Influence of Osmotic Pressure Changes on the Opening of Existing Cracks in 2 Intervertebral Disc Models:

Study Design. An experimental hydrogel model and a numerical mixture model were used to investigate why the disc herniates while osmotic pressure is decreasing. Objective. To investigate the influence of decreasing osmotic pressure on the opening of cracks in the disc. Summary of Background Data. In the degeneration process, the disc changes structure (i.e., cracks occur, and osmotic pressure decreases). Disc herniation typically develops when hydration declines, but, on the other hand, it is said that the anulus of a highly hydrated disc has a high risk of rupture. We hypothesized that disc herniation is preceded by the opening of cracks as a result of decreasing osmotic pressure. Methods. The osmotic pressure was changed in hydrogel samples with a crack, which was visualized with a confocal laser scanning microscope (Zeiss, Go¨ ttingen, Germany). A 3-dimensional finite element mixture model simulated a decrease in osmotic pressure around a crack in a swelling material. Results. Experiments and simulations show that a decrease in osmotic pressure results in the opening of cracks. The simulations show high effective stress concentrations around the crack tip, while the overall stress level decreases, indicating an increased risk of crack growth. Conclusions. Decreasing osmotic pressure in a degenerating intervertebral disc enhances the opening of existing cracks, despite the concomitant decrease in anular stresses. Key words: intervertebral disc, herniated disc, osmotic pressure, crack, finite element analysis, mixture theory, hydrogel. Spine 2006;31:1783–1788