Velísková Petra, Bashkuev Maxim, Shirazi-Adl Aboulfazl, Schmidt Hendrik
Julius Wolff Institut, Charité - Universitätsmedizin Berlin, Germany.
École Polytechnique, Montréal, Canada.
J Biomech. 2018 Mar 21;70:16-25. doi: 10.1016/j.jbiomech.2017.10.032. Epub 2017 Nov 3.
The intervertebral disc viscoelastic response is governed primarily by its fluid content and flow. Invivo measurements demonstrate that the disc volume, fluid content, height and nucleus pressure completely recover during resting even after diurnal loading with twice longer duration (16 vs. 8 h). In view of much longer periods required for the recovery of disc height and pressure in vitro, concerns have been raised on the fluid inflow through the endplates that might be hampered by clogged blood vessels post mortem. This in silico study aimed to identify fluid-flow dependent response of discs and conditions essential to replicate in vitro and in vivo observations. An osmo-poroelastic finite element model of the human lumbar L4-L5 disc-bone unit was used. Simulating earlier in vitro experiments on bovine discs, the loading protocol started with 8 h preload at 0.06 MPa followed by 30 high/low compression loading cycles each lasting 7.5min at 0.5/0.06 MPa, respectively. Three different endplate configurations were investigated: free in- and outflow, no inflow and closed endplates with no flow. Additionally, the preload magnitude was increased from 0.06 MPa to 0.28 MPa and 0.50 MPa, or the initial nucleus hydration was reduced from 83% to 50%. For 0.06 MPa preload, the model with no inflow best matched in vitro trends. The model with free inflow increased segment height and nucleus pressure while the model with no fluid inflow resulted in a relatively small recovery in segment height and a rather constant nucleus pressure during unloading periods. Results highlight an excessive mobile fluid content as well as a restricted fluid inflow through endplates as likely causes of the discrepancies between in vivo and in vitro studies. To replicate in vivo conditions in vitro and in silico, disc hydration level should be controlled by adequate selection of preload magnitude/period and/or mobile fluid porosity.
椎间盘的粘弹性反应主要由其液体含量和流动决定。体内测量表明，即使在持续时间延长一倍（16小时与8小时）的日常负荷后，椎间盘体积、液体含量、高度和髓核压力在休息时也能完全恢复。鉴于体外恢复椎间盘高度和压力所需的时间长得多，人们对通过终板的液体流入表示担忧，因为死后血管堵塞可能会阻碍这种流入。这项计算机模拟研究旨在确定椎间盘的流体流动依赖性反应以及复制体外和体内观察结果所必需的条件。使用了人腰椎L4-L5椎间盘-骨单元的渗透-多孔弹性有限元模型。模拟早期对牛椎间盘的体外实验，加载方案开始时在0.06MPa下进行8小时预加载，然后分别在0.5/0.06MPa下进行30个高/低压缩加载循环，每个循环持续7.5分钟。研究了三种不同的终板配置：自由流入和流出、无流入以及无流动的封闭终板。此外，预加载大小从0.06MPa增加到0.28MPa和0.50MPa，或者初始髓核水合作用从83%降低到50%。对于0.06MPa的预加载，无流入模型最符合体外趋势。自由流入模型增加了节段高度和髓核压力，而无液体流入模型在卸载期间节段高度恢复相对较小，髓核压力相当恒定。结果突出表明，过多的可移动液体含量以及通过终板的液体流入受限可能是体内和体外研究之间差异的原因。为了在体外和计算机模拟中复制体内条件，应通过适当选择预加载大小/时间和/或可移动液体孔隙率来控制椎间盘水合水平。
