Biomechanical effects of FNS on femoral neck fractures based on different reduction quality: finite element analysis

Abstract

Background and objective

The femoral neck system (FNS) has been extensively studied and applied for the treatment of young patients with femoral neck fractures. The purpose of this study was to explore the biomechanical impact variations in reduction qualities on femoral neck fractures, considering factors such as tip-apex distance, the positioning of the bolt in the cortical corridor of the femoral neck, and bone mineral density.

Materials and methods

A randomly selected volunteer was recruited, whose clinical data on the femur were collected to establish finite element models for positive reduction, anatomical reduction, and negative reduction respectively. Based on the constructed models, different scenarios were established by varying the tip-apex distance, bone mineral density, and positioning of the bolt in the cortical corridor of the femoral neck. Under a vertical load of 2100 N, the displacement and Von Mises stress (VMS) distribution of each group of models were evaluated through simulation testing.

Results

Under a load of 2100 N, the maximum VMS values of the femoral neck system and femoral head was recorded during negative reduction, 968.85 MPa and 80.09 MPa respectively. In addition, factors influencing the negative reduction of FNS and the femoral head were identified to be the tip-apex distance > 10 mm, the presence of osteoporosis, and the bolt positioned in the lower-middle to the third part of the cortical corridor of the femoral neck.

Conclusion

The displacement and stress of negative reduction were greater than those of positive reduction and anatomical reduction when the tip-apex distance > 10 mm, and the bolt was situated in the lower-middle to the third part of the cortical corridor of the femoral neck, and in the presence of osteoporosis. This means that we recommend positive repositioning over negative repositioning when anatomical repositioning is not clinically feasible.

Introduction

Femoral neck fractures in young patients are frequently caused by high-energy injuries such as car accidents or high-altitude falls [1]. At present, traditional internal fixation methods comprise three cannulated cancellous screws (CCS) and dynamic hip screws (DHS) [2]. The former is often faced with a large shear force that leads to the failure of internal fixation, whereas the latter is traumatic and inevitably damages the blood supply of the femoral head [3–5]. Therefore, the femoral neck system (FNS, DePuy Synthes, Switzerland), which utilizes bolt, anti-rotation screw, outer plate, and locking nails, stands as an optimized internal fixation technique for the treatment of femoral neck fractures in young patients. Besides, our previous study have found that the femoral neck system exhibits superior anti-rotation performance, effectively minimizing the risk of femoral neck shortening and internal fixation failure after fracture healing [6].

Recent studies reported that anatomical reduction of the injured femoral neck is beneficial for preserving the fragile blood supply of the femoral head and lowering the risk of postoperative complications such as the shortening and osteonecrosis of the femoral neck [7]. As is well documented, anatomical reduction is crucial to the early internal fixation of femoral neck fractures in young individuals. Gotfried first introduced the concept of reduction of femoral neck fractures and according to the relative position of the fractured end, the reduction results can be divided into positive fixation, anatomical reduction, and negative fixation [8]. Specifically, biomechanics analyses have verified that positive fixation and anatomical reduction are conducive to the stability of femoral neck fractures [9, 10].

At present, tip-apex distance, positioning of the bolt, and the bone condition have been identified as key factors affecting the prognosis of fractures [11, 12]. The majority of FNS surgeons postulate that the tip-apex distance should be as close to the bone surface as possible to achieve optimal stability [13]. According to FNS guidelines, an FNS bolt should be inserted along the center of the cortical corridor of the femoral neck [14]. In addition, joint replacement is typically the gold standard for elderly patients with femoral neck fractures suffering from osteoporosis or arthritis (over 65 years old), but the stability of FNS fixation in young patients remains to be elucidated [15, 16].

Currently, the impact of the above-mentioned factors (tip-apex distance, position of the bolt, and the bone condition) on the stability of femoral neck fractures with different reduction qualities remains unknown. Therefore, this study aimed to establish a finite element biomechanical analysis model to explore the biomechanical effects of these factors on FNS fixation of femoral neck fracture with different reduction qualities.

