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.