Is the sheep a suitable model to study the mechanical alterations of disc degeneration in humans? A probabilistic finite element model study.

Intervertebral disc degeneration is one major source of low back pain, which because of its complex multifactorial nature renders the treatment challenging and thus necessitates extensive research. Experimental animal models have proven valuable in improving our understanding of degenerative processes and potentially promising therapies. Currently, the sheep is the most frequently used large animal in vivo model in intervertebral disc research. However, despite its undoubted value for investigations of the complex biological and cellular aspects, to date, it is unclear whether the sheep is also suited to study the mechanical aspects of disc degeneration in humans. A parametric finite element (FE) model of the L4-5 spinal motion segment was developed. Using this model, the geometry and the material properties of both the human and the ovine spinal segment as well as different appearances of disc degeneration can be depicted. Under pure and combined loads, it was investigated whether degenerative changes to both the human and the ovine model equivalent caused the same mechanical response. Different patterns of degeneration resulted in large variations in the ranges of motion, intradiscal pressure, ligament and facet loads. In the human, but not in the ovine model, all these results differed significantly between different degrees of degeneration. This FE model study highlighted possible differences in the mechanical response to disc degeneration between human and ovine intervertebral discs and indicates the necessity of further, more detailed, investigations.