Finite element analysis of six distinct fixation devices for vertically oriented femoral neck fractures

Abstract

Background:

Vertically oriented femoral neck fractures are a challenge for orthopedic surgeons, and the complication rates are also high. Recently, several innovative devices have been proposed, such as the proximal femoral bionic nail, InterTAN, and medial buttress plate combined with cannulated screws, to increase the stability of fixation. However, the differences among these innovative devices need to be addressed.

Objective:

This study aimed to compare the stability of the proximal femoral bionic nail, InterTAN, and medial buttress plate combined with cannulated screws for vertically oriented femoral neck fractures. Additionally, traditional fixation devices—including three parallel cannulated screws, a compression hip screw system, and a proximal femoral nail—were included for comparison, resulting in a total of six distinct devices evaluated in this study.

Methods:

A finite element model of a femoral neck fracture fixed with the six internal fixation devices was created. Furthermore, two different fracture conditions—with and without a 1-mm fracture gap—were considered. The maximum loading during level walking was applied to the model for comparison.

Results:

The results indicated that the InterTAN has the best ability to maintain the gap and prevent collapse. Under the fracture gap condition, the peak displacement of the femoral head was smaller in the innovative devices compared to the traditional ones. Specifically, the peak displacements were 1.98 mm for the medial buttress plate combined with cannulated screws, 2.12 mm for the proximal femoral bionic nail, and 1.16 mm for the InterTAN system. The von Mises stress in the medial buttress plate was also the highest among the devices, with values of 1000 MPa with the gap and 1477 MPa without the gap.

Conclusion:

Based on the present results, the medial buttress plate combined with cannulated screws, proximal femoral bionic nail, and InterTAN are recommended for cases without a fracture gap, while the InterTAN is recommended for cases with a fracture gap to prevent bone shortening.

Keywords

vertically oriented femoral neck fractures proximal femoral bionic nail proximal femoral InterTAN medial buttress plate finite element analysis

Introduction

Femoral neck fractures are common injuries that typically occur in older adults due to falls and in younger individuals following high-energy trauma, such as motor vehicle accidents.¹ High-energy trauma often results in a more vertically oriented fracture pattern, producing greater shear forces at the fracture site.² These shear forces increase implant loading and elevate the risk of construct failure and non-union after fixation. Reported non-union rates for vertically oriented femoral neck fractures range from 10% to 30%.^2–6 Although total hip replacement is an option for restoring hip function in proximal femoral fractures, the range of motion of an artificial hip joint remains more limited than that of the native hip.^7–9 Therefore, in patients under 40 years of age, a femoral head-preserving approach is preferred over joint replacement in order to maintain function and quality of life.¹⁰ In such cases, preserving the integrity of the femoral head remains the primary objective of surgical fixation.

Many conventional fixation devices have been used to treat femoral neck fractures, including three parallel cannulated screws arranged in a triangular configuration, dynamic hip screw systems, proximal femoral nails, angle blade plates, and compression hip screw (dynamic hip screw combined with an additional compression screw).^11–13 In recent years, several innovative devices—such as the proximal femoral bionic nail, and InterTAN nail—have been developed to provide greater stability than traditional devices.^14–18 Additionally, the use of a medial buttress plate combined with cannulated screws has been shown to reduce failure rates in Pauwels type III fractures.^19–21 However, application of a medial buttress plate with cannulated screws is technically demanding due to the complex anatomy and limited operative space. In contrast, the surgical procedures for the bionic nail and InterTAN nail are relatively simpler. Nevertheless, the comparative effectiveness of the medial buttress plate with cannulated screws, the bionic nail, and the InterTAN nail remains controversial.

This study aimed to compare the stability of vertically oriented femoral neck fractures fixed with various internal fixation devices, including multiple cannulated screws, a compression hip screw system, a proximal femoral nail, a proximal femoral bionic nail, an InterTAN nail, and a medial buttress plate combined with cannulated screws. To eliminate variability between samples, a finite element model of the fractured femur with the implants was used in this study.

