Abstract: BACKGROUND: The treatment of Pauwels type III femoral neck fractures in young adults is extremely difficult. When treating Pauwels type III femoral neck fractures with internal fixation, it is not only necessary to ensure the mechanical stability of the internal fixation, but also to ensure that it can resist the impact of shear, compression and tension, so as to prevent the internal fixation from breaking or loosening due to fatigue during the fracture healing process. However, existing internal fixation methods do not fully meet our requirements for the treatment of this fracture.
OBJECTIVE: To provide a more reasonable and effective choice of internal fixation mode for clinicians in the treatment of young adults with Pauwels type III femoral neck fracture, so as to optimize the treatment effect, reduce the occurrence of complications, and improve the recovery speed and quality of life of patients. 
METHODS: A femoral neck fracture model with Pauwels angle of 70° was constructed using CT data extracted from volunteers, and three placement modes were set up: (1) femoral neck system group: the main nail of femoral neck system was located in the center of the femoral neck axis. (2) Combination group: The main nail of femoral neck system was located in the lower third of the femoral neck axis, and a cannulated compression screw was located in the upper third of the femoral neck axis. (3) Cannulated compression screw group: The arrangement of three cannulated compression screws was arranged in the classic inverted triangle shape to ensure the maximum fixed effect. Stress distribution and displacement of proximal femoral fracture fragments and internal fixation devices were analyzed under three different internal fixation methods.
RESULTS AND CONCLUSION: (1) The stress of the proximal femur models in the three groups was concentrated near the fracture line: combination group (201.10 MPa) < femoral neck system group (222.25 MPa) < cannulated compression screw group (271.25 MPa). (2) The peak value of proximal femur displacement was located at the top: combination group (6.33 mm) < femoral neck system group (9.37 mm) < cannulated compression screw group (9.92 mm). (3) The internal fixation stress was concentrated on the screw surface at the broken end of the fracture and gradually extended from there to both sides; cannulated compression screw group (136.67 MPa) < combination group (201.10 MPa) < femoral neck system group (222.25 MPa). (4) The maximum displacement of the internal fixation device model was located at the very top of the internal fixation. The three groups gradually decreased from the top to the far end. The peak displacement values of combination group (6.21 mm) < femoral neck system group (9.19 mm) < cannulated compression screw group (9.51 mm). (5) The stress of proximal bone mass in the three models was mainly concentrated in the fracture end, especially in the lower part of the fracture end. This indicates that these regions were subjected to greater stress and strain during mechanical transfer; combination group (24.96 MPa) < femoral neck system group (39.69 MPa) < cannulated compression screw group (44.47 MPa). (6) The results indicate that the internal fixation strategy of femoral neck system combined with cannulated compression screw arranged in parallel coronal surface shows higher biomechanical stability than single femoral neck system fixation or cannulated compression screw in inverted triangle arrangement fixation, and provides a more reliable and effective solution for the treatment of Pauwels type III femoral neck fracture in young adults.  

Key words: femoral neck fracture, femoral neck system, cannulated compression screw, finite element analysis, biomechanics