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Biomechanical properties of off-axis screw in the treatment of vertical femoral neck fractures: a finite element analysis

BMC Musculoskeletal Disorders volume 26, Article number: 479 (2025) Cite this article

Abstract

Purpose

This study aims to assess the biomechanical performance of the 7.3 mm fully threaded off-axis screw (7.3-FTOS) configuration in stabilizing vertical femoral neck fractures (FNSs), comparing its effectiveness with other fixation methods.

Methods

A vertical FNF model with an 80° Pauwels angle was simulated and stabilized using four internal fixation constructs: Three inverted cannulated compression screws (ICCS), 7.3 mm partially threaded off-axis screw (7.3-PTOS), 4.5 mm fully threaded off-axis screw (4.5-FTOS), and 7.3-FTOS. The fracture fixation models were developed, refined, and optimized using Geomagic for 3D reconstruction and surface smoothing, and SolidWorks for parametric modeling. Comparative finite element analysis was then conducted using Ansys to assess the biomechanical behavior of each fixation method.

Results

The maximum femoral displacements for ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS were 10.40 mm, 8.08 mm, 7.71 mm, and 7.15 mm, respectively. The peak internal fixation displacements were 9.76 mm, 7.55 mm, 7.25 mm, and 6.78 mm, respectively. The maximum gap displacement measured 2.73 mm, 1.19 mm, 0.94 mm, and 0.55 mm, respectively. The maximum displacement along the Z-axis, representing the shear force direction, was recorded as 2.93 mm, 2.69 mm, 2.66 mm, and 2.63 mm, respectively. The peak von Mises stress (VMS) values in the distal femur were 218.04 MPa, 121.99 MPa, 123.42 MPa, and 113.86 MPa, respectively. The peak VMS values of the internal fixation constructs were 10.40 mm, 8.08 mm, 7.71 mm, and 7.15 mm, respectively.

Conclusions

Finite element analysis results revealed that 7.3-FTOS configuration exhibited superior resistance to shear and rotational forces in vertical FNFs compared to ICCS, 7.3-PTOS, and 4.5-FTOS, demonstrating enhanced mechanical performance. These findings establish a foundation for advancing experimental investigations and potential future clinical translation.

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Introduction

Femoral neck fractures (FNFs) are common injuries in orthopedic practice, with vertical FNFs being the most unstable type [1]. According to the Pauwels classification, vertical FNFs are categorized as Pauwels Type III, characterized by huge shearing, bending, and tensile deformation force around the fracture site. To preserve the femoral head, achieving anatomic reduction with stable internal fixation is the preferred treatment approach [2]. However, the steep inclination of vertical FNFs generates substantial shear and rotational forces at the fracture site, making stable fixation particularly challenging. Prognoses for internal fixation are often poor, with failure rates reported as high as 41.9% and avascular necrosis rates ranging from 16–21% [3,4,5]. Developing an optimal fixation strategy for vertical FNFs is crucial to reducing postoperative complications, yet it remains a topic of ongoing debate in orthopedic practice.

The most commonly used implants to stabilize FNFs are fixed angle devices and percutaneous cannulated cancellous screws [6]. A recent meta-analysis revealed that cannulated screws have similar mortality, revision, and nonunion rates as DHSs and are superior with respect to avascular necrosis [7]. In addition, cannulated screw fixation remains a clinically favored option due to its reduced surgical dissection, enhanced rotational stability, and preservation of bone stock—features that align with modern minimally invasive orthopedic principles.

