1 Introduction

Femoral neck fracture is a relatively common subtype of hip fracture. Global epidemiological projections suggest that the number of people worldwide with hip fractures will reach 6.26 million by 2050, approximately half of these being femoral neck fractures.¹ Femoral neck fractures can occur at any age, but they are particularly common in elderly people. While femoral neck fractures in elderly people are often caused by low-energy trauma, younger patients generally incur these injuries through high-energy mechanisms. Patients undergoing conservative treatment are required to have prolonged bed rest, which can lead to multiple complications. Therefore, surgical treatment is commonly prioritized in clinical practice. When selecting treatment protocols, it is common for elderly patients to have characteristics such as osteoporosis and low fracture healing rates, which frequently lead to the adoption of hip arthroplasty.^2,3. In contrast, treatment strategies for young patients tend to prioritize preserving their native hip joint function. The preference for internal fixation stems from the good fracture healing capacity and longer life expectancy, as well as the service life of joint prostheses ^4,5. Notably, Pauwels type Ⅲ femoral neck fractures are characterized by a vertically oriented fracture line (angle>50°), which results in high shear forces that challenge stability and healing. The fracture type is most common in young and middle-aged patients. Complications including nonunion, avascular necrosis, and fixation failure are common, driving ongoing debate about the optimal fixation construct.⁶ Therefore, selecting an appropriate internal fixation is of significant clinical importance in improving treatment outcomes and the prognosis.

In recent years, research in this field has focused on novel internal fixators, finite element analysis, and biomechanical studies.^7,8. A growing number of researchers have used finite element and biomechanical methods to compare the mechanical stability of various internal implants. The biplane double-supported screw fixation (BDSF), with its "F"-shaped configuration, was first proposed by Filipov in 2011⁹ and improved in 2015.¹⁰ One distal screw of BDSF is inserted from the anteroinferior to the posterosuperior direction to support the posterior cortex of the femoral neck, while the other two screws fix the tension and pressure sides. A recent study showed that the total displacement of BDSF is higher than that of FNS, indicating that the biomechanical performance of BDSF is slightly inferior to FNS.¹¹ The four cannulated screws (FCS) comprise an inverted triangular arrangement of cannulated compression screws (CCS), combined with a lag screw inserted at the greater trochanter and oriented as perpendicularly as possible to the fracture line. In 2014, Gumustas et al.¹² conducted the first mechanical experiment to compare the use of FCS and CCS. The study showed that the biomechanical performance of FCS was superior to that of CCS. In 2023, Yu et al.¹³ performed a retrospective clinical study to compare the efficacy of FNS and FCS. Results demonstrated that the FNS group experienced fewer complications and achieved better recovery of hip joint function, indicating superior clinical efficacy of FNS versus FCS. In 2015, Tang X's team developed a novel type of internal fixation interlocking hip screw (IHS). The implant consists of a plate and dynamically locked screws. Comparisons of the results of mechanical tests and clinical efficacy between IHS and CCS showed that IHS has better biomechanical performance.¹⁴ In 2018, Synthes developed the femoral neck system (FNS), consisting of a sliding screw, a plate, and an anti-rotation screw. FNS combines angular stability with minimally invasive techniques. Studies indicate FNS offers enhanced biomechanical stability and clinical performance for unstable femoral neck fractures ^15,16. However, cases of FNS fixation failure in femoral neck fractures exist in clinical practice.¹⁷

Although previous studies have compared some of these implants, a comprehensive biomechanical comparison of IHS, FNS, BDSF, and FCS is lacking. This study aims to evaluate their relative biomechanical performance in stabilizing Pauwels type Ⅲ femoral neck fractures, providing evidence to inform clinical decision-making.

2 Materials and methods

2.1

2.1 Construction of the femoral and fracture model

This study has obtained informed consent from the patient and received approval from the ethics committee. A healthy male volunteer aged 35 years was used to obtain the CT data (height: 170 cm, weight: 70 kg). Digital Imaging and Communications in Medicine (DICOM) files were imported into Mimics 21.0 (Materialise, Belgium) to generate a three-dimensional model of the femur, which was saved in stereolithography (STL) format. The model was subsequently processed in Geomagic Studio 2021 (Geomagic, USA) to smooth, refine, and reconstruct the surface. The cortical and cancellous bone components were created and assembled in SolidWorks 2023 (Dassault Systèmes, France) via Boolean operations.

To simulate a Pauwels type Ⅲ femoral neck fracture, we created a plane through the femoral neck center at a 70° horizontal angle, performed by a virtual osteotomy in SolidWorks (Fig. 1).

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Fig. 1 Pauwels type Ⅲ femoral neck fracture. A: Intact femur; B: Proximal femur.

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2.2

2.2 Establishment of internal fixation models

Four internal fixation models were constructed in SolidWorks (Fig. 2).

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Fig. 2 Geometric models of four internal fixations. A: IHS; B: FNS; C: FCS; D: BDSF.

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2.2.1

2.2.1 IHS model

Comprised a lag screw (11.0 mm) and a compression screw (7.0 mm), which form a combined interlocking unit, attached to a two-hole 127° side plate. There are two locking screws (5.0 mm) distally.

