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Effect of surgical positioning on the risk of edge loading in Total Hip Replacement: in-vitro and in-silico simulation
Lin Wang1-2, Mazen Al Hajjar1-2, John Fisher2, Jonathan Thompson1-2
1 DePuy Synthes Joint Reconstruction, Leeds, United Kingdom
2 Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, United Kingdom
Keywords: Edge loading, experiential testing, computational modelling, artificial hip joint, MARATHON® Polyethylene Liner
Mismatch of artificial hip joint bearing component centres and tension of surrounding soft tissues can result in component separation during a gait cycle, where the femoral head contacts the rim of the acetabular liner. This phenomenon can potentially increase material wear, adversely impact durability and shorten the lifespan of an implant. Therefore, it is critical to evaluate the effect of surgical positioning on dynamic separation, contact and wear mechanics under edge loading condition.
Biomechanical and tribological tests of the 36mm diameter metal-on-polyethylene hip replacements (MARATHON® Liners, DePuy Synthes, UK) have been conducted with a hip joint simulator (ProSim EM13, Simulation Solutions, UK), to investigate the severity of edge loading (Cup inclination: 45° (n=3) and 65° (n=3); Medial-lateral mismatch: 1, 2, 3 and 4mm) and wear performance (4 mm translational mismatch over three million cycles (mc) with inclination of 45° (n=6) and 65° (n=6), respectively. Furthermore, 3D elastic-plastic Finite Element models have been developed using Ansys v18.2 (ANSYS, Inc. PA USA), where a spring element was pre-compressed to represent the corresponding medial-lateral mismatches between head and cup centres. Polyethylene wear and creep behaviours were predicted based on the Archard’s law and modified time hardening model.
With a 65° inclination and 4mm mismatch, the largest dynamic separation was measured, while this was substantially reduced in cases of lower mismatch and/or lower inclination. In comparison to the wear rates of the concentric conditions with inclinations of 45° and 65° (12.9±3.8 and 15.4±5.0 mm3/mc respectively), higher wear rates (21.5±5.5 and 23.0±5.7 mm3/mc) were observed with the 4mm mismatch. According to the Finite Element simulation, edge loading was predicted to occur in the swing phase and extend up to heel strike, resulting in a considerable increase of the contact pressure and resulting plastic deformation while increasing the value of mismatch 
Decreasing the occurrence and severity of edge loading, through optimal surgical positioning, could considerably improve biomechanical and tribological performance of a hip joint implant.
1. Ali, M., et al., Wear and deformation of metal-on-polyethylene hip replacements under edge loading conditions due to variations in surgical positioning. Bone & Joint Journal Orthopaedic Proceedings Supplement, 2017. 99-B(SUPP 3): p. 12.
2. Wang, L., D. Beedall, and J. Thompson. Effect of lateral mal-positioning on edge loading and wear of mop total hip replacement: A Finite Element Analysis. in 8th World Congress of Biomechanics. 2018. Dublin, Ireland.
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