National Centre for Advanced Tribology at Southampton (nCATS), Engineering Sciences, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK; Bioengineering Science Research Group, Engineering Sciences, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK.
DePuy Synthes Joint Reconstruction, 700 Orthopaedic Drive, Warsaw, IN, USA.
J Mech Behav Biomed Mater. 2014 Apr;32:62-75. doi: 10.1016/j.jmbbm.2013.12.023. Epub 2014 Jan 3.
Considerable progress has been made in understanding implant wear and developing numerical models to predict wear for new orthopaedic devices. However any model of wear could be improved through a more accurate representation of the biomaterial mechanics, including time-varying dynamic and inelastic behaviour such as viscosity and plastic deformation. In particular, most computational models of wear of UHMWPE implement a time-invariant version of Archard's law that links the volume of worn material to the contact pressure between the metal implant and the polymeric tibial insert. During in-vivo conditions, however, the contact area is a time-varying quantity and is therefore dependent upon the dynamic deformation response of the material. From this observation one can conclude that creep deformations of UHMWPE may be very important to consider when conducting computational wear analyses, in stark contrast to what can be found in the literature. In this study, different numerical modelling techniques are compared with experimental creep testing on a unicondylar knee replacement system in a physiologically representative context. Linear elastic, plastic and time-varying visco-dynamic models are benchmarked using literature data to predict contact deformations, pressures and areas. The aim of this study is to elucidate the contributions of viscoelastic and plastic effects on these surface quantities. It is concluded that creep deformations have a significant effect on the contact pressure measured (experiment) and calculated (computational models) at the surface of the UHMWPE unicondylar insert. The use of a purely elastoplastic constitutive model for UHMWPE lead to compressive deformations of the insert which are much smaller than those predicted by a creep-capturing viscoelastic model (and those measured experimentally). This shows again the importance of including creep behaviour into a constitutive model in order to predict the right level of surface deformation on a tibial insert. At high compressive loads, inelastic deformation mechanisms (creep and plasticity) dominate the mechanical response of UHMWPE components by altering the surface geometry (penetration depth and so contact area) and therefore the contact pressure. Although generic creep models can provide a good first approximation of material characteristics, for best accuracy both viscous and plastic effects must be captured, and model parameters must be founded upon specific experimental test data. Such high-fidelity numerical creep models will provide a better foundation for the next generation of more robust and accurate in-silico wear prediction tools.
在理解植入物磨损和开发新的骨科设备磨损预测数值模型方面已经取得了相当大的进展。然而，通过更准确地表示生物材料力学特性，包括时变动态和非弹性行为（如粘性和塑性变形），任何磨损模型都可以得到改进。特别是，UHMWPE 磨损的大多数计算模型都实施了 Archard 定律的时不变版本，该定律将磨损材料的体积与金属植入物和聚合物胫骨插入物之间的接触压力联系起来。然而，在体内条件下，接触面积是时变的，因此取决于材料的动态变形响应。从这个观察结果可以得出结论，当进行计算磨损分析时，UHMWPE 的蠕变变形可能非常重要，这与文献中的情况形成鲜明对比。在这项研究中，不同的数值建模技术在生理代表性环境下，通过对单髁膝关节置换系统进行实验蠕变测试进行了比较。使用文献数据对线性弹性、塑性和时变粘弹性模型进行基准测试，以预测接触变形、压力和面积。本研究的目的是阐明粘弹性和塑性效应对这些表面量的贡献。研究结果表明，蠕变变形对 UHMWPE 单髁插入物表面测量的（实验）和计算的（计算模型）接触压力有显著影响。对于 UHMWPE，仅使用弹塑性本构模型会导致插入物的压缩变形远小于粘弹性模型（和实验测量值）预测的变形。这再次表明，为了预测胫骨插入物表面的正确变形水平，必须在本构模型中包含蠕变行为。在高压缩载荷下，非弹性变形机制（蠕变和塑性）通过改变表面几何形状（穿透深度和接触面积）来改变 UHMWPE 部件的力学响应，从而改变接触压力。尽管通用蠕变模型可以很好地提供材料特性的初步近似，但为了获得最佳精度，必须捕获粘性和塑性效应，并且模型参数必须基于特定的实验测试数据。这种高保真数值蠕变模型将为下一代更健壮和准确的计算机磨损预测工具提供更好的基础。
