Stiffness properties of human lumbar intervertebral discs in compression and the influence of strain rate

There have been numerous researchers that have investigated the properties of human intervertebral discs. However, there has been no attempt to characterize the effects of dynamic loading on the compressive stiffness of human lumbar intervertebral discs. Therefore, the purpose of this study was to develop the compressive stiffness properties of lumbar intervertebral discs when subjected to various dynamic compressive loading rates. This was accomplished by performing a total of 33 axial compression tests on 11 human lumbar intervertebral discs dissected from 6 fresh frozen human cadavers, 5 male and 1 female. The adjacent vertebral bodies were fixed to a load cell with a custom aluminum pot and then subjected to three dynamic compressive loading rates using a servo-hydraulic Material Testing System: 6.8, 13.5, and 72.7 strain/ sec. The results show that the compressive stiffness of lumbar intervertebral discs is dependent on the loading rate. There was no significant correlation (p > 0.05) between functional spinal unit compressive stiffness and vertebral level at any of the three loading rates. Therefore, a linear relationship between loading rate and vertebral disc compressive stiffness was developed by curve fitting the stiffness data from the current study along with static compressive stiffness data reported by previous studies.