Biomechanical comparison of femoral neck system and cannulated screws coupled with medial plate for treating Pauwels III femoral neck fractures

Abstract

BACKGROUND:

The femoral neck system (FNS) has been considered as a novel strategy for femoral neck fracture. The diversity of internal fixation creates difficulties in choosing an effective option for Pauwels III type femoral neck fractures. Therefore, it is significant to investigate the biomechanical effects of FNS versus conventional approaches on bones.

OBJECTIVE:

To evaluate the biomechanical characteristics of FNS versus cannulated screws coupled with medial plate (CSS $+$ MP) for the treatment of Pauwels III type femoral neck fractures.

METHODS:

Through three-dimensional computer software (Minics, Geomagic – Warp), the proximal femur model was rebuilt. Based on the present clinical characteristics, models of internal fixation were reconstructed in SolidWorks, including cannulated screws (CSS), medial plate (MP) and FNS. After parameter setting and meshing, boundary conditions and loads were set up for the final mechanical calculation in Ansys Software. Under identical experimental conditions, such as the same Pauwels angle and force loading, the peak values of displacement, shear stress and equivalent (von Mises) stress were recorded.

RESULTS:

This study showed that the displacement of the models was CSS, CSS $+$ MP, and FNS in descending order of magnitude. The shear stress and equivalent stress of the models was CSS $+$ MP, FNS, and CSS in descending order. The principal shear stress of CSS $+$ MP was concentrated on the medial plate. The equivalent stress of FNS was more dispersed and distributed from the proximal main nail to the distal locking screw.

CONCLUSION:

CSS $+$ MP and FNS exhibited better initial stability compared to CSS. However, the MP was subjected to more shear stress, which could increase the risk of internal fixation failure. Due to its unique design, FNS may be a good choice for the treatment of Pauwels III type femoral neck fractures.

Keywords

Femoral neck system femoral neck fractures finite element analysis biomechanics

1. Introduction

Globally, the tendency of femoral neck fractures (FNFs) has increased continuously due to increased average age and osteoporosis, which accounts for approximately 50% of hip fractures [1]. By 2050, the cases of FNFs are estimated to be 6.7 million annually [2]. Of particular note is the morbidity and mortality of FNFs, especially in the developed countries [3]. FNFs can be divided into three types in compliance with the angle between the fracture face and the horizontal plane [4]. Of these fracture patterns, Pauwels type III is considered to be extremely unstable, which causes clinical nonunion and internal plant failure [5]. Therefore, for patients aged 65 and younger, the application for effective internal fixations has become a clinical research priority recently.

While the best treatment of FNFs remains controversial. Cannulated screws (CSS) and dynamic hip screw (DHS) are widespread adoptions to stabilize FNFs [6, 7, 8]. DHS and CSS have significantly similar complications, including non-union and revision rate, but CSS has superior to DHS on avascular necrosis [9]. It is universally acknowledged that fixation by CSS has been a standard method for stable and undisplaced femoral neck fracture [10]. However, for unstable fractures (Pauwels Type III), this strategy is prone to failure due to the shear forces mentioned above. To address this issue, researchers have successfully optimized fixation techniques by using a medial buttress plate (MP). Some finite element experiments have shown that this auxiliary technique has the effect of reducing the torsional force and shear stress while maintaining the axial compressive stress [11, 12, 13]. The recently applied implant Femoral Neck System (FNS) (DePuy Synthes, Zuchwil, Switzerland) can provide a minimally invasive surgical while allowing a reduced implant footprint. By offering angular stability and intraoperative compression, FNS has been considered as a novel strategy for femoral neck fracture (Fig. 1). In addition, based on excellent biomechanics behaviour, the stability of FNS has been proved to be comparable to DHS with anti-rotation screw and superior to cannulated screws [14, 15].

Figure 1.

Representative X-ray image of one clinical case who was treated for vertical femoral neck fracture using Femoral Neck System.

Some studies have shown the advantages of both CSS $+$ MP and FNS. To our knowledge, a few finite element analyses have been performed to evaluate the stability of these two fixation methods. However their findings are controversial [16, 17]. Accordingly, we observed the peak values of displacement, shear stress and equivalent (von Mises) stress of different internal fixations for treating Pauwels type III FNFs based on finite element analysis (FEA).