Methods and equipment

Methods

Reduction quality

The reduction quality after fracture was divided into three types: anatomical reduction, positive reduction, and negative reduction. Anatomical reduction refers to the lower edge of the proximal femoral neck fracture being neatly aligned with the distal medial upper edge without displacement. In contrast, the inferomedial edge of the distal femoral neck fracture protrudes to the superomedial edge of the proximal femoral neck fracture with positive reduction. Finally, negative reduction refers to the protrusion of the inferomedial edge of the proximal femoral neck fracture to the inferomedial edge of the distal femoral neck fracture [8]. The reduction qualities are illustrated in Fig. 1.

Fig. 1.

(A) Anatomical reduction (B) Negative reduction (C) Positive reduction

Modeling of the femoral neck and internal fixation

A 30-year-old male volunteer with a height of 175 cm and a weight of 70 kg was randomly recruited. Then, osteonecrosis of the femoral head, dysplasia, osteoporosis, and other hip joint diseases were ruled out via X-ray imaging. After obtaining the informed consent form of the volunteer, the proximal femur and femoral neck were scanned using computed tomography, and the data were captured. The 3D CT imaging data with a slice thickness of 0.625 mm were stored in the DICOM format and imported into Mimics medical image processing software after threshold segmentation, region growth, threshold editing. Then 3D reconstruction for 3D modeling of the femur, and the resulting file was saved in the STL format. Next, the Geomagic software was utilized to generate the surface model by processing the model mesh, extracting the curve, establishing the surface and grid, and fitting the surface. We validated the model to ensure its effectiveness. The results of our validation of the model are as follows: the compressive stiffness is 967.74 N/mm at a loading condition of 1500 N, which coincides with the results of Papini et al. with a compressive stiffness of (0.76 ± 0.26) KN/mm [17]. The results of the model validation are shown in Fig. 2.

Fig. 2.

The results of the model validation. ①: The displacement of the femur ②: The stress of the femur

Afterwards, the above-mentioned model was imported into Creo Parametric 5.0.5.0 software to construct a three-dimensional solid model and establish the FNS models respectively. An FNS internal fixator with a 1-hole steel plate and 5 mm bolt increment was chosen, followed by grouping the FNS and femur models for Boolean operation, and the assembly of each model was subsequently completed [14]. All the meshes were generated by ten-node tetrahedral elements. To ensure the sensitivity of the mesh, we carried out a mesh convergence study with a total of seven sets of analyses with different mesh sizes, the smallest being 1 mm and the largest being 5 mm, to study the deformation and stress distribution on the model under the seven mesh sizes. Through comparative analysis, considering that 1 mm mesh size is too small and the number of meshes is too high, which is too demanding on the capability of the analysis workstation, for all other mesh sizes, we found that when the mesh size is 2.5 mm, the size of the error is 1.6%, and therefore the best convergence is achieved by using this mesh size. Specific information on the mesh convergence study is shown in Table 1.

Table 1.

Information about mesh convergence

Mesh size（mm）	Displacement（mm）	VMS（MPa）	Error（%）	Nodes	Elements
1	2.17	41.31	0	9134151	6490950
1.5	2.17	40.57	1.8%	3491082	2457690
2	2.17	39.73	3.9%	1842459	1289577
2.5	2.16	40.64	1.6%	1374477	968385
3	2.16	39.58	4.3%	683985	470007
3.5	2.16	38.32	7.8%	443424	301395
5	2.16	38.13	6.6%	165323	106573

Femoral neck models under different factors are illustrated in Fig. 3.

Fig. 3.

A-C Standard position of FNS under different reduction qualities. D-E FNS is located in the lower third part of the cortical corridor of the femoral neck under different reduction qualities. G-I Tip-apex distance is greater than 10 mm but less than 15 mm. J-L Standard position of FNS in patients with osteoporosis under different reduction qualities

Element information consisting of finite elements models are shown in Table 2.

Table 2.