Methods

Solid model

A three-dimensional solid model of the hip joint was reconstructed based on computed tomography (CT) images obtained from a Caucasian male cadaver, aged 55 years, with a height of 165 cm and a weight of 70 kg. The bony geometries were delineated by extracting the outer contours of both cortical and cancellous bone structures. The cartilage was modeled in SolidWorks (v2020) to occupy the space between the femoral head and the acetabulum. To establish a vertically oriented femoral neck fracture model, a virtual cutting plane was introduced to completely transect the femoral neck. Specifically, this plane was oriented parallel to the sagittal plane and positioned at the femoral neck, close to the intertrochanteric region. To represent a worst-case scenario, the fracture plane was oriented at an angle of 80° relative to the horizontal plane (Figure 1A). Additionally, a 1-mm thick bone slice was removed along the fracture plane to simulate an unstable fracture pattern (Figure 1B). Consequently, two bone conditions—with and without a fracture gap—were considered for analysis.

Figure 1.

Physical models of vertically oriented femoral neck fracture: (A) with a gap, (B) without a gap, and (C) finite element model and loading conditions.

To evaluate and compare the stabilization performance of conventional and newly proposed internal fixation devices for femoral neck fractures, six distinct fixation configurations were investigated (Figure 2): (1) three parallel cannulated screws in an inverted triangular configuration (ITC), (2) a medial buttress plate combined with three parallel cannulated screws (MBPC), (3) a dynamic hip screw with an additional cannulated screw (CHS), (4) a proximal femoral intramedullary nail (PFN), (5) a proximal femoral bionic nail (PFBN), and (6) an InterTAN nail (InterTAN).

Figure 2.

The six distinct implants used in this study: ITC (inverted triangle configuration), CHS (compression hip screw), PFN (proximal femoral nail), PFBN (proximal femoral bionic nail), MBPC (medial buttress plate with cannulated screws), and InterTAN (proximal femoral InterTAN nail).

The dimensions of all implants were established, and variations in screw types and sizes were incorporated (Table 1), using values obtained from previously published literature.^12,22–25 The side plate in the CHS system was a 4-hole design, with a thickness of 5.8 mm and a width of 19 mm. Additionally, the barrel had a length of 38 mm and an outer diameter of 12.5 mm. The 4-hole medial buttress plate (MBPC) measured 50 mm in length and 1.2 mm in thickness. Since one of the holes was located at the fracture line, only three cortical screws were used for fixation. All implant configurations were determined based on clinical practice and relevant literature.

Table 1.

Dimensions of screws and nails used in this study.

Model	Implant	Diameter (mm)	Length (mm)	Thread length(mm)
ITC	Cannulated screw	6.5	100–110	20
MBPC	Cannulated screw	6.5	100–110	20
	Cortical screw	3.5	38	Full thread
CHS	Cannulated screw	6.5	105	60
	Lag screw	8	100	25
	Cortical screw	5	40	Full thread
PFN	Nail	16	255	-
	Lag screw	10	115	27
	Distal cortical screw	5	40	Full thread
PFBN	Nail	16	255	-
	Lag screw	10	115	27
	Distal cortical screw	5	40	Full thread
	Tension screw	5	60	10
InterTAN	Nail	16	255	-
	Lag screw	10	115	27
	Distal cortical screw	5	40	Full thread
	Compression screw	7	105	90

FE model

The solid models of the fractured femur with fixation devices were imported into ANSYS Workbench 2024 for subsequent simulations. Quadratic tetrahedral elements were employed to mesh the complex geometries of the bone and implants. The element edge length for the proximal femur and the fixation devices was set to 1 mm. Additionally, the contact regions between the bone and the screw threads were locally refined using the ‘face sizing’ function in ANSYS Workbench, with an element edge length of 0.5 mm. In contrast, to reduce the total element count and improve computational efficiency, the remaining hip bone and the distal femur, which do not directly contact the implants, were meshed with a coarser element size of 4 mm (Figure 1C). The same meshing strategy was applied to all models. As a result, the total number of nodes ranged from 1,127,876 for the ITC.

