The human body’s ability to bear weight and move forward depends on the proper functioning of the ankle joint. Ankle arthroplasty and ankle arthrodesis are two surgical procedures used to address paralytic deformity with muscle–tendon imbalance and end-stage joint diseases such post-traumatic arthritis of the ankle, chronic ankle instability, rheumatoid arthritis, tuberculous or suppurative ankle arthritis, and ischemic necrosis of the astragalus [10]. Recent years have seen significant advancements in ankle arthroplasty techniques; nevertheless, these have seen limited clinical application because of narrow indications, increased technical requirements, and problems including infection and loosening of prostheses [11, 12]. Ankle arthrodesis is a surgical procedure that realigns the bones around a joint to treat lesions, alleviate pain, restore proper alignment, strengthen the joint, and enhance its function. The compression effect must be powerful and dependable, and there must be sufficient contact area for it to work. Careful consideration must be given when choosing internal fixators. There have been numerous studies comparing the biomechanical and clinical efficacy of different internal fixation techniques for ankle arthrodesis [13,14,15]. Studies have found that fixation with screws had greater advantages in terms of the fusion rate and complication control, especially after the development and widespread use of fixation with three compression screws [8, 16, 17]. In arthroscopic fusion, most of the capsule is preserved, potentially adding to the stability of the fixation. Nonetheless arthroscopic fusion allows only for screw fixation.

Ankle arthrodesis has evolved in recent years to include the use of multiple locking plate types and orientations. The anterior plates have been shown through biomechanical testing to function as “tension bands” that efficiently resist ankle flexion and extension, hence minimizing micro-motion of the ankle arthrodesis surface. They can also aid in bone repair and greatly increase rotational resistance [18,19,20]. The ankle joint can be more thoroughly exposed, and a clear surgical field can facilitate the procedure. Although, based on the two crossed screws, the third posterior–lateral screw can significantly resist dorsiflexion and rotational stresses and increase the initial stability; its placement can easily damage the peroneal nerve and cause the screws to collide with each other. With the aid of image analysis software, So et al. [21] compared and analyzed the proportion of loss of articular surface area (SA) on the top of the astragalus between the traditional fixation with two screws and that with three screws. The results showed that the third posterior–lateral screw created an oval hole due to the small entry angle and non-perpendicular direction, which resulted in a loss of 6%–10% of the subtarsal joint SA; therefore, cautious use of this screw is required in patients with low bone mass. So et al. proposed using locking plates in combination with minimal screw fixation. We have been performing ankle arthrodesis using the fixation with two crossed screws (Ø6.5 mm) and anterior plates (Ø2.7 mm) through a small anterior incision since 2020, after carefully considering the benefits and drawbacks of various internal fixation models and the choice of access. The biomechanical features could not be identified, although there were satisfactory therapeutic results.

Finite-element analysis is a dynamic, widely utilized, practical, and efficient numerical analytical method that has swiftly extended from structural engineering strength analysis to practically all sectors of science and industry thanks to the rapid advancement and popularity of computer technology. In 1972, it was initially used in orthopedic biomechanics to measure bone stresses [22]. Since then, it has been increasingly applied in stress analysis of bones and bone prosthesis, fracture fixation devices, and non-bone tissues to assess the relationship between load-bearing function and morphology, and provide a theoretical basis for clinical practice, thus optimizing techniques for implant design and fixation. Finite-element analysis can simulate working conditions that cannot be produced in conventional biomechanical experiments and can be used for static or dynamic analytic research, with the advantages of short duration, low cost, repeatability, and comprehensive performance testing. Additionally, it can complement conventional biomechanical experiments, and provide more comprehensive, accurate, three-dimensional, and diversified mechanical data for clinical practice. In recent years, the finite-element method has been increasingly used in the study of biomechanics of ankle joints [23,24,25,26]. Finite-element analysis has been used in the analysis of ankle arthrodesis. Wang et al.'s study focused on the initial stability of three-screw fixation for ankle arthroscopic anthrosis. The screw configuration of the posteromedial home-run screw was found to avoid collisions and was biomechanically more stable than that of the posteromedial home-run screw [27]. Zhu et al. discussed initial stability and stress distribution of ankle arthroscopic arthrodesis with three kinds of two-screw configuration fixation [28]. However, there is no finite-element analysis focusing on the mechanical properties of the anterior plate in ankle arthrodesis for the time being. In addition, a cross-sectional comparison of the mechanical properties of multiple ankle arthrodesis internal fixation modalities by finite-element analysis is urgently needed to provide reliable evidence support for further clinical studies and clinical practice.

