1 Introduction

Fibula fractures are significant injuries in the realm of trauma care, affecting a diverse range of patients and having a notable impact on mobility and quality of life. The fibula, a slender bone located parallel to the tibia in the lower leg, plays a key role in supporting and distributing body weight, even though it does not bear as much load as the tibia. These fractures occur in various contexts, from high-impact sports to everyday accidents, and their incidence is influenced by factors such as age, gender, lifestyle, and pre-existing conditions. The prevalence of fibula fractures is particularly high among athletes involved in high-impact sports such as football, skiing, and basketball.¹ These activities carry a significant risk of falls, collisions, and abrupt changes in direction, which are common mechanisms of injury. Moreover, repetitive stress from endurance sports like long-distance running can lead to stress fractures. Everyday accidents, such as slips and falls, are also frequent causes, especially among the elderly and individuals with osteoporosis or reduced bone density. In these populations, even minor trauma can result in significant fractures.² Fibula fractures can result from various mechanisms, including direct trauma (e.g., impact from a fall or a blow during contact sports), indirect trauma (e.g., rotational forces leading to spiroid fractures), and torsional forces encountered during car accidents.³ These fractures may also arise from repeated stress or excessive strain, particularly in athletes. Pre-existing conditions like osteoporosis further increase the risk of fractures, highlighting the importance of bone health in injury prevention. Diagnosing fibula fractures typically involves a combination of clinical examination and imaging techniques. X-rays are the cornerstone for identifying the fracture and its characteristics, such as the location and type of break (e.g., transverse, spiroid, or comminuted). In complex cases, additional imaging modalities such as CT or MRI are used to assess associated injuries to soft tissues or joints. Early and accurate diagnosis is critical for determining the appropriate treatment and optimizing recovery outcomes.^4,5 This classification helps doctors determine the most appropriate treatment and predict recovery times, as depicted in Fig. 1. Classification by Location.•Fibula Head Fracture, Fig. 1 (a): these fractures occur at the top of the bone, near the knee. They can be the result of direct trauma to the knee or violent twisting of the leg. Fibula head fractures are particularly significant because they can be associated with cruciate ligament injuries and other knee joint damage. Treatment can range from simple immobilization to more complex surgeries, depending on the severity of the fracture and associated injuries.•Fracture of the Peroneal Malleolus, Fig. 1 (b): occur at the bottom of the fibula, near the ankle. This type of fracture is common in ankle sprains or following direct impacts on the ankle. Often, these fractures are associated with fracture of the tibia and ankle ligaments (such as injury to the anterior talus peroneal ligament). Treatment may require the use of a cast or, in severe cases, surgery to ensure proper alignment of bone fragments.•Diaphyseal fracture of the fibula, Fig. 1 (c): it concerns the diaphyseal portion, i.e. the central part of the bone. These fractures can result from direct impacts or from torsional forces applied to the leg. o The fibula shaft, being a relatively thin region and less protected than other parts of the bone, is susceptible to stress fractures especially in athletes or individuals who engage in intense physical activity. Treatment usually involves immobilization, but complex fractures may require surgery.

Close

Fig. 1 Classification of fibular fractures by location: 1 (a) - Fibula Head Fracture, 1 (b) - Fracture of the Peroneal Malleolus; 1 (c) - Diaphyseal fracture.

Export to PPT

A detailed understanding of the location and severity of a fibula fracture is essential for developing an effective treatment plan. The main goals of treatment include restoring bone alignment, preventing complications, and returning to the patient's normal daily and physical activities as quickly as possible. When a fibula fracture involves the joints, particularly the knee or ankle, specific surgery may be necessary to reconstruct the damaged joint. This surgery aims to restore the joint surface to ensure that the joint maintains its functionality and mobility. After surgery, the patient will have to go through a period of immobilization to allow the healing process to begin. This period can vary depending on the complexity of the surgery and the extent of the injury. The surgical treatment of fibula fractures is reserved for the most complex cases, where it is necessary to intervene directly to ensure proper healing and restoration of limb function.^6,7 These interventions are essential to deal with displaced fractures, complex joint fractures or situations in which conservative treatment has not led to the desired results. Below, an in-depth analysis of the main aspects of this type of treatment. This article aims to investigate the mechanical behavior of an implanted fibula affected by a diaphyseal fracture, focusing particularly on its torsional stiffness, and to compare this with the mechanical properties of a healthy, uninjured fibula. To achieve this, two detailed finite element (FE) models were developed. These models simulate the anatomical structure of the lower limb, including the resected femur, patella, tibia, fibula, and foot, as well as the associated tendons and ligaments,.⁸ The models were subjected to both compressive and torsional forces to replicate the stresses and movements typically experienced by the leg. Additionally, the foot was fixed at its extremity to mimic the conditions under which the mechanical behavior of the fractured and healthy fibula could be compared. This approach allows for a comprehensive understanding of the biomechanical differences between a fractured and intact fibula, with a particular emphasis on the effects of torsion on the overall stability and function of the lower limb.

