Development of a bespoke finite element wear algorithm to investigate the effect of femoral centre of rotation on the wear evolution in total knee replacements

Total Knee Replacements (TKRs) are a commonly used treatment to help patients suffering from severely damaged knee joints, which is normally brought on by osteoarthritis. The aim of the surgery is to reduce pain and regain function of the joint, however, some of these implants fail prematurely with implant wear being one of the main factors of failure. Computational analysis is an efficient tool that can provide an in-depth insight on the evolution of wear, before utilising experimental techniques which are time-consuming and costly. In this study, a bespoke finite element (FE) based wear algorithm has been further developed for TKRs and was used to investigate how location of femoral centre of rotation (CoR) affects the evolution of wear at the bearing surfaces. Three locations of femoral CoR have been investigated: international standards (ISO) CoR, being the location defined in ISO 14243-3, distal CoR being the centre of the femoral component's distal radius, and reference CoR being the middle ground between the two. All investigations were setup in accordance with ISO 14243-3 for displacement-controlled wear testing conditions for knee simulators. The wear algorithm extracts contact pressure and sliding distance from the FE analysis to determine wear depth, wear pattern, volumetric wear, and wear rates on the polymeric insert and femoral component's bearing surfaces using Archard's wear law. The polymeric insert volumetric wear rate after 5 million cycles (Mc) for ISO, reference, and distal CoR are 4.37mm³/Mc, 5.40mm³/Mc, and 6.83mm³/Mc respectively. Furthermore, the wear pattern's location on the bearing surfaces is dependent on the femoral CoR, with ISO CoR wear pattern being positioned more posteriorly, distal CoR being more anteriorly, and reference CoR in between ISO and distal. The ISO CoR investigation showed a region of minimal wear between two wear regions at the middle of the femoral component's wear pattern, on both medial and lateral condyles. This region of minimal wear reduces for the reference CoR and further reduces for the distal CoR. After 5 Mc, the average polymeric insert-femoral component contact area changes with femoral CoR, with the average contact area being 66.53mm², 68.35mm², and 71.21mm² for ISO, reference, and distal CoRs respectively, with distal having around 7% more contact area than ISO. The results from this study show that there is a wide range of wear values for different locations of femoral CoR. As such the choice of femoral CoR should be carefully considered when performing any wear investigation to ensure that the CoR location is consistent for all studies being compared.