Effect of disc degeneration on the mechanical behavior of the human lumbar spine: a probabilistic finite element study

Abstract

BACKGROUND CONTEXTIntervertebral disc degeneration has been subject to numerous in vivo and in vitro investigations and numerical studies during recent decades, reporting partially contradictory findings. However, most of the previous studies were limited in the number of specimens investigated and, therefore, could not consider the vast variety of the specimen geometries, which are likely to strongly influence the mechanical behavior of the spine. PURPOSETo complement the understanding of the mechanical consequences of disc degeneration, whereas considering natural variations in the major spinal geometrical parameters. DESIGN/SETTINGA probabilistic finite element study. METHODSA parametric finite element model of a human L4–L5 motion segment considering 40 geometrical parameters was developed. One thousand individual geometries comprising four degeneration grades were generated in a probabilistic manner, and the influence of the severity of disc degeneration on the mechanical response of the motion segment to different loading conditions was statistically evaluated. RESULTSVariations in the individual structural parameters resulted in marked variations in all evaluated parameters within each degeneration grade. Nevertheless, the effect of degeneration in almost all evaluated response values was statistically significant. With degeneration, the intradiscal pressure progressively decreased. At the same time, the facet loads increased and the ligament tension was reduced. The initially nonlinear load-deformation relationships became linear whereas the segment stiffness increased. CONCLUSIONSResults indicate significant stiffening of the motion segment with progressing degeneration and gradually increasing loading of the facets from nondegenerated to moderately degenerated conditions along with a significant reduction of the ligament tension in flexion.