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.

Study Design. Disc recovery behavior under hypo-and hyperosmotic pressure. Objective. To evaluate the effect of osmotic pressure on the unloaded recovery response of healthy discs. Summary of Background Data. The intervertebral disc is a poroviscoelastic material that experiences large fluctuations in water composition throughout a diurnal loading cycle. Fluid flow out of the disc occurs through mechanical loading, whereas fluid flow into the disc occurs through passive diffusion because of an imbalance of ions between the disc and its surrounding environment. Osmotic pressure has been used to alter water uptake and tissue hydration. Methods. Motion segments were prepared from the caudal spine sections of the skeletally mature bovines. A 300-N compressive load was applied for 2 hours before unloaded recovery for 12 hours. Hypo-and hyperosmotic pressure was used to alter the rate of water uptake and disc height recovery during unloaded recovery. A 5-parameter rheological model was used to describe the disc's time-dependent recovery behavior. Results. The elastic response was not altered by changes in osmotic pressure; however, viscoelastic recovery was highly dependent on saline osmolarity and recovery time. The fast response of viscoelastic recovery was not dependent on osmotic pressure. The time constant for the slow response decreased whereas the slow response stiffness increased as osmotic pressure increased.Conclusion. The fast response of viscoelastic recovery is governed by flow-independent recovery, whereas the slow response is related to flow-dependent recovery. The rate and magnitude of flow-dependent recovery are highly sensitive to changes in osmotic pressure of the saline bath. There is an osmotic pressure that reduces disc recovery behavior to an elastic response or flow-independent recovery.

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Bovine caudal discs have been widely used in spine research due to their increased availability, large size, and mechanical and biochemical properties that are comparable to healthy human discs. However, despite their extensive use, the radial variations in bovine disc composition have not yet been rigorously quantified with high spatial resolution. Previous studies were limited to qualitative analyses or provided limited spatial resolution in biochemical properties. Thus, the main objective of this study was to provide quantitative measurements of biochemical composition with higher spatial resolution than previous studies that employed traditional biochemical techniques. Specifically, traditional biochemical analyses were used to measure water, sulfated glycosaminoglycan, collagen, and DNA contents. Gravimetric water content was compared to data obtained through Raman spectroscopy and differential scanning calorimetry. Additionally, spatial distribution of lipids in the disc's collagen network was visualized and quantified, for the first time, using multi‐modal second harmonic generation (SHG) and Coherent anti‐Stokes Raman (CARS) microscopy. Some heterogeneity was observed in the nucleus pulposus, where the water content and water‐to‐protein ratio of the inner nucleus were greater than the outer nucleus. In contrast, the bovine annulus fibrosus exhibited a more heterogeneous distribution of biochemical properties. Comparable results between orthohydroxyproline assay and SHG imaging highlight the potential benefit of using SHG microscopy as a less destructive method for measuring collagen content, particularly when relative changes are of interest. CARS images showed that lipid deposits were distributed equally throughout the disc and appeared either as individual droplets or as clusters of small droplets. In conclusion, this study provided a more comprehensive assessment of spatial variations in biochemical composition of the bovine caudal disc.

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Stresses applied to the spinal column are distributed between the intervertebral disc and facet joints. Structural and compositional changes alter stress distributions within the disc and between the disc and facet joints. These changes influence the mechanical properties of the disc joint, including its stiffness, range of motion, and energy absorption under quasi-static and dynamic loads. There have been few studies evaluating the role of facet joints in torsion. Furthermore, the relationship between biochemical composition and torsion mechanics is not well understood. Therefore, the first objective of this study was to investigate the role of facet joints in torsion mechanics of healthy and degenerated human lumbar discs under a wide range of compressive preloads. To achieve this, each disc was tested under four different compressive preloads (300-1200 N) with and without facet joints. The second objective was to develop a quantitative structure-function relationship between tissue composition and torsion mechanics. Facet joints have a significant contribution to disc torsional stiffness (∼60%) and viscoelasticity, regardless of the magnitude of axial compression. The findings from this study demonstrate that annulus fibrosus GAG content plays an important role in disc torsion mechanics. A decrease in GAG content with degeneration reduced torsion mechanics by more than an order of magnitude, while collagen content did not significantly influence disc torsion mechanics. The biochemical-mechanical and compression-torsion relationships reported in this study allow for better comparison between studies that use discs of varying levels of degeneration or testing protocols and provide important design criteria for biological repair strategies. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

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Heart failure continues to be a common and deadly sequela of myocardial infarction (MI). Despite strong evidence suggesting the importance of myocardial mechanics in cardiac remodelling, many MI studies still rely on two-dimensional analyses to estimate global left ventricular (LV) function. Here, we integrated four-dimensional ultrasound with three-dimensional strain mapping to longitudinally characterize LV mechanics within and around infarcts in order to study the post-MI remodelling process. To induce infarcts with varying severities, we separated 15 mice into three equal-sized groups: (i) sham, (ii) 30 min ischaemia–reperfusion, and (iii) permanent ligation of the left coronary artery. Four-dimensional ultrasound from a high-frequency small animal system was used to monitor changes in LV geometry, function and strain over 28 days. We reconstructed three-dimensional myocardial strain maps and showed that strain profiles at the infarct border followed a sigmoidal behaviour. We also identified that mice with mild remodelling had significantly higher strains in the infarcted myocardium than those with severe injury. Finally, we developed a new approach to non-invasively estimate infarct size from strain maps, which correlated well with histological results. Taken together, the presented work provides a thorough approach to quantify regional strain, an important component when assessing post-MI remodelling.