McConnell ZA, Stambough JB, Barnes CL, et al. - This study attempted to explore cobalt levels and psuedotumor characteristics in a non-recalled modular femoral prosthesis. The relationships between serum cobalt levels and pseudotumors following modular and metal-on-metal (MoM) total hip arthroplasty (THA), the location and size of pseudotumors, and changes to pseudotumor grade over time were ascertained. A cohort of 190 primary THAs implanted with a dual modular femoral stem by one surgeon from 2005-2013 was retrospectively assessed. The results showed that pseudotumor characteristics differ between patients with CoCrMN designs vs those with a MoM articulation. This study’s findings demonstrate that normal serum cobalt levels did not exclude the presence of a pseudotumor and routine MARS MRI should be included in follow-up of patients with these CoCrMN prostheses.