Saweeres Emad S B, Kuiper Jan H, Evans Richard O, Richardson James B, White Stephen H
Institute for Science and Technology in Medicine, Keefe University, Staffordshire, UK.
Am J Sports Med. 2005 May;33(5):666-73. doi: 10.1177/0363546504271203. Epub 2005 Feb 8.
Anterior cruciate ligament graft fixations experience cyclic loads in vivo. Present cyclic loading studies testing fixation use "incremental cycling," "residual strength" protocols, or a combination. Industrial standards, however, rely on fatigue life testing and use tolerance limits to determine guaranteed minimum levels of cycles to failure.
Industrial standards of fatigue life and lower tolerance limits provide a more conservative assessment of cycles to failure than do currently used cyclic loading models, and they facilitate interpretation of data toward clinical performance.
Descriptive laboratory study.
Fatigue life curves and lower tolerance limits were calculated for porcine patellar tendon graft fixations that were tested to failure in single pulls or cycled to failure at 2 different load levels. A log curve was constructed so that the expected values at other load levels could be calculated. Comparison of a metal and a composite resorbable interference screw design was used as an example.
Cycles to failure of both screw designs varied widely at each load level. The guaranteed minimum number of cycles calculated by tolerance limits was therefore much lower than the mean cycles to failure at any given load level. For example, at a load of 250 N, the predicted mean life for the composite screw and the metal screw was 2513 and 1490 cycles, respectively, whereas the 80/80 tolerance limits (the value that 80% could be expected to achieve with 80% confidence) were only 63 and 68 cycles, respectively. Small reductions in load level gave very large increases in minimally expected life.
Fatigue testing of anterior cruciate ligament reconstructions better shows the wide variation between specimens and the large effect of load levels on expected life. Wide scatter makes mean levels over-optimistic and difficult to interpret. Tolerance limits give estimates that are more conservative and facilitate data interpretation.
A method for testing and analyzing fatigue properties was presented, results of which are more readily interpreted to clinical practice.
前交叉韧带移植物固定在体内承受周期性负荷。目前测试固定的周期性负荷研究使用“递增循环”“残余强度”方案或两者结合。然而，工业标准依赖疲劳寿命测试，并使用公差极限来确定保证的最低失效循环次数。
与目前使用的周期性负荷模型相比，疲劳寿命的工业标准和更低的公差极限对失效循环次数提供了更保守的评估，并且有助于对临床性能的数据进行解释。
描述性实验室研究。
计算猪髌腱移植物固定的疲劳寿命曲线和更低公差极限，这些固定在单次拉伸中测试至失效或在2个不同负荷水平下循环至失效。构建对数曲线以便能够计算其他负荷水平下的预期值。以金属和复合可吸收干涉螺钉设计的比较为例。
在每个负荷水平下，两种螺钉设计的失效循环次数差异很大。因此，通过公差极限计算的保证最低循环次数远低于任何给定负荷水平下的平均失效循环次数。例如，在250 N的负荷下，复合螺钉和金属螺钉的预测平均寿命分别为2513次和1490次循环，而80/80公差极限（80%的样本有80%的置信度能够达到的值）分别仅为63次和68次循环。负荷水平的微小降低会使最低预期寿命大幅增加。
前交叉韧带重建的疲劳测试更好地显示了样本之间的广泛差异以及负荷水平对预期寿命的巨大影响。广泛的离散性使得平均水平过于乐观且难以解释。公差极限给出的估计更保守且便于数据解释。
提出了一种测试和分析疲劳特性的方法，其结果更容易应用于临床实践。