To improve fixation stability in vertical FNFs, the off-axis screw technique was developed by Garden [8]. This method involves placing a third screw perpendicular to the fracture plane to neutralize shear forces, supplemented by two parallel screws positioned along the femoral neck axis. Since then, the off-axis screw technique has been widely studied for its biomechanical and clinical effects, but its effectiveness and reliability in practice remain debated. The transverse off-axis screw fixation for FNFs can be categorized into two distinct configurations based on screw size and threading: 4.5 mm fully threaded off-axis screws (4.5-FTOS), which are anchored in the calcar cortex, and 6.5 mm partially threaded off-axis screws (6.5-PTOS), which are positioned in the inferior-medial quadrant of the femoral head [9,10,11,12]. Recent biomechanical research using synthetic femurs found that 4.5-FTOS configuration exhibited significantly higher axial stiffness and ultimate failure load than 6.5-PTOS configuration [10]. However, 4.5-FTOS fixation is not appropriate for all vertical FNFs, such as subcapital fractures or transcervical fractures with medial cortex comminution. Meanwhile, recent studies have revealed that medial cortex comminution is present in 83.9–94% of vertical FNF cases [13, 14]. Consequently, 4.5-FTOS fixation may not be broadly recommended for clinical practice.

In recent years, the headless cannulated compression screw has emerged as a new option for treating FNFs. Its fully threaded design not only enhances static stability but also reduces pull-out forces [15]. By integrating the advantages of headless cannulated compression screws with off-axis fixation, we developed a novel technique utilizing 7.3 mm fully threaded off-axis screws (7.3-FTOS) positioned in the inferior-medial quadrant of the femoral head. Nevertheless, it remains uncertain whether the biomechanical properties of 7.3-FTOS configuration surpasses the 4.5-FTOS alternative. Consequently, this study aims to evaluate the biomechanical performance of the 7.3-FTOS configuration in stabilizing vertical FNFs by comparing it with ICCS, 4.5-FTOS, and 7.3-PTOS fixation methods.

Materials and methods

Building a geometric model

A 35-year-old healthy male volunteer, without any history of hip or systemic disease, was recruited for this study. Imaging was conducted using a Toshiba Aquilion 64-row spiral CT scanner, with a layer thickness of 1 mm for the femur. The acquired CT images were stored in DICOM format and subsequently processed using the medical three-dimensional reconstruction software Mimics 16.0. A three-dimensional model of the femur was generated based on tissue gray values and region segmentation, which was then exported as an STL file. This model underwent further refinement within Geomagic Studio 11, where smoothing, meshing, and surface fitting operations were performed before integration into SolidWorks 2014 (Dassault, France). The cortical and cancellous bone structures were developed through Boolean operations, resulting in a comprehensive assembly of the femoral bone model.

Building the vertical FNF model

SolidWorks software was utilized to simulate a vertical FNF. Initially, the axis of the femoral shaft was established, serving as the reference for the creation of a sagittal plane. Subsequently, a cutting plate was designed to intersect the center of the femoral neck at an angle of 10° relative to the sagittal plane of the shaft axis. The femoral neck was then sectioned by this cutting plane, thereby simulating a vertical FNF with a Pauwels angle of 80° (Supplementary Fig. S1).

Building the internal fixation model

Building the cannulated screws model

Cannulated screws with diameters of 7.3 mm and 4.5 mm were designed using SolidWorks software, based on the screw data from Synthes. Given that the primary focus of this study was the thread, the details of the thread were meticulously replicated. The threaded portion of the partially threaded cannulated screw was 16 mm. The total lengths of the screws were as follows: 105 mm for the 7.3 mm partially threaded screw, 90 mm for the 7.3 mm fully threaded screw, and 75 mm for the 4.5 mm fully threaded screw.

Building the four internal fixation models

The traditional ICCS model was constructed based on the surgical technique described in the literature [9]. Using SolidWorks software, the screws were arranged in a parallel inverted triangular configuration, with each screw oriented at a 135° angle relative to the longitudinal axis of the femur. The inferior screw was positioned along the inferior femoral neck within the calcar region, starting just above the lesser trochanter. The two cephalad screws were inserted at a more superior level, adjacent to either the anterior or posterior cortices of the femoral neck, ensuring a 5 mm clearance from the subchondral bone in the femoral head.

