Three dimensional finite element method was used to analyze the effect of FNS and CCS on the stability of Pauwels Ⅲ femoral neck fracture.

A 25-year-old healthy male volunteer who X-ray examination showed no abnormal hip development was selected to undergo Hip CT thin layer scan and collect the imaging data of the middle and upper segment of femur, then his data were imported into the medical 3D reconstruction software Mimics 19.0 to obtain the original 3D model of femur and performed in Geomagic Wrap 2017 software to simultaneously construct the cortical bone and cancellous bone structure of the femur. Then it was imported into Solidworks 2017 software to establish the three dimensional model of Pauwels Ⅲ femoral neck fracture. At the same time, using the Solidworks 2017 software to establish the FNS model and the hollow lag screw model according to the clinical internal fixation size, and the combination model with Pauwels Ⅲ type femoral neck fracture was established, so as to establish four treatment models for femoral neck fracture (model 1: clinical standard spot implantation model of FNS nail. Model 2: FNS main nail implantation model deviated from the standard point; Model 3: Three inverted triangle parallel hollow lag screw implantation model. Model 4:2 parallel hollow lag screw implantation model). Finally, ANSYS17.0 software was used to perform mesh division, load application and data calculation. Stress distribution, stress peak value, maximum displacement and displacement under external rotational stress of femur and internal fixation of each model were analyzed. 20 points were uniformly selected in each three dimensional finite element model within the corresponding region, and corresponding values of each site were recorded.

(1) In the 4 models under different pressures, the stress value of proximal and distal end of femoral neck fracture was the largest in Model 1, but the stress difference of fracture end was the smallest, followed by Model 3. (2) In the 4 models, the proximal femur stress of model 1 was the smallest under different pressures of different internal fixation models. (3) In the 4 models,the internal fixation stress of model 1 and Model 2 is obviously higher than that of the other two models. (4) In the 4 models, the axial (X-axis) and horizontal (z-axis) displacements of femoral neck under different pressures in different internal fixation models were larger with the increase of pressure, and the model 1 was the smallest, followed by Model 3. (5) In the 4 models, femoral neck sagittal (Y-axis) displacement model 3 was the smallest, followed by model 1. (6) In the 4 models, under the external rotational stress of lower limbs, the sagittal (Y-axis) displacement of femoral neck model 1 and 3 were smaller than the other two models.

Standard implant therapy of femoral neck fracture with FNS showed excellent support, varus resistance, resistance to rotation and extorsion stress resistance, three nabla parallel hollow tension screw also showed a good support, varus resistance, resistance to rotation and extorsion stress resistance, in its support and resistance to stress the femoral neck slightly less power cross nail system. At the same time, it was found that the improper placement of FNS had a great influence on the stability of the treatment of femoral neck fracture, and significantly reduced the anti-rotation and anti-external rotation ability of the screw.