Does a positioning rod or a patient-specific guide result in more natural femoral flexion in the concept of kinematically aligned total knee arthroplasty?

Flexion of the femoral component in 5A degrees increments downsizes the femoral component, decreases the proximal reach and surface area of the trochlea, delays the engagement of the patella during flexion, and is associated with a higher risk of patellar-femoral instability after kinematically aligned TKA. The present study evaluated flexion of the femoral component after use of two kinematic alignment instrumentation systems. We determined whether a distal cutting block attached to a positioning rod inserted perpendicular to the distal femoral joint line in the axial plane and 8-10 cm into the distal femur anterior and posterior to the distal cortex of the femur in the sagittal plane or a femoral patient-specific cutting guide sets the femoral component in more natural flexion. Flexion of the femoral component was measured with respect to the sagittal femoral anatomic axis of the distal diaphysis and the sagittal femoral axis on rotationally controlled long-leg lateral computer scanograms. Measurements were performed on 53 consecutive patients treated with a kinematically aligned TKA performed with a distal cutting block attached to a positioning rod, and 53 consecutive patients treated with a kinematically aligned TKA performed with a femoral patient-specific cutting guide. The average flexion and variability (+/- standard deviation) of the femoral component of patients treated with a positioning rod was 1A degrees A +/- 2A degrees and 7A degrees A +/- 4A degrees with respect to the anatomic and mechanical axes, respectively, which was 5A degrees less than the average flexion of the femoral component of patients treated with a femoral patient-specific cutting guide of 6A degrees A +/- 4A degrees and 12A degrees A +/- 5A degrees (p = 0.0001, p = 0.0001, respectively). Because a distal cutting block attached to a positioning rod sets the femoral component in 5A degrees less flexion and with less variability than a femoral patient-specific cutting guide, we prefer this instrumentation system when performing kinematically aligned TKA to reduce the risk of patellar-femoral instability. Each surgeon should determine the repeatability of setting the flexion of the femoral component with this instrumentation system.