Abstract: BACKGROUND: Compared with clinical tools based on facial bone mineral density, finite element model strength analysis under specific load conditions is more and more used to improve fracture risk assessment. However, most finite element models are limited to the estimation of bone strength and the location of possible fractures, but cannot model the fracture process itself.  
OBJECTIVE: The crack of femoral neck fracture was simulated by extended finite element method in ABAQUS, and the crack initiation position and crack propagation path were determined based on the criterion of maximum principal stress and fracture energy.
METHODS:  The CT data of the proximal femur of a healthy volunteer were collected and imported into Mimics to reconstruct the three-dimensional model in DICOM format. The corresponding material parameters of osteoporotic cortical bone and cancellous bone were given. Static analysis was initially performed to assess bone stress distribution and identify fracture risk areas. Based on the stress results and excluding the elements near the boundary, the enrichment area which allowed fracture to occur in extended finite element method analysis was defined. When the principal stress exceeded 116 MPa, the crack initiation occurred in the element, which could predict crack location and evolution process.  
RESULTS AND CONCLUSION: (1) Static analysis: Under the standing load, the stress concentration appeared in the femoral neck, and the maximum equivalent stress was 711.8 MPa. According to the fourth strength theory, the yield strength of the cortical bone of the femur was 116 MPA, and irreversible plastic failure would occur in the femoral neck under the current load. In addition, the maximum principal stress was concentrated on the lateral side of the femoral neck, and the maximum value was higher than that of 116 MPa. (2) Crack propagation analysis: The generation of crack was a process of accumulating energy. When the stress increased and exceeded the threshold, the unit damage began to appear at the upper and outer part of the femoral neck. At this time, it was in the state of viscous crack and still had the ability to resist fracture. As the stress continued to increase, the element failed completely. At this time, the stress around the crack surface and the tip decreased rapidly, and the stress concentration was distributed to both sides of the element. The crack extended to the anterior superior and posterior inferior direction of the femoral neck, and finally formed the fracture of the femoral neck. (3) The proximal femoral fracture model based on fracture mechanics could predict not only the elastic behavior, but also the fracture crack path under standing load. It emphasized the importance of fully calibrating fracture criteria and evaluated the effects of mechanical properties and critical principal stress. Compared with the modeling method of strength theory, this method provides more information about the mechanism of fracture crack propagation and is helpful to explore the potential risk of fracture.

Key words: femoral neck fracture, ABAQUS, extended finite element, fracture mechanics, crack propagation, fracture toughness, strength theory, biomechanics