Takeuchi type III hinge fracture is a serious intraoperative complication of medial open wedge high tibial osteotomy (MOWHTO), which can lead to a decrease in the mechanical stability of the osteotomy site and affect the prognosis. This study evaluated the changes in the mechanical stability of the osteotomy site after adding an auxiliary fixation system (AFS) to the original internal fixation, with an aim of providing new ideas for the treatment of type III hinge fractures.

CT scan data of human lower limb bones and TomoFix plates and screws were used to establish the intact hinge MOWHTO model A and type III hinge fracture model B. Model C was constructed by adding an AFS on the basis of model B. Sitting to standing, walking, and twisting states were simulated. Different loads were applied above the tibial plateau of the models. The maximum stress of the internal fixation and the maximum displacement of the medial osteotomy site and the lateral hinge part were measured to evaluate the changes in the mechanical stability of the osteotomy site.

From the sitting to standing state, compared with model A, the maximum stresses of the plates in models B and C increased by 85.5% and 25.3%, respectively; the maximum displacement at the medial osteotomy site increased by 6.0% and 3.7%, respectively; and the maximum displacement at the lateral hinge position increased by 19.2% and 10.1%, respectively. In the working state, the maximum stress of the plates in models B and C increased by 62.8% and 14.4%, respectively; the maximum displacement at the medial osteotomy site increased by -3.4% and -6.9%, respectively; and the maximum displacement at the lateral hinge position increased by 106.4% and 89.3%, respectively. Under torsional load, the maximum stresses of the plates in models B and C increased by 25.5% and -10.2%, respectively; the maximum displacement at the medial osteotomy site increased by 16.8% and 9.3%, respectively; and the maximum displacement at the lateral hinge position increased by 8.5% and 1.6%, respectively.

This study demonstrates that type III hinge fractures reduce the stability of the osteotomy site, leading to an increase in the stress of the plate internal fixation and the displacement of the fracture site on the lateral tibial plateau. It has the greatest impact on the stability of the osteotomy site during the sit-to-stand transition, necessitating either avoidance of the movement or implementation of protective measures. AFS can distribute the stress of the internal fixation device, reduce the displacement of the fracture site on the tibial plateau, improve the stability of the osteotomy site, theoretically reducing the incidence of failure of the internal fixation device, loss of the alignment, and non-union of the osteotomy site. Compared with the original internal fixation, AFS can partially bear weight earlier, provide safer early functional exercise, facilitate rapid recovery, and reduce complications. It can be considered for the treatment of type Ⅲ hinge fractures. However, further biomechanical studies are needed to confirm this finding.