The ligaments spanning the hip joint and contributing to its stability were modeled using tension-only spring elements. Each spring was defined by connecting the anatomical origin and insertion points of the respective ligaments. A total of three ligaments were included in the model: the iliofemoral, pubofemoral, and ischiofemoral ligaments. The stiffness of each spring element was assigned based on values reported in the literature²⁶ (Table 2). The interactions between the screws, intramedullary nail, and surrounding bone were modeled as frictional surface-to-surface contact in ANSYS Workbench. Similarly, the bone-to-bone interface at the fracture site was defined as frictional surface-to-surface contact. The contact behavior between the cartilage of the acetabulum and the femoral head was set to frictionless, while the cartilage of the acetabulum and the acetabulum cup was set to bound. The coefficients of friction were set to 0.45 for bone-to-bone contact, 0.3 for metal-to-bone contact, and 0.2 for metal-to-metal contact.^27–29 The contact behavior between the cortical screws and the side plate in the CHS configuration, as well as between the screws and the medial buttress plate, was defined as bonded.

Table 2.

Material properties of bone and titanium, and ligament stiffness used in this study.

	Elastic modulus (mPa)	Poisson's ratio	Yield strength	Tangent modulus (mPa)	Spring Stiffness (N/mm)	Spring number	Reference
Sawbone
Cortical bone	14,800	0.3	-	-	-	-	28
Cancellous bone	640	0.3	-	-	-	-	29
Human bone
Cortical bone	17,500	0.3	-	-	-	-	31
Cancellous bone	600	0.3	-	-	-	-	31
Cartilage	100	0.3	-	-	-	-	30
Titanium	110,000	0.3	900	3000	-	-	27
Ligament
Iliofemoral ligament	-	-	-	-	100	2	24
Pubofemoral ligament	-	-	-	-	100	1	24
Ischiofemoral ligament	-	-	-	-	37	3	24

Material property

Prior to the comparison, a validation process was conducted to confirm the reliability of the numerical model used in this study. During the validation phase, synthetic femurs were used as bone models and compared with experimental results obtained from sawbones; therefore, the material properties of sawbones were applied in the simulation.^30,31 In contrast, during the comparison of the six distinct fixation devices, the material properties of human bone were used. The cortical bone, cancellous bone, and articular cartilage were assumed to be linear elastic, isotropic, and homogeneous, with elastic moduli assigned based on values reported in the literature.^32,33 Material properties representative of young adult bone quality were adopted to reflect clinical fracture conditions. All metallic implants used in this study were assumed to be made of titanium, and a bilinear isotropic hardening model was applied to represent the plasticity behavior of titanium (Table 2).²⁹

Mesh convergence and validation

To verify the stability and convergence of the finite element model, the number of nodes in the ITC model was increased by globally reducing the edge length of the tetrahedral elements. The total number of nodes was increased from 419,073 to 3,274,792. The peak displacement of the femoral head and the total strain energy were used as indicators for mesh convergence assessment. Furthermore, the structural stiffness results for the ITC and CHS were compared with values reported in the literature to validate the numerical model. To maintain consistency with the referenced mechanical tests, the validation model excluded the pelvis and acetabulum. The structural stiffness was defined as the ratio of the applied load to the peak displacement of the femoral head.

Loading and boundary condition

To evaluate the stability of the fractured femur with the implanted fixation devices, the peak loading condition during level walking was applied to the model. A downward joint reaction force of 2800 N was applied to the uppermost surface of the partial hip bone model used in this study. Additionally, the force exerted by the gluteus medius muscle was simulated by applying a force of 625 N medially and 1300 N upward on the superolateral aspect of the greater trochanter (Figure 1C). The force magnitudes were defined according to the literature.³⁴ The degrees of freedom of the nodes on the inferior surface of the distal femur were fully constrained. Furthermore, the degree of freedom on the medial surface of the pubis body in vertical direction was set as free while degrees of freedom in the anterior-posterior and medial-lateral were set to zero.

