The goal of this study is to compare the fixed bearing (FB) Triathlon knee (Stryker, USA) and the mobile bearing (MB) Triathlon knee (Stryker, USA) and study the effect of implant design on kinematics and micromotion. During two tasks the kinematics measured with fluoroscopy (kinematics and movement of the polyethylene bearing). Roentgen Stereophotogrammetric Analysis (RSA) will be used to evaluate micromotion between prosthesis and the bone for the MB and FB Triathlon knee.