Abstract:

BACKGROUND: Cannulated screw fixation is the first choice for the treatment of fresh femoral neck fracture. However, in the patients with bone defect, the mechanical conduction and stability of the proximal femur are obviously changed, which easily leads to the failure of internal fixation, nonunion or delayed healing of the fracture, so the study of biomechanics has important clinical significance.

OBJECTIVE: To explore the biomechanical changes of proximal femur with posterior-inferior bone defect using finite element analysis, compare the biomechanical effect of cannulated screws with different configurations in treatment of adductive femoral neck fractures.

METHODS: Original DICOM data of CT scan of proximal femur in an adult healthy male volunteer were obtained. MIMICS 10.01 software and Rhino3D NURBS software were used to make the models of femoral neck fracture with different degrees of posterior-inferior bone defects (no defect model, small defect model, medium defect model and large defect model). Two configurations of cannulated screws (inverted triangle and positive triangle) were used in four models. Mesh generation and material property assignment were conducted after assembly. The coupling relationship was established between the center and the surface of the femoral head by ABAQUS 6.12 software. The load and constraints of slow walking were applied to all models. 

RESULTS AND CONCLUSION: (1) The inner side of the femoral neck was subjected to compressive stress, and the lateral side was subjected to tensile stress for the non-defect model. The stress distribution of the femoral head was more uniform. With the increase of the degrees of posterior-inferior bone defect, the stress peak value of femoral head, femoral neck pressure side and tension side, and cannulated screw tail were increased gradually. (2) With the increase of the defect degree, the peak value of the stress on the pressure side of the cannulated screw increased gradually. In the middle and large defect models, the stress in the inverted triangle group was higher than in the regular triangle group (P < 0.05), and there was no difference between the two groups in the other models (P > 0.05). (3) With the increase of the defect degree, the peak stress on tension side of cannulated screw increased gradually. In the model of no defect and small defect, the stress in the regular triangle group was higher than in the inverted triangle group (P < 0.05). In the large defect model, the stress in the regular triangle group was lower than in the reverted triangle group (P < 0.05). In the middle defect model, there was no difference between the two groups. (4) With the increase of the defect degree, the peak stress of femoral head increased gradually. There was no significant difference between the inverted triangle group and the regular triangle group in the three models (P > 0.05). (5) With the increase of the defect degree, the peak value of stress in the tail of cannulated screw increased gradually. In the small, medium and large defect models, the stress in the inverted triangle group was higher than in the regular triangle group (P < 0.05). No significant difference was found between the two groups in models without bone defect (P > 0.05). (6) Results suggested that different degrees of posterior-inferior bone defects could obviously affect the biomechanical properties of the proximal femur. For femoral neck fracture with no defect or lesser degree of defect, the biomechanical effect of inverted triangle was superior to regular triangle. For femoral neck fracture with big defect, the biomechanical effect of regular triangle was superior to inverted triangle. 

Key words: femoral neck fracture, bone defect, finite element, biomechanics, cannulated screw, arrangement, proximal femur, peak stress