Effect of different subtalar joint arthroereisis implants on pediatric flexible flatfoot: a finite element analysis

Introduction

Flexible flat foot is a relatively common disease in foot and ankle surgery and has a high incidence rate in adolescents and children. According to a report, the incidence of flat foot deformity is 7.1–32.0%, while the incidence of symptomatic flat foot is 0.8–3.7% (1). The etiology of flat foot has not been fully determined, but the condition is characterized (2) by the collapse of the medial longitudinal arch of the foot caused by excessive valgus (pronation) of the subtalar joint, rear foot valgus, and forefoot abduction. A portion of patients with flat foot deformity also have other deformities, such as gastrocnemius aponeurosis or Achilles tendon contracture, peroneal muscle spasm, and medial column instability. However, the treatment of flexible flat foot remains controversial (3). Patients with mild symptoms are generally recommended conservative treatment, including the use of orthotics. However, surgical treatment can be considered in patients who have severe foot pain, are easily fatigued, experience gait change, or have gastrocnemius or Achilles tendon contracture (4). Surgical procedures include tendon lengthening and transfer, osteotomy, subtalar arthroereisis (STA), and fusion of single or multiple joints.

At present, STA is more commonly used in the clinical treatment of pediatric flexible flat foot. STA involves the placement of implants in the sinus tarsi (typically via screws) to limit the excessive pronation of the subtalar joint, thereby correcting the subluxation of the subtalar joint, which when left untreated can lead to the collapse of the valgus of the calcaneus and of the medial longitudinal arch. In recent years, STA has gradually come to be favored by foot and ankle surgeons due to its advantages of simple operation, less trauma, and better prognosis (5). A variety of implants have been developed thus far, and they differ in their biomechanical mechanism, orientation of placement, and design characteristics. In 1987, according to biomechanical characteristics, Vogler classified implants into three types: axis-altering devices, impact-blocking devices, and self-locking wedges (6). With the in-depth understanding of flexible flat foot and the continuous update of implant products, self-locking wedge devices have gradually grown to prominence. In 2012, Graham and Jawrani proposed a novel classification system that also consisted of three types (7). Type IA includes cylindrical prostheses, which are inserted into the tarsal sinus laterally to medially and aid in limiting the excessive motion of subtalar the joint through the impact mechanism; its representative implant is the Maxwell-Brancheau arthroereisis (MBA). Type IB, aside from its conical in shape, has characteristics similar to those of type I; its main representative implant is Talar-Fit, which is widely used in clinic. Type II is completely different from type IA and type IB, not only in terms of geometric design but also in the implantation position. It has a medial-cylindrical and lateral-conical geometry, which is in line with the irregular anatomy of the tarsal sinus and canal. Additionally, it is inserted at an anterior-lateral-distal to posterior-medial-proximal orientation and anchored in the canalis portion of the tarsal sinus; the only product of this type is the HyProCure.

Several studies involving clinical follow-ups on patients with STA have been conducted, all of which indicate that STA is clinically effective in correcting the deformity of pediatric flexible flat foot both in Talar-Fit and HyProCure (8,9). However, there are few reports clarifying whether there are differences in the corrective effect between different types of implants, including across different sizes of the same type. So, it makes the performers confused to choose the best implant including the best size.

Therefore, the purpose of this study was dual. First, we compared the corrective ability of two representative implants: Talar-Fit (type IB) and HyProCure (type II). Second, we compared the corrective ability between different sizes. It will provide a guidance for performers to choose the best implant including the best size.

Methods

In this study, we reconstructed a finite element model of a foot derived from a pediatric patient diagnosed with flexible flat foot. Subsequently, we crafted several sinus tarsi implant models of varying dimensions, namely Talar-Fit (type IB) and HyProCure (type II).

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by Ethics Committee of The Second Affiliated Hospital and Yuying Children’s Hospital of Wenzhou Medical University (No. 2025LYYJ083). And informed consent was taken from the patients’ legal guardians.

Patient recruitment

In our investigation, we recruited a pediatric patient diagnosed with flexible flat foot. As part of the inclusion criteria, the patient was required to undergo X-ray and computed tomography (CT) examination of the foot and ankle to rule out the presence of fractures or other deformities. Based these preliminary evaluations, an additional foot magnetic resonance imaging (MRI) scan was performed to exclude related ligament and tendon injuries. Consequently, we included a 15-year-old male patient with flexible flatfoot, who measured 170 cm in height and 60 kg in weigh from the Department of Orthopedic Surgery at The Second Affiliated Hospital and Yuying Children’s Hospital of Wenzhou Medical University (Figure 1).

Figure 1 Appearance and X-ray examination of enrolled patient. (A) Lateral view with weightbearing showing the collapsed medial arch. (B) Lateral X-ray examination under weightbearing.