2. Material and methods

2.1 Building a normal proximal femur model

The volunteer we recruited (40-year-old, female, 60 Kg, 160 cm) did not have any hip and systemic disease. Written informed consent was acquired before the examination. The normal proximal femur of the volunteer was scanned using a computed tomography scanner (Philips, Netherlands, 128 spiral). A series of images (Slice Thk 0.5 mm) were exported and saved as Digital Imaging and Communications in Medicine (DICOM) format. These images were imported to Mimics 20. After predefined threshold, region growing, mask modifying and preliminary borders smoothing, the project was outputted in STL format. This model was imported into Geomagic – Wrap 2017 software for advanced smoothing, adjusting polygon mesh and fitting surface and also integrated into the SolidWorks 2017 software. The three-dimensional (3-D) model of proximal femoral bone was established by Boolean operation.

2.2 Modelling of FNFs and internal fixation

By using the part pattern, the model was divided into two parts, and the angle between the fracture face and horizontal plane was 60 (Fig. 2a). Based on the present clinical characteristics, models of internal fixation were reconstructed through schematic drawing function. The medial reconstructive plate was modeled utilizing equidistant offset surface method, and the three CSS (7.3 mm) were arranged in an inverted triangle. Under assembly pattern, models of FNFs and internal fixations were combined into one part, which could be edited directly (Fig. 3). In the end, all parts were detected by interference to evaluate their process quality, then were imported into Ansys 17.0 software.

Figure 2.

Details of proximal femoral bone model: Pauwels type III (a); Uneven meshes of femur model (b); load and muscle forces (c).

2.3 Parameter setting and meshing

According to the previously set material property parameters [11, 18], the model including cortical bone, cancellous bone, and internal fixations was assigned to different elastic modulus and Poisson’s ratio (Table 1). The connection between the fracture surface was set to friction, and the coefficient of friction was 0.46 [19]. The relationship between the internal fixations and bone was set as binding relation.

Table 1
Material properties of models in this study

Item	Poisson’s ratio	Elastic modulus (MPa)
Femoral cancellous bone	0.2	840
Femoral cortical bone	0.3	16800
Cannulated screw	0.3	110000
Medial plate	0.3	110000
Femoral neck system	0.3	110000

Figure 3.

Details of the modeling: CSS(a); CSS $+$ MP(b); FNS(c).

To improve the calculation accuracy, a mixed mesh method was adopted to build meshes of the femoral neck, and different mesh types were established (Fig. 2b). This method allowed for more nodes and elements, which resulted in higher accuracy of biomechanical analysis. The minimum effective size of a four-node tetrahedron body element was 1.5 mm, which was used for bone and all kinds of internal fixations.

2.4 Boundary conditions and loads

We created a simplified model in which the various materials were recognized to be homogeneous, isotropic linear. An 1800 N load corresponding to 3 times body weight [20, 21] was uniformly applied to the weight-bearing region of the femoral head along the specific direction. The load and muscle forces were simplified [12] (Fig. 2c).

2.5 Evaluation criteria

First, the statistics of all reconstructed models were recorded, including their nodes and elements. Second, the displacement, shear stress and equivalent (von Mises) stress of different internal fixations were observed.

Figure 4.

Details of displacement: CSS(a); CSS $+$ MP(b); FNS(c).

Figure 5.

Schematic diagram of shear stress of internal fixations: CSS(a); CSS $+$ MP(b); FNS(c).

3. Results

3.1 Maximal displacement of femur and internal fixators

From the vector display, the displacement of models was the same direction as the force. Meanwhile, this study showed roughly the same displacement direction in FNS and CSS $+$ MP. The distribution of displacement was showed in the contour bands, the peak value of displacement was recorded via the probe function. The maximum displacement of all models was located in the weight-bearing region of the femoral head and the proximal end of internal fixation. The maximal displacement in CSS, CSS $+$ MP, and FNS models was 0.7776 mm, 0.7753 mm, and 0.7719 mm, respectively. In addition, the displacement was gradually decreased from head to tail of internal fixators (Fig. 4).

3.2 Peak value and distribution of shear stress

As shown in Fig. 5, the maximum shear stress occurred at the contact surface between the internal fixation and the fracture. The shear stress of CSS was located in the middle of the distal screw; the maximum value was 13.47 Mpa. The principal shear stress of CSS $+$ MPS was concentrated on the medial plate, the maximum value was 31. 97 Mpa. The principal shear stress of FNS was concentrated on the middle of the primary nail; the maximum value was 15.75 Mpa.

The interfragmentary shear stress of CSS, CSS $+$ MP, FNS was 6.19 Mpa, 4.91 Mpa, 5.12 Mpa respectively.

