The effect of osteotomy depth and hinge axis orientation on biplanar surgical accuracy in medial opening-wedge high tibial osteotomy-a deeper understanding by 3D simulations.

Background: Not all surgical osteotomy steps have been properly investigated for their potential impact on surgical accuracy. The main study objective was to investigate the osteotomy parameters that have respectively major and minor impact on coronal and sagittal bony accuracy in medial opening-wedge high tibial osteotomy (MOWHTO).

Methods: Three tibias from an existing 3D MOWHTO osteotomy database were chronologically selected based on segmentation quality, tibial plateau size and the presence of tibial varus. The study consisted of three parts: (I) translating the hinge axis in the coronal plane and switching the osteotomy starting point (30-40 mm) and depth, (II) the hinge axis was rotated stepwise by 10° to perform five simulations, (III) the hinge axis was rotated in the axial plane stepwise by 10° towards anterolateral to perform four simulations (0°, +10°, +20°, +30°). The medial proximal tibial angle (MPTA) and lateral tibial slope were the primary outcomes. Simulations were performed with 5, 10 and 15 mm gap distraction.

Results: In the coronal plane, maximum difference in osteotomy depth was 10 mm which represented an MPTA difference of 0.8°-1.1° in 10 mm gap distraction and 1.2°-2.0° in 15 mm gap distraction. Tibial slope remained unchanged. Rotating the hinge axis in the sagittal plane delivered minor changes on both MPTA (<0.5°) and tibial slope (<1.5°) at 10 mm gap distraction. Per 10° of axial rotation of the hinge axis towards anterolateral, the tibial slope increased by 1.0°-1.3° in 10 mm gap distraction while the MPTA remains nearly unchanged.

Conclusions: The study showed that the medio-lateral osteotomy length is the main parameter for obtaining bony accuracy in the coronal plane and maintaining a strict perpendicular axial hinge axis position is crucial in preserving the native tibial slope. Correct axial alignment of the hinge axis can be obtained by creating an equal osteotomy depth of the anterior and posterior tibial cortices in the lateral metaphyseal area.