48 patients requiring unicompartmental knee arthroplasty in 56 knees were randomized to receive either a mobile bearing or fixed bearing implant in order to determine which style of implant brings about the best clinical outcomes and knee kinematics. The mobile bearing knees achieved increasingly better tibial internal rotation, less medial femoral condyle and contact point movement, and a reduced incidence of radiolucency at the bone implant interface compared to the fixed bearing knees. No significant differences were found in the incidence of lateral compartment osteoarthritis, progression of osteoarthritis at the patello-femoral joint, Knee Society Scores, WOMAC scores, and SF-36 scores between the two groups.