3.3 Peak value and distribution of equivalent (von Mises) stress

The distribution map of equivalent (von Mises) stress was showed in Fig. 6. The stress was scattered at the femoral medial cortical bone. However, the peak value of equivalent stress was located in the internal fixator. The maximum equivalent stress of CSS was concentrated on the middle of three screws; the peak value was 52.01 Mpa. The maximum equivalent stress of CSS $+$ MP was concentrated on the medial plate; the peak value was 128. 76 Mpa. The maximum equivalent stress of FNS was concentrated on the primary screw; the peak value was 53. 67 Mpa. In the contour bands, the stress range on screws near fracture was more expansive than other area. Compared to CSS, the stress of CSS $+$ MP was concentrated on the medial plate, while the stress of the three screws and fracture end was less. Among all the models, the stress distribution of FNS was more uniform and extensive.

Figure 6.

Schematic diagram of equivalent stress of internal fixations: CSS(a); CSS $+$ MP(b); FNS(c).

4. Discussion

In the treatment of FNFs, implant choice has been one of the most critical yet contentious questions. A meta-analyze showed the incidence of implant failure was 9.7% [22], which leading to a high rate of reoperation. It is significant to choose most effective internal fixation approach to treat unstable fractures (Pauwels Type III). A previous FEA evaluated the biomechanical stability of cannulated screws with different configurations [23]. The author considered inverted triangle configuration as the recommended choice. However, it is not particularly appropriate for unstable fractures because of vertical shear force. To address this issue, a medial buttress plate was recommended to assist in the operation, affording increased resistance of shear stress [24]. As an improvement in classic minimally invasive surgery, CSS $+$ MP not merely retains the sliding compression and anti-rotation ability of the CSS, but also directly resists the shear stress, especially exists in unstable fractures (Pauwels Type III) end. Some scholars have suggested the addition of medial support plates after anatomic reduction of vertically oriented femoral neck fractures to improve the high complication rate after severe injuries [24]. As a novel internal fixation, FNS was developed with various factors. A current finite element analysis showed that FNS had better stability than CSS [19]. Numerous advantages of existing clinical constructs were taken into consideration. The new concept of FNS still emphasizes the importance of biomechanical stability. The pressure load of the femoral head can be decomposed into compressive stress along the central axis of the femoral neck, and the shear stress downward along the fracture surface. The compressive stress has an evident effect in promoting the fracture healing. In fact, the compressive effect can be obtained through internal fixation during the operation. Due to the design between the locking bolt and anti-rotation screw, the maximum sliding pressure distance of FNS is up to 20 mm. The cylindrical design of sliding screw plays a role in whole support. In the structure of the femoral neck system, the angle between the sliding screw and the anti-rotation screw is 7.5 degree. In this respect, FNS was privileged with regard to rotational stability. Although FNS bolts are manufactured in 5 mm increments, some scholars have improved the technique to help precisely adjust the bolt depth for femoral neck fracture dislocations [25]. CSS $+$ MP also has the compressive function; in practice, we need to be aware of the placement sequence of internal fixation. After fracture reduction, it is necessary to fix the fracture with CSS first to form initial compression. Then, the medial plate is placed to maintain stability. Some scholars tried to use a medial plate without a proximal screw in finite element analysis; they thought that using a proximal screw on the buttress plate was an indispensable part in reducing stress and improving anti-shearing ability [13]. Additionally, although the stability of internal fixation represents a complex choice, it is essential to recognize that excessive tissue damage also leads to profound impacts on vascular flow in the femoral head. FNS was developed to combine stability and minimally invasive insertion technique. Moreover, the size of locking plate in FNS is smaller than the plate of other internal fixation, which means that the FNS has less soft tissue damage during the operation. Because of the locking effect of the outer steel plate, some complications will not happen, such as screw loosening. The main vascular flow in the femoral head is from the medial femoral artery [26]. In theory, using CSS $+$ MP may cause more tissue damage. When the medial plate is placed on the anteromedial side of the femoral neck during surgery, it will not damage the main vascular flow [27]. Clinical studies also reported that using medial plate will not interfere with fracture healing and not increase the tendency of ischemic necrosis [28]. It is absolutely accepted that the operation of CSS $+$ MP will take more time than CSS in surgery. In summary, both FNS and CSS are widely utilized in the treatment of femoral neck fractures.

