The Ideal Hinge Axis Position to Reduce Tibial Slope in Opening-Wedge High Tibial Osteotomy Includes Proximalization-Extension and Internal Rotation

Purpose: To evaluate the effect of the hinge axis position on the posterior tibial slope (PTS) in medial opening-wedge high tibial osteotomy. Methods: This study included adults with medial-compartment osteoarthritis who had computed tomography (CT) scans available that were amenable to Bodycad Osteotomy software analysis. Virtual osteotomies modeling a 10-mm medial opening-wedge gap were performed. The hinge axis was rotated internally and externally and was proximalized-extended and distalized-flexed with respect to the anterior tibial cortex for 5?, 10?, 15?, and 20?. Each resultant PTS was recorded and compared with the results obtained from the true lateral hinge position and with the preoperative PTS. Results: Computed tomography scans from 10 patients were used. Strong linear correlations were found with each hinge axis position change and the resultant PTS. The trend-line differences were statistically significant by single-factor analysis of variance (P < .001). The PTS decreased for an anterolateral hinge, whereas it increased for a posterolateral hinge. Linear regression analysis showed that rotating the hinge axis by 9.0? externally or angulating the hinge axis by 21.8? of distalization-flexion would result in increasing the tibial slope by 1? whereas rotating the hinge axis by 8.7? internally or angulating the hinge axis by 21.6? of proximalization-extension would decrease the tibial slope by 1?. Conclusions: Distalization-flexion and external rotation of the hinge axis position led to stepwise increases in the PTS, whereas proximalization-extension and internal rotation led to decreases in the PTS. Clinical Relevance: Our findings suggest that when performing medial opening-wedge high tibial osteotomy and aiming to decrease the PTS, the surgeon should aim to achieve maximal internal rotation (producing an anterolateral hinge), as well as proximalization-extension, of the hinge axis. This study quantifies and provides a model for the effect of the hinge axis position for a predetermined angular correction on the PTS.