Abstract Purpose Tibial deflexion osteotomy (TDO) is performed to correct an increased posterior tibial slope (PTS). Unintended rotation of the osteotomy and the hinge axis (HA) orientation can result in a postoperative deviation of the mechanical medial proximal tibial angle (mMPTA). This study aimed to investigate how internal and external HA rotations affect postoperative mMPTA. Methods Three‐dimensional (3D) bone models of ten patients with increased PTS were used to simulate infratuberositary TDO with different HA orientations and closing distances. Postoperative changes in mMPTA were analysed. Results In total, 440 TDOs were simulated. The PTS changed by 0.9 ± 0.0° per mm of closing distance. TDO perpendicular to the coronal plane of the long‐leg radiograph showed no significant change in the postoperative mMPTA. Internal and external rotation of the HA resulted in significant changes in postoperative mMPTA, with absolute changes up to 4.5° ± 0.5°. Conclusion A TDO oriented perpendicular to the leg's coronal plane preserves the preoperative mMPTA and therefore avoids unintended coronal correction. The mMPTA changed significantly with a rotation of the HA of only 5° and exceeded a postoperative change of ≥2° with 15° of HA rotation. Level of Evidence N/A.