Abstract: BACKGROUND: The age distribution of patients with distal femur fracture is bimodal, being more common in young adults with high-energy injuries and older individuals with low-energy injuries. In cases of complex distal femoral fractures, a single less invasive stabilization system or retrograde femoral nail is insufficient to provide a stable environment for fracture healing and combined fixation is often necessary. 
OBJECTIVE: A novel combined fixation method (less invasive stabilization system combined with retrograde tibial nail) was developed for the treatment of type A3 distal femur fractures. Using finite element analysis, the bio-mechanical properties of this method were compared with two common combined fixation methods—dual plate (less invasive stabilization system combined with locking compression plate) and nail plate combination (less invasive stabilization system combined with retrograde femoral nail). This study aims to provide a theoretical basis for the selection of clinical internal fixation methods for distal femur fracture. 
METHODS: CT images of a 23-year-old healthy male volunteer were utilized to reconstruct the femur three-dimensional model using Mimics and Geomagic Studio software. This model was then compared with in vitro finite element analysis data from existing literature to validate the accuracy of the femoral three-dimensional model. The three-dimensional model of type A3 distal femur fractures and internal fixation were then created using Creo 5.0 software. Three kinds of different combined fixation methods (less invasive stabilization system + retrograde tibial nail, less invasive stabilization system + locking compression plate, less invasive stabilization system + retrograde femoral nail) were assembled and subjected to Boolean operation to establish three sets of finite element models. These models were then imported into Abaqus finite element analysis software to assign material properties, apply consistent boundary conditions, and submit calculations under three loads (normal standing, slow walking, and descending stairs). The resulting stress distribution within the internal fixation as well as overall and local deformation of the femur was analyzed. Furthermore, the failure risk and anti-deformation ability of the new combined fixation method (less invasive stabilization system combined with retrograde tibial nail) were evaluated.
RESULTS AND CONCLUSION: (1) Although the anti-deformation ability of the less invasive stabilization system combined with retrograde tibial nail was approximately 10% lower than that of the less invasive stabilization system combined with locking compression plate, it was comparable to that of less invasive stabilization system combined with retrograde femoral nail and demonstrated good anti-deformation ability. (2) The stiffness of the femur with all three combined fixation methods remained consistent during gait, while femur displacement increased linearly with load, indicating high stability. (3) The failure risk associated with less invasive stabilization system combined with retrograde tibial nail was lower than that of the other two methods, with a 2.94% reduction in failure risk compared to less invasive stabilization system combined with locking compression plate. (4) This new combined fixation method offers distinct advantages in terms of anti-deformation ability, safety, and effectiveness, laying a theoretical foundation for further clinical application.  

Key words: distal femoral fracture, dual plate, retrograde femoral nail, retrograde tibial nail, finite element analysis, biomechanics