50 patients undergoing total knee replacement (TKA) were randomized to compare the P.F.C Sigma fixed and mobile bearing designs. Patients were monitored over 2 years, and were evaluated for implant migration using radiostereometric analysis (RSA), bone mineral density using dual-energy X-ray absorptiometry (DEXA), and outcome on the Oxford Knee Score. Results demonstrated significantly higher migration of the tibial tray with fixed bearings when compared to the mobile bearings, predominantly within the first 3 postoperative months. Bone mineral density and Oxford Knee Scores were comparable between groups. Stable fixation was noted by 3 months in both groups, and both groups displayed migration within satisfactory limits.