The 7.3-PTOS model was constructed using SolidWorks software, incorporating two parallel 7.3 mm partially threaded cannulated screws along with a 7.3 mm partially threaded off-axis screw. The two parallel screws were positioned across the superior and inferior portions of the femoral neck, both inserted at a 135° angle relative to the femoral shaft, with entry points slightly above the lesser trochanter on the lateral femur. The third 7.3 mm partially threaded cannulated screw was introduced from the lateral aspect of the greater trochanter, directed perpendicular to the fracture line toward the inferior portion of the femoral head to enhance stability.

The 4.5-FTOS model was constructed using SolidWorks software, incorporating two parallel 7.3 mm partially threaded cannulated screws along with a 4.5 mm fully threaded off-axis calcar screw. The two parallel screws were positioned across the superior and inferior portions of the femoral neck. To enhance stability, a 4.5 mm fully threaded screw was inserted perpendicularly to the fracture site, anchoring into the calcar region. This configuration aimed to improve resistance against shear forces and enhance overall biomechanical stability.

The 7.3-FTOS model was constructed using SolidWorks software, incorporating two parallel 7.3 mm partially threaded cannulated screws along with a 7.3 mm fully threaded off-axis trochanteric screw. In this model, the partially threaded off-axis cannulated screw from the 7.3-PTOS configuration was replaced with a 7.3-mm fully threaded screw. This configuration aimed to improve resistance against shear forces and enhance overall biomechanical stability.

Specific models are illustrated in Fig. 1. Subsequently, these models were imported into ANSYS software (ANSYS Inc., USA) for meshing and mechanical analysis, enabling a detailed evaluation of their biomechanical performance.

Fig. 1
figure 1

Geometric modeling of four internal fixation of femoral neck fracture. A1, A2: ICCS model. B1, B2: 7.3-PTOS model. C1, C2: 4.5-FTOS model. D1, D2: 7.3- FTOS model. ICCS: the traditional inverted triangular cannulated screw model. 7.3-PTOS: two cancellous screws and a 7.3 mm partially threaded transverse screw inserted into the inferior head model. 4.5-FTOS: two cancellous screws along with a 4.5 mm fully threaded transverse screw placed into the calcar model. 7.3-FTOS: two cancellous screws and a 7.3 mm fully threaded transverse screw placed into the inferior head model

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Conditional assumptions and material parameter settings

The fracture surface was assumed to be fully fractured and anatomically reduced, ensuring complete closure of the fracture gap. Following the screw-bone interface sensitivity analysis, the threaded regions of the screws were bonded to the bone using tie constraints, while frictional contact was applied to interfaces between the bone and the partially-threaded screw bodies, with a coefficient of friction set at 0.3 [16]. The friction coefficient for bone-bone interaction was established at 0.46 [16]. All materials within the model were assumed to be homogeneous, isotropic linear elastic materials. The elastic modulus and Poisson’s ratio for the various structural materials are presented in Table 1 [17].

Table 1 Bone and internal fixation material properties
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Boundary conditions and loading force settings

All nodes on the surface of the distal femur were constrained with zero degrees of freedom to prevent rigid body motion during the analysis (Supplementary Fig. S2A). This study simulated the forces applied to the hip during the stance phase of walking. The finite element models were subjected to a load of 2100 N, corresponding to 300% of body weight (Supplementary Fig. S2B). The force vector was applied laterally at a 13° angle relative to the femoral shaft axis in the coronal plane and posteriorly at an 8° angle in the sagittal plane, with the force directed at the center of the femoral head [16].

Evaluation criteria

Mesh convergence was assessed by evaluating von Mises stress in the model, with a convergence criterion of < 5% variation in stress results [18]. After the mesh sensitivity analysis, a 1 mm mesh size was chosen for all models. The finite element model consists of a 3D tetrahedron mesh. The number of elements and nodes of the models was listed in Table 2. Initially, the von Mises stress distributions and peak stresses in the femur and four internal fixations were analyzed. Subsequently, the maximum gap displacement on the fracture surfaces and the Z-axis displacements of the four models were measured. In this study, the Z-axis was defined by projecting the load direction onto the fracture plane, thereby representing the direction of the shear force.

