Finite element analysis of the Femoral neck system for different placement positions in the fixation of Pauwels type Ⅲ femoral neck fractures

Objective

This study aimed to identify the optimal position for the femoral neck system (FNS) device when fixing Pauwels III #NOF, by analyzing the mechanical stability and stress distribution of FNS at different screw placement positions for the fixation of Pauwels III femoral neck fractures.

Methods

We employed finite element analysis to create a 3D model of a Pauwels type III femoral neck fracture. Six models were designed, each with varied FNS screw placement positions. Axial stresses of 600 N, 1200 N, and 1800 N were applied to simulate physiological loads during different activities: standing on two legs, standing on one leg, and walking. The mechanical properties of these screw placements were assessed by comparing stress distribution, displacement, and fracture stability across models under varying load conditions.

Results

Model 1, with the power rod aligned along the femoral neck axis, showed the best stability, with 42.40 % lower maximum VMS and 18.49 % less femoral displacement, compared to the worst model. Displacement of the internal fixation and fracture surface decreased by 21.72 % and 19.16 %, respectively. It also had superior results for internal fixation VMS and fracture surface compressive stress. Model 2, with the head screw centered axially, demonstrated good stability but had higher stress concentrations under 1800 N load. The displacement of the fracture surface and femur in model 2 increased by 18.37 % and 17.26 %, respectively, compared to model 1. Models 5 and 6, with the FNS nail near the lateral femoral cortex, showed significant stress concentrations, with compressive and shear forces rising by about 33 %. Model 5′s maximum VMS increased by 46.68 %, and model 6′s maximum compressive stress of the fracture surface increased by 46.37 %, compared to model 1. Models 3 and 4, with the power rod shifted up or down, displayed moderate stability, reducing displacement in some tests.

Conclusion

This finite element analysis highlights that centring the FNS power rod along the femoral neck axis significantly enhances fracture stability and minimises postoperative displacement. Conversely, poor screw placement may result in mechanical stress concentration, raising the risk of nonunion or malunion. Clinicians should prioritise screw placements with more excellent mechanical stability to optimise treatment outcomes.