Experimental Investigation of an Influence of Coupled Compressive Loading on Porcine Spine Specimens

Abstract

The human spine shortens approximately by 1% of its height during the daily activities and returns to its primary height during the night rest. Cyclic loading is important in order to ensure diffusion and convention of the nutrients and metabolites within the intervertebral discs. On the other hand, cyclic loading could lead to the damage of the intervertebral discs and the vertebra bodies if the magnitude and frequency of the loads applied to the spine exceed the allowable limits. As most of the in vitro studies that investigate the influence of cyclic loading deal with functional spinal units consisting of single intervertebral disc the purpose of this study is to investigate an influence of cyclic compression and flexion on the structural integrity and geometrical parameters of the spinal specimens consisting of more than one intervertebral disc. Two specimens consisting of four adjacent vertebrae and three intervertebral discs were scanned by using computed tomography then loaded with combined cyclic compression and flexion and then scanned for the second time in order to capture the current condition of the specimens. Obtained images were used to evaluate the changes of structural integrity and geometrical parameters of the discs. A significant decrease of the specimen height was observed during loading, mainly due to the loss of the fluid content within intervertebral discs. In total, the difference of the height of the two specimens immediately after the loading was 1.577 mm. No obvious damage to the specimens was observed when comparing images before and after the loading. A statistically significant differences between the height of the intervertebral discs before and after loading in both the first specimen (p = 0.0224) and the second specimen (p = 0.0155) were calculated with the lowest disc of both specimens decreasing the most and obviously losing the highest water content. The cross-sectional area of the lowest disc in both specimens also decreased the most. This once again confirms that lower part of the spine such as intervertebral discs L4-L5 and L5-S1 are the least prone to the injuries and degeneration due to disturbed nutrition and loss of water content.