Margin-of-safety Algorithm Used with EOS Imaging to Interpret MHRA Warning for 46-48mm MOM Arthroplasty


  • Ian Clarke
  • Jean-Yves Lazennec Department of Orthopaedics, La Pitie Hospital, UPMC University, Paris, France



BHR Resurfacing Arthroplasty, cup inclination, edge wear, MHRA alert, EOS, MOM


The Medical Healthcare Products Regulatory Agency (MHRA June-2015) warned of higher risks with 46-48mm sizes of BHR hip resurfacing arthroplasty (HRA). The most common condemnation of adverse results in MOM bearings has been termed edge loading. We originally developed a margin-of-safety (MOS) algorithm to define edge loading of cups in simulator studies. This method integrated simulator wear-patterns with respect to cup diameters and cup designs. The algorithm’s simplicity lay in the fact that with wear-patterns and rim-profile angles predetermined, the only input required was the cup inclination-angle. The algorithm demonstrated that the margin-of-safety decreased in smaller cups due to the tribo-mechanics of spherical CoCr bearings, a previously unrecognized feature. For the 46mm and 48mm cups highlighted in the MHRA alert, the critical cup inclinations where edge-wear became a risk occurred at 65-66°, revealing an insignificant difference with respect to diameters. The MOS-algorithm also indicated that lower lateral-inclination angles were particularly beneficial, i.e. a 46mm cup positioned at 50° inclination would exhibit a higher margin of safety than either 48mm or 50mm sizes positioned at 55° inclination. This evidence supported clinical studies that recommended BHR cup inclinations up to 50-55° and lower as optimal for reducing metal-ion concentrations. In a patient with normal spine mobility, our EOS imaging demonstrated that the inclination in the 46mm cup steepened by 9° from standing to the seated position while margin-of-safety was reduced by 50%. Our 2nd patient with a stiff spine sat with the same component orientations as in his standing posture. Thus MOM impingement and subluxation in different functional postures may also provoke rim-damage mechanisms. Here the combination of EOS imaging and the MOS-algorithm may aid understanding of such risks. Thus the margin-of-safety algorithm confirmed and helped explained the relative risks in the 46mm and 48mm cups highlighted by the MHRA. The algorithm’s stratification by cup rim-profile, inclination angle and cup diameter may assist the surgeon determine which patients may be more at risk for edge wear with the smaller BHR cups. 

Margin-of-Safety Algorithym


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How to Cite

Clarke, I., & Lazennec, J.-Y. (2015). Margin-of-safety Algorithm Used with EOS Imaging to Interpret MHRA Warning for 46-48mm MOM Arthroplasty. Reconstructive Review, 5(3).



Original Article