Model reconstruction

Finite element model of pediatric flexible flat foot

A CT scan was performed on the patient’s left foot to obtain Digital Imaging and Communications in Medicine (DICOM) data. It was crucial during this process to maintain the foot and ankle in a neutral position. We segmented and reconstructed the DICOM images using Mimics 21.0 (Materialise, Leuven, Belgium) software and converted these into an STL file. This file was then imported into Geomagic Studio (3D Systems, Rock Hill, SC, USA) for surface fitting and smoothing, ultimately resulting in a three-dimensional geometric model of a flat foot that encompassed the skin, soft tissue, bones, ligaments, and tendons. The finite element model of pediatric flexible flat foot is presented in Figure 2.

Figure 2 Finite element model of the pediatric flexible flat foot and different sinus tarsi implants. (A) Finite element model of the pediatric flexible flat foot. (B) Finite element model of the 6.5-mm HyProCure implant. (C) Finite element model of the 8-mm Talar-Fit implant. (D) Finite element model of the 8.5-mm HyProCure implant. (E) Finite element model of the 10-mm Talar-Fit implant.

Finite element model of sinus tarsi implant

Different implant models of varying dimensions were created using the reconstructed geometry provided by Rhino 7.0 software (Robert McNeel & Associates, Seattle, WA, USA) (Figure 2). The dimensions of the implants were carefully chosen based on the size of the sinus tarsi and canal. Ultimately, the Talar-Fit 8-mm model and the d HyProCure 8.5-mm model emerged as the optimal choices, while the Talar-Fit 10-mm model and HyProCure 6.5-mm model were considered as secondary options. To prevent impingement into the joint space, the boundaries of the implants were filleted. These sinus tarsi implant models were then integrated into the flat foot finite element model in a manner that mirrored the procedure used in clinical surgical implantation (Figure 3).

Figure 3 Finite element model of the different sinus tarsi implants in the talotarsal joint. (A) Lateral view of the implant in the talotarsal joint. (B-E) Bottom view of the different sinus tarsi implants in the talotarsal joint. (B) The 6.5-mm HyProCure implant. (C) The 8.5-mm HyProCure implant. (D) The 8-mmTalar-Fit implant. (E) The 10-mm Talar-Fit implant.

Mesh creation

In this study, we used Hypermesh 13.0 software (Altair Engineering, Troy, MI, USA) to generate meshes, adhering to the guidelines established by previous work (10-12).

Material properties

In our modeling framework, the bones, articular cartilage, and ground were treated as homogeneous and linearly elastic materials, whereas the skin and soft tissues were considered hyperelastic materials. Ligaments and tendons were modeled as one-dimensional linear spring units. The material properties_—specifically the Poisson ratio and Young modulus—of the soft tissue, cartilage, and skeleton components, were defined in alignment with parameters reported in previously published literature (13) (Table 1).

Boundary settings and loading

Boundary settings and loading conditions were established based on previous studies (14-16). We specified nonlinear contact behavior between cartilaginous layer and binding contact behavior among bone, cartilage, and skin. However, the contact interaction between the plantar skin and the ground was designated as frictional, characterized by a friction coefficient of 0.6 (17). Ligaments and tendons were implemented as one-dimensional spring elements, created by directly connecting nodes on the surface of the cortical bone mesh of the flat foot to the corresponding attachment points of the ligaments indicated by MRI data, thereby serving as traction unit for the flat foot’s ligaments and tendons (18).

The fibula and tibia ends were stabilized. A vertical load of 600 N in total (500 N on the tibia and 100 N on the fibula) was applied on the surface of the tibia and fibula (19). Additionally, the reaction force of the corresponding tendons ranged from 6% to 50% of the body weight, as detailed in Table 2.

Model output and analysis

For the simulation, we employed the standard quasistatic solver within Abaqus 6.11 (Dassault Systèmes Simulia Corp., Providence, RI, USA). We analyzed the plantar pressure distribution along with the von Mises stress of the medial and lateral columns. Additionally, the strain on the medial ligaments and the metrics for assessing the degree of deformity correction were evaluated.

Results

Validation of finite element model

In this study, we evaluated the calcaneus pitch angle, the talar-first metatarsal angle, and the talonavicular coverage angle in both weight-bearing radiographs and the finite element model, as presented in Table 3. A comparative assessment of these parameters yielded a measurement error within the 4.2% range, thus providing initial validation for our finite element model.

Furthermore, following the application of body weight loading, we also examined the distribution of plantar stress, as depicted in Figure 4. A peak stress was evident under the head of the first metatarsal, with additional high stress under the heel. These observations align with findings from prior research (19,20), further corroborating the accuracy of our finite element model.

