Role of load history in intervertebral disc mechanics and intradiscal pressure generation

Abstract

Solid-fluid interactions play an important role in mediating viscoelastic behaviour of biological tissues. In the intervertebral disc, water content is governed by a number of factors, including age, disease and mechanical loads, leading to changes in stiffness characteristics. We hypothesized that zonal stress distributions depend on load history, or the prior stresses experienced by the disc. To investigate these effects, rat caudal motion segments were subjected to compressive creep biomechanical testing in vitro using a protocol that consisted of two phases: a Prestress Phase (varied to represent different histories of load) followed immediately by an Exertion Phase, identical across all Prestress groups. Three analytical models were used to fit the experimental data in order to evaluate load history effects on gross and zonal disc mechanics. Model results indicated that while gross transient response was insensitive to load history, there may be changes in the internal mechanics of the disc. In particular, a fluid transport model suggested that the role of the nucleus pulposus in resisting creep during Exertion depended on Prestress conditions. Separate experiments using similarly defined load history regimens were performed to verify these predictions by measuring intradiscal pressure with a fibre optic sensor. We found that the ability for intradiscal pressure generation was load history-dependent and exhibited even greater sensitivity than predicted by analytical models. A 0.5 MPa Exertion load resulted in 537.2 kPa IDP for low magnitude Prestress compared with 373.7 kPa for high magnitude Prestress. Based on these measurements, we developed a simple model that may describe the pressure-shear environment in the nucleus pulposus. These findings may have important implications on our understanding of how mechanical stress contributes to disc health and disease etiology.

Osmotic Pressure Alters Time-dependent Recovery Behavior of the Intervertebral Disc

“…13, 18 These models have been valuable in demonstrating significant differences in material properties with loading response, recovery behavior, injury, and degeneration. 11,[19][20][21][22] A diurnal loading cycle includes large compressive loads that are sustained throughout the day and reduced during bed-rest recovery. Previous studies demonstrated that the rate of disc height recovery during unloading is 3 to 4 times slower than the rate of disc height loss during loading, suggesting that passive diffusion of water molecules is not sufficient for full recovery within 8 hours.…”

Poroelastic behaviour of the degenerating human intervertebral disc: a ten-day study in a loaded disc culture system

“…This affects the water-binding capacity, thereby reducing the hydrostatic pressure (Sato et al, 1999), and altering the response to compressive loads (Hwang et al, 2012;Vergroesen et al, 2014). The influence of proteoglycan content on IVD mechanics has been shown indirectly, as enzymatic digestion of proteoglycans, induced by injection of chondroitinase ABC (CABC), caused disc narrowing (Fry et al, 1991;Hoogendoorn et al, 2007;Lü et al, 1997), and changed flexion mechanics (Lü et al, 1997) and intradiscal pressure (Sasaki et al, 2001).…”

“…Other studies used complete motion segments for biomechanical in vitro research, but the IVDs used were removed from their physiological conditions and unloaded for a while, which may have influenced the results of the research performed so far (e.g. Hwang et al, 2012;Wilke et al, 1998). The recent development of animal disc culture systems may benefit research relating degeneration to mechanical function, as they allow extended loading of animal IVDs in order to approximate physiological conditions (Jim et al, 2011;Korecki et al, 2007;Paul et al, 2012).…”

“…However, the direct relation between proteoglycan content and whole-disc mechanics has to our knowledge not yet been studied. This relation however may be the core of the degeneration process, because the loss of proteoglycans and the related reduced hydrostatic pressure increases the shear stresses in the nucleus (Hwang et al, 2012). The increase in shear stresses in turn influences cell activity, as hydrostatic pressure leads to different gene expression in the cells than when subjected to shear stresses, the latter further reducing proteoglycan content (Carter and Wong, 2003;Hsieh and Twomey, 2010;Paul et al, 2013).…”

“…The reduction of poroelastic behaviour in degenerated discs implies a reduction in hydrostatic pressure. The reduction in hydrostatic pressure will increase the shear stresses in the IVD (Carter and Wong, 2003;Hwang et al, 2012), which is known to have a mechanobiological effect on the IVD cells. The cells respond to shear stress by activation of remodelling genes, which leads to increased production of collagen type-I (Carter and Wong, 2003, Paul et al, 2013).…”