We hypothesized that the tibial plafond revealed a novel anatomical reference axis associated with the axial alignment through an image processing technique. The plafond axis (PLA) was defined as the line connecting the midpoints of the medial and lateral plafond margin. In terms of intraclass correlation coefficient, the reproducibility to identify the PLA was not less than that to identify a conventional trans-malleolar axis (TMA). When the proximal medial–lateral axis was applied, the tibial torsion with reference to the PLA and the TMA was 12.0°±8.4°, and 23.3°±8.6°, respectively. The PLA can be considered a reproducible reference and is expected to be applicable in preoperative planning as well as postoperative evaluation of the implant alignment in total knee arthroplasty.