Finite element analysis of biomechanical effects of residual varus/valgus malunion after femoral fracture on knee joint

Objective Post-operative femoral shaft fractures are often accompanied by a residual varus/valgus deformity, which can result in osteoarthritis in severe cases. The purpose of this study was to investigate the biomechanical effects of residual varus/valgus deformities after middle and lower femoral fracture on the stress distribution and contact area of knee joint. Methods Thin-slice CT scanning of lower extremities and MRI imaging of knee joints were obtained from a healthy adult male to establish normal lower limb model (neutral position). Then, the models of 3 degrees, 5 degrees, and 10 degrees of varus/valgus were established respectively by modifying middle and lower femur of normal model. To validate the modifying, a patient-specific model, whose BMI was same to former and had 10 degrees of varus deformity of tibia, was built and simulated under the same boundary conditions. Result The contact area and maximum stress of modified models were similar to those of patient-specific model. The contact area and maximum stress of medial tibial cartilage in normal neutral position were 244.36 mm(2) and 0.64 MPa, while those of lateral were 196.25 mm(2) and 0.76 MPa. From 10 degrees of valgus neutral position to 10 degrees of varus, the contact area and maximum stress of medial tibial cartilage increased, and the lateral gradually decreased. The contact area and maximum stress of medial meniscus in normal neutral position were 110.91 mm(2) and 3.24 MPa, while those of lateral were 135.83 mm(2) and 3.45 MPa. The maximum stress of medial tibia subchondral bone in normal neutral position was 1.47 MPa, while that of lateral was 0.65 MPa. The variation trend of medial/lateral meniscus and subchondral bone was consistent with that of tibial plateau cartilage in the contact area and maximum stress. Conclusion This study suggested that varus/valgus deformity of femur had an obvious effect on the contact area and stress distribution of knee joint, providing biomechanical evidence and deepening understanding when performing orthopedic trauma surgery or surgical correction of the already existing varus/valgus deformity.