Element information consisting of finite elements models

Group	Nodes	Elements
A	1475077	930777
B	1461870	924818
C	1466810	928538
D	1428599	903977
E	1467499	929788
F	1471779	932948
G	1412128	992372
H	1405385	988353
I	1409680	891317
J	1401032	903698
K	1412350	912536
L	1436528	905968

Grouping

A: anatomical reduction, tip-apex distance is 2.5 mm, FNS is located in the center of the cortical corridor of the femoral neck, healthy bone.

B: positive reduction, tip-apex distance is 2.5 mm, FNS is located in the center of femoral neck cortical corridor, healthy bone.

C: negative reduction, tip-apex distance is 2.5 mm, FNS is located in the center of the cortical corridor of the femoral neck, healthy bone.

D: anatomic reduction, tip-apex distance is 2.5 mm, FNS is located in the lower third part of the cortical corridor of the femoral neck, healthy bone.

G: anatomic reduction, 10 mm < tip-apex distance < 15 mm, FNS is located in the center of the cortical corridor of the femoral neck, healthy bone.

J: anatomical reduction, tip-apex distance is 2.5 mm, FNS is located in the center of femoral neck cortical corridor, osteoporotic bone.

Parameter settings

The femoral neck system was made of titanium alloy. The femur is composed of cortical bone, cancellous bone, and a hollow medullary cavity. Referring to data from previous research, different elastic moduli and Poisson’s ratios were assigned to these parameters in Hypermesh 2017(Altair, USA) [18–20]: the elastic modulus and Poisson’s ratio of the cortical bone were 16,800 MPa and 0.30, respectively; the elastic modulus and Poisson’s ratio of the cancellous bone were 840 MPa and 0.20, respectively; the elastic modulus and Poisson’s ratio of osteoporotic cortical bone were 11,760 MPa and 0.30, respectively; the elastic modulus and Poisson’s ratio of osteoporotic cancellous bone were 588 MPa and 0.20, respectively; the elastic modulus and Poisson’s ratio of the femoral neck system were 105,000 MPa and 0.35, respectively. Parameters of elastic moduli and Poisson’s ratio are presented in Table 3.

Table 3.

Parameters of elastic moduli and poisson's ratio

	Elastic modulus	Poisson's ratio
cortical bone	16800	0.30
cancellous bone	840	0.20
osteoporotic cortical bone	11760	0.30
osteoporotic cancellous bone	588	0.20
FNS	105000	0.35

1. Lu H, Shen H, Zhou S, Ni W, Jiang D. Biomechanical analysis of the computer-assisted internal fixation of a femoral neck fracture. Genes Dis. 2020;7(3):448-55

2. Morgan EF, Bayraktar HH, Keaveny TM. Trabecular bone modulus-density relationships depend on anatomic site. J Biomech. 2003;36(7):897-904

3. Nolte D, Bull AMJ. Femur finite element model instantiation from partial anatomies using statistical shape and appearance models. Med Eng Phys. 2019;67:55-65

Mechanical load and boundary conditions

The degrees of freedom of all nodes of the distal femur were set to 0 in the X, Y and Z directions and constrained to prevent rigid body motion [13, 21, 22]. And a load of three times that of the body weight, that is, 2100 N, was applied above the femoral head to simulate the load caused by exercise in the normal gait cycle. The boundary and loading direction of femur are shown in Fig. 4. All interfaces between the implant and the two fracture ends are considered friction contact, and the friction coefficients between bone-bone, bone-implant, and implant-implant are 0.46, 0.42, and 0.20, respectively [3, 13].

Fig. 4.

The boundary and loading direction of femur

Equipment and software

The Siemens Medical Solutions Sensation 64 spiral CT scanner (Forchheim, Germany) was used to perform thin-slice CT scanning of the volunteer’s pelvis and lower limbs at a voltage of 120 kV, a current of 150 mA, and a scan layer thickness of 0.625 mm.

Graphics processor workstation: T7910, Intel (R) Xeon (TM), CPU Zhiqiang E5-2650 * 2,24G memory, Hard disk: 2000G.

The software used in this study were Mimics 21.0 (Materialise, Belgium), Geomagic Studio 2021 (Geomagic, USA), Creo Parametric 5.0.5.0 (PTC, USA), Hypermesh 2017(Altair, USA), and ANSYS 19.0 (ANSYS, USA).

