Bonifasi-Lista Carlos, Lake Spencer P, Small Michael S, Weiss Jeffrey A
Department of Bioengineering, University of Utah, 50 S Central Campus Drive, Rm. 2480, Salt Lake City, UT 84112, USA.
J Orthop Res. 2005 Jan;23(1):67-76. doi: 10.1016/j.orthres.2004.06.002.
Ligament viscoelasticity controls viscous dissipation of energy and thus the potential for injury or catastrophic failure. Viscoelasticity under different loading conditions is likely related to the organization and anisotropy of the tissue. The objective of this study was to quantify the strain- and frequency-dependent viscoelastic behavior of the human medial collateral ligament (MCL) in tension along its longitudinal and transverse directions, and under shear along the fiber direction. The overall hypothesis was that human MCL would exhibit direction-dependent viscoelastic behavior, reflecting the composite structural organization of the tissue. Incremental stress relaxation testing was performed, followed by the application of small sinusoidal strain oscillations at three different equilibrium strain levels. The peak and equilibrium stress-strain curves for the longitudinal, transverse and shear tests demonstrate that the instantaneous and long-time stress-strain response of the tissue differs significantly between loading conditions of along-fiber stretch, cross-fiber stretch and along-fiber shear. The reduced relaxation curves demonstrated at least two relaxation times for all three test modes. Relaxation resulted in stresses that were 60-80% of the initial stress after 1000 s. Incremental stress relaxation proceeded faster at the lowest strain level for all three test configurations. Dynamic stiffness varied greatly with test mode and equilibrium strain level, and showed a modest but significant increase with frequency of applied strain oscillations for longitudinal and shear tests. Phase angle was unaffected by strain level (with exception of lowest strain level for longitudinal samples) but showed a significant increase with increasing strain oscillation frequency. There was no effect of test type on the phase angle. The increase in phase and thus energy dissipation at higher frequencies may protect the tissue from injury at faster loading rates. Results suggest that the long-time relaxation behavior and the short-time dynamic energy dissipation of ligament may be governed by different viscoelastic mechanisms, yet these mechanisms may affect tissue viscoelasticity similarly under different loading configurations.
韧带粘弹性控制着能量的粘性耗散,进而影响损伤或灾难性失效的可能性。不同加载条件下的粘弹性可能与组织的结构和各向异性有关。本研究的目的是量化人体内侧副韧带(MCL)在纵向和横向拉伸以及沿纤维方向剪切时,应变和频率依赖性的粘弹性行为。总的假设是人体MCL会表现出方向依赖性的粘弹性行为,这反映了组织的复合结构。进行了增量应力松弛测试,随后在三个不同的平衡应变水平施加小幅度正弦应变振荡。纵向、横向和剪切测试的峰值和平衡应力-应变曲线表明,在沿纤维拉伸、跨纤维拉伸和沿纤维剪切的加载条件下,组织的瞬时和长期应力-应变响应存在显著差异。所有三种测试模式的简化松弛曲线都显示至少有两个松弛时间。松弛1000秒后,应力降低至初始应力的60-80%。在所有三种测试配置中,增量应力松弛在最低应变水平下进行得更快。动态刚度随测试模式和平衡应变水平变化很大,并且在纵向和剪切测试中,随着施加应变振荡频率的增加而有适度但显著的增加。相位角不受应变水平的影响(纵向样本的最低应变水平除外),但随着应变振荡频率的增加而显著增加。测试类型对相位角没有影响。在较高频率下相位增加从而能量耗散增加,这可能会在更快的加载速率下保护组织免受损伤。结果表明,韧带的长期松弛行为和短期动态能量耗散可能受不同的粘弹性机制控制,但这些机制在不同加载配置下可能对组织粘弹性产生类似影响。