稳定时，在韧带前方添加一个支点可减轻椎间盘负荷。其次，确定支点和韧带的长度与刚度之间的关系对椎间盘卸载、脊柱前凸和运动保留的影响。
与活动相关的下腰痛可能归因于椎间盘负荷异常或运动异常。脊柱融合术解决了这两种机制，但存在局限性。使用格拉夫韧带进行软性稳定可限制异常运动，但会增加椎间盘压力。戴尼斯系统在韧带周围使用一个塑料圆柱体来防止椎间盘过载，但它限制伸展并导致脊柱前凸消失。
开发了一种新型动态稳定系统（支点辅助软性稳定或FASS），其中在椎弓根螺钉之间的韧带前方放置一个柔性支点。假设该支点应将后方韧带的压缩力转化为前方的牵张力并减轻椎间盘负荷。使用木块模拟椎体，用不同硬度的氯丁橡胶模拟椎间盘，制作了三个脊柱模型。测试了它们的载荷-变形特性，并在脊柱测试仪中与尸体脊柱的特性进行比较。然后对具有最接近尸体脊柱载荷-变形特性的脊柱模型测试了FASS系统的效果，该系统由高密度聚乙烯棒作为支点和橡胶“O”形环作为韧带组成。用位于中心的应变片记录脊柱模型中的椎间盘压力。
仅在椎弓根螺钉之间应用韧带会增加椎间盘压力，产生脊柱前凸，并减小运动范围。应用支点可减轻椎间盘压力并维持脊柱前凸。增加支点长度会导致椎间盘逐渐卸载，但会增加运动节段的刚度。当支点长度接近运动节段的高度时，脊柱前凸消失，椎间盘完全卸载。降低支点的侧弯刚度对椎间盘卸载和运动节段刚度的影响最小。
新型FASS系统可减轻椎间盘负荷，控制运动范围，并维持脊柱前凸。这些参数可通过韧带和支点系统的适当组合来控制。该研究为未来开发临床适用原型时支点和韧带所需的生物力学特性提供了一个方向。
