All patients receive a Columbus knee prosthesis and in both groups an OrthoPilot navigation system is used to control the alignment. Pain intensity using the Visual Analogue Scale (VAS) and concomitant pain medication intake are documented daily during the first 20 days after operation. As long as the patient is hospitalized the Range of Motion will also be documented daily. Knee society score, oxford score, as well as the WOMAC score are monitored pre- and postoperatively.