50 patients undergoing total hip arthroplasty (THA) were randomized to undergo THA with either a metal-on-polyethylene (MPE) bearing with a titanium shell or a metal-on-metal (MoM) bearing with an all-cobalt chrome shell. The goal of this study was to evaluate the effect of these two types of modular neck systems on serum ion levels. Results at 2 years indicated that the MoM group had significantly increased chromium and cobalt levels, but significantly lower titanium levels, when compared to the MPE group. There were no adverse events due to the increase in serum ion levels reported.