Direction-dependent resistance to flow in the endplate of the intervertebral disc: an ex vivo study.

A comparison of the higher hydrostatic pressure in the nucleus of the healthy intervertebral disc during daily loading with the relatively lower osmotic swelling pressure in the disc during rest suggests the existence of direction-dependent flow resistance such that all of the fluid exuded from the disc during loading is recovered during rest. In this study, this direction-dependent resistance was demonstrated for flows through the cartilage endplates and the underlying marrow contact channels in the bony endplates. Using an ex vivo sheep endplate model, the resistance ratio (Rout/Rin) for flow through the endplate was 39.0+/-3.8 (mean +/- S.E.). In addition, a path of fluid flow through the marrow contact holes was revealed using fluorescent staining.

Direction-dependent constriction flow in a poroelastic solid: the intervertebral disc valve.

We hypothesize that a direction-dependent flow resistance exists in the intervertebral disc due to constriction flow in the cartilage endplates. A comparison of the hydrostatic pressure in the nucleus of the healthy intervertebral disc during daily loading with the relatively low osmotic swelling pressure during rest, suggests the necessity of such direction-dependent flow resistance to ensure that all the fluid exuded from the disc during loading is recovered during rest. A physical model demonstrating the direction-dependent resistance of constriction flow in a poroelastic solid is presented. A finite element model was developed and validated against this physical model. The finite element model showed that decrease of the constriction hole area not only increases the resistance to fluid flow, but also causes the direction-dependency of flow resistance to decrease. Through this mechanism, endplate sclerosis could affect normal daily fluid exchange in the intervertebral disc, resulting in decreased mass transport and/or dehydration of the disc.