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 20 h of free swelling, effects of two preload magnitudes (0.06 and 0.28 MPa) on nucleus pressure were investigated under 8 h of constant preloads followed by 10 cycles of high-low loads each lasting 15 min 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.

实验和计算研究通常假定未退化的人类腰间盘的髓核起充液腔的作用，并且整个过程中的静液压均随时间，位置和方向而变化。然而，最近同时测量椎间盘核内多个位置的压力已显示出时间依赖性和非均匀的压力分布。这项结合了体外和计算机模拟的研究旨在重新检查椎间盘核内压力的时空变化，特别关注组织水合作用的影响。

在自由溶胀20小时后，在6个盘状牛标本中，在恒定预紧力8 h下研究了两个预紧力大小（0.06和0.28 MPa）对细胞核压力的影响，然后进行了10次高-低负载循环，每次持续15分钟。记录了3个不同核位置处压力的变化，以作为圆盘内流体流动的替代量度。为了确定在体外观察到的可能机制，在模拟前述方法和其他加载方案的同时，采用了人椎间盘的有限元模型（L4-L5）。

体外计算结果表明，核内存在明显且显着的压力梯度，尤其是在较高载荷和水化圆盘中施加载荷后的早期。压力在原子核中心达到最大，轴向朝着端板减小，径向朝着核-环隙界面减小。这些会导致压力梯度随着时间的推移和较低的水合水平而大大降低。随着时间的推移以及随着孔隙压力的下降，原子核的贡献增加，直到达到平衡为止。在支撑施加的载荷时，环空的相对份额不仅随时间显着增加，而且在更高的载荷和更低的水合作用下也显着增加。因此，在各种静态-动态载荷下，阀瓣的水合状态对于阀瓣压力分布和内部响应至关重要。体外和体内条件下的复制。