The effect of knee joint rotation in the sagittal and axial plane on the measurement accuracy of coronal alignment of the lower limb

Background Although the measurement of coronal alignment of the lower limb on conventional full-length weight-bearing anteroposterior (FLWAP) radiographs was reported to be influenced by the knee joint rotation, no comparative analysis was performed considering the effects of knee joint rotation on the sagittal and axial planes simultaneously using the three-dimensional images while taking into account the actual weight-bearing conditions. The aim of this study was to investigate the effect of knee joint rotation on the measurement accuracy of coronal alignment of the lower limb on the FLWAP radiograph. Methods Radiographic images of 90 consecutive patients (180 lower limbs) who took both the FLWAP radiograph and the EOS image were retrospectively reviewed. The relationship among delta values of mechanical tibiofemoral angle (mTFA) between the FLWAP radiographs and the EOS images (Delta mTFA), knee flexion/extension angle (sagittal plane rotation) on the EOS images, and patellar rotation (axial plane rotation) on the FLWAP radiographs were analyzed. Further, subgroup analysis according to each direction of knee joint rotation was performed. Results There was a significant correlation between Delta mTFA and sagittal plane rotation (r = 0.368,P < 0.001), whereas axial plane rotation was not correlated. In the analysis according to the direction, statistically significant correlation was observed only in the knee flexion group (r = 0.399,P < 0.001). The regression analysis showed a significant linear relationship between Delta mTFA and sagittal plane rotation (r(2) = 0.136,P < 0.001). Additional subgroup analysis in patients with the patellar rotation greater than 3% showed a similar result of a linear relationship between Delta mTFA and sagittal plane rotation (r(2) = 0.257,P < 0.001), whereas no statistically significant relationship was found in patients with the patellar rotation less than 3%. Conclusion The measurement accuracy of coronal alignment of the lower limb on the FLWAP radiographs would be influenced by knee flexion, specifically when there is any subtle rotation of the knee joint in the axial plane. A strict patellar forward position without axial plane rotation of the knee could provide accurate results of the measurement even if there is a fixed flexion contracture of the knee.