Overloading effect on the osmo-viscoelastic and recovery behavior of the intervertebral disc.

In vitro studies investigating the effect of high physiological compressive loads on the intervertebral disc mechanics as well as on its recovery are rare. Moreover, the osmolarity effect on the disc viscoelastic behavior following an overloading is far from being studied. This study aims to determine whether a compressive loading-unloading cycle exceeding physiological limits could be detrimental to the cervical disc, and to examine the chemo-mechanical dependence of this overloading effect. Cervical functional spine units were subjected to a compressive loading-unloading cycle at a high physiological level (displacement of 2.5 mm). The overloading effect on the disc viscoelastic behavior was evaluated through two relaxation tests conducted before and after cyclic loading. Afterward, the disc was unloaded in a saline bath during a rest period, and its recovery response was assessed by a third relaxation test. The chemo-mechanical coupling in the disc response was further examined by repeating this protocol with three different saline concentrations in the external fluid bath. It was found that overloading significantly alters the disc viscoelastic response, with changes statistically dependent on osmolarity conditions. The applied hyper-physiological compressive cycle does not cause damage since the disc recovers its original viscoelastic behavior following a rest period. Osmotic loading only influences the loading-unloading response; specifically, increasing fluid osmolarity leads to a decrease in disc relaxation after the applied cycle. However, the disc recovery is not impacted by the osmolarity of the external fluid.

How Osmoviscoelastic Coupling Affects Recovery of Cyclically Compressed Intervertebral Disc.

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.

How pre-strain affects the chemo-torsional response of the intervertebral disc.

BACKGROUND: The role of the axial pre-strain on the torsional response of the intervertebral disc remains largely undefined. Moreover, the chemo-mechanical interactions in disc tissues are still unclear and corresponding data are rare in the literature. The paper deals with an in-vitro study of the pre-strain effect on the chemical sensitivity of the disc torsional response. METHODS: Fifteen non-frozen 'motion segments' (two vertebrae and the intervening soft tissues) were extracted from the cervical spines of mature sheep. The motion segments were loaded in torsion at various saline concentrations and axial pre-strain levels in order to modulate the intradiscal pressure. After preconditioning with successive low-strain compressions at a magnitude of 0.1 mm (10 cycles at 0.05 mm/s), the motion segment was subjected to a cyclic torsion until a twisting level of 2 deg. at 0.05 deg./s while a constant axial pre-strain (in compression or in tension) is maintained, the saline concentration of the surrounding fluid bath being changed from hypo-osmotic condition to hyper-osmotic condition. FINDINGS: Analysis of variance shows that the saline concentration influences the torsional response only when the motion segments are pre-compressed (p < .001) with significant differences between hypo-osmotic condition and hyper-osmotic condition. INTERPRETATION: The combination of a compressive pre-strain with twisting amplifies the nucleus hydrostatic pressure on the annulus and the annulus collagen fibers tensions. The proteoglycans density increases with the compressive pre-strain and leads to higher chemical imbalances, which would explain the increase in chemical sensitivity of the disc torsional response.