School of Biomedical Engineering, Colorado State University, Fort Collins, CO, USA.
McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, USA.
Acta Biomater. 2018 Mar 1;68:78-89. doi: 10.1016/j.actbio.2017.12.024. Epub 2017 Dec 26.
Despite efforts to simulate the in vivo environment, post-mortem degradation and lack of blood perfusion complicate the use of ex vivo derived material models in computational studies of spinal cord injury. In order to quantify the mechanical changes that manifest ex vivo, the viscoelastic behavior of in vivo and ex vivo porcine spinal cord samples were compared. Stress-relaxation data from each condition were fit to a non-linear viscoelastic model using a novel characterization technique called the direct fit method. To validate the presented material models, the parameters obtained for each condition were used to predict the respective dynamic cyclic response. Both ex vivo and in vivo samples displayed non-linear viscoelastic behavior with a significant increase in relaxation with applied strain. However, at all three strain magnitudes compared, ex vivo samples experienced a higher stress and greater relaxation than in vivo samples. Significant differences between model parameters also showed distinct relaxation behaviors, especially in non-linear relaxation modulus components associated with the short-term response (0.1-1 s). The results of this study underscore the necessity of utilizing material models developed from in vivo experimental data for studies of spinal cord injury, where the time-dependent properties are critical. The ability of each material model to accurately predict the dynamic cyclic response validates the presented methodology and supports the use of the in vivo model in future high-resolution finite element modeling efforts.
Neural tissues (such as the brain and spinal cord) display time-dependent, or viscoelastic, mechanical behavior making it difficult to model how they respond to various loading conditions, including injury. Methods that aim to characterize the behavior of the spinal cord almost exclusively use ex vivo cadaveric or animal samples, despite evidence that time after death affects the behavior compared to that in a living animal (in vivo response). Therefore, this study directly compared the mechanical response of ex vivo and in vivo samples to quantify these differences for the first time. This will allow researchers to draw more accurate conclusions about spinal cord injuries based on ex vivo data (which are easier to obtain) and emphasizes the importance of future in vivo experimental animal work.
尽管努力模拟体内环境，但死后降解和缺乏血液灌注使离体衍生材料模型在脊髓损伤的计算研究中变得复杂。为了量化离体表现出的机械变化，比较了体内和离体猪脊髓样本的黏弹性行为。使用一种称为直接拟合方法的新型特征化技术，根据每种条件的应力松弛数据拟合非线性黏弹性模型。为了验证所提出的材料模型，使用每种条件下获得的参数来预测各自的动态循环响应。离体和体内样本都表现出非线性黏弹性行为，随着施加应变，松弛显著增加。然而，在所比较的三个应变幅度下，离体样本经历的应力和松弛都高于体内样本。模型参数之间的显著差异也显示出明显不同的松弛行为，特别是与短期响应（0.1-1 s）相关的非线性松弛模量成分。这项研究的结果强调了在研究脊髓损伤时，从体内实验数据中利用材料模型的必要性，因为时间依赖性特性是至关重要的。每种材料模型准确预测动态循环响应的能力验证了所提出的方法，并支持在未来的高分辨率有限元建模工作中使用体内模型。
神经组织（如大脑和脊髓）表现出时变或黏弹性的机械行为，使得难以模拟它们对各种加载条件（包括损伤）的反应。旨在描述脊髓行为的方法几乎完全依赖于离体尸体或动物样本，尽管有证据表明死后时间与活体动物（体内反应）的行为不同。因此，这项研究首次直接比较了离体和体内样本的机械响应，以量化这些差异。这将使研究人员能够根据更容易获得的离体数据更准确地得出关于脊髓损伤的结论，并强调未来体内实验动物工作的重要性。
