The results of this study show that compared with the other three configurations, the modified F-technique screw fixation configuration are the smallest with respect to the maximum femoral and internal fixation displacement, the maximum displacement in the direction of shear force, the maximum crack distances of the fracture surfaces and the maximum femoral stress, indicating that the performance of modified F structure in anti-shearing force and anti-varus of femoral head is stronger than that of dynamic hip screw plus anti-rotation screw model, F-technique screw model and inverted triangle parallel screw model. Therefore, the modified F-technique configuration fixes Pauwels type Ⅲ FNFs getting the best stability performance. When encountering Pauwels type Ⅲ FNFs that require internal fixation in practice, the modified F-technique screw fixation configuration can be considered first, followed by dynamic hip screw plus anti-rotation screw, F screw fixation, and parallel cannulated screw fixation.
Pauwels type III FNFs usually occur in young patients with high-energy injuries [2, 20]. In such patients, hip-preserving treatment with reduction and internal fixation is usually the gold standard [21]. In Pauwels type III FNFs, because the fracture line is more inclined, the shear force is dominant, and the varus force is large, which easily leads to fracture displacement and varus collapse [22]. The existence of these two forces must be fully considered when choosing internal fixation [21, 22]. It has been reported in the literature that the nonunion rate of Pauwels type III FNFs is 16%-59%, and the incidence of femoral head necrosis is 11%-86% [4]. Poor prognosis such as femoral head necrosis and fracture nonunion will bring catastrophic consequences to patients. However, at present, the best treatment for Pauwels Ⅲ FNFs is still inconclusive, exploring the best fixation methods for FNFs is one of the current research focuses [4].
In order to overcome the greater shearing force of Pauwels type III FNFs, some authors have proposed a treatment plan that uses screws and medial femoral neck plates. Related mechanical studies have shown [23, 24] that the medial femoral neck plate can effectively resist shear forces. However, it is difficult to install a steel plate on the inner of the femoral neck because the femoral neck is deep and the area is close to the joint capsule of the hip joint. Placing the steel plate will inevitably destroy the joint capsule structure. It may also damage the blood supply of the femoral head, especially the lower retinaculum artery, which is considered to play an important role in the blood supply of the femoral head after a FNF [25]. The need to increase the medial steel plate is bound to increase an incision, and the operation time and trauma will be greater. Therefore, the model of the medial femoral neck plate was not used in this study.
Parallel screws have been used for the treatment of FNFs for many years. They have the advantages of minimal invasion, low blood loss, short hospital stay, and short operation time [26, 27]. However, among the four fixation models in this study, the three parallel screws of the inverted triangle have the weakest resistance to shear force and the varus of the femoral head when fixing the Pauwels type III FNFs. So, this configuration is not recommended to be used in fixing Pauwels type III FNFs. Research by Gurusamy et al. showed that within the first 3 months after surgery, the failure rate of 3 parallel threaded screws was as high as 39% [28]. Research by Liporace et al. also showed [4] that the nonunion rate of Pauwels type III FNFs fixed with 3 parallel cannulated screws was 19%. Stankewich et al. concluded in their cadaveric study that the use of parallel screws to fix Pauwels type III FNFs is not recommended [29].
The shear resistance and anti-varus stability of the F-technique screw configuration is weaker than that of the dynamic hip screw plus anti-rotation screw model, but stronger than the parallel cannulated screw configuration. Filipov initially proposed to fix osteoporotic FNFs with the F screw fixation technique with dual-plane and dual-support [20]. Clinical studies have shown that the failure rate of this configuration in the treatment of FNFs is only 3.4% [14]. The finite element study of Lin et al. using F screw fixation configuration to treat Pauwels type III fractures showed that F screw fixation has higher stability than parallel cannulated screw fixation [23]. Consistent with the conclusions of this study, however, the stability of F screw fixation is still lower than that of the dynamic hip screw plus anti-rotation screw fixation configuration.
In this study, the modified F configuration does not require additional incisions, and its shear resistance and anti-varus stability is the strongest among the four models. The distal screw adopts a full-thread setting, and the contact area between the bone and the screw is larger, so that it can obtain better shear resistance and mechanical stability. Therefore, the modified F configuration can obtain better early mechanical stability of FNF fixation. The biomechanical study of Zhang et al. [30] showed that the larger the Pauwels angle of FNFs, the more obvious the advantages of full-threaded screws to fix FNFs. Improving the fixation stability of FNF can improve the blood supply of femoral head and reduce the poor prognosis such as fracture nonunion and femoral head necrosis, which is the key factor for the fixation of unstable FNF [31]. The modified F configuration only replaces the distal half-threaded screw with a full-threaded screw. The upper two half-threaded screws still have a partial sliding pressure effect [19, 32–34], and the addition of full-threaded screws can effectively prevent the femoral neck from shortening. The modified F configuration provides a new ideal option for the treatment of Pauwels type Ⅲ FNF.
Moreover, stresses distribution in the subtrochanteric region of modified F configuration was lower than that in a and b models, indicating that modified F configuration may not increase the risk of subtrochanteric fractures. The small distance of less than 7 mm between the three parallel cannulated screws, used in traditional inverted triangular cannulated screw configuration, may be a significant stress-riser in subtrochanteric region. However, the rather wide distance between screws in F configuration (20–40 mm) might not weaken the subtrochanteric femur bone, because the tensile forces acting on the lateral cortex are spread over a larger area[14]. Filipov et.al conducted a study containing 207 femoral neck fractures treated with F configuration, he did not observe any iatrogenic subtrochanteric fracture after surgery[14].
The novelty of this study is that we propose a modify F configuration with strong stability in fixing Pauwels type Ⅲ FNF. Moreover, finite element analyses were used with the primary goal of comparing various fixation methods by measuring the interfragmentary motions to assess more important parameters that are difficult and almost unlikely to experimentally measure, for example, the stress or strain distribution within the bone and implant [35]. The limitations of this study include simplifying the fracture model and internal fixation details as well as ignoring the effects of surrounding muscles and ligaments on fracture stability. The study is only a biomechanical analysis, and a randomized controlled trial is needed to further verify our results.
Through the present finite element analyses, it was found that the modified F cannulated screw technique is capable of eliminating the varus stress and shear stress while maintaining the axial compressive stress at the fracture end. Thus, this configuration can create a good mechanical environment for fracture healing. Accordingly, for the Pauwels III FNF, the use of modified F configuration is recommended, followed by dynamic hip screw plus anti-rotation screw, F screw fixation, and parallel cannulated screw fixation.
Statement:
All the authors confirm that all experimental protocols were approved by the Ethics Committee of The Fifth Affiliated Hospital of Sun Yat-Sen University, affiliated to Sun Yat-Sen University.
All the authors confirm that all methods were carried out in accordance with relevant guidelines and regulations.
All the authors confirm that informed consent was obtained from the participant.