Figure 4 Plantar stress distribution in different conditions. (A) Plantar stress distribution before implantation. (B-E) Plantar stress distribution after placement of different sinus tarsi implants, including the 6.5-mm HyProCure implant, the 8.5-mm HyProCure implant, the 8-mm Talar-Fit implant, and the 10-mm Talar-Fit implant, respectively.

Evaluation of the STA simulation

Plantar pressure distribution

Our flat foot model revealed specific regional stress distributions in peak plantar pressures, notably at the first metatarsal head and heel. Following implantation, there was a decrease in peak plantar pressures at these two regions, yet an increase was observed in the area of the fifth metatarsal (Figure 4). These findings indicate that arthroereisis has the capacity to redistribute the plantar pressure, leading to a more uniform distribution. However, the effect was more pronounced with the HyProCure implant than with the Talar-Fit implant. Additionally, larger implant size exhibited even more significant effects.

von Mises stress of the medial and lateral columns

Comparative analysis of the medial and lateral columns revealed alterations in peak von Mises stresses following sinus tarsi implantation, as illustrated in Figure 5. Notably, significant reductions in peak von Mises stresses were observed in the talus, medial cuneiform, first metatarsal, and calcaneus, with decreases ranging from 4.64% to 63.13%. These reductions were more substantial with the HyProCure implant, with the larger size resulting in a more pronounced decrease. However, the peak von Mises in the lateral column (fifth metatarsal) significantly increased, ranging from approximately 6.95 to 9.44 MPa, equivalent to an increment of about 86.33–153.08%. Here too, the HyProCure implant surpassed the Talar-Fit implant in terms of stress increase, and the larger implant size displayed a greater impact.

Figure 5 Comparison of von Mises stress of bones in the medial and lateral columns of the finite element model under weight-bearing conditions after the placement of different sinus tarsi implants. H, HyProCure; STA, subtalar arthroereisis; T, Talar-Fit.

Peak strain of ligaments

Table 4 and Figure 6 collectively illustrate the alterations in peak strain that various ligaments, including the anterior talotibial, posterior talotibial, tibiocalcaneal, tibionavicular, spring, and plantar fascia ligaments, were subjected to following sinus tarsi implantation; prior to implantation, their respective strain values were 1.13, 0.52, 0.85, 1.50, 3.19, and 2.34 MPa. Each of these values underwent alterations postimplantation, with reductions ranging from approximately 3.53% to 47.02%. Notably, the spring ligament underwent decreases of 33.54% (implant HyProCure 6.5-mm), 35.74% (implant HyProCure 8.5-mm), 15.67% (Talar-Fit 8-mm), and 25.71% (Talar-Fit 10-mm). The HyProCure sinus tarsi implant elicited more pronounced effects than did the Talar-Fit. Furthermore, the larger implant resulted in more significant changes as compared to its smaller counterpart.

Figure 6 Comparison of the peak strain of ligaments after the placement of different sinus tarsi implants. ATTL, anterior talotibial ligament; H, HyProCure; PF, plantar fascia; PTTL, posterior talotibial ligament; SL, spring ligament; T, Talar-Fit; TCL, tibiocalcaneal ligament; TNL, tibionavicular ligament.

The indexes of deformity correction

To ascertain the extent of deformity correction, we re-evaluated the talar-first metatarsal angle, talonavicular coverage, and calcaneus pitch postimplantation. As illustrated in Table 5, all indexes exhibited significant alterations following sinus tarsi implantation. Notably, both the HyProCure and Talar-Fit implants led to significant reductions in the talar-first metatarsal angle and talonavicular coverage, whereas the calcaneus pitch increased. It was evident that the HyProCure implant induced a more pronounced effect as compared to the Talar-Fit implant. Furthermore, larger implants consistently yielded better results than did their smaller counterparts, irrespective of whether they were HyProCure or Talar-Fit.

Similar observations were noted in the evaluation of arch height. The model without an implant had an arch height of 8.32 mm, which rose to a range of 13.67–16.85 mm postimplantation, an increase of 64.30–102.52%. Notably, the HyProCure implant proved more effective in enhancing the arch height compared to Talar-Fit. However, when the effect of different sizes within the same product line were assessed, the results were not uniform. The large size exhibited a superior improvement as compared to the smaller one, although the difference was not significant.