2 Material and methods

This research approaches the finite element (FE) method to investigate a comparison between the healthy fibula versus a fractured and implanted one, both simulated in a complete model which includes: femur, knee, tibia, and foot linked by the related soft tissue connections (tendons, ligaments, etc.), evaluating the different stress distribution of the lower extremity under body weight loading and torsional adjunctive loads. A comprehensive three-dimensional (3D) solid model of the lower extremity was developed using computed tomography (CT) imaging data. Following the construction of this model, it was transformed into a finite element (FE) model, enabling the application of specific loading and boundary conditions to analyze stress distribution, see Fig. 2(a) and (b). Two distinct numerical models of the lower extremity were generated by integrating soft tissue images obtained through magnetic resonance imaging (MRI) with bone images captured via CT scans from a healthy adult patient. One model represented the complete lower extremity, while the other one reproduce a fractured fibula for comparative analysis. The FE model simulating the diaphyseal fibular fracture was implanted with a Stryker VariAx 2 One-Third Tubular Plating System. The Stryker VariAx 2 One-Third Tubular Plating System is a specialized medical device designed for the internal fixation of fractures in adult patients. The geometric properties and characteristics of the modeled components are quite similar those reported in literature in previous papers.^9,10 In Table 1 are reported material properties of the Stryker VariAx 2 One-Third Tubular Plating System, as reported in literature.^11,12 The plates and non-locking screws are produced from titanium alloy (Ti6Al4V ELI), whereas the locking screws are produced from cobalt-chrome alloy (CoCr). Table 1 reports also mechanical properties of all the other physiologic tissues. The material properties of the bone were assumed to be linearly elastic, isotropic, and homogeneous, with a clear differentiation between cortical and trabecular bone structures. The contact interfaces were defined using a penalty-based method incorporating a weight factor and a friction coefficient of 0.4.¹³ To simulate realistic biomechanical conditions, an axial compressive load of 700 N was applied to the upper extremity of the femur, along with a torque of 6000 Nmm to induce rotational movement of the tibia and fibula. Additionally, a fixed constraint was applied to the foot, as shown in Fig. 2 (c). The rotational direction was configured to mimic external rotation of the proximal femur, simulating the toe-off phase of normal gait. During this phase, the tibia and femur rotate synchronously in the same direction, resulting in external rotation of the foot. This stage of walking involves the knee in an extended position, with the foot and ankle functioning as a rigid lever to facilitate the propulsion phase of gait. Non-linear finite element analyses were conducted using Abaqus version 5.4 (Hibbitt, Karlsson, and Sorensen, Inc., Pawtucket, RI) on both the complete lower extremity model and the fibula-excluded model. These analyses employed geometric non-linearity and automatic time-stepping options to enhance accuracy. The primary goal was to explore how various bony components respond to the combined effects of axial and torsional loads, particularly in scenarios involving the absence of a critical structural element like the fibula. Eight specific points, see Fig. 2 (d), were chosen for evaluation to assess the mechanical behavior in terms of von Mises equivalent stress and angular displacement. These points were strategically selected to provide a comprehensive view of stress distribution and deformation. They included points on the proximal and distal femur (points 1 and 2), the medial portion of the patella (point 3), the proximal and distal regions of the tibia (points 4 and 5), the proximal and distal sections of the fibula (points 6 and 7), and the anterior region of the cuboid bone in the foot (point 8). Fig. 3 depicts a detailed representation of the fracture location on the Y-X and Y-Z views, see Fig. 3 (a), while Fig. 3 (b) depicts geometric characteristics of the Stryker VariAx 2.

Close

Fig. 2 Comprehensive three-dimensional (3D) solid model of the lower extremity, bony and soft tissues in anterior view (a); lateral view(b); loading conditions (c); and points of interest (d).

