Iatridis J C, Setton L A, Foster R J, Rawlins B A, Weidenbaum M, Mow V C
McClure Musculoskeletal Research Center, Department of Orthopaedics and Rehabilitation, University of Vermont, Burlington, USA.
J Biomech. 1998 Jun;31(6):535-44. doi: 10.1016/s0021-9290(98)00046-3.
Axial and radial specimens of non-degenerate and degenerate human anulus fibrosus (AF) were tested in confined compression to test the hypothesis that degeneration significantly affects the compressive properties of AF. Due to the highly oriented structure of AF, a secondary objective was to investigate anisotropic behaviors of AF in compression. Uniaxial swelling and stress relaxation experiments were performed on site-matched samples of anulus from the anterior outer region of L2-3 intervertebral discs. The experimental stress-relaxation behavior was modeled using the finite deformation biphasic theory and a finite-difference approximation scheme. Significant effects of degeneration but not orientation were detected for the reference stress offset, sigma(offset), and parameters describing the compressive stiffness (i.e. reference aggregate modulus, H(A0), and nonlinear stiffening coefficient, beta). Average values were 0.13+/-0.06 and 0.05+/-0.05 MPa for sigma(offset), 0.56+/-0.21 and 1.10+/-0.53 MPa for H(A0) and 2.13+/-1.48 and 0.44+/-0.61 for beta for all normal and degenerate specimens, respectively. No significant effect of degeneration or orientation were detected for either of the parameters describing the strain-dependent permeability (i.e. reference permeability, k0 and strain-dependent permeability coefficient, M) with average values for all specimens of 0.20+/-0.10 x 10(-15) m4/N-s and 1.18+/-1.30 for k0 and M, respectively. The loss of sigma(offset) was compensated with an elastic stiffening and change in the shape of the equilibrium stress-strain curve with H(A0) for degenerate tissues almost twice that of normal tissues and beta less than one sixth. The increase in reference elastic modulus with degeneration is likely related to an increase in tissue density resulting from the loss of water content. The significant effects of degeneration reported in this study suggested a shift in load carriage from fluid pressurization and swelling pressure to deformation of the solid matrix of the AF. The results also suggest that the highly organized and layered network of the anulus fibrosus, which gives rise to significant anisotropic effects in tension, does not play a major role in contributing to the magnitude of compressive stiffness or the mechanisms of fluid flow of the anulus in the confined compression configuration.
对非退变和退变的人纤维环（AF）的轴向和径向样本进行了受限压缩测试，以验证退变会显著影响AF压缩特性的假设。由于AF具有高度定向的结构，第二个目标是研究AF在压缩过程中的各向异性行为。对L2-3椎间盘前外侧区域的纤维环进行了现场匹配样本的单轴肿胀和应力松弛实验。使用有限变形双相理论和有限差分近似方案对实验应力松弛行为进行建模。对于参考应力偏移量σ(offset)以及描述压缩刚度的参数（即参考总体模量H(A0)和非线性硬化系数β），检测到退变有显著影响，但取向没有显著影响。所有正常和退变样本的σ(offset)平均值分别为0.13±0.06和0.05±0.05MPa，H(A0)平均值分别为0.56±0.21和1.10±0.53MPa，β平均值分别为2.13±1.48和0.44±0.61。对于描述应变相关渗透率的参数（即参考渗透率k0和应变相关渗透率系数M），未检测到退变或取向有显著影响，所有样本的k0和M平均值分别为0.20±0.10×10(-15)m4/N-s和1.18±1.30。对于退变组织，σ(offset)的损失通过弹性硬化以及平衡应力-应变曲线形状的变化得到补偿，其中H(A0)几乎是正常组织的两倍，β小于正常组织的六分之一。随着退变，参考弹性模量的增加可能与由于含水量损失导致的组织密度增加有关。本研究中报道的退变的显著影响表明，载荷承载从流体加压和肿胀压力转变为AF固体基质的变形。结果还表明，纤维环高度有序且分层的网络在拉伸时会产生显著的各向异性效应，但在受限压缩配置中，它对AF压缩刚度的大小或流体流动机制的贡献并不显著。
