Fractures of the limbs are one of the most common and severe types of injuries in trauma orthopaedics. Their rapid treatment and stable fixation directly affect the pain control of the injured, the prevention of further injuries, and the subsequent treatment process (Medici et al., 2023). In the treatment of multiple injuries, fractures of the limbs caused by high-energy traumas such as traffic accidents and falls from heights have a relatively high incidence (Almigdad et al., 2022; Hoogervorst et al., 2020; Weil and Mosheiff, 2011). In military war injury treatment, limb injuries have accounted for a major proportion for a long time. Modern battlefield statistics show that limb injuries account for up to 76 %–90 % of war injuries, and about 40 % of them are severe limb fractures (Belmont et al., 2013; Stern et al., 2019; Yun et al., 2016). In addition, in disaster rescue scenarios such as earthquakes, building collapses and crush impacts can cause a large number of fracture casualties in a short period of time (Sever et al., 2024; Wolfson et al., 2016). The common characteristics of these scenarios are a complex environment, scarce medical resources, and difficult transportation of the injured. Therefore, there is a higher demand for emergency fixation equipment: it should be portable, with a wide range of applications, fast assembly speed, and reliable fixation, in order to meet the urgent need for treating a large number of injured people within a short period of time (Bliven et al., 2019).

The external-fixation technique, due to its advantages such as its being minimally invasive, its rapidity, and the ability to be implemented on site, plays an irreplaceable role in the aforementioned complex scenarios. It can achieve fracture stabilization in the early stage, limit abnormal movement of the fracture ends, reduce secondary soft tissue injuries, and secure time for subsequent transfer and staged treatment. Additionally, its open structure makes it convenient for observing the wound, changing the dressing, and conducting the subsequent surgical procedures (Bull et al., 2025; Fernando et al., 2021). After years of development, clinical external fixations have formed several mature configurations, mainly including unilateral, bilateral, circular (Ilizarov type), and mixed basic forms. Different configurations have significant differences in load-bearing paths, bending and torsion resistance, applicable areas, and operational complexity (see Table 1). However, the existing external fixations often have a single fixation dimension and limited application scope, and their specialized designs are often unsuitable for multi-site fractures or joint fixation requirements, as well as emergency tasks with significant variations in the patient's body type. In such cases, it is often necessary to replace the equipment or to repeatedly adjust, which can result in restricted applicability, complex procedures, and increased time consumption. Therefore, for tasks with high timeliness such as large-scale casualties and battlefield rescue, how to achieve a new type of reconfigurable external-fixation framework with multiple dimensions and applicability to various fracture scenarios while ensuring mechanical stability becomes a key challenge in the development of the external-fixation system.

Addressing this challenge necessitates the integration of advanced reconfigurable mechanism designs. While current research in reconfigurable mechanisms and multi-modal robots has enabled robotic systems with remarkable adaptability to diverse environments and tasks (Liu et al., 2022; Wang et al., 2026), their application within the specific domain of external-fixation systems remains largely unexplored.

The innovation of this study lies in that it does not simply optimize the dimensions or local structure of a conventional external fixator but also introduces a reconfigurable mechanism concept into the design of an orthopaedic external-fixation system. Based on a seven-link topological framework, the proposed device integrates multiple fixation modes into a unified structural platform so that one system can adapt to different fracture sites and fixation postures. In this way, the proposed design differs from traditional task-specific external fixators and provides a new solution that combines multi-dimensional adaptability, multi-scenario applicability, and mechanical stability for emergency fracture fixation.

Reconfigurable and multi-mode mechanisms have been widely investigated in robotics, deployable structures, and adaptive mechanical systems because of their capability to improve task versatility and environmental adaptability. By changing configuration, topology, or motion mode, a single mechanism can perform different functions or adapt to different working conditions. However, in the field of orthopaedic external fixation, the application of reconfigurable mechanism theory remains relatively underexplored. Most currently available external-fixation devices are still designed for specific anatomical regions or relatively single fixation tasks, and few studies have systematically explored the use of a single reconfigurable mechanism to rapidly adapt to multiple fracture sites, fixation postures, and body size differences. Therefore, introducing reconfigurable mechanism synthesis into external-fixation system design is meaningful not only for improving clinical applicability in emergency rescue but also for extending the application scope of reconfigurable mechanism theory in medical engineering.

