Clinical studies have demonstrated that FNS is a safe and effective strategy for internal fixation [19, 20]. Although manufacturers provide relatively clear steps and standards for operation, application in actual clinical practice is inconsistent. According to the clinical feedback, in order to ensure that the locking nail is located in the medullary cavity of the bone stem, it is recommended to confirm the position of the anti-spin nail before implanting the anti-spin nail. This may be due to individual patient differences and the proficiency of the physician. Meanwhile, many scholars have optimized the application of FNS. For increased stability, Fan  suggested using a two-hole FNS when the fracture line angle exceeded 70°. Jung  studied the surgical variations of using FNS to stabilize Pauwels type III femoral neck fractures. In his opinion, a gap between the femoral stem and the plate is an effective method of controlling the bolt length. In studies on intertrochanteric fractures, TAD (Tip-Apex Distance) was used to assess the stability of internal fixation . However, the biomechanical performance of femoral neck fractures is not identical to that of intertrochanteric fractures. External forces acting on the femur are directed from the center of the femoral head to the femoral spine medial to the lesser trochanter, and then through the femoral stem. Is it necessary to improve the method of evaluating the position of the FNS in femoral neck fractures? Researchers have enhanced the technique of precisely adjusting the bolt depth for femoral neck fracture dislocations using FNS bolts manufactured in 5 mm increments . Previous studies have shown that a position in the middle of the coronal plane may be preferable. A further investigation of similar findings in the sagittal plane is necessary.
As a result, we would like to examine the biomechanical properties of variation of FNS in the sagittal plane with the aid of finite element analysis. As opposed to traditional mechanical experiments, FEA provides the same mechanical environment. Mechanical results are influenced by the morphology of the femoral head, bone density, anterior inclination of the femoral neck, etc. Although it does not fully simulate the in vitro situation, this homogeneous setup can still improve the credibility and repeatability of the experiment. In our study, all models and working conditions were the same, and the only variable was the position in the sagittal plane. Moreover, meshing is a very selective process in finite element analysis. There is no doubt that the calculations of the hexahedron are fast, whereas the calculations of the tetrahedron are slow. Since the proximal femur is irregular, further simplification and cutting of the model are required if a hexahedral mesh is employed. In previous studies, we applied hexahedra in the analysis of laminar-like structures such as intervertebral discs, while irregular bones were analyzed using tetrahedra more. Since the bolt diameter of FNS is much larger than a normal screw, excessive refinement of sagittal positions in the femoral head is not necessary.
Equivalent stress, shear stress, and total deformation were recorded in our study. Equivalent stress cloud diagrams can help us understand the force distribution very well. It uses stress contours to represent the stress distribution within the model, which clearly depicts how a result varies throughout the model, allowing the analyst to quickly identify the most dangerous areas of the model. In the model, the role of internal fixation is to take up and distribute the stress. In other words, the lower the stress value of the screw, the lighter the color of the stressed area, the higher the stability, because excessive stress increases the fatigue of the material. In our research, equivalent stress primarily occurred at the contact between the fracture surface and the internal fixation, which was consistent with the force situation (Fig. 4). However, we found that there was no stress concentration region in the superior model, indicating that the internal fixation did not play a role in dispersing the stress. Combined with the stresses on the fracture surface, our hypothesis was further confirmed. The internal fixation stress of the superior model was 4.3% smaller than that of the central model. While, the interfragmentary stress of the superior model was 4.2% larger than that of the central model. We also observed that the superior model had the largest range of forces on the fracture surface. Generally, the larger the area under stress, the more bone deformation occurs and the less stable it is. Also, our data showed the displacement of the superior model increased by 9% compared to the central model. Clearly, the superior model's internal fixation system was ineffective.
Femoral head pressure load can be separated into compressive and shear stresses based on the central axis of the neck and fracture surface. Axial compressive stress can promote fracture healing. The presence of shear stress increases the relative slip between fracture surfaces, which can lead to the failure of the fixation model. Nonetheless, previous finite element analyses did not include shear stress results. In our study, the area of concentration of shear stress varied with the location of the internal fixation. This eccentric placement may lead to uneven forces, thus weakening the compression effect on the fracture surface. The superior placement kept the screw away from the pressure trabeculae and did not effectively support and share the pressure from the femoral head. Generally, the shear stress area reflects the eccentricity of the screw. In the anterior position, the shear resistance area was the largest, indicating the greatest eccentricity at the moment (Fig. 4). According to our hypothesis, this was due to the anteversion of the femoral neck. As the bolt was placed anteriorly, the plate was located on the posterior side of the proximal femur, resulting in a large angle between the bolt and the axis of the femoral neck. Results indicated that the superior model had the lowest shear stress, but its shear resistance area was larger than that of the central and inferior model. Combined with the shear resistance area and shear stress, the central location was more preferable.We combined all the results into a dotted line graph so that we could find patterns (Fig. 5). Based on the previous analysis, we considered the superior model to be the most unstable. And the central model exhibited better biomechanical performance in terms of equivalent stress, shear stress, and total deformation. This result was in accordance with the previous findings in the coronal plane.
Nevertheless, this study does have some limitations. First, the proximal femur rather than the entire length of the femur was used for modeling, but it could reflect the trend of change. Second, we ignored the force variation in the healing process of femoral neck fracture. Third, biomechanical analysis of stable fractures was not been studied. Whether the pattern of the present study is consistent in stable fractures requires further analysis.