Effect of Hydration on Healthy Intervertebral Disk Mechanical Stiffness

The intervertebral disk has an excellent swelling capacity to absorb water, which is thought to be largely due to the high proteoglycan composition. Injury, aging, degeneration, and diurnal loading are all noted by a significant decrease in water content and tissue hydration. The objective of this study was to evaluate the effect of hydration, through osmotic loading, on tissue swelling and compressive stiffness of healthy intervertebral disks. The wet weight of nucleus pulposus (NP) and annulus fibrosus (AF) explants following swelling was 50% or greater, demonstrating significant ability to absorb water under all osmotic loading conditions (0.015 M–3.0 M phosphate buffered saline (PBS)). Estimated NP residual strains, calculated from the swelling ratio, were approximately 1.5 × greater than AF residual strains. Compressive stiffness increased with hyperosmotic loading, which is thought to be due to material compaction from osmotic-loading and the nonlinear mechanical behavior. Importantly, this study demonstrated that residual strains and material properties are greatly dependent on osmotic loading. The findings of this study support the notion that swelling properties from osmotic loading will be important for accurately describing the effect of degeneration and injury on disk mechanics. Furthermore, the tissue swelling will be an important consideration for developing biological repair strategies aimed at restoring mechanical behavior toward a healthy disk.

Osmotic or Chemical Activation of the TRPV4 Ion Channel Enhances the Development of Chondrocyte-Based Tissue Engineered Cartilage

Dynamic mechanical loading can enhance the formation of engineered cartilage, potentially through secondary biophysical effects such as changes in interstitial osmolarity. This study examined the effects of daily osmotic loading, as well as direct activation of the osmosensitive ion channel TRPV4, on the biochemical and functional properties of chondrocyte-laden cartilage constructs. Osmotic loading, as well as exposure to the TRPV4-specific agonist GSK1016790A, enhanced extracellular matrix (ECM) accumulation, and TRPV4 activation enhanced the functional properties of the constructs. This study implicates the Ca ++-permeable TRPV4 ion channel in the metabolic response of articular chondrocytes to osmotic and mechanical loading. Furthermore, these results suggest that targeting TRPV4, either directly with channel agonists, or indirectly via osmotic loading, may provide a novel strategy for enhancing tissue engineered cartilage construct maturation.