Outcome Measures

The peak displacement of the femoral head, the deformation of the fracture gap, and the von Mises stress in the implants and cancellous bone were used as indicators to quantify the differences among the various fixation devices. The peak displacement of the femoral head was used as an index to compare the stability of different fixation devices.

Results

Mesh convergence and validation

The differences in the peak displacement of the femoral head and the total strain energy between the models with 1,127,876 and 3,274,792 nodes were 1.48% (increasing from 2.63 to 2.67 mm) and 0.14% (decreasing from 273.24 to 272.86 mJ), respectively (Figures 3A and 3B). The structural stiffness of the present ITC model was 700 N/mm, compared to 531 N/mm (SD 181) reported in the literature (Figure 3C). Additionally, the structural stiffness of the present CHS configuration was 970 N/mm, while the corresponding published value was 1179 N/mm (SD 221).

Figure 3.

Convergence of displacement (A) and strain energy (B), model validation (C), and gap distance under loading (D).

Stability

The MBPC provided the greatest stability, resulting in the smallest displacement of the femoral head in the absence of a fracture gap, whereas the InterTAN exhibited the highest stability when a 1-mm fracture gap was present (Figures 4 and 5). The lowest stability was observed in the PFN without a gap and in the CHS with a gap. Specifically, the peak displacement of the femoral head was 0.91 mm for the MBPC without a gap and 1.16 mm for the InterTAN with a gap. The InterTAN also demonstrated superior capability in maintaining the gap distance under loading conditions, with an increase of 0.28 mm at the superior edge of the fracture gap and a decrease of 0.32 mm at the inferior edge (Figure 3D). Although the CHS and PFBN effectively maintained the superior edge of the fracture gap, the inferior edge nearly collapsed. In contrast, in the ITC, PFN, and MBPC, the fracture gap was almost completely closed.

Figure 4.

Total displacement of the fractured femur without gap fixed with different fixation implants.

Figure 5.

Total displacement of the fractured femur with gap fixed with different fixation implants.

Von Mises stress

The von Mises stress within the implant of the MBPC was substantially higher than that of the other fixation devices, with values of 1000 and 1477 MPa for the models with and without a fracture gap, respectively (Figures 6 and 7). In general, the von Mises stress of the implants was higher in the presence of a gap than in its absence. For the cancellous bone in the proximal femoral fragment, the highest von Mises stress was observed in the ITC with a gap (38.6 MPa) and in the CHS model without a gap (44.8 MPa) (Figures 8 and 9). The von Mises stress in the cancellous bone for the InterTAN model was lower than that observed for the PFBN and MBPC configurations. Among all constructs, the PFN demonstrated the lowest von Mises stress in the cancellous bone.

Figure 6.

Von Mises stress of the implants for fractured femur without gap under loading.

Figure 7.

Von Mises stress of the implants for fractured femur with gap under loading.

Figure 8.

Von Mises stress of the cancellous bone in the proximal femur fragment without gap under loading.

Figure 9.

Von Mises stress of the cancellous bone in the proximal femur fragment with gap under loading.

Discussion

In the present study, the fracture pattern was defined according to the Pauwels classification, a widely referenced biomechanical system for categorizing femoral neck fractures. This classification is based on the inclination angle of the fracture line relative to the horizontal plane, which directly influences the magnitude of shear forces acting across the fracture site during physiological loading. In type III fractures (inclination angle >50°), the shear force increases with the angle, resulting in greater instability and a higher risk of complications such as non-union and avascular necrosis.^35–37 Under such conditions, more robust fixation strategies are required to counteract the elevated shear forces and ensure adequate stability for bone healing. In the present simulation, InterTAN, PFBN, and MBPC demonstrated superior resistance to applied loading in the presence of a fracture. Therefore, these fixation methods are considered preferable for treating vertically oriented femoral neck fractures.