To meet the emergency fixation needs for large-scale fracture patients, this paper proposes a reconfigurable external-fixation frame based on a seven-link mechanism, which forms a new type of external-fixation frame with multi-dimensionality, adaptability to postures, and applicability to multiple fracture scenarios. It can achieve rapid and stable external fixation for fractures in the femur, tibia, elbow joint, knee joint, and ankle joint.

The multiple configuration transitions of the seven-link mechanism are primarily applied in joint-spanning fixation scenarios. Based on the design principles of kinematic chain locking and virtual hinges, the configurations for elbow-spanning, ankle-spanning, and knee-spanning fixation are pre-adjusted. At the corresponding joint-spanning positions, a controllable kinematic chain with a single degree of freedom is formed by releasing specific hinges. Functionally, this kinematic chain is equivalent to a lockable “virtual hinge”; by adjusting its spatial arrangement and axis orientation, it can be aligned with the physiological axis of the human joint, as shown in Fig. 4. This design enables the external fixator, in its locked state, to provide both axial and torsional stability.

The key to achieving multi-modal reconfiguration lies in the configuration-switching mechanism. According to the operation specifications and screw placement requirements of orthopaedic external fixators (Bible and Mir, 2015; Chao and Hein, 1988; Fragomen and Rozbruch, 2007), four external-fixator configurations of the linkage system are predesigned in this study to correspond to three target joint-spanning fixation modes, as shown in Fig. 4. For each configuration mode, specific combinations of hinges are either pre-locked or kept with designated degrees of freedom. During actual use, reconfiguration of the mechanism does not require disassembly of any core links; the linkage structure is simply adjusted to the target configuration, after which the predefined key hinges are tightened.

In the elbow-spanning fixation mode, the upper limb is flexed to 90° and fixed in front of the chest according to fracture fixation requirements. Depending on the distance of the fracture site from the elbow joint, two postural configurations of the external fixator are constructed, as schematically illustrated in Fig. 4A and B. When the fracture site is relatively far from the elbow joint, the external fixator is rotated 90° about R2, and the remaining hinge angles and link lengths are adjusted to configuration (Fig. 4A). When the fracture site is closer to the elbow joint, the external fixator is rotated 90° about R1, and the remaining hinge angles and link lengths are adjusted, folding the links to a shorter overall length to achieve configuration (Fig. 4B). Both configurations can be used for elbow-spanning fixation.

According to the requirements for knee-spanning external fixation in long-bone fracture management, the screws are to be inserted on the lateral side of the thigh and the medial side of the lower leg. A schematic and structural diagram are shown in Fig. 4C. The entire external fixator must span the knee joint in space, with the screw fixation modules located on opposite sides of the limb. By adjusting the spatial position of the key hinge C, as well as the lengths and angular positions of the various links, an asymmetric spatial configuration is achieved, as illustrated in Fig. 4C, allowing the fixator to span the knee joint coaxially while remaining on different sides of the limb. For fractures located near the ankle joint that require ankle-spanning fixation, the corresponding spanning configuration is shown in Fig. 4D.

Since both the tibia and the femur are non-hinged long bones fixed in place, their structural topologies, constraint methods, and mechanical action mechanisms are the same. The differences mainly lie in the geometric scale and load parameters rather than the change in the configuration itself. Therefore, the finite-element analyses of these two configurations are classified into the same category. To verify the rationality of the configuration design of the seven-link reconfigurable external fixator, finite-element analysis was performed to conduct biomechanical simulations for the four configurations and to establish a static analysis environment. The cortical bone, the cancellous bone, and all components in the four configurations were simplified as continuous, homogeneous, isotropic, linear elastic materials, and the corresponding material parameters were defined for the simulations (Hadeed et al., 2019; Leonardo-Diaz et al., 2020; Russo et al., 2018).

An oblique fracture of 20° was set in the fracture model, with a 5 mm defect at the fracture end and a 5 mm gap between the two fracture fragments to simulate an unstable fracture (Raja Izaham et al., 2012). The distance between the external fixator and the bone was set to 30 mm. Since the morphology of the bone is approximately cylindrical, particularly in the diaphyseal region, a cylinder was used to approximate its geometric characteristics. The materials involved in the simulation and their properties are listed in Table 4.

The boundary conditions for axial loading were defined as follows: to prevent displacement during the analysis, the distal end of the fractured bone was fixed, and an axial load of 1500 N was applied to the proximal end to represent axial compressive loading (Abdul Wahab et al., 2020).

The boundary conditions for torsional loading were defined as follows: the distal bone end was fixed, and the proximal bone end was subjected to a torque (T = 15 N m).

