Department of Mechanical Engineering University of California, Berkeley, United States.
Department of Chemical and Biomolecular Engineering University of California, Berkeley, United States.
J Mech Behav Biomed Mater. 2020 Dec;112:104089. doi: 10.1016/j.jmbbm.2020.104089. Epub 2020 Sep 22.
The intervertebral disc is an avascular composite structure, comprised of the nucleus pulposus (NP) and the annulus fibrosus (AF). Previous tissue-level experiments either examined relative differences in swelling capacity of the two disc regions at a single time point or tested explant structures that did not replicate in situ boundary conditions. Previous joint-level studies that investigated time-dependent fluid flow into the disc provided limited information about swelling-induced intradiscal strains with respect to time and boundary constraints. Therefore, the objective of this study was to investigate time-dependent swelling behavior of the intervertebral disc ex situ. The first study investigated time-dependent free-swelling response of the whole disc and the disc's subcomponents separately (i.e., NP and AF). Findings from this study showed that the swelling rate and swelling capacity of NP explants under free-swelling conditions were greater than AF explants. The second study evaluated the effect of boundary conditions on in-plane strain distributions of intact discs and AF rings. Swelling-induced strain was highly heterogeneous in AF rings, where negative circumferential strains were observed in the inner AF and tensile circumferential strains were observed in the outer AF. Radial strains in AF rings were an order of magnitude greater than circumferential strains. Restricting fluid flow only to the outer AF periphery reduced the swelling of the inner AF. Swelling of intact discs affected both NP and AF swelling behaviors, where NP hydration decreased by 60%. Furthermore, the presence of the NP reduced peak radial strains in the AF and resulted in uniform strain distribution throughout the AF. In conclusion, these studies highlight that tissue hydration and swelling-induced strains largely depend on regional biochemical composition and geometric boundary constraints.
椎间盘是一种无血管的复合材料结构，由核髓（NP）和纤维环（AF）组成。以前的组织水平实验要么在单个时间点检查两个椎间盘区域的肿胀能力的相对差异，要么测试不能复制原位边界条件的外植体结构。以前研究关节水平的研究调查了时间依赖性的流体流入椎间盘，提供了关于肿胀引起的椎间盘内应变的有限信息，这些信息涉及时间和边界约束。因此，本研究的目的是体外研究椎间盘的时间依赖性肿胀行为。第一项研究分别研究了整个椎间盘及其子组件（即 NP 和 AF）的时间依赖性自由肿胀反应。这项研究的结果表明，NP 外植体在自由肿胀条件下的肿胀速率和肿胀能力大于 AF 外植体。第二项研究评估了边界条件对完整椎间盘和 AF 环的面内应变分布的影响。在 AF 环中，肿胀诱导的应变具有高度的非均匀性，在内侧 AF 中观察到负的周向应变，在外侧 AF 中观察到拉伸的周向应变。AF 环中的径向应变比周向应变大一个数量级。仅将流体流动限制在 AF 外周的外侧会减少内 AF 的肿胀。完整椎间盘的肿胀会影响 NP 和 AF 的肿胀行为，NP 水合作用减少了 60%。此外，NP 的存在降低了 AF 中的峰值径向应变，并导致 AF 中的应变分布均匀。总之，这些研究强调了组织水合作用和肿胀诱导的应变在很大程度上取决于区域生化组成和几何边界约束。