Evaluation criterion

The maximum displacement value and peak von Mises stress were selected as indicators in the finite element analysis to evaluate the stability and risk of internal fixation failure of the FNS devices for the femur. We also compared the von Mises stress and maximum displacement of the femoral head with different reduction qualities under the load generated during the normal gait cycle.

Results

The Von Mises stress (VMS) distribution analysis of FNS and femoral head.

• I.Standard fixation: The maximum VMS values of internal fixation appeared during negative reduction, and the peak VMS during anatomical reduction, negative reduction, and positive reduction was 590.60 MPa, 968.85 MPa, and 644.13 MPa, respectively.

  More importantly, the maximum VMS values of the femoral head was noted during negative reduction, and the peak VMS during anatomical reduction, negative reduction, and positive reduction was 15.62 MPa, 80.09 MPa, and 15.69 MPa, respectively.

• II.FNS was located in the lower third part of the cortical corridor of the femoral neck: the the peak VMS of internal fixation during anatomical reduction, negative reduction, and positive reduction were 658.66 MPa, 948.59 MPa, and 821.05 MPa, respectively. Additionally the peak VMS of the femoral head during anatomical reduction, negative reduction, and positive reduction was 21.95 MPa, 26.34 MPa, and 25.02 MPa, respectively.
• III.10 mm < tip-apex distance < 15 mm: the maximum VMS values of internal fixation was observed during negative reduction, and the peak VMS during anatomical reduction, negative reduction, and positive reduction was 773.68 MPa, 989.12 MPa, and 945.82 MPa, respectively. In contrast, the maximum VMS values of the femoral head was detected during negative reduction, and the peak VMS during anatomical reduction, negative reduction and positive reduction was 19.50 MPa, 72.35 MPa, and 20.94 MPa, respectively.
• IV.Osteoporosis: The the peak VMS of internal fixation during anatomical reduction, negative reduction, and positive reduction was 663.34 MPa, 1109.44 MPa, and 730.73 MPa, respectively. Additionally, the peak VMS of the femoral head during anatomical reduction, negative reduction and positive reduction was 12.43 MPa, 58.76 MPa, and 12.55 MPa, respectively. It is worth noting that the peak FNS von Mises stress was found at the junction of the internal fixation and the fracture surface from the stress cloud, and the maximum stress in the femoral head was found at the lower edge of the fracture surface cortex. The VMS figure of FNS and femoral head are displayed in Figs. 5 and 6.

Fig. 5.

Stress nephogram of FNS. A-C Standard position of FNS under various reduction qualities. D-E FNS is located in the lower third part of the cortical corridor of the femoral neck under different reduction qualities. G-I Tip-apex distance is greater than 10 mm but less than 15 mm. J-L Standard position of FNS in patients with osteoporosis under different reduction qualities

Fig. 6.

Stress nephogram of the femoral head. A-C Standard position of FNS under different reduction qualities. D-E FNS is located in the lower third part of the cortical corridor of the femoral neck under different reduction qualities. G-I Tip-apex distance is greater than 10 mm but less than 15 mm under. J-L Standard position of FNS in patients with osteoporosis under different reduction qualities

The displacement distribution analysis of FNS and femur.

• I.Standard fixation: the maximum displacement of internal fixation was observed during anatomical reduction; the maximum displacement during anatomical reduction, negative reduction, and positive reduction was 7.36 mm, 7.00 mm, and 7.32 mm, respectively, and the maximum displacement of the femoral head during anatomical reduction, negative reduction, and positive reduction was 7.62 mm, 7.20 mm, and 7.62 mm, respectively.
• II.FNS was located in the lower third part of the cortical corridor of the femoral neck: the maximum displacement of internal fixation was noted during anatomical reduction, and the displacement during anatomical reduction, negative reduction, and positive reduction was 7.86 mm, 6.60 mm, and 6.92 mm, respectively, whereas the maximum displacement of the femoral head appeared during anatomical reduction, and the displacement of the femoral head during anatomical reduction, negative reduction, and positive reduction was 8.61 mm, 7.14 mm, and 7.59 mm, respectively.
• III.10 mm < tip-apex distance < 15 mm: the maximum displacement of internal fixation was identified during anatomical reduction; the maximum displacement during anatomical reduction, negative reduction, and positive reduction was 7.03 mm, 6.68 mm, and 7.01 mm, respectively, whereas the maximum displacement of the femoral head during anatomical reduction, negative reduction, and positive reduction was 7.65 mm, 7.22 mm, and 7.66 mm, respectively.
• IV.Osteoporosis: The maximum displacement of internal fixation was observed during anatomical reduction, and the maximum displacement during anatomical reduction, negative reduction, and positive reduction was 10.39 mm, 9.91 mm, and 10.35 mm, respectively. Additionally, the maximum displacement of the femoral head during anatomical reduction, negative reduction, and positive reduction was 10.77 mm, 10.17 mm, and 10.76 mm, respectively.

