# Significant changes of the mechanical medial proximal tibial angle in dependence of internal and external rotation of the hinge axis in slope correcting infratuberositary tibial deflexion osteotomy

## TL;DR

This study shows how rotating the hinge axis during a tibial osteotomy can significantly change the mechanical angle of the tibia, affecting the outcome of the surgery.

## Contribution

The study quantifies how small rotations of the hinge axis during tibial osteotomy can lead to significant changes in the mechanical medial proximal tibial angle.

## Key findings

- TDO perpendicular to the coronal plane preserves the preoperative mMPTA.
- Internal and external HA rotations caused up to 4.5° changes in mMPTA.
- A 15° HA rotation exceeded a postoperative change of ≥2° in mMPTA.

## Abstract

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.

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.

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°.

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.

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