Zec Michelle L, Thistlethwaite Paul, Frank Cyril B, Shrive Nigel G
Department of Orthopaedics, Faculty of Medicine, University of British Columbia, 3415-910 West 10th Avenue, Vancouver, BC, V5Z 4E3, Canada.
J Biomech Eng. 2010 Jan;132(1):011001. doi: 10.1115/1.4000108.
Ligaments are regularly subjected to repetitive loading in vivo. Typically, mechanical studies focus on repetitive loading protocols of short duration, while those characterizing damage accumulation over a longer duration (i.e., fatigue studies) are lacking. The aims of this study were as follows: (a) to demonstrate that damage does accumulate in ligament tissue subjected to repetitive loading and (b) to evaluate existing and new methods for characterizing fatigue damage accumulation. It was hypothesized that ligaments would accumulate damage with repetitive loading as evidenced by failure at stresses well below ultimate tensile strength, creep curve discontinuities, and by reductions in stiffness during loading. Eight normal medial collateral ligaments from female New Zealand white rabbits were cycled in tension, between 0 MPa and 28 MPa, to failure or until 259,200 cycles, whichever came first. Medial collateral ligaments that did not fail were subsequently loaded to failure. Displacement rates (dl(max)/dt) as well as primary, secondary, and tertiary creeps were monitored as indices of damage accumulation and impending mechanical failure. Additionally, the relative utilities of tangent, secant, and chord stiffness parameters were critically evaluated. Finally, new uses for the second derivative of force-displacement data were explored. Three out of eight ligaments failed during testing, demonstrating that ligaments can fail in fatigue under moderate tensile stress in vitro. The evaluation of displacement rates (dl(max)/dt), as well as primary through tertiary creep patterns, were not well suited to predicting failure in normal ligaments until rupture was all but imminent. Tangent stiffness, which was calculated from a mathematically defined start of the "linear region," was surprisingly constant throughout testing. Secant stiffness dropped in a predictable fashion, providing a global indicator of tissue stiffness, but did not provide any insight into fiber mechanics. Chord stiffness, on the other hand, appeared to be sensitive to fiber recruitment patterns. The second derivative of force-displacement data proved to be a useful means of (a) objectively defining the start of the linear region and (b) inferring changes in fiber recruitment patterns within ligament tissue. Tangent, secant, and chord stiffnesses highlight different attributes of ligament responses to loading; hence these parameters cannot be used interchangeably. Additionally, the second derivative of the force-displacement curve was introduced as a useful descriptive and analytical tool.
在体内，韧带经常受到重复性负荷作用。通常，力学研究聚焦于短时长的重复性负荷方案，而对于表征较长时间内损伤累积情况的研究（即疲劳研究）则较为缺乏。本研究的目的如下：（a）证明在承受重复性负荷的韧带组织中会累积损伤；（b）评估用于表征疲劳损伤累积的现有方法和新方法。研究假设，韧带会随着重复性负荷而累积损伤，这可通过在远低于极限抗拉强度的应力下发生失效、蠕变曲线间断以及加载过程中刚度降低得以证明。取自雌性新西兰白兔的八条正常内侧副韧带在0兆帕至28兆帕之间进行拉伸循环，直至失效或达到259,200次循环，以先达到者为准。未失效的内侧副韧带随后加载至失效。监测位移速率（dl(max)/dt）以及初级、次级和三级蠕变，作为损伤累积和即将发生机械失效的指标。此外，还对切线、割线和弦刚度参数的相对效用进行了严格评估。最后，探索了力 - 位移数据二阶导数的新用途。八条韧带中有三条在测试过程中失效，表明韧带在体外中等拉伸应力下会发生疲劳失效。对位移速率（dl(max)/dt）以及从初级到三级蠕变模式的评估，直到接近断裂时才非常适合预测正常韧带的失效。从数学定义的“线性区域”起始点计算得出的切线刚度在整个测试过程中出人意料地保持恒定。割线刚度以可预测的方式下降，提供了组织刚度的总体指标，但未提供对纤维力学的任何洞察。另一方面，弦刚度似乎对纤维募集模式敏感。力 - 位移数据的二阶导数被证明是一种有用的手段，（a）可客观定义线性区域的起始点，（b）可推断韧带组织内纤维募集模式的变化。切线、割线和弦刚度突出了韧带对负荷反应的不同属性；因此这些参数不能互换使用。此外，力 - 位移曲线的二阶导数被引入作为一种有用的描述和分析工具。