Table 2 The statistics of four assembly elements and the total amount of nodes
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Results

Displacement changes

The maximum femoral displacements were recorded as 10.40 mm, 8.08 mm, 7.71 mm, and 7.15 mm for the ICCS, 7.3-PTOS, 4.5-FTOS, and 7.3-FTOS models, respectively (Fig. 2). Similarly, the maximum implant displacements measured 9.76 mm, 7.55 mm, 7.25 mm, and 6.78 mm for these models, respectively (Fig. 3). Fracture gap movement also varied significantly across models, with the greatest displacement observed in ICCS (2.73 mm), followed by 7.3-PTOS (1.19 mm), 4.5-FTOS (0.94 mm), and finally 7.3-FTOS (0.55 mm) (Fig. 2). Notably, the 7.3-FTOS construct exhibited the least fracture gap movement, underscoring its superior ability to resist varus displacement of the femoral head.

Fig. 2
figure 2

The displacement of the femur and gap displacement in four models. The maximum femoral displacements in ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS models were 10.40 mm, 8.08 mm, 7.71 mm, and 7.15 mm, respectively. The maximum gap displacement measured 2.73 mm, 1.19 mm, 0.94 mm, and 0.55 mm, respectively

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Fig. 3
figure 3

The displacement of the implants in four models. The maximum implant displacements for ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS models were recorded as 9.76 mm, 7.55 mm, 7.25 mm, and 6.78 mm, respectively

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The maximum Z-axis displacements, representing shear force along the load-projected fracture plane, for the ICCS, 7.3-PTOS, 4.5-FTOS, and 7.3-FTOS models were 2.93 mm, 2.69 mm, 2.66 mm, and 2.63 mm, respectively (Fig. 4). Notably, the 7.3-FTOS model exhibited the least displacement, indicating its superior resistance to shear forces compared to the other three models.

Fig. 4
figure 4

The displacement of Z axis, representing the shear force direction, in four models. The maximum Z axis displacements for ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS models were 2.93 mm, 2.69 mm, 2.66 mm, and 2.63 mm, respectively

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The von mises peak stress distribution

Differences in stress distribution were observed among the implants and femurs across the four models. The peak von Mises stresses at the distal femur were 218.04 MPa, 121.99 MPa, 123.42 MPa, and 113.86 MPa for the ICCS, 7.3-PTOS, 4.5-FTOS, and 7.3-FTOS models, respectively (Fig. 5). Higher femoral stress is associated with an increased risk of screw cut-out. Similarly, the peak von Mises stresses recorded for the implants were 397.55 MPa, 202.35 MPa, 216.61 MPa, and 164.69 MPa for the ICCS, 7.3-PTOS, 4.5-FTOS, and 7.3-FTOS models, respectively (Fig. 6).

Fig. 5
figure 5

The stress of the distal femur in four models. The peak von Mises stresses at the distal femurs were 218.04 MPa, 121.99 MPa, 123.42 MPa, and 113.86 MPa for ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS models, respectively

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Fig. 6
figure 6

The stress of the implants in four models. The peak von Mises stresses for the implants were recorded as 397.55 MPa, 202.35 MPa, 216.61 MPa, and 164.69 MPa for ICCS, 7.3-PTOS, 4.5-FTOS and 7.3-FTOS models, respectively

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Detailed results are presented in Table 3.

Table 3 Parameters results
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Discussion

In this study, we innovatively proposed and evaluated the biomechanical stability of the 7.3-FTOS configuration for treating vertically oriented FNFs, comparing it with three other configurations. The findings revealed that the 7.3-FTOS fixation significantly improved resistance to varus displacement and shear forces at the femoral head compared to the ICCS, 7.3-PTOS, and 4.5-FTOS fixations. Notably, ICCS fixation demonstrated the weakest biomechanical performance and is therefore not recommended for managing vertical FNFs.