MBPC provides good stability for vertically oriented femoral neck fractures, it does not offer significant advantages over intramedullary fixation methods such as PFBN and InterTAN in terms of surgical complexity.²³ Clinically, the medial plate requires joint exposure to allow direct access to the medial aspect of the femoral neck, resulting in a larger incision and reduced minimally invasive benefits compared to intramedullary approaches. However, the medial buttress plate with cannulated screws is more cost-effective than newer devices like PFBN and InterTAN. Given the relatively lower cost of plates and screws, MBPC may be a more affordable option for economically disadvantaged patients, offering a reasonable balance between performance and affordability.

Regarding fracture stability, when no fracture gap is present, the bone cross-section at the fracture site can still bear normal force, i.e., the component of the load perpendicular to the fracture surface.³⁸ As a result, under the same fixation method, the displacement of the femoral head is smaller compared to cases with a fracture gap. In this scenario, the MBPC provides direct support beneath the fracture site, resulting in the most effective fixation.²² In the present study, the PFBN and InterTAN systems, both of which utilize dual-screw support, demonstrated stability second only to the MBPC. The PFN system, with only a single lag screw, demonstrates the lowest stability. Although the CHS system also employs two screws, its side plate provides extramedullary support with a longer lever arm, leading to inferior stability compared to intramedullary devices with two screws. Traditional ITC fixation still provides reasonable stability, outperforming both CHS and PFN, but remaining less effective than PFBN, InterTAN, and MBPC.

In cases where a fracture gap is present, the implant must bear the entire load until the bone fragments on either side of the fracture come into contact. Under such conditions, the construct is less stable, which may negatively affect the clinical outcome.³⁹ In the present simulation, the InterTAN system demonstrated the highest stability when a fracture gap was present. Its advantage lies in the combination of a large-diameter lag screw and a fully threaded compression screw. The threaded design provides excellent resistance to sliding, allowing InterTAN to maintain the fracture gap effectively. As a result, its stability surpasses that of the MBPC, whose fracture gap has nearly completely collapsed. In comparison, although the femoral head displacement in the Bionic nail is slightly greater than that of the MBPC, it still maintains a fracture gap on one side. The CHS system preserves the gap at the upper edge of the fracture due to the presence of a screw thread crossing the fracture site, while the lower portion has fully collapsed. The PFN system, equipped with only a single lag screw and lacking anti-sliding capability, fails to maintain the gap, resulting in complete collapse and the largest displacement. Similarly, traditional ITC fixation also lacks anti-sliding features, leading to complete gap closure; however, its overall stability remains superior to that of CHS.

In clinical practice, achieving complete contact at the fracture site without any gap is often challenging, and partial contact frequently occurs after reduction. However, replicating the irregular morphology of fracture surfaces in a computational model is difficult. To address this limitation, the present study simulated two extreme conditions: complete reduction (gap = 0 mm), representing the most stable scenario, and incomplete reduction (gap = 1 mm), representing the least stable scenario. Although incomplete reduction was modeled, loading caused the fragment to rotate, resulting in partial contact between the fracture surfaces. This condition may approximate the clinical scenario of partial reduction, where load sharing occurs between the fracture interface and the fixation device, and thus provides useful reference for understanding implant performance under suboptimal reduction.

The use of screw threads to prevent bone slippage and maintain gap stability is a common approach in various fracture fixation applications.^40,41 In this study, similar to the InterTAN system, the Compression Hip Screw (CHS) also demonstrates this effect. By incorporating an additional screw that traverses the fracture site—compared to the conventional Dynamic Hip Screw (DHS)—CHS helps reduce changes in the fracture gap. Although the tension screw in the Proximal Femoral Bionic Nail (PFBN) does not feature threads across the fracture site, the addition of a screw on the tension side still contributes to gap maintenance and collapse prevention.