Discussion

Flexible flat foot, despite its controversial but nearly uniform treatment, has attracted increased research attention. In recent years, several flat foot models have been created, with several ligaments (such as the talocalcaneal interosseous ligament, spring ligament, and the long and short plantar ligaments) of cadaver specimens being dissected to generate these models (21). However, this method entails certain drawbacks. For one, it involves the dissection of multiple ligaments in the foot, thereby impairing their physiological function. Moreover, cadaver foot specimens may not accurately mimic the physiological conditions of living specimens. Furthermore, obtaining cadaver specimens, particularly pediatric ones, pose distinct challenges that add to the complexities of studying pediatric flexible flat foot. Although a number of studies have managed to create flat foot models through finite elements and to simulate the biomechanical analysis of two different types of STA implants, the majority have used adult acquired flat foot as a modeling template (20,22). To the best of our knowledge, only a small number of scholars have developed a pediatric flexible flat foot finite element model or investigated its biomechanics. Hence, our study is the first attempt to create a finite element model of pediatric flexible flat foot and also further examined the biomechanics of various STA implants for flexible flatfoot.

Our study used a finite element model based on a 15-year-old male patient with flat foot. The model was validated by a comparison of the parameters obtained from radiographs of a weight-bearing model with those from a deformed finite element model. STA, a treatment gaining traction in the management of pediatric flexible flat foot and likely to become even more frequent in the future, was investigated using our model. There are an increasing number of options available for implants, and Graham and Jawrani (7) has categorized these into three types: type IA, type IB, and type II. HyProCure and Talar-Fit, corresponding to type IB and type II respectively, are among the most frequently used products in the market. Although numerous clinical follow-up reports on these two implant types and their efficacy in improving foot arch have been conducted, few biomechanical comparisons between them have been published. Although there is literature suggesting that type II implants are more effective than type I implants, a related study has primarily included adult participants (23).

Our results indicate that both implant types are advantageous in the treatment of pediatric flexible flat foot. The benefits were primarily observed as changes in plantar pressure distribution, reduced stress in foot ligaments, increased load-bearing by the lateral column, and decreased load-bearing by the medial column, together with restoration of the medial longitudinal arch. Notably, the HyProCure implant exhibited a more potent effect, which is in alignment with Wang et al.’s study (23). Their cadaver study analyzed the mechanisms of the two implant types, revealing superior correction ability of type II implants in both the transverse and sagittal planes. This differential correction ability could be attributed to the varied mechanisms of the implants (7). The type I implant is only inserted in the lateral region, whereas the type II implant is inserted centrally in the sinus tarsi, which not only restricts excessive talus motion but also elevates the head of the talus. Furthermore, the anatomical similarity of type II implants to the tarsal canal-sinus structure makes them more physiologically appropriate.

Although the application of STA is becoming increasingly prevalent, there have been reports of postoperative complications (24,25), such as sinus tarsi pain, dislocation of implant, and periprosthetic fracture. The most common complication is sinus tarsi pain, with an incidence rate between 10% and 40% (25). However, the etiology of this pain remains unclear and is likely multifactorial. Cook et al. conducted a prospective study and identified the size of the implant as a risk factor. They suggested that if both small and large sizes yield satisfactory results, choosing the larger size could potentially reduce postoperative pain (26). The impact of implant sizes on correcting ability could vary, particularly in terms of altering ligament strain and redistributing the load from the medial column to other bone blocks. High strain in ligaments and load on bone blocks could potentially cause sinus tarsi pain (27). In our study, we observed a significant reduction in the strain of ligaments associated with flat foot, notably in the spring ligament, from 3.19 Mp to a range between 2.05 and 2.19 MPa, marking a peak reduction of 35.74%. Simultaneously, load distribution between the medial and lateral columns were adjusted, leading to a more uniform overall load distribution. These changes were more pronounced with an increase in implant size. However, some researchers argue that an increase in implant size is not always beneficial (23). As the implant bears load upon insertion, there is contact stress between the bone and the implant. This stress could increase with the size of the implant, potentially leading to sinus tarsi pain.

Although our study provides valuable insights, it is not without limitations. First, our finite element model is based on a single patient, precluding any statistical analysis across multiple samples. Second, our data collection exclusively focused on static loading conditions, thereby disregarding any dynamic influences during the gait cycle. Finally, the properties of the ligaments were determined based on data from previously published research, which may not precisely align with our patient's condition. Each of these factors could potentially impact the results obtained from our study.

Conclusions

Despite these limitations, our study, to the best of our knowledge, is the first to compare the impact of different STA implants, accounting for both type and size, in the treatment of pediatric flexible flat foot. Our findings suggest that the HyProCure implant provides a superior effect as compared to the Talar-Fit implant. Additionally, a larger size may also be more effective than a smaller one. Future studies should seek to increase the sample size, refine the pediatric flexible flat foot finite element model, and incorporate clinical follow-up data to enhance the study quality.

Acknowledgments

None.

Footnote

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-352/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-352/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-352/coif). The authors have no conflicts of interest to declare.

Ethics Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by Ethics Committee of The Second Affiliated Hospital and Yuying Children’s Hospital of Wenzhou Medical University (No. 2025LYYJ083). And informed consent was taken from the patients’ legal guardians.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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