Abstract: BACKGROUND: With the development of intramedullary nail technique, use of intramedullary nailing to treat proximal tibia fractures has gradually increased. However, it is not so clear about the biomechanical stability of intramedullary nails and double locking plate fixation of proximal tibial fractures.   
OBJECTIVE: To simulate extra-articular comminuted proximal tibia fracture using three-dimensional finite element, and to analyze biomechanical characteristics of intramedullary nailing and bilateral locking plate fixation under different forces. 
METHODS: The finite element model of the extra-articular comminuted proximal tibia fracture was established by using the CT tibial data. According to the principle of internal fixation, the proximal tibia bilateral locking plate and proximal tibia intramedullary nail were assembled. The finite element analysis method was used to analyze the displacement and stress of the two internal fixation methods of the comminuted proximal tibia fracture under different loading conditions. 
RESULTS AND CONCLUSION: (1) Under the axial load, in the intramedullary nail model, the stress of internal fixation and the tibia was minimal. The displacement of the fractured end was smallest. The stress of the internal fixation was mainly concentrated near the fractured end, and the stress distribution was more uniform in the intramedullary nail model. (2) Under the three-point bending load, in the intramedullary nail model, the stress of internal fixation and the tibia was minimal, but the displacement of the fractured end was higher than the double-plate model. Stress concentration occurred at the intersection of the third locking nail and the main nail at the proximal end of the intramedullary nail. The internal fixation stress was mainly concentrated around the locking screw on both sides of the steel plate. (3) Under the torsional force, the internal fixation system of the intramedullary nail model bore the smallest stress and generated the smallest fracture end displacement. The internal fixation stress was evenly distributed at the intersection of each locking nail and the main nail in the intramedullary nail model. The stress of the double steel plate was mainly concentrated on the contact surface of the lateral steel plate and the proximal tibia. (4) The results demonstrate that the intramedullary nail has better resistance to axial loading and torsional loading for comminuted extra-articular fractures of the proximal tibia. However, the double locking plate is more advantageous when it is against bending. 

Key words: bone, fractures, proximal tibial fracture, three-dimensional reconstruction, intramedullary nail, locking plate, displacement, stress, finite element analysis