Method: Patients age 50 through 85 years old, with gonarthritis, in need of knee replacement, are included in the trial. Radiostereometric analysis (RSA), CT-scans, X-rays, clinical evaluation score systems and laboratory measures are used in the evaluation process. A cost-effective analysis is performed based on data from Norwegian life tables, data from SINTEF and from the Norwegian arthroplasty register. Data from the Norwegian arthroplasty register will be statistically analysed separately for all knee replacements done with computer navigation in Norway in the last 5 years. Four Norwegian hospitals will collaborate in this trial (Haukeland university hospital, Haugesund hospital, Haugesund sanitetsforenings hospital for rheumatic diseases and Lovisenberg diakonale hospital) and patients are recruited from all four hospitals.