Export to PPT

Close

Fig. 3 Fracture location on the Y-X and Y-Z views (a), and geometric characteristics of the Stryker VariAx 2 System (b).

Export to PPT

3 Results

This paper aims to perform a numerical comparison among three different FE models developed to assess the torsional stiffness of the lower extremity subjected to normal and torque loading. In order to carry on this kind of comparison the results obtained in a previous paper,⁸ related to a complete lower extremity FE model compared with a partial one without fibula, were compared with the present model of a complete lower FE model with an implanted fibula. Fig. 4 shows the equivalent von Mises contours of stress on the three models.

Close

Fig. 4 Contour maps of Equivalent Von Mises Stress on the numerical models: (a) complete model; (b) model without fibula; (c) model with implanted fibula.

Export to PPT

The lower average level of stress about 42 MPa, is reached by the a) configuration (healthy complete lower limb model), followed by the (b) configuration, lower limb without fibula, about 51 MPa. The third c) configuration reaches the higher level of Equivalent Von Mises stress, about 65 MPa, any case these values are reached on the plate and its screws and are quite below the strength limits of the material. The effects of fibula in total stiffness of the lower limb are evident, enduring a consistent quote of stress, and its absence or partial efficiency involves a distribution of stress on other parts such as ligaments and tendons. In Table 2 are reported the equivalent von mises stresses evaluated in different part of the lower extremity for each of the three numerical models, the inclusion of percentage differences relative to (a) helps in understanding the variations. As it is possible to notice femur, patella, and tibia show increased values in models (b) and (c), indicating increased stress or load-bearing. However, the tibia experiences a slightly lower increase of stress in (c) (17 %), compared to (b) (20 %), suggesting that implanting the fibula might help distribute the load better. The foot, knee ligaments, and foot ligaments follow a similar pattern, where model (b) experiences higher stress than (a), and model (c) has slightly lower values than (b), but still higher than (a). This suggests that reintroducing the fibula partially restores normal load distribution. In model (c), the fibula has a value of about 14 MPa, with a 27 % increase compared to (a), indicating additional strain or load distribution changes after implantation. Concerning Fibula's Ligaments and Tendons, in model (c), a small 6 % increase is noted compared to (a), which may indicate altered ligament behavior due to the implant. Finally the Stryker VariAx 2 System appears in model (c) with a value of 65 MPa, confirms a very good performance of the systems in terms of stress adsorbing capability, otherwise the stress levels achieved in the plate and screws are well below the yield level of the material.

Fig. 5 presents a comparative contour map of displacements for the three FE models. The maximum displacement in configuration (a) is approximately 5.86 mm, while in configuration (b), it increases to 7.23 mm, an increase of about 23 %. Configuration (c) exhibits a displacement of 6.4 mm, demonstrating the role of the VariAx 2 System. Although stress levels increase in the area of the screws, there is no risk of rupture, and the VariAx 2 System effectively reduces displacements.

Close

Fig. 5 Contour maps of Displacements on the numerical models: (a) complete model; (b) model without fibula; (c) model with implanted fibula.

Export to PPT

In Table 3, and Fig. 6, are summarized results obtained at the eight specific anatomical points, see Fig. 2 (d, under the three different analyses performed, in terms of Angular Displacement [°], which indicates the degree of rotation or bending at that particular point, and Equivalent von Mises Stress [MPa], to assess the intensity of stress in a material, serving as an indicator for potential yield or failure. As it is possible to notice, point 1 shows that Model (a) has an angular displacement of 6.4° with a von Mises stress of 23.6 MPa, Model (b) registers 7.3° and 25.4 MPa, and Model (c) records 6.5° and 24.5 MPa; similarly, at femur point 2, the angular displacements are 5.2° for Model (a), 6.1° for Model (b), and 5.5° for Model (c) with corresponding stresses of 20.6 MPa, 23.8 MPa, and 22.6 MPa respectively, indicating that Model (b) consistently results in slightly higher values both in terms of displacement and stress, which might suggest differences in assumed material properties or boundary conditions.

Close

Fig. 6 Results obtained at the eight specific anatomical points in terms of angular displacements and Eq. Von Mises Stress.

