Use of the Cusum technique for evaluation of a CT based navigation system for total knee replacement (TKR) Praemer et al30reported that in 1996, 245,000 total knee replacements (TKRs) were done in the United States. They estimated that the projected increase in TKRs in the United States will be 209,000 between 1996 and 2030. Most of the early failures of TKRs are related to technical flaws. Experimental and clinical data indicate that to achieve optimal midterm and long-term results of a TKR, good alignment in /the frontal plane of the lower limb is mandatory. In an experimental study, Hsu et al 14showed that a femoral component in 7° valgus, with the tibial component placed at 90° to the long axis of the tibia, was the best position for the total condylar knee and produced equal force distribution between the medial and lateral plateaus. For the Kinematic knee, the best position was 9° valgus for the femoral component with 2° varus of the tibial component. A clinical association between failure of TKR and malalignment seems likely. Ritter et al 32 reported that varus knees had a lower survival at 10 years than aligned or valgus knees. Jeffery et al,16 with the first design of the Denham prosthesis, reported that at 12 years the incidence of loosening was 3% when, on long-leg standing radio-graphs, the line joining the center of the femoral head to the center of the body of the talus passed through themiddle1⁄3of the knee prosthesis. The incidence of loosening was 24% when the line was not in this range. Alignment in a narrow range around the neutral axis is a major objective of TKR even if the exact range is not clearly identified. To achieve this goal, different systems are available based on intramedullary or extramedullary rods that allow targeting. From experimental data18, 26, 31 and clinical studies, 4, 7, 13, 21 it has been shown that these systems