5) Method: We will randomize Profix cemented bicompartmental total knee implanted conventionally vs the same prosthesis implanted with the assistance of computer navigation. Profix is the standard implant in Helse Vest and it has good 5-year implant survival data in our Norwegian arthroplasty register (15).Tantalum markers will be injected into the bone and the implant for radiostereometric analysis (RSA). The radiographic technique is somewhat challenging and we have recruited specially trained and educated radiographers to obtain these images.The images allow us to localize every marker in a three dimensional coordinate system. Mathematic models will then calculate differences in position from one image to another. Micromotion down to 0,1mm and 0,2 degrees will be detected. Micromotions within the first two years correlate with long term implant survival. The radiation dose is low (10-20% of a regular x-ray of the knee). The image processing and the calculations are time consuming and expensive. Special software is needed and we therefore collaborate with Kompetansesenter for ortopediske implantater by Norges teknisk-naturvitenskapelige universitet (NTNU).