Abstract
Patient imaging and explant analysis has shown evidence of edge loading of hard-on-hard hip replacements in vivo. Experimental hip simulator testing under edge loading conditions has produced increased, clinically-relevant, wear rates for hard-on-hard bearings when compared to concentric conditions. Such testing, however, is time consuming and costly. A quick running computational edge loading model (Python Edge Loading (PyEL) - quasi-static, rigid, frictionless), capable of considering realistic bearing geometries, was developed. The aim of this study was to produce predictions of separation within the typical experimental measurement error of ∼0.5 mm. The model was verified and validated against comparable finite element (FE) models (including inertia and friction) and pre-existing experimental test data for 56 cases, covering a variety of simulated cup orientations, positions, tissue tensions, and loading environments. The PyEL model agreed well with both the more complex computational modelling and experimental results. From comparison with the FE models, the assumption of no inertia had little effect on the maximum separation prediction. With high contact force cases, the assumption of no friction had a larger effect (up to ∼5% error). The PyEL model was able to predict the experimental maximum separations within ∼0.3 mm. It could therefore be used to optimise an experimental test plan and efficiently investigate a much wider range of scenarios and variables. It could also help explain trends and damage modes seen in experimental testing through identifying the contact locations on the liner that are not easily measured experimentally.
Highlights
Separation of the femoral head and acetabular cup in total hip replacements (THRs) has been associated with increased wear and device damage (Kovochich et al, 2018; Manaka et al, 2004; Sanders et al, 2012)
The experimental testing was performed on a six station ProSim Electromechanical hip simulator (EM13, Simulation Solutions, Stockport, UK) with three stations used per case
Changing the number of contact position points evaluated by the Python model (PyEL) model for the four cases from 500 to 1000 points caused changes to the minimum and maximum separation of less than 0.004 mm
Summary
Separation of the femoral head and acetabular cup in total hip replacements (THRs) has been associated with increased wear and device damage (Kovochich et al, 2018; Manaka et al, 2004; Sanders et al, 2012). Experimental THR simulator testing can include a transverse force to cause this separation, representing soft tissue tension from slightly misaligned (mismatched) components, as well as a superoinferior (SI) force (ISO, 2018). The intention of these standardised edge loading tests is to capture an important aspect of in vivo contact area and pressure conditions, providing a method for comparison of new devices with well performing predicate devices. The degree of head-liner separation, and the subsequent damage modes, are dependent on various interacting factors, such as the cup orientation, translational mismatch between the components, tension force, and loading/activity.
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