An Important Contribution to Our Understanding of the Performance of the Current Generation of Metal-on-Metal Hip Replacements: Commentary on an article by A.J. Hart, MA, MD, FRCSG(Orth), et al.: “Which Factors Determine the Wear Rate of Large-Diameter Metal-on-Metal Hip Replacements? Multivariate Analysis of Two Hundred and Seventy-six Components”

Joshua J. Jacobs, Markus A. Wimmer

It is hard to overstate the importance of implant retrieval analysis in the development of successful orthopaedic devices. Given the enormous complexity of the musculoskeletal system, no amount of preclinical testing, joint simulation, and analytic modeling can fully predict the performance of orthopaedic implants. Despite the application of the most sophisticated methodologies available during the 1990s when so-called alternative bearings were being developed for total hip arthroplasty, there was insufficient information to predict, among other things, squeaking of ceramic-on-ceramic bearings and the adverse local tissue reactions that have been widely reported in association with metal-on-metal bearings. Implant retrieval analyses provide important and unique insights into the in situ performance of orthopaedic devices and provide critical clues into mechanisms of failure and success, the latter being determined by postmortem retrieval analyses. The findings from implant retrieval studies also provide the necessary end points for implant designers to develop preclinical testing models. Biomechanical testing, joint simulations, and analytic modeling will only be predictive of implant performance if the failure mechanisms, or end points, of the testing and modeling are congruent with the failure mechanisms observed on retrieved devices.

The study by Hart et al. is an important contribution to our understanding of the performance of the current generation of large femoral head metal-on-metal devices. The large number of components analyzed in this study adds considerably to the utility and generalizability of their findings. The authors point out that the extant implant retrieval literature for contemporary large femoral head metal-on-metal devices is limited to the analysis of components obtained from only sixty-nine patients. In order to provide the statistical power necessary for a multivariate statistical analysis of factors that may contribute to failure, it is necessary to have relatively large numbers of implants available for study. This has been particularly challenging in the United States because of the litigation environment.

The salient finding of this study is that the presence of edge-loading is the most important factor responsible for the variation of wear rate in the retrieved devices. Another important finding was that there was a strong positive correlation between blood metal levels and wear rate. Both of these observations are quite useful. The minimization of edge-loading becomes an important design criterion for the development of improved implants and the development of protocols replicating such adverse conditions in hip simulations that can be used in preclinical testing. Furthermore, it is informative for the surgeon regarding component positioning. Metal testing can be a useful adjunct in postoperative monitoring of patients with metal-on-metal bearings since there is no other noninvasive methodology to determine bearing surface performance; unlike the situation with ultra-high molecular weight polyethylene bearings, radiographs are not helpful in the assessment of wear of metal-on-metal bearings.