A parametric study on mechanical competence of nucleus pulposus for severely degeneration condition of a lumbar intervertebral disc

The highest degree of degradation for the intervertebral discs (IVDs) is expected at the final stage of degeneration, called severe degeneration which aimed to be simulated in this study using the computational model, verified by the results of stress profilometry. The computational model was generated according to the geometry of a two-level lumbar vertebral segment and its load carrying performance was verified according to the results of an In-Vitro compression test. To degrade the confining influence of annulus fibrosus (AF) in a degenerated condition, the influence of collagen fibers was ignored while for degeneration of nucleus pulposus (NP), its Young's modulus increased besides decreasing its Poisson's ratio, according to the scheme, proposed in the literature. Since the mechanical properties, adopted in the models of this study were referred to the valid experimental reports, a minor influence of patient specificity was expected to influence the results. The computational model showed that the load carrying stiffness of the vertebral segment under compression load decreased by 60% in a severely degenerated condition, due to a similar negative influence, observed for dysfunction of AF and NP. The results of the intradiscal pressure in normal and degenerated conditions, simulated by the computational model, had similar trends with experimental results, reported by stress profilometry. This study revealed a major influence on performance of IVDs due to reduction of Poisson's ratio in NP, because by reduction of this parameter equal to 10%, the stiffness of the simulated vertebral segment under the compressive load decreased by 23%.