Fluid-flow dependent response of intervertebral discs under cyclic loading: On the role of specimen preparation and preconditioning.

In vivo during the day, intervertebral discs are loaded mainly in compression causing fluid and height losses that are subsequently fully recovered overnight due to fluid inflow under smaller compression. However, in vitro, fluid flow through the endplates, in particular fluid imbibition, is hampered possibly by blood clots formed post mortem. Despite earlier in vitro studies, it remains yet unclear if and how fluid flow conditions in vitro could properly emulate those in vivo. Effects of various preload magnitudes (no preload, 0.06 and 0.28 MPa) and disc-bone preparation conditions (e.g., w/o bony endplates) on disc height and nucleus pressure were investigated using 54 bovine specimens. Changes in specimen height and pressure at different nucleus locations were used as surrogate measures to assess the fluid content and flow within the discs. Under all investigated preparation conditions and preload magnitudes, no significant pressure recovery could be obtained during low loading phases, even without bony endplates. On the contrary, partial to full displacement recovery were reached in particular under 0.28 MPa preload. Results highlight the significant role of disc preload magnitude in disc height recovery during low loading periods. Attention should hence be given in future studies to the proper selection of preload magnitude and duration as well as the animal models used if in vivo response is intended to be replicated. Findings also indicate that flushing the endplates or injection of bone cement respectively neither facilitates nor impedes fluid flow into or out of the disc to a noticeable degree in this bovine disc model.