Difference between the epicondylar and cylindrical axis of the knee

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Medical Imaging

Background:

• Accurate component placement in total knee arthroplasty (TKA) is crucial for optimal outcomes.
• Computer-assisted surgical navigation (CASN) enhances precision in TKA.
• The flexion-extension axis is a key reference for component alignment.

Purpose of the Study:

• To evaluate the accuracy and reproducibility of the transepicondylar axis (TEA) as a substitute for the true knee flexion-extension axis (FEA).
• To investigate the discrepancy between these axes in three-dimensional (3D) space and its implications for TKA.
• To determine if differences are masked in traditional two-dimensional (2D) imaging.

Main Methods:

• 3D reconstruction of 23 fresh-frozen cadaveric distal femurs using computed tomography (CT).
• Comparison of the TEA with a line equidistant from the femoral condylar articular surfaces (FEA).
• Multiple measurements by three observers to assess interobserver and intraobserver variability.

Main Results:

• A consistent difference of approximately 5 degrees was found between the TEA and FEA in 3D space.
• Interobserver and intraobserver variations were minimal, indicating high reproducibility.
• This discrepancy was reduced when projected to 2D planes, potentially explaining prior underestimation.

Conclusions:

• The TEA is not an accurate substitute for the true FEA in 3D space.
• The 5-degree difference may contribute to midrange instability in TKA.
• Failure to account for this discrepancy with CASN may lead to component malposition and suboptimal TKA outcomes.