The Effect of Creep on Human Lumbar Intervertebral Disk Impact Mechanics

摘要

The intervertebral disk (IVD) is a highly hydrated tissue, with interstitial fluid making up 80% of the wet weight of the nucleus pulposus (NP), and 70% of the annulus fibrosus (AF). It has often been modeled as a biphasic material, consisting of both a solid and fluid phase. The inherent porosity and osmotic potential of the disk causes an efflux of fluid while under constant load, which leads to a continuous displacement phenomenon known as creep. IVD compressive stiffness increases and NP pressure decreases as a result of creep displacement. Though the effects of creep on disk mechanics have been studied extensively, it has been limited to nonimpact loading conditions. The goal of this study is to better understand the influence of creep and fluid loss on IVD impact mechanics. Twenty-four human lumbar disk samples were divided into six groups according to the length of time they underwent creep (t_(creep) = 0, 3, 6, 9, 12, 15 h) under a constant compressive load of 400 N. At the end of t_(creep), each disk was subjected to a sequence of impact loads of varying durations (t_(imp) = 80, 160, 320, 400, 600, 800, 1000 ms). Energy dissipation (ΔE), stiffness in the toe (k_(toe)) and linear (k_(lin)) regions, and neutral zone (NZ) were measured. Analyzing correlations with t_(creep), there was a positive correlation with ΔE and NZ, along with a negative correlation with k_(toe). There was no strong correlation between t_(creep) and k_(lin). The data suggest that the IVD mechanical response to impact loading conditions is altered by fluid content and may result in a disk that exhibits less clinical stability and transfers more load to the AF. This could have implications for risk of diskogenic pain as a function of time of day or tissue hydration.