Temporal and spatial variations of pressure within intervertebral disc nuclei.

Experimental and computational studies often presume that nuclei pulposi of non-degenerated human lumbar discs function as fluid-filled cavities with single hydrostatic pressures throughout that vary neither with time nor location and orientation. Recent simultaneous measurements of the pressure at multiple locations within disc nuclei have however shown time-dependent and nonhomogeneous pressure distributions. This combined in vitro and in silico study aims to re-examine the temporal and spatial variations of the pressure within disc nuclei with special focus on the effect of tissue hydration. After 20h of free swelling, effects of two preload magnitudes (0.06 and 0.28MPa) on nucleus pressure were investigated under 8h of constant preloads followed by 10 cycles of high-low loads each lasting 15min using 6 disc-bone bovine specimens. Changes in pressure at 3 different nucleus locations were recorded as surrogate measures of fluid flow within the discs. To identify the likely mechanisms observed in vitro, a finite element model of a human disc (L4-L5) was employed while simulating foregoing plus additional loading protocols. In vitro and computed results show a clear and substantial pressure gradient within the nucleus, especially early after the load application under higher loads and in more hydrated discs. The pressure reaches its maximum in the nucleus center reducing axially toward endplates and radially toward the nucleus-annulus interface. These cause pressure gradients that substantially diminish with time and at lower hydration levels. With time and as the pore pressure drops, the contribution of the nucleus bulk increases till it reaches equilibrium. The relative share of the annulus bulk in supporting the applied loads markedly increases not only with time but at higher loads and lower hydrations. The hydration state of the disc is hence crucial in the disc pressure distribution and internal response under various static-dynamic loads in vitro and in the replication of in vivo conditions.