Role of endplates in contributing to compression behaviors of motion segments and intervertebral discs

Abstract

The purpose of this study was to gain an improved understanding of the mechanical behavior of the intervertebral disc in the presence and absence of the vertebral endplates. Mechanical behaviors of rat caudal motion segments, vertebrae and isolated disc explants under two different permeability conditions were investigated and viscoelastic behaviors were evaluated using a stretched-exponential function to describe creep and recovery behaviors. The results demonstrated that both vertebrae and discs underwent significant deformations in the motion segment even under relatively low-loading conditions. Secondly, disruption of the collagenous network had minimal impact on equilibrium deformations of disc explants as compared to disc deformations occurring in the motion segments provided that vertebral deformations were accounted for; however, differences in endplate permeability conditions had a significant effect on viscoelastic behaviors. Creep occurred more quickly than recovery for motion segment and explant specimens. In addition, disc explants and motion segments both exhibited non-recoverable deformations under axial compression under lowand high-loading conditions. Results have important implications for interpreting the role of vertebral endplates in contributing to disc mechanical behaviors and direct application to mechanobiology studies involving external loading to rodent tail intervertebral discs.

Different Effects of Static Versus Cyclic Compressive Loading on Rat Intervertebral Disc Height and Water Loss In Vitro

“…Finally, this study provides in vitro mechanical and water content data that may be used for comparison with the growing numbers of in vivo and organ culture studies using rat lumbar and caudal models. 6,7,10,12,13,[15][16][17]19,27,29 A quantitative association of relative contributions of disc bulging and volume loss would require a 3-dimensional multiphasic finite element model with intrinsic solid phase viscoelasticity, which is beyond the scope of this manuscript, although some similar models on human lumbar disc exist. 34…”

“…Skin and soft tissues were removed from the tails, and 6 adjacent motion segments were harvested using methods similar to those previously described. 19 Briefly, the vertebral bodies of each motion segment were potted in aluminum tubes using cyanoacrylate. Potted motion segments were then frozen in liquid nitrogen, wrapped in phosphate-buffered saline soaked gauze, double-wrapped in plastic, and stored at −20°C for 3 to 7 days until testing.…”

A biochemical/biophysical 3D FE intervertebral disc model

“…Also, using a discrete number of fibers and assuming the primary fibril direction is constant with an angle of ±30 • to the transversal plane is an approximation of the actual, slightly varying fiber angle in the annulus fibrosis (Holzapfel et al 2005). This is of relevance in the tissue mechanical behavior test as the fiber orientation of the isolated samples was not measured directly but is based on other experimental studies (Guerin and Elliott 2006a;Skaggs et al 1994).The simplified geometry of the 3D disc model, which does not account for the convex curvature of the endplates and their deformability (Brinckmann et al 1983;Iatridis et al 2005;MacLean et al 2007) might have also influenced model predictions.…”

“…Nevertheless, evaluation of the compressive disc properties calls for cautious interpretation of the results, as in vitro experiments may not be able to maintain the correct poroelastic properties or boundary conditions as experienced in vivo (Jones and Wilcox 2008). Several studies pointed out that the fluid flow through the endplate might be hampered through blood clotting in vitro (Adams and Hutton 1983;Ayotte et al 2001;Broberg 1993;MacLean et al 2007;van der Veen et al 2005), creating a resistance. Variation of the outflow boundaries (free fluid flow, no fluid flow, flow resistance) on the vertebra-disc interface of the 3D OVED model allowed a closer evaluation of the predicted creep response.…”

“…The latter simulates a phenomenon often seen with in vitro experiments, the clogging of endplates. The clogging of endplates was simulated through creating a resistance by reducing the permeability (k = 1.0e −48 m 4 /Ns), in an extra thin layer of elements added on the vertebra-disc interface (Ayotte et al 2001;MacLean et al 2007; van der Veen et al 2007). …”