Mid-Term Analysis of Serum Metal Ion Levels Comparison Between Monoblock and Modular Dual Mobility Components: A Source for Concern?

Background: Dislocation is one of the most prevalent complications in total hip arthroplasty. A leading recommendation to prevent dislocation is dual mobility (DM) constructs. The monoblock DM design features a one-piece shell with a mobile polyethylene liner, whereas the newer modular DM design holds a fixed liner within a standard cup. Despite several advantages, an additional bearing surface is concerning for fretting and metal ion release. Previously, we found the frequency of serum metal ions was greater in modular than monoblock at the 1-year time point. Elevated metal ion values may be associated with adverse local tissue reactions; therefore, we continued monitoring at 5 years. Methods: Three high-yield surgeons who used DM constructs included metal ion tests in their clinical practice. Patients with a monoblock/modular construct implanted between 2015 and 2019 were asked to complete metal ion testing at 5 years. Results were interpreted based on standardized metal ion ranges. Results: We contacted 98 patients from our previous study. Three patients had died, seven were lost to follow-up, leaving 88 patients eligible for continued monitoring. Fifty-two (57%) patients returned (monoblock, N = 22; modular, N = 30). Five-year mean metal ion values were the following: cobalt (0.83 ± 0.412), chromium (0.91 ± 1.3), and titanium (5.4 ± 4.3). Monoblock had higher chromium levels than modular (1.2 vs 0.69) but there was no significant difference in mean metal ions. Conclusions: We found no metal ion level differences between monoblock/modular constructs at 5 years. Future studies are needed to determine if normal metal ion results between constructs equate to similar clinical outcomes.