Wear of metal-on-metal 'off-the-shelf' HA-coated/beaded hip bearings

Introduction Wear studies of metal-on-metal (MOM) bearings in hip simulators have historically involved ‘custom’ acetabular cups, i.e. having neither beaded layers nor biological coatings [1]. Such custom designs improve the accuracy of the gravimetric wear assessments. However, it is not always feasible to obtain custom cups for various reasons. Thus, another possibility would be to machine off the implant coatings on ‘off-theshelf’ cups [2]. However, this raises questions regarding metallurgical changes or cup deformation [3] and even the possibility of third-body wear damage, all potentially leading to erroneous predictions. Therefore removing these beaded layers may be impractical [3]. To date, no studies have attempted to use the gravimetric method with ‘off-the-shelf’ beaded and hydroxyapatite (HA) coated MOM cups. Therefore, the aim of this study was to investigate wear using such MOM bearings and evaluate the potential for error in the gravimetric assessment. Primary wear of Co-Cr bearings was estimated from Co and Cr ion concentrations in the serum lubricant [4]. The study hypothesis was that the gravimetric analysis would create unacceptably large errors in predicted MOM wear-rates with beaded cups (> 25%).