How Osmoviscoelastic Coupling Affects Recovery of Cyclically Compressed Intervertebral Disc

Abstract

Study Design. Osmoviscoelastic behavior of cyclically loaded cervical intervertebral disc. Objective. The aim of this study was to evaluate in vitro the effects of physiologic compressive cyclic loading on the viscoelastic properties of cervical intervertebral disc and, examine how the osmoviscoelastic coupling affects time-dependent recovery of these properties following a long period of unloading. Summary of Background Data. The human neck supports repetitive loadings during daily activities and recovery of disc mechanics is essential for normal mechanical function. However, the response of cervical intervertebral disc to cyclic loading is still not very well defined. Moreover, how loading history conditions could affect the time-dependent recovery is still unclear. Methods. Ten thousand cycles of compressive loading, with different magnitudes and saline concentrations of the surrounding fluid bath, are applied to 8 motion segments (composed by 2 adjacent vertebrae and the intervening disc) extracted from the cervical spines of mature sheep. Subsequently, specimens are hydrated during 18 hours of unloading. The viscoelastic disc responses, after cyclic loading and recovery phase, are characterized by relaxation tests. Results. Viscoelastic behaviors are significantly altered following large number of cyclic loads. Moreover, after 18-hour recovery period in saline solution at reference concentration (0.15 mol/L), relaxation behaviors were fully restored. Nonetheless, full recovery is not obtained whether the concentration of the surrounding fluid, that is, hypo-, iso-, or hyper-osmotic conditions. Conclusion. Cyclic loading effects and full recovery of viscoelastic behavior after hydration at iso-osmotic condition (0.15 mol/L) are governed by osmotic attraction of fluid content in the disc due to imbalance between the external load and the swelling pressure of the disc. After removal of the load, the disc recovers its viscoelastic properties following period of rest. Nevertheless, the viscoelastic recovery is a chemically activated process and its dependency on saline concentration is governed by fluid flow due to imbalance of ions between the disc tissues and the surrounding fluid. Level of Evidence: 3

Relevance of a novel external dynamic distraction device for treating back pain

“…The biomechanics of the disc soft tissues should be considered in furthermore accurate models 30,31 along with deeper understanding of the disc functionality. 32–34 In the current study, it was therefore necessary to carry out direct measurements “in vivo” in order to limit these biases.…”

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“… 29,30,275,276,301‐306 Studies on disc recovery face similar challenges with reaching equilibrium, where full disc height recovery is often not achieved in 0.15 M phosphate‐buffered saline, even after 24 hours of unloaded or low‐load recovery 275,302 . Recent studies have shown that the recovery environment and previous loading history will greatly alter disc recovery mechanics, and the ability to achieve equilibrium during recovery 303,307 . That is, disc recovery does achieve equilibrium when under higher osmotic conditions that prevent fluid flow into biological tissues, but the direct representation of in vivo osmotic loading remains unknown.…”

“…275,302 Recent studies have shown that the recovery environment and previous loading history will greatly alter disc recovery mechanics, and the ability to achieve equilibrium during recovery. 303,307 That is, disc recovery does achieve equilibrium when under higher osmotic conditions that prevent fluid flow into biological tissues, but the direct representation of in vivo osmotic loading remains unknown.…”

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“…The results showed that shear differentially loaded the oblique counter-fibers in lateral annulus, thus increasing its vulnerability to disruption and herniation. From the perspective of chemo-mechanical coupling effects, both the osmotic pressure of the surrounding liquid [ 55 ], magnitudes of preloads [ 56 ], and loading rates [ 57 ] have coupling effects on the mechanical responses of IVDs. Feki et al [ 55 ] also revealed that 18 h of unloading was still not enough to regain the original height of discs, either in hypo-osmotic, iso-osmotic, or in hyper-osmotic conditions.…”

“…From the perspective of chemo-mechanical coupling effects, both the osmotic pressure of the surrounding liquid [ 55 ], magnitudes of preloads [ 56 ], and loading rates [ 57 ] have coupling effects on the mechanical responses of IVDs. Feki et al [ 55 ] also revealed that 18 h of unloading was still not enough to regain the original height of discs, either in hypo-osmotic, iso-osmotic, or in hyper-osmotic conditions. Recently, through scanning electron microscopy (SEM), Tavakoli et al [ 58 ] firstly revealed the elastic fibers network that the straight and thick parallel fibers formed across the NP.…”