SPINE Volume 31, Number 16, pp 1783–1788 ©2006, Lippincott Williams & Wilkins, Inc. Inﬂuence of Osmotic Pressure Changes on the Opening of Existing Cracks in 2 Intervertebral Disc Models Silvia Wognum, MSc, Jacques M. Huyghe, PhD, and Frank P. T. Baaijens, PhD 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 de- velops 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 decreas- ing osmotic pressure. Methods. The osmotic pressure was changed in hy- drogel samples with a crack, which was visualized with a confocal laser scanning microscope (Zeiss, Go ¨ ttingen, 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 de- crease in osmotic pressure results in the opening of cracks. The simulations show high effective stress con- centrations around the crack tip, while the overall stress level decreases, indicating an increased risk of crack growth. Conclusions. Decreasing osmotic pressure in a degen- erating intervertebral disc enhances the opening of exist- ing 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 Low back pain is the most common cause of disability in individuals between 20 and 50 years of age. Disorders associated with low back pain impose an economic bur- den similar to that of coronary heart disease and higher than that of other major health problems, such as diabe- tes, Alzheimer disease, and kidney diseases. 1 It is still largely unknown from what pathologies low back pain occurs, but it is believed that many cases occur from intervertebral disc problems. Two commonly known pa- thologies are intervertebral disc degeneration and inter- vertebral disc herniation. Knowledge of the causes and exact underlying processes of disc degeneration and her- niation is increasing, thanks to new imaging and model- ing techniques, and advances in cell biology and genet- ics. 1 However, the intervertebral disc seems to be poorly researched, even in comparison with other musculoskel- etal systems. Because of the limited knowledge, no ade- quate treatment exists for low back pain occurring from disc degeneration or herniation. The intervertebral disc, which is the primary connec- tion between 2 subsequent vertebrae, is a complex tissue consisting of different distinct structures: the gelatinous nucleus pulposus in the center, a ﬁbrous outer ring called anulus ﬁbrosis, and the cartilaginous endplates that con- nect the disc to the vertebrae. Both the nucleus and the anulus consist of a dense collagen network embedded in a gel-like hydrophilic material, with a high water content of 70% to 80% wt/wt on average and a high internal pressure of 0.1 MPa on average in supine position. 1a This pressure is caused by the swelling properties of the pro- teoglycans being resisted by tension in the anulus ﬁbro- sus and ligamentum ﬂavum. The ﬁxed charges in the material cause a difference in ion concentration between the material and solution, resulting in a Donnan osmotic pressure gradient. This gradient attracts water into the disc, resulting in a high water content. 2 The collagen network, especially that of the anulus, prevents the disc from swelling freely, leading to the high internal pressure and tensile osmotic prestressing of the anulus ﬁbers. The lower the amount of ﬁxed charges and the higher the concentration of the external salt solution, the lower the osmotic pressure and the lower the tensile stresses in the solid material. 3 Intervertebral disc tissue has been successfully modeled as a material consisting of a solid matrix with ﬁxed charges, representing the colla- gen ﬁbers and proteoglycans, together with interstitial ﬂuid with dissolved ions. The intervertebral disc ages faster than most other tis- sues because nutrition is hampered in an avascular tissue. During aging, the disc changes its structure and composi- tion. 1,4,5 A major structural change that occurs in the pro- cess of degeneration is a decrease in water content and os- motic pressure, mainly in the nucleus and inner anulus, 4,6,7 leading to a loss of prestressing of the collagen network in the whole disc. A possible reason for this decreasing hydra- tion could be the loss of and changes in distribution of proteoglycans and, consequently, ﬁxed charges. 7 A more localized form of an intervertebral disc pa- thology is intervertebral disc herniation. Intervertebral disc herniation typically is a problem in individuals 30 –50 years of age, which is the period of life in which From the Department of Biomedical Engineering, Eindhoven Univer- sity of Technology, Eindhoven, The Netherlands. Acknowledgment date: March 28, 2005. First revision date: August 31, 2005. Acceptance date: October 5, 2005. The manuscript submitted does not contain information about medical device(s)/drug(s). Federal funds were received in support of this work. No beneﬁts in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Jacques M. Huyghe, PhD, Eindhoven University of Technology, Department of Biomedical Engineering, Wh. 4.127, PO Box 513, 5600 MB Eindhoven, The Neth- erlands; E-mail: j.m.r.huyghe@tue.nl 1783