Rotation or flexion alters mechanical leg axis measurements comparably in patients with different coronal alignment

Purpose Flexion and rotation of the knee joint are supposed to alter the measurement of the mechanical leg axis on long leg radiographs. However, in patients with varus or valgus alignment it has not been systematically analyzed so far. The hypothesis is that measurement of the mechanical leg axis is more influenced by flexion and rotation in presence of varus or valgus alignment compared to patients with a straight coronal alignment. Methods 3D surface models of the lower extremities of seven individuals with varying degrees of coronal alignment were created based on CT data. The coronal alignment of the seven individuals captured the range between 9 degrees varus and 9 degrees valgus with equal steps of 3 degrees. Combinations of internal and external rotations of 10 degrees, 20 degrees, and 30 degrees with flexion of 5 degrees, 10 degrees, 15 degrees, 20 degrees, and 30 degrees were simulated. The mechanical leg axis was measured for each combination as the antero-posterior (ap)-projected hip-knee-ankle (HKA-) angle. Results 294 simulations with all combinations of rotation and flexion were performed. Ranges of deviation of HKA never showed a critical deviation of more than 3 degrees from median values. Deviations from baseline appeared normally distributed for all flexion and rotation combinations (p < 0.05) and the probability for a deviation from the mean mechanical leg axis of more than 3 degrees was less than 0.03 for all combinations. Comparability of the models, therefore, could be assumed. Conclusion Deviations in HKA-angle measurements, caused by rotation or flexion, does not vary relevantly through the range of coronal alignment of 9 degrees varus to 9 degrees valgus. As a clinical relevance, deviations in HKA-angle measurements can be considered as comparable in patients with different coronal alignment.