椎体复位器治疗胸腰椎骨折过程中的生物力学参数变化

Biomechanical alterations during the treatment of thoracolumbar vertebral compression fractures with repositor

目的:对胸腰椎压缩性骨折的复位目前主要是通过Harrington棒的撑开或Dick等器械压缩,间接地使压缩的椎体得到复位。但复位的力量不易掌握,效果往往也不够理想。因此,需要了解和掌握复位器未发生变形时所能承受的最大应力及椎体复位器在椎体内撑开复位的应力与位移的关系。方法:实验于1998-10/2003-04在上海市材料力学研究所、上海市第九人民医院生物力学研究室及上海第二医科大学解剖教研室市生物力学研究室完成。自行设计的椎体复位器为圆柱形,顶端的上下开启由尾端的圆形手柄控制。实验方法:①椎体复位器的抗压力实验:将椎体复位器撑开到最大后进行轴向压缩实验,当复位器刚好发生形变时的轴向压应力,即是复位器进行轴向撑开时所能承受最大应力。②新鲜小牛骨椎体抗轴向压力实验:在36个新鲜小牛骨椎体上制作压缩性骨折模型时,了解椎体形变时的应力形变的关系。实验前测出每个椎体的高度值,发生压缩形变后,再测量每个椎体的高度值,并在机器上测出相应的压应力值。③椎体复位器复位压缩性小牛骨椎体的生物力学实验:将20个新鲜小牛骨胸腰椎椎体分成2组,每组10个椎体。在轴向载荷下压缩椎体,制作压缩性骨折的模型,1/2椎体高度压缩组其压缩后高度接近原有椎体高度的1/2,2/3椎体高度压缩组其压缩后高度接近原有椎体高度的2/3。从椎体椎弓根两侧放入复位器,将压缩的椎体予以复位。了解复位器在椎体内的撑开应力与复位时位移关系。结果:①复位器未发生变形时所能承受的最大力为281N。②36个新鲜小牛骨椎体在承受轴向载荷发生变形时屈服点的平均应力为74.96kPa,各数据符合正态分布(t=16.044,P<0.01)。③1/2椎体高度压缩组和2/3椎体高度压缩组复位器在椎体内撑开后应力值差异无显著性意义(t=0.835,P>0.05)。结论:椎体压缩程度与椎体内撑开复位的力量没有直接关系,自行设计的椎体复位器在椎体内撑开复位的力量远小于复位器的屈服点应力。

AIM: The reposition of thoracolumbar vertebral compression fracture is always achieved by Harrington rob or Dick compression to reset the compressed vertebral body indirectly. However, the strength of reposition is very difficult to control, consequently, the curative effect is always dissatisfactory. Therefore, it is necessary to explore the maximal stress that the repositor can bear before its distortion occurs and investigate the correlation between the force and displacement caused by the repositor when it opens in fractured vertebrae. METHODS: The experiment was carried out in Shanghai Institute of Material Mechanics, Institute of Biomechanics of Shanghai Ninth People's Hospital and Department of Anatomy, Shanghai Second Medical University from October 1998 to April 2003. The self-designed vertebral repository was cylindrical and the up and down unlock was controlled by the caudal end round handle. ①The repositor was distracted completely and put on the platform of a dynamometer. The axial stress that the repositor was distorted served as the maximal stress it could bear. ②The compression fracture models were established in 36 fresh calf vertebra and the correlation between the axial load and distortion was explored. The vertebral height was measured before experiment and after distortion. Meanwhile, the corresponding stress values were recorded. ③Twenty fresh calf thoracolumbar vertebra were divided into 2 groups (n =10). The models of compression fracture were constructed by increasing the axial loads on the vertebrae. In 1/2 compression group, the vertebrae were compressed to 1/2 of their original heights and 2/3 in 2/3 compression group. The repositor was inserted from two sides of neck of vertebra to reset the compressed vertebra. The correlation between the force and displacement caused by the repositor when it opens in fractured vertebrae was observed. RESULTS: ①The maximal stress which the repositor could bear before its distortion occurred was 281 N. ②The minimal axial loads causing the distortion of fresh calf vertebrae averaged out to 74.96 kPa. And the result represented a normal distribution (t =16.044, P < 0.01). ③The average minimal force used to restore compressed vertebra in 1/2 and 2/3 compression groups showed no statistical difference (t =0.835, P > 0.05). CONCLUSION: There is no direct correlation between the severity of compressive fracture and the force to restore the compressed vertebra. The force of self-designed repositor to restore the compressed vertebra is much smaller than the stress to cause the distortion of repositor.