External Rotation of Femoral Component in Valgus Knee TKA
In valgus knee deformity, the femoral component should be rotationally aligned using the anteroposterior axis (Whiteside line) rather than the posterior condylar axis, which typically results in greater external rotation than the standard 3° used in neutral knees. 1
Recommended Rotational Alignment Strategy
Primary Landmark for Valgus Knees
Use the anteroposterior axis (Whiteside line) as the primary reference for femoral component rotation in valgus knees, defined by a line through the deepest part of the patellar groove anteriorly and the center of the intercondylar notch posteriorly. 1
The posterior condylar axis is unreliable in valgus knees because the lateral condyle is typically hypoplastic, making this landmark inappropriate for rotational alignment. 1
Using the anteroposterior axis in 107 valgus knees resulted in Q angles <10° and acceptable patellar tracking in all but one case, compared to 8 intraoperative patellar realignments and 4 postoperative dislocations when using the posterior condylar axis in 46 valgus knees. 1
Alternative Reference Points
The transepicondylar axis can serve as a secondary reference, with the femoral component positioned parallel to this axis. 2
The femoral component rotation may be assessed relative to the transepicondylar axis, Whiteside line, or posterior femoral condyles on postoperative CT imaging. 3
Critical Pitfalls to Avoid
Excessive External Rotation Risks
Do not externally rotate beyond what the anteroposterior axis dictates, as excessive external rotation (>5°) causes medial collateral ligament slackening of 2mm and can lead to valgus instability in flexion. 4
External rotation of 5° from neutral tightens the lateral collateral ligament by 2mm while slackening the MCL by 2mm, creating asymmetric soft tissue tension. 4
Attempting to correct patellar tracking solely through femoral external rotation worsens tibial abduction (up to 5° at 90° flexion) and cannot restore normal kinematics. 5
Internal Rotation Complications
Avoid any internal rotation of the femoral component in valgus knees, as this dramatically increases patellar instability risk. 2
Excessive combined internal rotation of femoral and tibial components is directly proportional to the severity of patellofemoral complications. 2
Internal malrotation is the most common cause of patellofemoral instability after TKA, occurring in 1-12% of cases. 2
Postoperative Assessment
Verification of Rotation
CT without IV contrast is the gold standard for measuring component rotation when malrotation is suspected postoperatively. 3
Weight-bearing axial radiographs demonstrate patellar tilt or subluxation and provide superior assessment of patellofemoral kinematics. 3, 6, 7
The tibial component should be positioned in approximately 18° internal rotation relative to the tibial tubercle. 2, 3
Expected Outcomes
With proper anteroposterior axis alignment in valgus knees, medial tibial tubercle transfer should rarely be needed (1 in 107 cases vs 8 in 46 cases with posterior condylar referencing). 1
Patellar instability requiring revision should be eliminated at 2-year follow-up when using the anteroposterior axis technique. 1