  The displacement of FNS and femoral head are depicted in Figs. 7 and 8.

Fig. 7.

The displacement of FNS. A-C Standard position of FNS under different reduction qualities. D-E FNS is located in the lower third part of the cortical corridor of the femoral neck under different reduction qualities. G-I Tip-apex distance is greater than 10 mm but less than 15 mm. J-L Standard position of FNS in patients with osteoporosis under different reduction qualities

Fig. 8.

The displacement of the femoral head. A-C Standard position of FNS under different reduction qualities. D-E FNS is located in the lower third part of the cortical corridor of the femoral neck under different reduction qualities. G-I Tip-apex distance is greater than 10 mm but less than 15 mm. J-L Standard position of FNS in patients with osteoporosis under different reduction qualities

The resulting variation curves of VMS and displacement are shown in Fig. 9.

Fig. 9.

①:The peak VMS of internal fixation ②:The peak VMS of femoral head ③:The maximum displacement of internal fixation ④:The maximum displacement of femoral head. I:Standard fixation II:10 mm < tip-apex distance < 15 mm III:FNS was located in the lower third part of the cortical corridor of the femoral neck IV:Osteoporosis

Discussion

It is pivotal to achieve better reduction quality and prevent postoperative complications in patients with femoral neck fractures who are younger than 65 years old [23]. Femoral Neck System (FNS), a novel type of internal fixation, has been widely adopted in the clinical settings in recent years [24]. Indeed, the application of FNS has brought many benefits to patients. Huang et al. concluded that FNS has good anti-rotation properties that lowers the incidence of complications such as femoral neck shortening and internal fixation failure after fracture healing [6]. Although the manufacturer provides standard instructions for the surgical intervention and recommends that the FNS bolt should be inserted in the central trajectory of the cortical corridor, the procedure cannot be completely consistent with the standard procedure in clinical practice owing to differences in both patient fracture types and levels of surgical expertise. Ascribed to the fragile blood supply of the femoral neck, the quality of fracture reduction prior to internal fixation impacts its healing rate [25]. Therefore, this study established finite element models of femoral neck fracture with different reduction qualities and analyzed the mechanical effects of different surgical procedure on internal fixation and bone.

Finite element analysis revealed that when the tip-apex distance was > 10 mm, the stress of the femoral head in Group G was increased by 25.6% compared to Group A, and similarly, the stress during internal fixation was increased by 31.0%. In addition, when FNS was inserted in the middle and lower 1/3 of the cortical corridor of the femoral neck, the stress of the femoral head in Group D was increased by 41.4% compared with Group A; likewise, the stress of internal fixation was also increased by 11.5%. Notably, the stress nephogram illustrated that the stress of the femoral head was primarily concentrated in the femoral head and the fractured edge, whereas the stress during internal fixation was chiefly concentrated in the area between the bolt and the anti-rotation screw. It insinuates that an excessively large tip-apex distance or inadequate placement of the FNS outside the center of the femoral neck cortical corridor could result in excessive stress on the femoral head, thus affecting the blood supply of the femoral head [26, 27]. This increased the probability of postoperative complications such as femoral head necrosis and nonunion of fractures, as well as the stress of internal fixation, thereby elevating the risk of internal fixation fracture [28, 29].

