We aim to reveal a difference of 0,5 degrees in the two groups with alignment in the frontal plane as measured on the CT-scan. Earlier research in this field indicates a greater variation in the conventional group (standard deviation=1,3) than in the computer navigated group (standard deviation=0,9) (10). With 80% statistical power and a significance level of 0.05, a power calculation suggests 79 patients in each group. Further, to be able to detect a difference of 10 units in our functional score (KOOS)(13) with a common standard deviation of 20 (14), our power calculations suggested 94 patients in each group to reach a power of 80% at a significance level of 0,05. From these calculations we chose a study design with 100 patients in each group, assuming there will be some drop-outs. The study is recommended by the Regional ethics committee, Personvernombudet for forskning (Norsk samfunnsvitenskapelig datatjeneste) and Statens strålevern.