Abstract: BACKGROUND: Opening wedge high tibial osteotomy is a well-established method for treating medial compartment knee osteoarthritis with varus deformity. There is still controversy over where the lower limb force correction should be placed after opening wedge high tibial osteotomy.
OBJECTIVE: To investigate the mechanical characteristics of the knee joint and the internal fixation device after opening wedge high tibial osteotomy for knee osteoarthritis with varus deformity by finite element analysis of the mechanical loading of different force line patterns on the structures of the knee joint.
METHODS: A 57-year-old female volunteer weighing 60 kg, diagnosed with left knee osteoarthritis, was selected. Multislice spiral CT scanning was performed on the left knee joint to obtain imaging data. Finite element analysis software was used for mechanical loading to obtain analysis results. The hinge point of the opening wedge high tibial osteotomy was set approximately 15 mm above the tibial plateau at the fibular head, with a 5 mm lateral tibial cortex preserved at the hinge. The medial cortex osteotomy was positioned 30 mm from the medial tibial plateau and expanded by 5 mm, 10 mm, and 15 mm, respectively. Load lines were set vertically at 25%, 50%, 62.5%, and 75% of the tibial plateau. An 8-hole Tomofix plate with a thickness of 2 mm and eight screws (diameter 4 mm; lengths: 60 mm/60 mm/55 mm/50 mm/38 mm/34 mm/32 mm/20 mm) was placed. The osteotomy gap was filled with cancellous bone. Stress and displacement diagrams for various parts of the knee joint three-dimensional model were obtained. 
RESULTS AND CONCLUSION: (1) Before opening wedge high tibial osteotomy, knee joint stress was primarily concentrated on the lateral femoral condyle and medial tibial plateau, areas prone to cartilage wear. Meniscal stress was concentrated on the medial meniscus body and the anterior horn of the lateral meniscus. As the load-bearing region shifted outward, stress on the lateral cartilage and meniscus increased, while medial stress decreased. (2) In the 5 mm osteotomy model post-opening wedge high tibial osteotomy, as the stress loading position moved laterally, stress on the lateral cartilage and meniscus of the femur and tibia gradually increased. Stress on the graft bone was minimal at the 50% load position, and stress on the titanium plate and screws decreased with lateral movement of the load line. (3) Analysis of different expansion heights showed that with increased expansion, stress on the lateral cartilage and meniscus increased, and stress concentration on the titanium plate intensified, while screw stress changes were insignificant. (4) Fatigue fracture of the Tomofix titanium plate and screw breakage at the screw head were prone to occur at the posterior-medial aspect of the plate and at the D and 1 holes. (5) It is concluded that opening wedge high tibial osteotomy can effectively transfer the pressure in the medial compartment of the knee joint, but attention should be paid to the risk of increased stress in the lateral cartilage and meniscus. The posterior medial side of the Tomofix titanium plate and the screws at holes D and 1 are weak areas of stress concentration. During the operation, excessive correction should be avoided and the internal fixation design should be optimized to reduce the risk of complications.

Key words: opening wedge high tibial osteotomy, knee osteoarthritis, Tomofix titanium plate, force line, knee-preserving surgery, finite element analysis, arthroplasty, internal fixation