Should sagittal osteotomy line be parallel to tibial posterior slope in high tibial osteotomy?

Background: The reason why the osteotomy line in the sagittal view should be parallel to the medial tibial posterior slope in open wedge high tibial osteotomy (OWHTO) remains unclear. In addition, previous study reported that a posterolateral hinge position led to an increase in tibial posterior slope (TPS) after OWHTO. Our aims were to examine the relationships between angles among the tibial plateau and osteotomy planes or the hinge point and the change in WS, and the location of the hinge position after OWHTO using three-dimensional computed tomography (3DCT). We hypothesized that the sagittal angle between the tibial plateau and osteotomy planes with an anterior-widening proximal tibial fragment resulted in increased TPS, and the hinge position located posterolaterally. Methods: Preoperative planning anticipated a weight-bearing line ratio of 62% on the radiograph. The anterior gap was 67% of the posterior gap in OWHTO. We identified the tibial plateau and upper and lower osteotomy planes on 3DCT of 82 patients with symptomatic medial osteoarthritic knee after OWHTO. The osteotomy plane angles between the tibial plateau and upper osteotomy planes, and opening gap angles between both osteotomy planes in the coronal and sagittal views were measured. The anteroposterior (AP) and lateral hinge position was displayed as a percentage on the upper osteotomy plane. We assessed the relationships among them. Results: The TPS significantly increased after OWHTO (p = 0.002). There was no significant difference between the sagittal osteotomy plane angle and the change in WS. The sagittal opening gap angle and the AP hinge position ratio were significantly correlated with the change in the WS (r= 0.477 p < 0.001 and r= - 0.342, p = 0.002, respectively). The hinge position was located a mean of 16.0% from the lateral and 48.6% from the posterior tibial edge in the upper osteotomy plane. Conclusions: Contrary to our expectation, the osteotomy plane did not need to be parallel to the tibial plateau plane in the sagittal view. However, the osteotomy gap should be rectangular in the sagittal view. The hinge position located nearly in the center of the sagittal view.