When FNFs occur in younger individuals, they often present with a more vertical fracture line, characterized by a high Pauwels angle. This vertical alignment increases shear forces at the fracture site, leading to potential complications such as hip varus, collapse, nonunion, avascular necrosis of the femoral head, or failure of internal fixation [19, 20]. As a result, the management of FNFs in young adults poses significant challenges in orthopedic trauma surgery [21]. Despite advancements in fixation techniques, such as dynamic hip screws (DHS), femoral neck systems (FNS), configurations of three or four cannulated screws, and medial buttress plates, determining the optimal fixation approach for young patients with FNFs remains a critical and unresolved issue in orthopedic practice [9, 22, 23]. Cannulated screw fixation offers advantages, including smaller surgical incisions, enhanced rotational stability, and minimal bone destruction, making it a preferred option for treating FNFs [19]. However, controversy persists regarding the ideal fixation configuration and choice of screws for vertical FNFs.

Many biomechanical studies present evidence that off-axis screw technique can provide improved stiffness, compression strength, and decreased interfragmentary motion compared to the classic parallel-screw technique [11, 24]. A biomechanical study reported a 70% increase in stiffness and 43% increase in force required for displacement with off-axis screws compared with traditional parallel screws [25]. Consistent with prior research, our biomechanical analysis demonstrates that off-axis screw configurations provide superior resistance to both varus displacement and shear forces compared to ICCS fixation. Parallel screw configurations, typically inserted at acute angles to the vertical FNF line, generate shear forces that may contribute to secondary displacement-a phenomenon clinically referred to as the “sliding effect”. In contrast, off-axis screw is oriented more perpendicularly or at obtuse angles relative to the fracture line, creating a compressive “dragging force” that counteracts shear stresses [26]. This fundamental mechanical advantage explains why the off-axis screw technique consistently demonstrates superior interfragmentary stability compared to parallel screw configurations.

However, there is still no consensus on the clinical impact of the off-axis screw. A retrospective comparative study by Jiang et al., with at least two years of follow-up, found that off-axis screws were associated with a significantly lower femoral neck shortening rate compared to parallel screws [26]. Another study also suggested that using the off-axis screw technique to treat vertical FNFs significantly improved hip functional recovery and reduced the postoperative femoral neck shortening rate [27]. In contrast, a retrospective clinical study by Hoshino et al. compared off-axis screws with fixed-angle devices and reported a failure rate of 60% in the off-axis screw group versus 21% in the fixed-angle group [28]. According to the findings of the current study, the inconsistent results observed in previous studies may be attributed to the lack of standardization in the application of off-axis screws.

Our study demonstrated that the 4.5-FTOS configuration provided superior biomechanical stability compared to the 7.3-PTOS configuration, consistent with the findings of Kuan et al. In the 7.3-PTOS configuration, a 7.3 mm off-axis screw was advanced unicortically into the inferior femoral head. In contrast, the 4.5-FTOS configuration involved inserting a 4.5 mm off-axis screw from the greater trochanter to the nonarticular portion of the femoral neck, achieving bicortical screw purchase, which resulted in greater stability than the 7.3-PTOS configuration. However, it is important to note that 4.5-FTOS fixation may not be suitable for all vertical FNFs, particularly subcapital and transcervical fractures with medial cortex comminution. Recent studies have reported that comminution is present in 83.9–94% of vertical FNF cases [13, 14]. As a result, the 4.5-FTOS configuration cannot be universally recommended for clinical practice.