2.2.2

2.2.2 FNS model

Included a sliding screw-type power rod, a 6.4 mm anti-rotation screw angled at 7.5° to the rod, and a one-hole locking plate with a 130° plate angle. There is a 5.0 mm locking screw distally.

2.2.3

2.2.3 BDSF model

Consisted of three 7.3 mm partially threaded cannulated screws arranged in an "F" configuration according to Filipov's improved method.¹⁰

2.2.4

2.2.4 FCS model

Incorporated four 7.3 mm partially threaded cannulated screws; three were placed in an inverted triangle parallel to the femur neck axis, and the fourth was inserted transversely from the greater trochanter, perpendicular to the fracture line. Finally, all models were meshed in Ansys Workbench 2023 (ANSYS, USA), and mesh quality was verified.

2.4

2.4 Contact parameters, boundary constraints, and loading

Fracture surfaces were assigned a friction coefficient of 0.46. Implant threads were bound to the bone, while non-threaded regions had non-separation contacts. The distal femur was fully constrained.¹⁹ The femoral model was positioned at 10° adduction and 9° extension to simulate single-leg stance.²⁰ Static vertical loads of 700 N, 1400 N, and 2100 N (equivalent to one to three times body weight) were applied to the center of the femur head (Fig. 3).

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Fig. 3 Distal femur constrained with axial load applied to the femoral head center.

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3 Results

3.1

3.1 Femoral displacement

Femoral displacement decreased gradually from the load application area at the femur head toward the distal fixation. Peak displacement occurred at the femoral head center (Fig. 4). Under all loading conditions, the IHS group exhibited the smallest peak femoral displacement (Fig. 5).

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Fig. 4 Femoral displacement cloud diagram under 2100 N load: A: IHS group; B: FNS group; C: BDSF group; D: FCS group.

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Fig. 5 Peak femoral displacement diagram of four internal fixations under 700 N, 1400 N, and 2100 N loads.

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3.2

3.2 Femoral Von Mises stress

The Von Mises stress distribution in the femur was similar across all groups, concentrated medially at the femoral neck (Adams' arch) and the proximal shaft. Peak stress was localized to the medial aspect of the fracture site (Fig. 6). Peak femoral stress values were comparable among the four groups (Fig. 7).

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Fig. 6 Femoral stress cloud diagram under 2100 N load: A: IHS group; B: FNS group; C: BDSF group; D: FCS group.

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Fig. 7 Peak femoral stress diagram of four internal fixations under 700 N, 1400 N, and 2100 N loads.

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3.3

3.3 Implant displacement

Implant displacement decreased from the anterior to the posterior end, with maximum values observed at the device tips (Fig. 8). The peak displacement of the implants increased progressively with the applied load. The IHS group demonstrated the lowest peak implant displacement across all load levels, followed by FCS, FNS, and BDSF (Fig. 9).

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Fig. 8 Internal fixation displacement cloud diagram under 2100 N load: A: IHS group; B: FNS group; C: BDSF group; D: FCS group.

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Fig. 9 Peak displacement diagram of internal fixations in four groups under 700 N, 1400 N, and 2100 N loads.

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3.4

3.4 Implant von mises stress

Implant stress was concentrated near the fracture line (Fig. 10). Stress distribution patterns were similar between IHS and FNS, as well as between BDSF and FCS. The IHS and FNS groups had significantly higher peak stress values than the BDSF and FCS groups (Fig. 11).

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Fig. 10 Internal fixation stress cloud diagram under 2100 N load: A: IHS group; B: FNS group; C: BDSF group; D: FCS group.

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Fig. 11 Peak stress diagram of internal fixations in four groups under 700 N, 1400 N, and 2100 N loads.

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4 Discussion

Pauwels type Ⅲ femoral neck fractures are characterized by significant shear and rotational forces at the fracture site. These forces can easily cause the proximal femur to swing and rotate, thereby increasing the risk of postoperative complications.^21,22. Clinically, multiple internal fixation methods are available for femoral neck fractures, with the core objectives being to achieve anatomical reduction and provide strong fixation support.²³

