The risk of disc prolapses with complex loading in different degrees of disc degeneration - a finite element analysis.

BACKGROUND Disc prolapses can result from various complex load situations and degenerative changes in the intervertebral disc. The aim of this finite element study was to find load combinations that would lead to the highest internal stresses in a healthy and in degenerated discs. METHODS A three-dimensional finite element model of a lumbar spinal segment L4-L5 in different grades of disc degeneration (healthy, mild, moderate, and severe) were generated, in which the disc height reduction, the formation of osteophytes and the increasing of nucleus' compressibility were considered. The intradiscal pressure in the nucleus, the fiber strains, and the shear strains between the annulus and the adjacent endplates under pure and complex loads were investigated. RESULTS In all grades of disc degeneration the intradiscal pressure was found to be highest in flexion. The shear and fiber strains predicted a strong increase under lateral bending+flexion for the healthy disc and under axial rotation and lateral bending+axial rotation for all degenerated discs, mostly located in the postero-lateral annulus. Compared to the healthy disc, the mildly degenerated disc indicated an increase of the intradiscal pressure and of the fiber strains, both of 25% in axial rotation. The shear strains showed an increase of 27% in axial rotation+flexion. As from the moderately degenerated disc all measurement parameters strongly decreased. INTERPRETATION The results support how specifically changes associated with disc degeneration might contribute to risk of prolapse. Thus, the highest risk of prolapses can be found for healthy and mildly degenerated discs.

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