Accordingly, we would like to analyze the biomechanical properties of CSS and FNS by the finite element method [29] in order to provide help for clinical selection. Compared to traditional biomechanical analysis, the finite element analysis can provide more convenient conditions, such as various loading methods, multivariate test indexes and relatively low research costs. At present, the FEA of FNFs mainly focuses on the comparison of internal fixation methods of different types to optimize surgical approaches. In our study, models with the same Pauwels angle were rebuilt through finite element analysis, which enhanced the accuracy and repeatability of experiments. A more elaborate model of FNS was rebuilt with high amount of details. Meanwhile, the shear stress was recorded for the first time to evaluate the stability of models.

In this finite element analysis, we evaluated the biomechanical characteristics of FNS and CSS $+$ MP. Compared with default mesh pattern, the amount of nodes and elements in the mixed mesh pattern was obviously increased. It is universally acknowledged that the number of elements determines the accuracy of experiment. Whereas, more elements need complex computing tasks. In a previous study, the stress was intensely focused on the fracture surface of femoral neck [30]. Thus, models with very fine mesh were created to evaluate stress distribution and displacement. The displacement of the proximal femur is complex and often includes femoral neck shortening and femoral head collapse. In general, the displacement of the model determines the overall stability. In our study, the displacement of FNS and CSS $+$ MP was smaller than that of CSS. We believe that displacement and stiffness are negatively related under the same force. Due to the integrated design of the FNS, the stiffness of the FNS is higher, which results in a smaller total displacement of the model. In other words, excellent initial stability helps maintain femoral head length and reduce the collapse of femoral head. In unstable fractures, the shear stress at the fracture end is not negligible.

Our results showed that the internal fixation shear stress of FNS increased by 16% compared to CSS, while the shear force of MP increased by 137%. It indicates that the medial plate also continues to be subjected to more shear forces as time increases, which raises the risk of internal fixation failure. From the results, the CSS $+$ MP had the lowest interfragmentary shear stress. Due to the multi-planar fixation of the CSS $+$ MP, the internal fixation material shares the main shear stress. Compared to CSS $+$ MP, the interfragment stress increased by 4% in FNS and 65% in CSS. This shows that the CSS model has the worst biomechanical stability. For the equivalent stress, FNS increased by 3% compared to CSS, while MP increased by 147%. In addition, in CSS $+$ MP, more equivalent forces were transferred from the fracture end to the medial plate, greatly increasing the peak stress of internal fixation. The stress cloud showed that the stress in FNS was more dispersed and distributed from the proximal main nail to the distal locking screw. Dispersed contact area results in less localized pressure. Conversely, over-concentrated stress can jeopardize the fatigue resistance of internal fixation.

However, this study has some limitations. First, the simplified finite element models were reconstructed without muscles, joint capsules and ligaments. Therefore, the accuracy of the experimental results was slightly lower than the actual results, but it could reflect the trend of change. Second, this study was designed to simulate the immediate mechanical environment after the operation. After all, it’s hard to evaluate the force variation in the healing process of femoral neck fracture. Third, different bone mass, running and other different conditions were not taken into consideration. Thus, further biomechanical tests and clinical assessments need to be performed.

5. Conclusions

Ethics approval and consent to participate

This study was approved by the Medical Ethics Committee of the Baoding No.1 Central Hospital ([2020]038). All methods were carried out in accordance with relevant guidelines and regulations. The volunteer agreed to the trial protocol and her informed consent was obtained.

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Funding

The study was financially supported by the Baoding City Science and Technology Plan Project (2041ZF103).

Author contributions

CN: Conceptualization, Methodology, Software, Writing-review & editing. YL: Conceptualization, Methodology, Software, Editing. YL: Visualization, Investigation. LM: Software, Validation. ZM: Supervision, Writing – review & editing. All authors read and approved the final manuscript. CN and YL contributed equally to this work and should be considered co-first authors.

Footnotes

Acknowledgments

Not applicable.

Conflict of interest

The authors declare that they have no competing interests.

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Biomechanical comparison of femoral neck system and cannulated screws coupled with medial plate for treating Pauwels III femoral neck fractures

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2.1 Building a normal proximal femur model

2.2 Modelling of FNFs and internal fixation

Table 1 Material properties of models in this study

2.5 Evaluation criteria

3.1 Maximal displacement of femur and internal fixators

3.2 Peak value and distribution of shear stress

3.3 Peak value and distribution of equivalent (von Mises) stress

5. Conclusions

Ethics approval and consent to participate

Availability of data and materials

Funding

Author contributions

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Material properties of models in this study