Is the ovine intervertebral disc a small human one? A finite element model study.

The sheep is one of the most frequently used animal models for experimental intervertebral disc research questions. Although there are large differences in size between human and ovine discs, recent in vivo and in vitro studies indicate similarities in the internal disc stresses. The present finite element model study, therefore, intended to detect the parameters that, despite the different geometry, ensure mechanical comparability between both species. At first, a finite element model of the human L4-L5 lumbar intervertebral disc was developed. The predicted displacement and nucleus pressure response were validated with experimental in vivo and in vitro data. Starting with adapting the model geometry from the human to the ovine disc, several material and biochemical parameters, which might contribute to the preservation of the mechanical disc response across both species, were successively adapted to ovine properties. Replacing the geometry yielded a substantially higher disc stiffness and lower nucleus pressure compared to in vitro measurements performed on ovine discs. Additional reduction of annulus and nucleus elasticity led to an improved correlation between model predictions and measurements. Changes in the glycosaminoglycan content and endplate permeability improved the predicted pressure, but only slightly affected the displacement response. Only the combination of all parameters resulted in a good agreement between the predictions and measurements. This study demonstrated that there are profound differences between model predictions and in vitro results if an ovine simulation is run with human material properties. However, once the species-specific material properties are included, the predictions fit the in vitro results. Therefore, it seems that the human and ovine disc is functionally adapted to produce similar internal stresses, despite the large variation in geometry.