Axial But Not Sagittal Hinge Axis Affects Posterior Tibial Slope in Medial Open-Wedge High Tibial Osteotomy: A 3-Dimensional Surgical Simulation Study

Purpose The purpose of this 3-dimensional (3D) surgical simulation study was to investigate the effects of axial and sagittal hinge axes (hinge axes in the axial and sagittal planes) on medial and lateral posterior tibial slope (PTS) in medial open-wedge high tibial osteotomy (OWHTO), and evaluate the quantitative relationship between hinge axis and PTS change. Methods Preoperative computed tomography data from patients with varus knee deformity were collected. A standard hinge axis (0°) and 12 different hinge axes (6 axial hinge axes and 6 sagittal hinge axes: ±10°, ±20°, and ±30°) were defined in a 3D surgical simulation of OWHTO using a bone model. The differences between before and after simulation surgery in medial and lateral PTS, medial proximal tibial angle, opening gap, and opening wedge angle were measured. Results In total, 93 varus knees in 93 patients were included for study. Compared with the standard hinge axis, axial hinge axis significantly affected medial and lateral PTS (P < .001). In contrast, sagittal hinge axis had no significant effect on medial and lateral PTS (P > .05). Every 10° change in axial hinge axis with a mean coronal valgus correction of 10° might result in approximately 1.6° of alteration in PTS. Stepwise regression analysis showed that axial hinge axis is the most significant factors affecting PTS (β coefficient = 0.78, P < .001), followed by opening wedge angle (β coefficient = 0.36, P < .001) and gap ratio (β coefficient = 0.12, P < 0.001). Conclusion Based on our findings of 3D OWHTO simulation, axial hinge axis significantly influences medial and lateral PTS in OWHTO, but sagittal hinge axis has no effect on change in PTS. Every 10° change of axial hinge axis with a 10° coronal valgus correction caused approximately 1.6° change of PTS. Clinical Relevance Hinge axis in the axial plane significantly affects PTS, but hinge axis in the sagittal plane has no effect on PTS. To maintain PTS, surgeons should make hinge axis at the true lateral position of the tibia in the axial plane. To intentionally alter PTS, an anterolateral axial hinge axis could be used to decrease PTS or a posterolateral axial hinge axis could be used to increase PTS. Opening wedge angle or gap ratio is also useful for intentional modification of PTS. The purpose of this 3-dimensional (3D) surgical simulation study was to investigate the effects of axial and sagittal hinge axes (hinge axes in the axial and sagittal planes) on medial and lateral posterior tibial slope (PTS) in medial open-wedge high tibial osteotomy (OWHTO), and evaluate the quantitative relationship between hinge axis and PTS change. Preoperative computed tomography data from patients with varus knee deformity were collected. A standard hinge axis (0°) and 12 different hinge axes (6 axial hinge axes and 6 sagittal hinge axes: ±10°, ±20°, and ±30°) were defined in a 3D surgical simulation of OWHTO using a bone model. The differences between before and after simulation surgery in medial and lateral PTS, medial proximal tibial angle, opening gap, and opening wedge angle were measured. In total, 93 varus knees in 93 patients were included for study. Compared with the standard hinge axis, axial hinge axis significantly affected medial and lateral PTS (P < .001). In contrast, sagittal hinge axis had no significant effect on medial and lateral PTS (P > .05). Every 10° change in axial hinge axis with a mean coronal valgus correction of 10° might result in approximately 1.6° of alteration in PTS. Stepwise regression analysis showed that axial hinge axis is the most significant factors affecting PTS (β coefficient = 0.78, P < .001), followed by opening wedge angle (β coefficient = 0.36, P < .001) and gap ratio (β coefficient = 0.12, P < 0.001). Based on our findings of 3D OWHTO simulation, axial hinge axis significantly influences medial and lateral PTS in OWHTO, but sagittal hinge axis has no effect on change in PTS. Every 10° change of axial hinge axis with a 10° coronal valgus correction caused approximately 1.6° change of PTS.