Purpose: The transepicondylar axis (TECA) is an important landmark for positioning the femoral component in correct the rotation during total knee arthroplasty. In vivo studies have shown that the TECA corresponds to the flexion-extension axis of the knee joint. Two TECA have been defined in the literature depending on the landmark used for the medial epicondyle: the eminence for the “clinical” TECA and the depression for the “surgical” TECA. The purpose of the present study was to investigate in vivo the relations between the TECA and the mechanical axis of the femur (FA) and the tibia (TA) measured on computed tomography (CT) scans of the flexed knee, analysing separately the two TECAs.

Material and methods: CT scans of the right knees of ten volunteers were studied. Goniometric data was acquired on the scans. Five controls with genu varum and five with genu valgum were also studied. Images were acquired at 0°, 45° and 90° flexion. The epicondyles were identified on the horizontal sections and three frontal sections parallel to the posterior cortical of the tibia were reconstructed. Superoposition of these three sections, for each flexion angle, gave a frontal section with TECA-clin, TECAsurg, TA, and the posterior bicondylar line (PBL). The angles between TECA and TA, FA and PBL were analysed during flexion. Angles were measured by the medial side.

Results: TECAsurg remained perpendicular to the TA throughout flexion but with considerable interindividual variability. The mean variation during flexion was 3.4±1.5°. The FA-TECA angle was 88.5±0/8° and did not vary with morphotype. The TECA/PBL and TECA/TA angles varied with morphotype but less with flexion.

Conclusion: The surgical TECA maintains constant relations with the tibial axis during knee flexion. It can thus be used as a landmark for positioning the femoral component for total knee arthroplasty in order to optimise femorotibial kinematics. The relations between the clinical TECA and the TA are variable and preoperative identification on the main medial epicondylar eminence may give variable results.