Three Degrees External to the Posterior Condylar Axis Has Little Relevance in Femoral Component Rotation: A Computed Tomography-Based Total Knee Arthroplasty Simulation Study

Abstract

Background: Femoral component rotation in total knee arthroplasty (TKA) has a significant impact on balance and patellofemoral kinematics. However, normal anatomic relationships between rotational axes are poorly understood. As such, we sought to characterize anatomic femoral rotational axes in patients undergoing primary TKA.

Methods: We identified 100 patients who underwent a primary TKA with a preoperative computed tomography scan. The angles between the surgical epicondylar axis (SEA) and the anterior-posterior (AP) axis to the posterior condylar axis (PCA) were measured independently by a musculoskeletal fellowship-trained radiologist and a fellowship-trained arthroplasty surgeon. We simulated an ideal TKA in which the femoral component was placed exactly 3° external to the PCA and measured resulting rotation.

Results: The SEA was on average 1.5° externally rotated to the PCA (range 3.1° internal to 7.0° external). The AP axis was on average 4.5° externally rotated to the PCA (range 2.3° internal to 10.3° external). The AP axis was a mean 2.7° externally rotated to the SEA (range 6.3° internal to 10.3° external). Routinely setting femoral rotation 3° external to the PCA would result in only 51 (51%) TKAs within ±2° of the SEA and 23 (23%) femoral components internally rotated relative to the SEA.

Conclusion: Normal anatomic rotational axes of arthritic knees are highly variable, with a 10° range in the SEA and 16° range in the AP axis. Routinely setting femoral rotation 3° external to the PCA will yield significant error in aligning the femoral component with either the SEA or AP axis.

Keywords: posterior condylar axis; rotation; surgical epicondylar axis; total knee; whiteside’s line.