PURPOSE: Objective of this study was to evaluate the effect of functional alignment on femoral component rotation compared to mechanical alignment in knees with a constitutional tibial varus alignment classified as coronal plane alignment of the knee (CPAK) types I, II and IV. METHODS: This retrospective study included patients undergoing conventional total knee replacement (TKR) ( = 64) and robot-assisted TKR ( = 84). Coronal and axial measurements were performed manually and automatically using pre- and postoperative computed tomography images in the conventional group and robot-assisted group respectively. RESULTS: Femoral component rotation was statistically significant more external rotated in the conventional group versus robot-assisted group (1.3° ± 2.7° vs. 0.76° ± 1.4°,  < 0.001). Also, the tibial plateau was placed statistically significant more in varus in the robot-assisted group compared to the conventional group (87.6° ± 2.5° vs. 88.9° ± 1.4°,  < 0.001). CONCLUSION: Functional alignment leads to more varus of the tibial component and less external rotation of the femoral component compared to mechanical alignment in TKR in patients with a constitutional tibial varus alignment. Since this is a radiological analysis, further research is needed to understand how these differences affect clinical outcomes as faster recovery, adequate patellar tracking and longer survival. LEVEL OF EVIDENCE: Level III.