Reliability of the Posterior Condylar Axis as an Alternative Reference for Femoral Rotation in Japanese Patients with Varus Knee Osteoarthritis Undergoing Total Knee Arthroplasty

Background: This study aimed to assess the reliability of the posterior condylar axis (PCA), the most widely used alternative reference for the surgical transepicondylar axis (sTEA), as a reference for appropriate femoral rotational alignment in total knee arthroplasty (TKA) by comparing variances with other alternative reference axes. Methods: A total of 305 knees in 233 Japanese patients who underwent TKA due to varus knee osteoarthritis were assessed in this study. Variances and relationships between alternative references were determined. Angles of alternative reference axes relative to the sTEA in the axial plane were measured using computer software based on computed tomography. Results: The PCA line was 3.0°±1.8° (range, -2.1° to 8.8°) internally rotated relative to the sTEA. No significant differences were observed in the PCA angle relative to the sTEA angle between males and females. The variance with respect to the sTEA was significantly smaller in the order of anatomical transepicondylar axis (aTEA), PCA, and antero-posterior axis (APA). The variance of the APA and that of the femoral anterior tangent line (FAT) did not significantly differ, while the variance of the FAT was significantly smaller than that of the trochlear anterior line (TAL). The proportion of outliers for PCA (>3° away from the average value) was 9.8%. The PCA was weakly correlated with the aTEA, APA, FAT, and TAL. Conclusions: In terms of variance with respect to the sTEA, the PCA was second to aTEA in reliability as an alternative reference axis, and was on average 3.0°±1.8° internally rotated relative to the sTEA. Our findings suggest that the PCA is a reliable and reproducibly identifiable alternative axis for accurately determining femoral rotational alignment in TKA.