How Osmo-Viscoelastic Coupling Affects Recovery of Cyclically Compressed Intervertebral Disc.

STUDY DESIGN Osmo-viscoelastic behavior of cyclically loaded cervical intervertebral disc. OBJECTIVE Evaluate in-vitro the effects of physiologic compressive cyclic loading on the viscoelastic properties of cervical intervertebral disc and, examine how the osmo-viscoelastic 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 eight motion segments (composed by two 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-hours recovery period in saline solution at reference concentration (0.15 M), relaxation behaviors were fully restored. Nonetheless, full recovery is not obtained whether the concentration of the surrounding fluid, i.e. hypo-, iso- or hyper-osmotic conditions. CONCLUSION Cyclic loading effects and full recovery of viscoelastic behavior after hydration at iso-osmotic condition (0.15 M) is 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, being a chemically activated process, 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.