 Joint arthroplasty is an effective surgical intervention, however prosthetic survivorship is finite. New or modified prosthetic components are routinely introduced, partly in an effort to improve survivorship. When new technologies emerge for total knee replacement it is not always evident that they are an improvement over existing options. This determination requires long-term follow-up studies. Conventional studies require a large number of patients and approximately 10 years of follow-up. By using RSA to measure micromotion of prostheses over time problems can be elucidated with a much smaller sample over a shorter time period. RSA involves using tantalum markers 0.5-1.0 mm beads inserted in the bone and prosthesis during surgery to measure motion in subsequent bi-planar x-rays of the joint. The relatively new Medial Pivot Knee ADVANCE Knee theoretically reproduces the natural kinematics of the knee but it is unclear what effect this design may have on the forces applied to the tibial component-bone interface. The amount of micromotion of the tibial component is directly related to the amount of force exerted on it and inversely related to its fixation. In this study we propose to compare in vivo micromotion at the tibial component-bone interface with the ADVANCE Medial-Pivot Knee experimental group versus a more conventional prosthesis control, the ADVANCE Traditional Knee. Patients n60; age 45-80 years old with primary osteoarthritis of the knee and mono-articular disease will be randomized to receive either the ADVANCE Traditional or ADVANCE Medial-Pivot Knee using Simplex cement. Eight tantalum markers will be placed in both the tibia and femur proximal to the prosthesis and in the polyethylene tray of the tibial component. Pre-operatively and at 6, 12 and 24 months all patients will complete general health SF-12 and joint specific questionnaires Oxford -12 Item Knee Score, have their body mass index calculated, and have the range of motion, alignment, extensor function and ligamentous competency of their index knee recorded. On the first weight-bearing day after surgery bi-planar standing x-rays will be taken and used as a reference point for tibial component position. At 6, 12 and 24 months post-operatively patients will undergo more bi-planar standing x-rays incorporating provocative loading tests. Micromotion will be analyzed through serial comparison of digital x-rays by blinded study personnel using RSA software. Parametric statistics unpaired Students t-test will be used to compare the 2 groups. 