High tibial osteotomy (HTO) is a versatile surgical procedure which has been utilized in the treatment of medial compartment osteoarthritis, ligamentous instability, anterior cruciate ligament (ACL) deficiency, meniscal deficiency, and focal cartilage defects. Hinge axis position has been identified as a significant determinant in altering posterior tibial slope (PTS) during high tibial osteotomy (HTO). Therefore, when preparing for medial opening wedge HTO, careful preoperative planning is essential in order to determine not only the degree of correction in the coronal plane, but also to properly assess the change in PTS that will arise as a result of this coronal plane correction, a variable that can be challenging to predict. The purpose of this study was to evaluate the effect of hinge axis position on PTS in medial opening wedge HTO.
Adults with medial compartment osteoarthritis who had CT scans available that were amenable to Bodycad Osteotomy software analysis were included. Virtual osteotomies were performed modeling a 10 mm medial opening wedge gap. The hinge axis was rotated internally and externally and proximalized/extended and distalized/flexed with respect to the anterior tibial cortex for 5, 10, 15 and 20 degrees. Each resultant PTS was recorded and compared with the results obtained from the true lateral hinge position and with the preoperative PTS.
CT scans from ten patients were utilized. There were strong linear correlations with each hinge axis position change and the resultant PTS. The trendline differences were statistically significant by single factor ANOVA (p<0.001). PTS decreased for an anterolateral hinge, while it increased for a posterolateral hinge. Linear regression analysis demonstrated 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°.
Distalization/flexion and external rotation of the hinge axis position led to stepwise increases in PTS, whereas proximalization/extension and internal rotation led to decreases in PTS. This suggests that when performing medial opening wedge HTO and aiming to decrease 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 hinge axis position for a predetermined angular correction on PTS.