Recent studies have demonstrated that the use of fully threaded cannulated screws for vertical FNF fixation provides superior biomechanical stability and significantly reduces complication rates compared to partially threaded cannulated screws [15, 29,30,31]. Consequently, the 7.3-FTOS configuration was introduced for treating vertical FNFs. In our study, a 7.3 mm fully threaded off-axis screw was positioned near the distal cortex, directed from the lateral side to the inferior quadrant of the femoral head. Our results indicate that the 7.3-FTOS configuration offers greater shear resistance and interfragmentary stability than the 4.5-FTOS configuration. Mechanically, the partially threaded design of two parallel screws at the fracture site allows for high pullout strength to secure the fragment, while the fully threaded design of the larger off-axis screw, with enhanced trabecular anchorage, provides more stable support to withstand high shear forces in unstable fractures. The combination of these two screw types maximizes mechanical advantage. Additionally, the off-axis screw is positioned in the inferior part of the femoral neck, an area characterized by denser cancellous bone and closer proximity to the femoral calcar [26]. This placement enhances screw anchoring and cortical support. These characteristics of 7.3-FTOS fixation effectively ensure superior interfragmentary stability compared to parallel screws and the 4.5-FTOS configuration. Micromotion around the fracture site, particularly in the shear direction, disrupts bone callus formation and hinders revascularization across the fracture line [32]. According to Jiang et al., a fixation strategy that enhances interfragmentary stability is associated with improved clinical outcomes [27].

The novelty of this study lies in proposing the 7.3-FTOS configuration as a robust solution for fixing vertical FNFs. However, we acknowledge several limitations in our study. First, only static analysis was performed in this model, while dynamic analysis was not considered. Future research should incorporate various motion forms to provide a more comprehensive assessment. Second, this study did not include statistical analysis, a common limitation in similar simulation-based research. Third, our results were not validated with experiments in this study. Instead, the validity of the finite element model was considered to be acceptable based on the convergence study. Lastly, additional real-world biomechanical testing and large-sample randomized controlled clinical studies are necessary to further validate our findings and address the study’s limitations.

Through our finite element analyses, we established that the 7.3-FTOS configuration effectively reduces varus and shear stresses while maintaining axial compressive stress at the fracture surface. As a result, this configuration promotes an optimal mechanical environment for fracture healing. For vertical FNFs, we recommend the 7.3-FTOS configuration as the preferred choice, followed by the 4.5-FTOS and the 7.3-PTOS configurations. In contrast, the ICCS construct is not recommended for vertical FNFs.

Data availability

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

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Acknowledgements

Not applicable.

Funding

Shenzhen Science and Technology Projects (Grant Number:

JCYJ20220530152214032; JCYJ20220530152213030; JCYJ20190806160014794). Heyuan City Social Development Science and Technology Plan Project (Grant Number: 230613171602486).

Author information

Author notes

  1. Yimiao Lin, Fengting Cui and Zhaofeng Jia contributed equally to this work.

Authors and Affiliations

  1. Department of Trauma Orthopedic, Shenzhen People’s Hospital, The Second Clinical, Medical College of Jinan University and The First Affiliated Hospital of Southern University of Science and Technology, No. 1017 Dongmen North Road, Luohu District, Shenzhen, Guangdong, 518035, China

    Yimiao Lin, Fengting Cui, Zhaofeng Jia, Xinjia Hu & Shiyuan Lin

Authors
  1. Yimiao Lin
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  2. Fengting Cui
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  3. Zhaofeng Jia
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  4. Xinjia Hu
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  5. Shiyuan Lin
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Contributions

SYL and XJH wrote the main manuscript. YML, FTCand ZFJ prepared Figs. 1, 2, 3, 4, 5 and 6. All authors reviewed the manuscript.

Corresponding authors

Correspondence to Xinjia Hu or Shiyuan Lin.

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Ethics approval and consent to participate

We state that informed consent to participate was obtained from all of the participants in the study. We confirm that all experiments were performed in accordance with relevant guidelines and regulations. The study was approved by the Ethics Committee of the Shenzhen People’s Hospital. Moreover, our study adhered to the Declaration of Helsinki.

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Not applicable.

Competing interests

The authors declare no competing interests.

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Lin, Y., Cui, F., Jia, Z. et al. Biomechanical properties of off-axis screw in the treatment of vertical femoral neck fractures: a finite element analysis. BMC Musculoskelet Disord 26, 479 (2025). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12891-025-08722-9

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  • DOI: https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12891-025-08722-9

Keywords

BMC Musculoskeletal Disorders

ISSN: 1471-2474

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