Does Tibial Slope Affect Perception of Coronal Alignment on a Standing Anteroposterior Radiograph?

Background: A standing anteroposterior (AP) radiograph is commonly used to evaluate coronal alignment following total knee arthroplasty (TKA). The impact of coronal alignment on TKA outcomes is controversial, perhaps due to variability in imaging and/or measurement technique. We sought to quantify the effect of image rotation and tibial slope on coronal alignment. Methods: Using a standard extramedullary tibial alignment guide, 3 cadaver legs were cut to accept a tibial tray at 0 degrees, 3 degrees, and 7 degrees of slope. A computed tomography scan of the entire tibia was obtained for each specimen to confirm neutral coronal alignment. Images were then obtained at progressive 10 degrees intervals of internal and external rotation up to 40 degrees maximum in each direction. Images were then randomized and 5 blinded TKA surgeons were asked to determine coronal alignment. Continuous data values were transformed to categorical data (neutral [0], valgus [L], and varus [R]). Results: Each 10 degrees interval of external rotation of a 7 degrees sloped tibial cut (or relative internal rotation of a tibial component viewed in the AP plane) resulted in perception of an additional 0.75 degrees of varus. Conclusion: The slope of the proximal tibia bone cut should be taken into account when measuring coronal alignment on a standing AP radiograph. (C) 2017 Elsevier Inc. All rights reserved.