# The transepicondylar axis or the posterior condylar axis: Which is the best reference for femoral component rotation in robotic-assisted total knee arthroplasty?

## TL;DR

This study compares two anatomical references for femoral component rotation in robotic-assisted knee surgery, finding that one provides more consistent results between surgeons.

## Contribution

The study introduces a quantitative comparison of inter-surgeon consistency between two anatomical axes in robotic-assisted TKA planning.

## Key findings

- The posterior condylar axis (PCA) showed significantly higher inter-surgeon consistency compared to the transepicondylar axis (TEA).
- Over 25% of patients had more than a 2° difference in TEA-reference angles between surgeons, while less than 5% showed such differences for PCA-reference angles.

## Abstract

For robotic-assisted total knee arthroplasty (TKA), accurate identification of anatomical landmarks directly affects the displayed value for femoral component rotation. This study aimed to quantify the inter-surgeon consistency of the TEA-reference angle (the angle between the transepicondylar axis and the femoral component axis) and the PCA-reference angle (the angle between the posterior condylar axis and the femoral component axis).

The anatomical data of 56 patients who underwent robotic-assisted TKA at our institution were analyzed. Two surgeons independently identified the transepicondylar axis (TEA) and posterior condylar axis (PCA) landmarks on the 3D femoral models generated by the MAKO TKA system. The TEA-reference angle was recorded as α with the PCA-reference angle standardized to 0°, and the PCA-reference angle was recorded as β with the TEA-reference angle standardized to 0°. The measured values were α1β1 for Surgeon-1 and α2β2 for Surgeon-2. The differences between surgeons for α (∆α = α₁ – α₂) and β (∆β = β₁ – β₂) were calculated. The values of α and β are defined as positive for external rotation and negative for internal rotation.

The inter-surgeon intraclass correlation coefficient (ICC) for α was 0.761 (95% CI: 0.592–0.860), and that for β was 0.943 (95% CI: 0.902–0.966). The absolute difference between surgeons (∆α) was > 2° in 15/56 (26.8%) patients and ≤ 1° in 24/56 (42.9%) patients. With respect to ∆β, 3/56 (5.4%) patients had a difference > 2°, whereas 45/56 (80.4%) patients had a difference ≤ 1°.

The inter-surgeon consistency of the PCA was significantly greater than that of the TEA in robotic-assisted TKA planning. To mitigate the risk of inappropriate femoral component rotation, surgeons should verify landmark positions, particularly in patients with anatomical abnormalities of the distal femur, and consider cross-referencing both axes.

Video Abstract

Video Abstract

The online version contains supplementary material available at 10.1186/s42836-025-00362-7.