Abstract: Objective The mechanical stability of the osteotomy site significantly decreases in patients with Takeuchi type Ⅱ hinge fractures after medial open wedge high tibial osteotomy (MOWHTO), which may affect prognosis. The purpose of this study is to explore the effect of auxiliary fixation system (AFS) supplemented on the basis of the original internal fixation on the mechanical stability of the osteotomy site, and to provide a reference for the treatment and rehabilitation of type Ⅱ hinge fractures. 
Methods The MOWHTO finite element models A and B, representing intact hinges and Takeuchi Ⅱ hinge fractures, were established using bone data from CT scans of the human lower extremities and incorporating TomoFix plates and screws. On the basis of model B, AFS was supplemented to construct model C. Different axial compressive loads were applied above the tibial plateau of the models, replicating states ranging from sitting to standing and walking. Additionally, torsional loads were applied to mimic rotational state. The maximum stress on the internal fixator, as well as the maximum displacement at the medial osteotomy site and the lateral hinge position under these conditions, was measured to evaluate the changes in mechanical stability at the osteotomy site. 
Results From the sitting to the standing state, compared with model A, the maximum stresses of the plates in models B and C increased by 178.3% and 95.9% respectively, the maximum displacement at the medial osteotomy site increased by 33.2% and 29.7% respectively, and the maximum displacement at the lateral hinge position increased by 51.0% and 43.7% respectively. The maximum stresses of the plate in the walking state increased by 113.1% and 91.9% respectively, and the maximum displacement at the medial osteotomy site increased by 17.2% and 13.8% respectively. The maximum displacement at the lateral hinge position increased by 389.4% and 385.1% respectively, and the maximum stresses of the plate under torsional load increased by 261.4% and 122.0% respectively. The maximum displacement at the medial osteotomy site increased by 362.6% and 242.1% respectively, and the maximum displacement at the lateral hinge position increased by 338.8% and 204.7% respectively. Compared with model A, the maximum stress of the plate in models B and C increased under various states, and the increase of model B was more obvious. Compared with the sitting to standing and walking states, the difference of the maximum stress increase of the plate between models B and C was the largest (139.4%) under the torsional load state. 
Conclusion This study demonstrates that type Ⅱ hinge fractures may result in a reduction of stability at the osteotomy site after MOWHTO, particularly during transitions from sitting to standing and twisting states. During the operation, the stability around the hinges needs to be strengthened, and corresponding actions should be performed under protective conditions after the operation. The supplemented AFS can enhance the stability of the osteotomy site and distribute the stress on the internal fixator. Theoretically, this approach could decrease the incidence of internal fixator failure, postoperative alignment deviation, and nonunion at the osteotomy site. Compared with the original internal fixation alone, it allows for earlier partial weight-bearing, which facilitates rapid recovery and minimizes complications. While AFS appears promising for the treatment of hinge fractures, further biomechanical investigations are warranted to confirm its efficacy. 

Key words:  , tibia, knee joint, osteotomy, finite element