In BDSF, the three screws increase the insertion angles, which is beneficial for resisting shear force and rotational force while enhancing the resistance to varus deformity.²⁴ A cadaveric mechanical study demonstrated that BDSF fixation provides superior biomechanical stability, suggesting its potential as a preferred implant for unstable femoral neck fractures.¹⁰ However, studies have suggested that the non-parallel screws in BDSF may reduce the sliding compression effect of CCS.⁹ Additionally, the BDSF screw insertion method is relatively complex and requires a high level of clinical technical proficiency. This is because achieving optimal treatment outcomes requires precise needle entry points and insertion trajectories. Some researchers argue that the current advantages of BDSF are primarily supported by biomechanical studies and require extensive clinical validation. In FCS, the four screws are arranged in a non-parallel pattern. Three screws are distributed in an inverted triangular pattern, while the fourth screw is oriented transversely to the fracture line. This configuration stabilizes and compresses the fracture site while enhancing frictional force at the fracture ends, thereby efficiently resisting shear forces. As a non-sliding implant, the FCS demonstrates superior biomechanical performance compared to CCS, with patients exhibiting better hip joint function recovery post-surgery.^12,25 However, current biomechanical and clinical research on FCS remains limited, and its stability needs further verification. In FNS, the anti-rotation screw and bolt enable up to 20 mm of sliding compression to mitigate femoral neck collapse. The sliding compression plate induces less soft tissue trauma compared to the dynamic hip screw (DHS).²⁶ The angle formed by the anti-rotation screw with the bolt is 7.5°, which enhances angular stability, but the small angular offset results in suboptimal rotational resistance. Relevant studies have shown that FNS fixation for Pauwels type Ⅲ femoral neck fractures can decrease the incidence of postoperative complications and exhibit good clinical outcomes.^27,28. In IHS, inserting the compression screw generates significant linear compression at the fracture site. This compression is direct, continuous, and controllable, and provides an effective environment for fracture healing. The steel plate has a smaller curvature radius than the femoral shaft. The two lateral edges of the steel plate make contact with the bone, forming a double-line contact. This design minimizes the contact area to prevent compression of the periosteum and damage to the local blood supply. Locking screws are used to secure the combined interlocking screws, which prevents them from sliding and enables the internal fixation system to form a unified mechanical unit for biomechanical stabilization. Compared with CCS, IHS offers distinct advantages, including superior biomechanical stability, operational simplicity, shorter operative time, reduced intraoperative fluoroscopy frequency, and lower postoperative complication rates.¹⁴ However, as newly developed implants, both IHS and FNS have limitations in biomechanical research and clinical data. Their long-term efficacy requires further follow-up studies.

Most previous studies have compared different implants with CCS to analyze their biomechanical stability. In this study, we use the finite element method to present the first comparative models of Pauwels type Ⅲ femoral neck fractures with IHS, FNS, BDSF, and FCS fixation, analyzing their biomechanical performance. In the study, the IHS showed the lowest displacements for both the femur and the implant. This finding suggests that the IHS is capable of reducing motion at the fracture site, which in turn creates a stable environment to support healing. The design of the IHS includes two key features: an interlocking screw mechanism used in combination with a low-profile plate that makes dual-line contact with the bone.¹⁴ This design may not only explain why the IHS performs better mechanically but also help preserve blood supply to the area.^29,30. It is worth noting that both the IHS and FNS groups had higher implant stress levels than the BDSF and FCS groups. Even so, these stress values stayed within the acceptable range for titanium alloy—and importantly, the IHS group had lower peak stress than the FNS group. These results match what earlier studies have found, which support using angle-stable devices to treat unstable femoral neck fractures.^11,31 That said, we need to consider the clinical significance of higher implant stress: this issue could be linked to fatigue failure of the implant over the long term. On the other hand, while the BDSF and FCS designs produced less stress on the implant, they led to more displacement at the fracture site. This greater displacement might end up interfering with the healing process.

The study has certain limitations. First, the mechanical tests did not incorporate the soft tissues that are crucial for stabilizing the joint, such as the surrounding musculature, joint capsule, and ligaments. This is also a common limitation across many biomechanical experiments. Second, it was not possible to fully simulate the complex stress conditions of the hip joint in patients after surgery. This made it hard to assess biomechanical performance accurately in dynamic scenarios. Third, the outcomes of this finite element analysis still need to be confirmed through mechanical studies on fresh frozen cadavers, as well as further clinical trials.

5 Conclusions

The study reveals that the IHS offers superior biomechanical stability for Pauwels type Ⅲ femoral neck fractures when compared to FNS, BDSF, and FCS. Specifically, it leads to less displacement of both the femur and the implant, along with a more favorable stress distribution. These findings indicate that the IHS could be a promising choice for treating these difficult-to-manage fractures. However, additional clinical studies are still required to verify its actual effectiveness.

Ethics approval and consent to participate

This study was approved by the Ethics Committee of the First Hospital of Dalian Medical University (PJ-KS-KY-2024-472). This study was conducted in accordance with the Declaration of Helsinki. The patient consented to participate in this study, and informed consents were signed by him in all instances.

Patients consent

All patients involved in the study have been fully informed of the purpose, procedures, potential risks, and benefits of the research. Written informed consent was obtained from each patient prior to the collection of any relevant clinical data or imaging materials used in this study. All patients voluntarily participated in the study and had the right to withdraw from the study at any time without any adverse impact on their clinical treatment. The personal privacy and medical information of all patients have been strictly protected, and no personal identifiable information is disclosed in this manuscript.

Authors’ contributions

CXH: Manuscript writing; Experimental procedures; Use of ANSYS workbench program.

ZCM: Literature review and manuscript writing; Evaluation of experimental results.

TX: Evaluation of experimental results and statistical analysis.

FGY: Experimental procedures; Data availability; Manuscript revision and submission.

Availability of data and materials

The data for this study may be obtained by contacting the corresponding author.

Funding

Not applicable.