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PurposeLong-leg-radiography (LLR) is commonly used for measurement of lower limb alignment. However, limb rotations during radiography may interfere with the alignment measurement. This study examines the effect of limb rotation on the accuracy of measurements based on the mechanical and anatomical axes of the femur and tibia, with variations in knee flexion and coronal deformity. MethodsForty-five lower limbs of thirty patients were scanned with CT. Virtual LLRs simulating 5 rotational positions (neutral, ±10°, and ±20° internal rotation) were generated from the CT images. Changes in the hip-knee-ankle angle (HKA) and the femorotibial angle (FTA) were measured on each image with respect to neutral values. These changes were related to knee flexion and coronal deformity under both weight- and non-weight-bearing conditions. ResultsThe measurement error of the HKA and FTA derived from limb rotation were up to 4.84±0.66° and 7.35±0.88° respectively, and were correlated with knee flexion (p＜0.001) and severe coronal deformity (p≤0.001). Compared with non-weight-bearing position, coronal deformity measured in weight-bearing condition was 2.62° greater, the correlation coefficients between the coronal deformity and the deviation ranges of HKA and FTA were also greater. ConclusionFlexion and severe coronal deformity have significant influence on the measurement error of lower limb alignment. Errors can be amplified in the weight-bearing condition compared with the non-weight-bearing condition. The error of measurement of the anatomic axis is greater than the mechanical axis. Considering LLR is the gold standard image modality, attention should be paid to the measurement of knee alignment. Especially for the possible errors derives from weight-bearing long leg radiographs of patients with severe knee deformities.

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