How do osteotomy and distraction affect distraction angles of opening wedge high tibial osteotomy?

Trigonometric formulae have been derived to correlate the distraction angle (height) and tibial specifications. However, the assumption-induced simplifications are inherent in the formulae such as the rigid tibia and the specific orientations of cutting plane and lateral hinge. This study aimed to evaluated the accuracy of the trigonometric formulae.

The 3D printed tibiae were used as the specimens for which the hinge orientations and distraction sites were systematically varied. Hinge orientation was determined by wedge inclination of the bone saw into the medial tibia and saw progression near the lateral cortex. The specimens were distracted at different distraction sites to measure the distraction angles that were compared with literature formulae.

In cases of the same distraction height, the wedge inclination, saw progression, and distraction site had various impacts on the coronal angles, indicating that the ideal formula should consider these parameters. Averagely, the predicted angles of the literature formulae were 15% higher than the testing results. The differences in these results may be attributed to the deformable property of the specimen material, non-ideal hinge orientation, and differences in distraction sites. Saw progression and distraction site had greater impacts than wedge inclination on the distraction angle.

Variations in three surgical indices constitute a complicated mechanism that affects the 3D hinged rotation of the distracted plateau. The non-middle distraction further deforms the tibial rotation and reduces the accuracy of the trigonometric formulae. The trigonometric formulae might underestimate the distraction angle; thus. Appropriate corrections are necessary for clinical application.