We evaluated the relationship among the various rotational reference axes of femoral component in TKA, and the potential change in rotational alignment when a posterior condylar axis (PCA) was used as a reference for different cartilage erosions between the medial and lateral posterior femoral condyles.

The study subjects included twenty male and twenty female volunteers. Axial MR image of the distal femur were used to measure the angles among the clinical transepicondylar axis (cTEA), surgical transepicondylar axis (sTEA), the PCA, and the AP axis. We then evaluated the difference in the amount of rotation between a normal and osteoarthritic model when the PCA was used as a rotational reference.

The mean angle between cTEA and PCA was 5.4 degrees, and it was significantly greater in the female (6.5 degrees) than in the male group (4.3 degrees). The cTEA demonstrated a perpendicular relationship with the AP axis in both groups. The average angles between cTEA and sTEA were 3.2 degrees in the male group and 3.8 degrees in the female group. When we assumed complete erosion of the cartilage on the medial posterior condyle with preservation on the lateral side, the angles between the cTEA and PCA decreased by an average of 1.9 degrees in both groups.

This study suggests that the two transepicondylar axes, which demonstrated greater than 3 degrees in angle-difference, should be defined clearly in scientific reports and in their application as rotational references in TKA. Some significant gender differences in the rotational profile were noted. When the PCA is used as a rotational reference, potential errors caused by the different amounts of cartilage erosion between the medial and lateral posterior condyles should be considered.