The Effect of Operative Factors on Outlier Ion Levels in Patients with Metal-on-Metal Hip Arthroplasties

Metal-on-metal (MoM) hip arthroplasties comprised of cobalt and chromium alloys continue to be successful alternatives to conventional bearings in younger patients with osteoarthritis (OA). A small proportion of patients have unusually high levels of cobalt and chromium ions postoperatively. Given the increasing prevalence of MoM bearings and the potential for cellular toxicity, the purpose of our study was to determine whether patient or surgical factors could account for abnormally elevated ion levels following MoM hip arthroplasty. Materials and Methods: Cobalt and chromium levels were analyzed from whole blood in 761 patients with MoM hip arthroplasties. Patient outliers were defined as those who had ion levels greater than or equal to three-fold the median value. Thirty-four patients (4.5%) met this criteria. This included 20 patients who underwent standard total hip arthroplasty (THA) and hip resurfacing, respectively. Patients were followed prospectively with the Harris hip scores (HHS) and the University of California Los Angeles (UCLA) activity scores. Serial radiographs and ion levels were analyzed at regular intervals. Results: At a mean follow-up of 2 years, the median values for outlier cobalt and chromium ions were 13.7 mu g/L and 6.0 mu g/L, respectively. Postoperative HHS and UCLA activity scores improved significantly when compared to pre-operative values. There was no statistical correlation between outlier ion levels, patient demographics, and HHS and UCLA activity scores. Acetabular inclination correlated with chromium values. Outlier cobalt levels were higher in patients after THA. Conclusions: We identified that various patient and surgical factors could account for some of the abnormally high metal ion levels in outliers. Further studies are necessary to better understand the effect of abnormal elevations in metal ions, given the recent concerns of adverse local soft tissue reactions following MoM hip implants.