It is worthwhile emphasizing that the reduction quality of femoral neck fracture also considerably impacts fracture healing and the incidence of postoperative complications [30]. The study conducted by Gotfried et al. [8] illustrated that both anatomical reduction and positive reduction can achieve favorable clinical results and are superior to the negative reduction in preventing postoperative complications such as shortening of the femoral neck and necrosis of the femoral head [9]. Notably, the sliding compression of the femoral head would be impacted during positive reduction; consequently, the medial cortex of the distal end of the fracture spans over the medial support bridge of the neck of the femur under the influence of the patient’s own weight [31]. This specific structure can play a decisive role in stabilizing the fracture end, thus promoting fracture healing. Furthermore, finite element analysis displayed that during negative reduction, the stress of internal fixation and femoral head in Group C was increased by 64.0% and 416.0% compared to Group A during anatomical reduction, respectively. On the contrary, during positive reduction, the stress of internal fixation and femoral head in Group B were increased by merely 0.01% and 0.09% compared to Group A during anatomical reduction. On the one hand, our results demonstrated that positive and anatomical reductions exerted similar biomechanical effects. For the surgeon, our study further confirms the mechanical stability and clinical reliability of positive repositioning, providing more clinical options at the time of surgery. On the other hand, negative reduction further increased the stress on the femoral head and internal fixation, leading to poor stability of internal fixation and increasing the incidence of complications such as shortening of the femoral neck or necrosis of the femoral head [16, 25].

Additionally, earlier studies reported that some patients under 65-years-old with neck femur fractures may experience osteoporosis or low bone density in the neck of the femur. Bartels et al. concluded that these patients have a high probability of femoral head necrosis and neck or femur shortening, which would lead to poor hip joint function [32, 33]. Therefore, the biomechanical effect of these patients was explored using FNS for fixation of the neck of femur fractures through finite element analysis. Our results demonstrated that the stress of internal fixation and the femoral head was increased by 41.1% and 41.2% in the osteoporotic neck of the femur fracture model compared with the healthy bone model, respectively. From the view of biomechanics, excessive displacement and stress may increase the likelihood of neck or femur shortening in osteoporotic patients [34].

Therefore, our study concludes that in the clinical management of femoral neck fractures, the FNS benefits the patient with positive reduction, whereas negative reduction is associated with inferior loading. Thus, when anatomical reduction is not feasible, we advocate for the optimal positioning of the FNS to be as follows: a positive reduction, with the screw centered within the cortical corridor of the femoral neck and maintaining tip-apex distance of less than 10 mm.

There were still some limitations in this study that merit acknowledgment. To begin with, both femur and internal fixation require to employ uniform and continuous materials, which differs from the actual situation in the clinical setting. No generalized model was used but a patient specific, also only one type of condition was tested, which might not be the worst case scenario. Secondly, the fractured end was also need to be smooth and continuous, which was distinct from the actual fractured end. However, these were reasonably acceptable for a preliminary biomechanical study. In the future, we hope to establish more realistic biomechanical models to further validate our experimental results.

Conclusion

In conclusion, we found that positive reduction had similar mechanical results to anatomical reduction in cases where the tip-apex distance was > 10 mm, the bolt was located in the lower third of the cortical corridor of the femoral neck, and in cases of osteoporosis. Therefore, if anatomical reduction is clinically difficult to achieve, we recommend positive reduction instead. This might provide a new option for trauma surgeons to deal with unstable femoral neck fractures.

Authors’ contributions

X. Z. , and Y. Z. drafted the manuscript. X. Z., X. Q. and S. H. collected and analysed the data. Y. W. and Z. Z. made the study design. Y. L. and W. L. revised and supervised the manuscript. X. Z. andY. Z. were co-first authors. All authors reviewed the manuscript.

Funding

This work was supported by the Xuzhou Special fund for promoting scientific and technological innovation (Grant No.KC22202).

Data availability

The datasets used and analysed during the current study available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval for this study was obtained from the Medical Human Experimental Ethics Committee of Xuzhou Medical University, and the patient signed a written informed consent before recruitment. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.