Abstract: BACKGROUND: Irreducible femoral neck fracture was difficult to obtain anatomic reduction. As a new type of internal fixation, the femoral neck system is still blank for the treatment of non-anatomical reduced femoral neck fractures.
OBJECTIVE: To explore the biomechanical stability of femoral neck system internal fixation under nonanatomical reduction in the treatment of femoral neck fractures based on finite element analysis.
METHODS: CT data of the hip joint of a healthy female adult were obtained. Anatomical reduction of femoral neck fracture models with Pauwels angles of 30°, 50°, and 70° were established using Mimics 21.0, Geomagic Wrap 2021, and SolidWorks 2020. The fracture proximal ends of the three anatomical reduction models were shifted upward by 2 mm along the fracture line, and three positive buttress models with different Pauwels angles were obtained. In the same way, three negative buttress models were acquired by shifting downward by 2 mm. SolidWorks 2020 was used to make the femoral neck system internal fixation, and the nine femoral neck fracture models were assembled with the femoral neck system. Then Ansys 19.0 was used for finite element analysis. The displacement distribution and maximum displacement, stress distribution and maximum stress of the femur and femoral neck system were recorded under 2 100 N stress. 
RESULTS AND CONCLUSION: (1) When Pauwels angles were 30°, 50°, and 70°, the maximum stresses of the femoral neck system appeared to be concentrated at the junction of the sliding hip screw and anti-rotation screw. The maximum femur stresses appeared to be concentrated in the medial cortex of the femur. The maximum displacement was concentrated at the upper of the femoral head and femoral neck system. (2) When Pauwels angles were 30° and 50°, the maximum displacement and maximum stress of the femoral neck system and femur were: negative buttress > anatomical reduction > positive buttress. (3) When Pauwels angle was 70°, the maximum displacement and maximum stress of the femoral neck system were: negative buttress > anatomical reduction > positive buttress; the maximum displacement and maximum stress of the femur were: negative buttress > positive buttress > anatomical reduction. (4) With the increase of Pauwels angle, the biomechanical advantage of the positive buttress was weakening. However, it was better than a negative buttress. When Pauwels angle was 30°, positive buttress was more stable than anatomical reduction. When Pauwels angle was 50°, the biomechanical difference between positive buttress and anatomical reduction became smaller. When Pauwels angle was 70°, the stability of anatomical reduction was slightly better than positive buttress. (5) If it was difficult to achieve anatomical reduction of femoral neck fracture during operation, but the positive buttress had been displaced within 2 mm, the femoral neck system could be used to offer stable mechanical fixation. It is necessary to avoid negative buttress reduction. 

Key words: femoral neck fracture, femoral neck system, internal fixation, nonanatomical reduction, positive buttress, finite element analysis