Experimental and computational evaluation of knee implant wear and creep under in vivo and ISO boundary conditions

Experimental knee implant wear testing according to ISO 14243 is a standard procedure, but it inherently possesses limitations for preclinical evaluations due to extended testing periods and costly infrastructure. In an effort to overcome these limitations, we hereby develop and experimentally validate a finite element (FE) based algorithm, including a novel cross-shear and contact pressure dependent wear and creep model, and apply it towards understanding the sensitivity of wear outcomes to the applied boundary conditions. Specifically, we investigated the application of in vivo data for level walking from the publicly available "Stan" dataset, which contains single representative tibiofemoral loads and kinematics derived from in vivo measurements of six subjects, and compared wear outcomes against those obtained using the ISO standard boundary conditions. To provide validation of the numerical models, this comparison was reproduced experimentally on a six-station knee wear simulator over 5 million cycles, testing the same implant Stan's data was obtained from. Experimental implementation of Stan's boundary conditions in displacement control resulted in approximately three times higher wear rates (4.4 vs. 1.6 mm3 per million cycles) and a more anterior wear pattern compared to the ISO standard in force control. While a force-controlled ISO FE model was unable to reproduce the bench test kinematics, and thus wear rate, displacement-controlled FE models accurately predicted the laboratory wear tests for both ISO and Stan boundary conditions. The credibility of the in silico wear and creep model was further established per the ASME V&V-40 standard. The model is thus suitable for supporting future patient specific models and development of novel implant designs.  ### Competing Interest Statement MJD, SHHN, BW, and WRT declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. PF and FA are employed at Zimmer Biomet, the company producing the Innex implant investigated here. ### Clinical Protocols ### Funding Statement Zimmer Biomet provided access to the INNEX implant CAD files and PE material data, material samples and implant components, and ran the physical test on the knee simulator. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study used only aggregated, summarized human data which is available at https://cams-knee.orthoload.com. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The Stan dataset used as input in this study can be found at https://cams-knee.orthoload.com. The wear prediction algorithm can be requested at https://www.empa.ch/web/s304/wearpy. Data pertaining to the investigated implant, such as geometry or material data, is not available due to its confidential nature.