In this study, we collected the CT scan image data of a healthy adult male volunteer and imported this into the Mimics software. Based on this, we created a rough 3D ankle joint model. We performed smoothing and denoising of the model using the Geomagic software, to assemble a solid model. Then, we imported the data into the Solidworks software for assembly and cutting, and simulating procedures such as ankle arthrodesis. With this, we created the experimental models for four types of ankle arthrodesis, for finite-element analysis. Finally, we imported the models into the finite-element analysis software Ansys, and set the analysis parameters and properties, as well as the loading conditions, to obtain the final analysis results. The 3D finite-element model established in this study can fully obtain spatial information from different angles, and the design of various parameters approximates clinical reality; therefore, this study has a high degree of simulation and validity.

The microdisplacement of the arthrodesis surface refers to the deformation of the arthrodesis surface of the ankle under the intorsion, extorsion, dorsiflexion torsion, and neutral stresses based on the gait during walking. The displacement value can indicate the effectiveness and stability of internal fixators for ankle arthrodesis. The results of this study revealed that in the fixation model with anterior plates alone, the displacement of the arthrodesis surface under neutral and dorsiflexion torsion was significantly smaller than that under extorsion and intorsion, indicating that anterior plates had significant advantages against neutral and dorsiflexion torsion; however, it was slightly weak in resisting intorsion and extorsion. In the model with a 6.5 mm posterior–lateral screw based on anterior plates, the displacement under neutral stress, dorsiflexion intorsion, extorsion, and intorsion was significantly smaller than that in the fixation model with anterior plates, specifically under intorsion and extorsion, indicating that the placement of the posterior–lateral screw can produce a better fixation effect. The findings of Clifford [29] and Xie [30] also confirm this theory. As for the fixation model with three screws, there was a small difference in the degree of deformation of the arthrodesis surface under the four forces, indicating that this model can provide more balanced resistance to external forces. The fixation model with three screws was more advantageous than the fixation model with anterior plates and posterior–lateral screws in resisting intorsion and extorsion; the anterior plates performed satisfactorily in resisting neutral stress and dorsiflexion intorsion, specifically the dorsiflexion intorsion. The fixation model with anterior plates and posterior–lateral screws performed better in terms of displacement of the arthrodesis surface.

Our analysis of the experimental results revealed that the fixation model with two crossed screws and anterior plates was undoubtedly the most stable configuration among the four models, and the displacement of the arthrodesis surface against neutral stress, dorsiflexion intorsion, intorsion, and extorsion was smaller than that in the other three models. Based on the stress distribution and stress peak, most forces were concentrated in the central sections of the compression screws, plate joints and bending parts of the plates. This indicates that the components used in these parts should be thickened and reinforced, to prevent fractured screws and plates after ankle arthrodesis. The fixation model with two crossed screws and anterior plates and the fixation model with three screws were superior to the fixation model with anterior plates and the fixation model with anterior plates and posterior–lateral screws with respect to resistance to intorsion and extorsion. The fixation model with two crossed screws and anterior plates, and the fixation model with anterior plates and posterior–lateral screws were superior to the fixation model with three screws and the fixation model with anterior plates with respect to resistance to neutral (vertical) stress and dorsiflexion intorsion.