Export to PPT

Moving to the patella (point 3), all models show very low angular displacements (0.6° for Models (a) and (c) and 0.7° for Model (b)) with low stress values (ranging from 4.2 MPa to 5.0 MPa), reflecting the patella's design for minimal movement under load. For the tibia, point 4 demonstrates moderate angular displacements of 4.6° in Model (a), 4.7° in Model(b), and 4.3° in Model (c) with von Mises stresses of 24.3 MPa, 27.4 MPa, and 26.6 MPa respectively; point 5 on the tibia shows displacements of 3.5° for Model (a), 4.2° for Model (b), and 3.8° for Model (c) with stress values of 26.9 MPa, 30.8 MPa, and 28.2 MPa, suggesting that the tibia, as a major load-bearing structure, experiences relatively high stress levels, particularly under the assumptions of Model (b). In the case of the fibula, point 6 provides data only for Models (a) and (c) with Model (a) showing 3.8° and 21.0 MPa and Model (c) indicating 4.0° and 25.6 MPa, while point 7 follows a similar trend with 3.0° and 24.7 MPa for Model (a) and 4.2° and 27.4 MPa for Model (c); these observations imply that even slight increases in angular displacement can lead to noticeably higher stress values, especially in Model (c). Finally, the foot at point 8 exhibits very low angular displacements (0.2° for Model (a) and 0.3° for both Models (b) and (c)) along with correspondingly low von Mises stress values (6.8 MPa, 7.1 MPa, and 8.3 MPa respectively), which is consistent with the foot's role in maintaining stability and absorbing loads with minimal deformation. Overall, the data suggest that Model (b) tends to produce higher responses in both angular displacement and stress across several regions compared to the other models, while regions like the patella and foot consistently display lower values; these differences likely arise from variations in modeling assumptions such as material properties, geometry, and loading conditions, and they provide valuable insight into the biomechanical performance of each region, highlighting that areas with higher stresses, particularly the femur and tibia, might be more susceptible to mechanical failure under extreme conditions and thus require careful consideration in biomechanical assessments and potential design improvements.

