The 3D FE-model (left) simulates the free swelling experiment (right) of the intervertebral disc. Usually (body), the disc is gripped between two vertebrae and surrounded with a physiological salt solution. This restrains the disc from swelling freely and allows the disc to develop its osmotic pressure. The tissue starts to swell because ions flow out and water flows in. However, when the vertebrae are removed the tissue can swell freely. The nucleus especially because the ion concentration is higher than in the Annulus.