Malrotation of whole-leg radiograph less than 10 degrees does not influence preoperative planning in open-wedge high tibial osteotomy

Coronal whole-leg radiography is generally used for preoperative open-wedge high tibial osteotomy (OWHTO) planning. Nevertheless, malrotational whole-leg radiographs could affect the planning, and the effects could possibly be strengthened by the knee flexed position. Consecutive 51 varus osteoarthritis knees that underwent OWHTO were included. The digitally reconstructed radiography (DRR) images parallel to the surgical epicondylar axis (neutral rotation; NR), 5° and 10° external rotation (ER) or internal rotation (IR), were reconstructed from preoperative CT. Preoperative weight-bearing line percentage (WBL%), medial proximal tibial angle (MPTA), planned opening angle, and planned postoperative MPTA were measured with OWHTO planning in each DRR image. Correlations among the measured differences relative to NR images and knee flexion angle were investigated. As the DRR image shifted from ER to IR, the differences in preoperative WBL% and MPTA gradually increased, whereas those in the opening angle gradually decreased, although all differences in the opening angle were within 0.5° on an average. Opening angle differences remarkably correlated with knee flexion angle, and knees with >10° flexion contracture had >1° difference in 10° ER or IR images. Planned postoperative MPTA had relatively high consistency regardless of whole-leg malrotation. The opening angle measurement using malrotated radiographs less than 10° would be clinically reliable in cases without knee flexion contracture. Nevertheless, extreme care should be taken in cases with >10° knee flexion contracture. The MPTA after medial opening would be a consistent and reliable parameter in whole-leg alignment evaluation.