Abstract: BACKGROUND: The more vertical the femoral neck fracture line, the more shear force the femoral neck fracture may bear, so it may be prone to internal fixation failure, nonunion, or necrosis of the femoral head. At present, there is controversy as to which hollow nail fixation mode is ideal. 
OBJECTIVE: To evaluate different configurations and numbers of cannulated screw configurations to learn more about the biomechanical differences in vertical fractures of the femoral neck using finite element analysis. 
METHODS: Femoral CT data were collected from a 24-year-old healthy male volunteer and imported into Mimics software to build a three-dimensional geometric model of the femur. Model refinement and surface fitting processing were carried out in Geomagic software and imported into SolidWorks software to establish a vertical fracture model of the femoral neck. Six cannulated screw models were established, including three triangle configuration, three inverted triangle configuration, three double-plane double-support configuration (F scheme), three transverse configuration, four diamond configuration and four Alpha configuration. The peak stress, strain and displacement of internal fixation and femoral fracture models under different loads (350, 700, 1 400, and 2 100 N) were compared among different configurations.
RESULTS AND CONCLUSION: (1) Under four kinds of loads, screw stress conditions were as follows: biplanar double-supported configuration (F scheme) > transverse configuration > inverted triangle configuration > positive triangular configuration > Alpha configuration > rhombus configuration. The peak value of Von mises stress was concentrated in the screw close to the fracture line. (2) Under four kinds of loads, the screw displacement was Alpha configuration > inverted triangle configuration > positive triangular configuration > biplanar double-supported configuration (F scheme) > rhombus configuration > transverse configuration, and the peak displacement was mainly concentrated on the hollow screw head. (3) Under four kinds of loads, the stress conditions of the proximal femoral bone block were biplanar double-supported configuration (F scheme) > transverse configuration > inverted triangle configuration > Alpha configuration > positive triangular configuration > rhombus configuration, and the stress peak mainly concentrated in the lower neck of femur. (4) Under 350 N load, the displacement of the proximal femur bone block was transverse configuration > biplanar double-supported configuration (F scheme) > positive triangular configuration > Alpha configuration > inverted triangle configuration > rhombus configuration. In the other three loads, the peak displacement of the inverted triangle configuration was smaller than that of the rhombus configuration. The peak displacement was mainly concentrated in the head. (5) The rhombus configuration was the most dispersed in the stress distribution of the proximal femoral bone. The rhombus configuration was the smallest in the peak displacement of the femur. The stress, displacement and peak displacement of the fracture end of each internal fixed model increased gradually with the increase of load. (6) The biomechanical performance of the four diamond-shaped models in the internal fixation of vertical femoral neck fractures is better than that of other groups of models. The four rhomboid models have stable fixation, small displacement value of fracture end and dispersed stress, which can help resist shear force and prevent varus collapse and create a good mechanical environment for fracture healing.

Key words: vertical femoral neck fracture, partially threaded cannulated screw, screw configuration, finite element, stress, displacement