title: Influence of osmotic pressure changes on the opening of existing cracks in 2 intervertebral disc models.
abstract: METHODS
  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.
  
  
  BACKGROUND
  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, Göttingen, Germany). A 2-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.
authors.affiliation: Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.
authors.forename: Silvia
authors.initials: S
authors.lastname: Wognum
authors.forename: Jacques M
authors.initials: JM
authors.lastname: Huyghe
authors.forename: Frank P T
authors.initials: FP
authors.lastname: Baaijens
mesh_terms: D004487:Edema / Q000150:complications*; D006801:Humans; D007403:Intervertebral Disc / Q000293:injuries*; D009997:Osmotic Pressure; D013122:Spinal Diseases / Q000150:complications*; D016103:Spinal Fractures / Q000209:etiology*