Dickinson A S, Browne M, Roques A C, Taylor A C
Bioengineering Science Research Group, University of Southampton, Southampton, UK; Aurora Medical Ltd., Southampton Science Park, Chilworth, Southampton, UK.
Bioengineering Science Research Group, University of Southampton, Southampton, UK.
Med Eng Phys. 2014 Jan;36(1):72-80. doi: 10.1016/j.medengphy.2013.09.009. Epub 2013 Oct 19.
Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing. A modular cup's stress distributions were predicted computationally, under assembly and in vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material's yield strength. For verification against failure in high-cycle-fatigue (HCF) each element's reserve factor was calculated using the conservative Soderberg infinite life criterion. Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl=0.1) produced a high cyclic stress ratio (Rs=0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation. Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.
骨科植入物承受着巨大的循环载荷，因此需要进行临床前分析以确保它们能够承受数百万次的加载循环。髋臼杯的研发旨在减小壁厚以保护骨质，这会在模块化植入物中产生较高的预应力。作为植入物研发过程的一部分，我们提出了一种用于模块化植入物临床前疲劳强度评估的技术，该技术考虑了平均应力、应力集中特征和材料加工因素。通过计算预测了模块化髋臼杯在组装和体内载荷下的应力分布，并计算了其循环残余应力和应力幅值。为了验证在低周疲劳（LCF）中是否会引发损伤，将峰值应力与材料的屈服强度进行了比较。为了验证在高周疲劳（HCF）中是否会失效，使用保守的索德伯格无限寿命准则计算了每个单元的储备系数。结果表明了考虑平均应力的重要性。预计髋臼杯会承受高循环平均应力和低幅值应力：低循环载荷比（Rl = 0.1）会产生高循环应力比（Rs = 0.80）。此外，最高循环平均应力和应力幅值的位置并不重合。预测的最小储备系数Nf为1.96（HCF）和2.08（LCF）。如果忽略平均应力或假设应力比等于载荷比，储备系数将大大降低，可能导致过度设计，减少骨质保护。疲劳强度评估只是更广泛研发过程中的一步，该过程应包括一系列针对各种正常和创伤性生理载荷情况、具有代表性的边界条件和代表性环境的验证。本研究提出并论证了一种疲劳分析方法，该方法可应用于早期开发中的各种模块化和预应力假体概念，并且在植入物研发旨在最大化模块化和骨质保护的情况下尤为相关。
