Relationship of the posterior femoral axis of the "kinematically aligned" total knee arthroplasty to the posterior condylar, transepicondylar, and anteroposterior femoral axes

Background: A recent proposed modification in surgical technique in total knee arthroplasty (TKA) has been the introduction of the "kinematically aligned" TKA, in which the angle and level of the posterior joint line of the femoral component and joint line of the tibial component are aligned to those of the "normal," pre-arthritic knee. The purpose of this study was to establish the relationship of the posterior femoral axis of the "kinematically aligned" total knee arthroplasty (TKA) to the traditional axes used to set femoral component rotation. Methods: One hundred and fourteen consecutive, unselected patients with preoperative MRI images undergoing TKA were retrospectively reviewed. The transepicondylar axis (TEA), posterior condylar axis (PCA), antero-posterior axis (APA) of the trochlear groove, and posterior femoral axis of the kinematically aligned TKA (KAA) were templated on axial MRI images by two independent observers. The relationships between the KAA, TEA, APA, and PCA were determined, with a negative value indicating relative internal rotation of the axis. Results: On average, the KAA was 0.5° externally rotated relative to the PCA (minimum of - 3.6°, maximum of 5.8°), - 4.0° internally rotated relative to the TEA (minimum of - 10.5°, maximum of 2.3°), and - 96.4° internally rotated relative to the APA (minimum of - 104.5°, maximum of - 88.5°). Each of these relationships exhibited a wide range of potential values. Conclusions: Using a kinematically aligned surgical technique internally rotates the posterior femoral axis relative to the transepicondylar axis, which significantly differs from current alignment instrument targets.