Determination of the strain-dependent hydraulic permeability of the compressed bovine nucleus pulposus

The hydraulic permeability, k, of the nucleus pulposus (NP) is crucial, both in withstanding compressive stress and for convective transport of nutrients within the disc. Permeability has previously been determined using biphasic mathematical models, but has not been found by direct permeation experiments, which is the objective of this study. Bovine coccygeal nucleus samples (n=64), φ10 mm and thickness 683±49 μm (mean±S.D.) were compressed axially to one of λ=1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4 or 0.3, where λ is the stretch ratio. Ringer's solution was permeated through the sample, with an o-ring ensuring axial flow. During stress equilibrium, k was determined and fitted to four permeability-strain equations. Permeability decreased exponentially with compression, and was best described byk(λ)=1.59×10-15(λ-0.20.8)1.13e[-0.02(λ2-1)/2].Values of k were comparable to those arising from mathematical models, lending confidence to permeability being determined from such models.