Effect of fixed charge density on water content of IVD during bed rest: A numerical analysis

The fixed charge density (FCD) in the intervertebral disc (IVD) matrix is essential for its capacity of absorbing water, particularly during overnight bed rest. However, the FCD decreases with IVD degeneration, reducing the disc propensity to swell and the related convective transport of molecules across the IVDs. The objective of this study was to investigate the effects of the FCD on water intake in the IVD during bed rest. A multibody musculoskeletal model was extended to include the osmotic properties of the IVDs, and used for the analysis of IVD swelling and its water content in a human subject during bed rest. The simulations were conducted with both healthy lumbar IVDs and lumbar IVDs with a reduced FCD. It was predicted that a decrease in the FCD had a considerable impact on the IVDs swelling during bed rest. A 20% and a 45% reduction in the FCD resulted respectively in an average 25% and 55% reduction of disc water intake overnight. This study provided an additional, quantitative information on IVD swelling in human subjects during bed rest. The computational model presented in this paper may be a useful tool for estimating disc hydration at different loading and pathological conditions.