The Relationships between the Rotational Axes of the Distal Femur in the Arthritic Caucasian Knee: A Magnetic Resonance Imaging Analysis

Abstract

Four distal femoral axes have been described to aid in the accuracy of transverse plane component alignment in total knee arthroplasty (TKA). These include the surgical epicondylar axis (SEA), the posterior condylar axis (PCA), and the anteroposterior (AP) axis. Therefore, the primary purpose of this study is to identify the axial femoral relationship with the least variation in a Caucasian population awaiting TKA for osteoarthritis. Secondarily, we further plan to determine if these femoral axis relationships differ with respect to sex or preoperative coronal alignment. These anatomic relationships were measured using preoperative magnetic resonance imaging, which was performed within 2 months of the planned operation date for patient specific instrumentation templating. In terms of our primary outcome, the mean SEA/AP angle was 92.8 degrees (standard deviation [SD]: 2.5 degrees), the mean PCA/AP angle was 95.7 degrees (SD: 2.8 degrees), and the mean PCA/SEA angle was 3.4 degrees (SD: 1.8 degrees). Overall, the PCA/SEA relationship had the smallest variance, while the PCA/AP had the most variance for all comparisons. In terms of our secondary outcome, there was no statistical difference between femoral axis relationships based on preoperative coronal plane alignment. In terms of sex differences, the PCA/SEA was significantly higher in female knees compared with males. Females had a PCA/SEA relationship of 4.7 degrees (SD: 1.3 degrees) compared with 2.6 degrees (SD: 1.2 degrees) for males (p < 0.05). By using the PCA/AP axes, the AP axis was externally rotated by 96.7 degrees (SD: 2.3 degrees) in females, compared with 93.2 degrees (SD: 2.1 degrees) in males, from the PCA. In conclusion, our results demonstrate that the PCA/SEA relationship should be used to determine axial rotation in TKA as it shows the least variation. When using the PCA, approximately 5 degrees of external rotation for Caucasian women and 3 degrees of external rotation for the Caucasian men should be dialed into the femoral cutting block to restore anatomic axial rotation. Further evaluation is needed to determine to role of preoperative coronal alignment on distal femoral axial alignment.