Degeneration of the intervertebral disc results in patent cracks [1] and a decrease in osmotic pressure associated with loss of fixed charges. The relationship between mechanical load and damage in the disc is very poor [2]. This finding is at odds with physical intuition. The subject of this study is relationship between the development of patent cracks and the decrease in osmotic pressure in the degenerating disc in the light of the physics of swelling [3–7]. We restrict the experimental part of this study to hydrogel, thus avoiding complications associated with biological variability. The finite element modelling [6,7] used in this study catches salient features of stress profiles measured by Mc Nally and al.

Thin hydrogel samples with a crack of 5 mm are used. The crack opens as a result of decreasing osmotic pressure in the experiments and in the simulation. The initial uniform stress distribution turns into a distribution with a decreased average stress level and a high stress around the crack tip. A decrease in osmotic pressure opens an existing crack in swelling materials independently from external mechanical load. Hence, disc degeneration causes the overall stress to decrease, while local stress around a crack tip increases. This mechanism may explain why damage in the disc is so poorly correlated with mechanical load [3] and why the degenerated disc is characterized by patent cracks [1]. The process of crack opening in the degenerating disc is comparable to the crack development in an aging oaken beam, while loosing its turgor.