Using finite-element analysis software in combination with SolidWorks for the 3D model, the total deformation and overall stress distribution of the four configurations under axial load and torque were analysed, as shown in Fig. 5, and the corresponding numerical results are presented in Table 5.

It is well known that, for ductile materials, yielding occurs when the stress reaches the stress limit (Abd Aziz et al., 2020). The stress distribution and deformation of each fixation configuration under axial load and torque are shown in Fig. 5.

According to the specific values in Table 5, under an applied axial load of 1500 N and a torsional moment of 15 N m, the total deformation and equivalent stress of the external fixator were evaluated for different fixation configurations. For the femur/tibia configuration, the total deformation ranged from 0 to 4.99 mm, and the equivalent stress ranged from 9.55 × 10⁻⁸ to 952.52 MPa. For the elbow joint configuration, the total deformation ranged from 0 to 16.95 mm, with equivalent stress values ranging from 2.49 × 10⁻¹¹ to 377.84 MPa. For the knee joint configuration, the total deformation ranged from 0 to 4.59 mm, and the equivalent stress ranged from 2.65 × 10⁻¹⁰ to 678.11 MPa. For the ankle joint configuration, the total deformation ranged from 0 to 11.03 mm, with equivalent stress values ranging from 1.72 × 10⁻⁹ to 680.72 MPa. The highest stress in the external fixator in this study is concentrated at the interface between the bone pins and the bone. However, the maximum stress in each mode is lower than the allowable stress of the titanium alloy, indicating that the stability of the designed external fixator is acceptable.

Under the applied ultimate load and torque, the overall deformation remains within a range that allows the external fixator to be used for temporary fixation. In summary, based on finite-element simulations of the four modes under loading, although the biomechanical performance of the proposed external fixator is slightly inferior to that of a circular external fixator, it provides sufficient stability and structural rationality for temporary fixation.

This paper aims to solve the problems of the traditional external fixator, such as a single fixed dimension, limited application range, long fixation time, and heavy weight. A single-function external fixator is unable to meet the various needs of fracture fixation in scenarios involving a large number of injured patients. Therefore, this paper proposed a reconfigurable external fixator based on a seven-link mechanism. The configuration of the seven-link mechanism was comprehensively analysed, and a configuration design was proposed. A human–fixator coupled biomechanical model was established, and finite-element analysis was conducted to analyse the mechanical and stability performance of fractures in five typical reconfigurable configurations under an axial load of 1500 N and a torsional load of 15 N m. The finite-element simulation mechanical analysis shows that, under the action of 1500 N axial load and 15 N m torsional load, all configurations can control the relative displacement of the fracture ends within the clinical allowable range, and the overall structure remains stable. Subsequently, the performance, mechanics, and fixation time of this reconfigurable external fixator were compared with those of the single-function external fixator, and, finally, a comparative experiment was conducted based on dogs to verify. When the relative displacement of the fracture end reaches 3 mm, the load required by this reconfigurable external fixator is 274.2 N, and the corresponding structural stiffness is 95.2 N mm⁻¹, both of which are higher than those of the Orthofix external fixator (237.63 N, 72.4 N mm⁻¹); when the relative rotation angle of the fracture end reaches 30°, the required torque is 14.66 N m, which is also superior to the control device. The comparison results of fixation time further indicate that the overall fixation time of this system is less than 10 min, significantly shorter than that of the traditional single-function external fixator. The results of the dog cross-knee joint experiment verified the fixation stability of this external fixator under axial-traction load at the experimental level.

Overall, the seven-link reconfigurable external fixator proposed in this paper achieves rapid adaptation in multiple dimensions, multiple postures, and multiple fracture scenarios while ensuring structural stability and fixation reliability. By integrating multiple fixation modes into a unified topological framework, this design reduces the reliance on instrument replacement and module reconfiguration in emergency scenarios to a certain extent, providing a new structural path for improving the efficiency of temporary fracture fixation under complex conditions. At the same time, beyond its direct clinical application value, this study also demonstrates the feasibility of introducing reconfigurable mechanism concepts into orthopaedic external-fixation system design. Although reconfigurable and multi-mode mechanisms have been widely studied in robotics, deployable structures, and adaptive mechanical systems, their application in external-fixation devices remains relatively underexplored. Therefore, the proposed seven-link reconfigurable external fixator can also be regarded to be a new application case of reconfigurable mechanism theory in medical engineering, which helps extend the application scope of reconfigurable mechanism design in orthopaedic fixation systems.