4 Discussion

The Stryker VariAx 2 One-Third Tubular Plating System represents a significant advancement in orthopedic fixation devices, offering versatility in application across multiple anatomical regions. Designed for use in the clavicle, scapula, olecranon, humerus, radius, ulna, distal fibula, and the small bones of the ankle, including the forefoot, midfoot, and hindfoot, it addresses a broad spectrum of clinical scenarios. Its indications extend to osteotomies, where precise bone cutting and reshaping correct deformities, and to the management of non-unions, a challenging condition in which fractures fail to heal properly.^14,15 The system's adaptability also makes it suitable for stabilizing fractures in both normal and osteopenic bone, acknowledging the growing need for effective solutions in populations with compromised bone density.¹⁶ Despite these strengths, the use of the VariAx 2 system must be judiciously balanced against several contraindications. Active or latent infections and significant local inflammation are critical factors, as they may hinder proper healing and elevate the risk of postoperative complications.¹⁷ Additionally, compromised vascularity and inadequate bone stock, whether due to previous surgeries, disease, or infection, can jeopardize the implant's stability and long-term success.¹⁸ Material sensitivity, including documented or suspected allergic reactions, further limits its application, and obesity is another important contraindication since excessive weight can increase the mechanical load on the implant, potentially leading to failure. Moreover, inadequate soft tissue coverage, interference with anatomical structures, and patient factors such as mental or neuromuscular disorders must be carefully evaluated to ensure optimal outcomes. This underscores how these misalignments influence the leg's overall stability and load distribution. Additional studies,^{19,20, and21} offer compelling insights into knee joint biomechanics by exploring the combined impact of the quadriceps and medial retinaculum on patellar instability during knee flexion, particularly when an imbalance in medial retinaculum loading occurs, showing that even slight variations in ligament tension can result in significant biomechanical changes. Additional investigations,^22,23 have demonstrated that various implant configurations can alter how stress and strain are distributed throughout the tibia and adjacent structures, including the fibula. This type of comparative analysis is crucial for selecting surgical implants that not only stabilize the tibia but also preserve its functional relationship with the fibula. Furthermore, another study,²⁴ examines tibio-talar contact stress through both experimental and computational approaches, offering valuable insights into stress distribution across the ankle joint. Biomechanical investigations offer additional insights into the role of the fibula in load distribution, which further contextualizes the importance of devices like the VariAx 2 System. For example, when the fibula is removed (as represented in model b), the tibia experiences a notable 20 % increase in load, a phenomenon that aligns with the understanding that the fibula typically shares load with the tibia under normal conditions.²⁵ This compensatory effect is not confined solely to the tibia; the femur, patella, and foot also demonstrate increased stress, by approximately 14 %, 13 %, and 6 % respectively, suggesting a systemic redistribution of forces throughout the lower limb.²⁶ Furthermore, the ligaments and tendons of the knee and foot show increased strain, with rises of 18 % and 21 % respectively, underscoring potential alterations in joint stability and overall biomechanics. Reintroducing the fibula, as in model c, appears to mitigate some of these adverse effects by redistributing load, yet it does not entirely restore the natural, healthy state. Although the implanted fibula reduces stress on adjacent structures, resulting in a decrease from 20 % to 17 % additional load on the tibia compared to the intact model, this partial recovery suggests that the implant, while beneficial, does not perfectly replicate the biomechanical behavior of the natural fibula. Notably, the fibula in the implanted model endures a 27 % higher stress than its natural counterpart, and the associated ligaments and tendons, although reengaged, still exhibit a 6 % increase in stress. These findings indicate that while the Stryker VariAx 2 System plays a unique role in stabilizing fractures and compensating for lost structural support, the biomechanical dynamics remain altered, potentially elevating the risk of long-term wear or failure in certain tissues. Accurately diagnosing a fibula fracture is essential for determining the best treatment and predicting recovery. The process starts with a physical exam and evaluation of symptoms, followed by imaging to confirm the fracture and assess its specifics. X-rays are commonly used, but if the fracture is complex or involves surrounding tissues, computed tomography (CT) may be necessary to provide detailed, three-dimensional images. While magnetic resonance imaging (MRI) is not typically used as the primary diagnostic tool, it can be valuable in cases of stress fractures or when evaluating soft tissue injuries. In some cases, ultrasound may also provide additional insights, ensuring the most effective treatment and recovery plan. Fibula fractures are classified based on their location along the bone and the severity of the injury, both of which help guide treatment decisions and predict recovery times. Fractures can occur at the fibula head near the knee, often due to direct trauma or twisting, and may be associated with knee joint injuries. In terms of severity, compound fractures involve the bone breaking into pieces but remaining aligned, while displaced fractures require surgery to realign bone fragments. Treatment approaches for fibula fractures depend on the type and severity of the injury. For less severe cases, conservative management, including immobilization with a cast or brace and pain management, is sufficient. In more complex cases, such as displaced or comminuted fractures, surgical intervention may be necessary to realign and stabilize the bone using plates, screws, or rods.^27,28 Rehabilitation plays a pivotal role in the recovery process, helping patients regain strength, mobility, and functionality in the affected limb.²⁹ Recent studies provide valuable insights into the etiology, management, and prevention of fibula fractures, highlighting the biomechanical factors contributing to fibula fractures in athletes,^30,31 while other studies emphasize the importance of tailored rehabilitation protocols in ensuring optimal recovery.^32,33 For fractures involving joints, surgery may be necessary to reconstruct the joint and restore functionality. Surgical treatment is used for complex fractures, particularly displaced fractures or cases where conservative treatment has failed.³⁴ Recovery from fibula fractures varies, with some requiring immobilization and others necessitating surgery and physiotherapy to restore strength and mobility. Fibula fractures can be classified according to their location along the bone or their severity.

5 Conclusions

In essence, the Stryker VariAx 2 One-Third Tubular Plating System is a versatile implant used for various fractures and osteotomies—even in osteopenic bone—but its application must be carefully considered in patients with contraindications such as infection, compromised vascularity, poor bone quality, material sensitivities, or obesity. Biomechanical studies reveal that removing the fibula significantly increases stress on the tibia, femur, patella, and foot, and while reintroducing a fibular implant helps, it only partially restores normal load distribution. Additionally, fibula fractures, which can result from diverse trauma mechanisms, require prompt diagnosis, tailored treatment strategies, and comprehensive rehabilitation to prevent complications. Overall, optimal outcomes depend on meticulous preoperative planning, individualized care, and ongoing interdisciplinary research.

Guardian/patient's consent

None.

Ethical consideration

Ethical considerations are not relevant to this study. This is because it is an experimental study involving finite element analysis and this study does not involve human subjects.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.