Abstract: BACKGROUND: At present, the mainstream treatment strategy for femoral shaft fracture is surgical treatment, including intramedullary nail fixation, steel plate fixation, and Bridge combined fixation system, which is becoming more and more popular in recent years. However, there are relatively few studies on the biomechanical properties of the three internal fixation methods.  
OBJECTIVE: To simulate the fracture of the middle femur using finite element technique, and analyze the biomechanical characteristics of locking plate, Bridge combined fixation system, and intramedullary nail internal fixation under different working conditions.
METHODS:  The CT data of left femur of a healthy volunteer from the Affiliated Hospital of Zunyi Medical University meeting the inclusion and exclusion criteria were collected in this study. The digital model of middle femoral fracture with defect was constructed by Mimics and Geomagic software. According to the principle of internal fixation, the fracture model was assembled with appropriate size locking plate, Bridge combined fixation system, and intramedullary nail respectively after introducing the fracture model into Pro/E software. After geometric cleaning and gridding, the model was incorporated into Abaqus software to analyze the displacement and stress of three internal fixation methods of middle femoral fracture under different stress conditions by finite element analysis.  
RESULTS AND CONCLUSION: In the results of finite element analysis, three kinds of femoral internal fixation models showed different mechanical properties under different working conditions. (1) Under the loading of vertical stress, the force of the intramedullary nail was uniformly distributed along the direction of the main nail, and the stress was the least, and the Z-shaped deformation of the two connecting rods in the bridging system near the fracture end distributed the overall stress, resulting in the smallest displacement and the best stability. (2) Under the loading of horizontal stress, the stress and displacement of the bridging system were the smallest, and the stability was the best. Stress was concentrated on the 1-3 fixing block on the bridging system connection rod and its adjacent metal components. (3) Under the loading of torsional stress, the displacement of the intramedullary nail was the smallest and the stability was the best, while the stress and displacement of the bridging system were the largest, and the stress was concentrated on the interface between the proximal locking nail and the bone, and the stability was the worst. (4) The results show that the femoral bridging system has good mechanical properties of anti-compression and anti-bending, while the femoral interlocking intramedullary nail has more advantages in anti-torsion force. As a conventional eccentric fixation, locking plate is less stable in vertical compression and bending, but better resistant to torsion than double-bar bridging system and inferior to the interlocking intramedullary nail.

Key words: femoral shaft fracture, middle femoral fracture, locking plate, bridge combined fixation system, intramedullary nail, finite element analysis