McConnell ZA, Stambough JB, Barnes CL, et al. - This study attempted to ascertain associations 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. A cohort of 190 primary THAs implanted with a dual-modular femoral stem by one surgeon from 2005 to 2013 was retrospectively evaluated. For all patients, serum iron levels were recorded. by Via musculoskeletal-trained radiologists, pseudotumor grade and location were assessed. This study’s findings demonstrate that pseudotumor characteristics differ between patients with cobalt-chromium modular neck (CoCrMN) designs vs those with an MoM articulation. It has been reported 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.