Implant cut-out, bone penetration, non-union, and avascular necrosis are common complications associated with femoral neck fracture fixation.^42,43 Excessive bone stress is strongly associated with these complications and should therefore be a key consideration when selecting a fixation strategy. In this study, the highest cancellous bone stress was generally observed at the fracture interface. When no fracture gap was present, the ITC fixation method resulted in the highest von Mises stress within the cancellous bone. However, in the presence of a fracture gap, the CHS system exhibited the highest stress values, indicating a greater risk of cancellous bone failure compared to the other fixation methods. Interestingly, the region of peak stress in the InterTAN system differed from the others, occurring at the interface between the compression screw and the proximal cancellous bone. This may be attributed to its limited sliding capacity. Although the stress level in InterTAN was moderate among the six devices analyzed, it still suggests a non-negligible risk of cancellous bone damage, highlighting the need for careful postoperative protection.

Implant failure required to revision is another complication after the femoral neck.^36,44 Although the incidence is low, its occurrence can impose additional medical burdens on the patient. Among all implants in the present study, the medial buttress plate (MBPC) exhibited significantly higher von Mises stress levels compared to the other devices. These stress values exceeded the yield strength of titanium alloy, entering the plastic deformation region, which indicates a high risk of implant failure. In contrast, the maximum von Mises stress observed in the other implants remained well below the yield threshold, except for the CHS lag screw under conditions with a fracture gap, where the stress value (849 MPa) approached the yield strength. The elevated stress in the MBPC is closely associated with its reduced thickness. Due to the high surgical complexity at this location and the need for plate contouring to match the bone surface, a thicker plate would be difficult to shape appropriately. However, the trade-off of using a thinner plate is increased von Mises stress, which raises concerns about potential plate fracture under postoperative loading.

In this study, the extremely high von Mises stresses observed in the medial buttress plate should be interpreted as an indicator of elevated risk rather than a definitive prediction of implant failure. High stress values generally imply a greater likelihood of fatigue fracture under cyclic loading. Although stress itself does not directly influence bone healing, implant breakage caused by excessive stress could potentially result in nonunion or delayed union. It is important to note that the finite element model cannot simulate actual plate fracture; therefore, these stress values serve only as a reference for identifying regions of high mechanical demand and potential failure risk. Future investigations incorporating fatigue analysis or experimental validation would be necessary to confirm whether these stress levels translate into clinically significant failure.

It is important to note that the majority of elderly patients sustaining this type of fracture are typically managed with arthroplasty rather than internal fixation, primarily due to the presence of osteoporosis and compromised bone quality. In contrast, internal fixation is generally indicated for younger patients with better bone stock. Therefore, this study focused on simulating the biomechanical conditions of younger individuals, and the bone material properties were assigned accordingly. While this approach improves the clinical relevance for the intended treatment strategy, we acknowledge that individual variability and the exclusion of osteoporotic bone conditions represent limitations of the present model. These factors should be considered when interpreting the results and may warrant further investigation in future studies.

There are several limitations in the present simulation due to simplifications in material properties and bone architecture. Additionally, the simulation was unable to model bone and implant fracture behavior. Since implant breakage typically results from cyclic loading over time, this phenomenon could not be captured, as the current simulation applied only a single loading cycle. Moreover, only the peak loading condition during level walking was considered; other functional activities, such as stair ascent and descent, were not included. Finally, the bone morphology was retrieved from an elderly cadaver, but the material properties were assigned to those of young bone; therefore, a slight discrepancy in cortical bone thickness may exist.

Conclusion

This study compared the fixation effectiveness of six fixation devices that are widely used clinically for the treatment of femoral neck fractures. Based on the present findings, the Proximal Femoral Bionic Nail, InterTAN, and the medial buttress plate combined with cannulated screws are recommended for femoral neck fractures without a fracture gap. For fractures with a gap, InterTAN is preferred due to its ability to help prevent bone shortening. Furthermore, because the medial buttress plate combined with cannulated screws bears the highest von Mises stress and has the greatest risk of implant breakage, the magnitude of weight bearing after fixation must be carefully controlled to reduce the likelihood of implant failure.

Footnotes

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Show Chwan Memorial Hospital under grant number SRD-108033.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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