Flexible flatfoot (FFF) is a prevalent orthopedic condition in clinical settings, particularly among the pediatric population, with a reported incidence rate of 5% (1). However, the etiology and pathological process of flexible flatfoot remain unclear (2). Flexible flatfoot is often characterized by the depression of the medial longitudinal arch, abduction of the forefoot, and eversion of the hindfoot, which may be caused by the excessive eversion of the subtalar joint (3). Other features, such as spasm of the peroneus and contracture of the gastrocnemius aponeurosis or Achilles tendon, are usually observed either individually or in conjunction with all of the aforementioned factors. Moreover, the majority of patients are often asymptomatic with a normal medial longitudinal arch. However, the main characteristics become more evident in weight-bearing, and some patients also suffer from subjective symptoms such as foot or ankle pain, gait changes, clumsiness, and easy fatigue after standing for long periods of time or walking long distances.

Treating flexible flatfoot still remains controversial (4). If not treated appropriately, some sequelae may occur, such as developing rigid flat feet and osteoarthritis, which can potentially destroy the mechanical axis of the lower limb (5). Most researchers recommend that patients exhibiting mild symptoms generally should be treated with conservative measures, such as utilizing orthopedic shoes or arch pads, while patients with persistent symptoms (such as persistent pain, gait changes, and easy fatigue) (6) should consider surgical treatment. Surgical procedures include subtalar arthroereisis (STA), osteotomies, arthrodesis, and soft tissue surgeries including tendon lengthening and transfer (7, 8). Currently, subtalar arthroereisis is gaining popularity in clinical settings due to its notable advantages of simple operation, less trauma, and better prognosis (9). STA refers to the insertion of implants into the sinus tarsi to correct excessive pronation of the subtalar joint and sustain its neutral position, consequently correcting the deformity of the flatfoot. Chambers first introduced the procedure in 1946, which gained popularity in the 1970s (10, 11). Through clinical observation, Fernández de Retana pointed out that the STA is indicated four or more of the following criteria are met: (1) no radiographic or clinical improvement after 2 years of conservative treatment; (2) valgus angle of hindfoot >10°; (3) FFF combined with Achilles tendon contracture; (4) Viladot footprint type II, III, or IV; (5) Meary angle <10°; (6) Moreau–Costa–Bartani angle >130° or Kite angle >25° (12). A wide variety of implants are now available on the market as a result of technological improvements. Vogler classified them into three types in 1987 based on their biomechanical characteristics: axis-altering devices, impact-blocking devices, and self-locking wedges. Among these types, self-locking wedges are the most commonly used in clinical practice (8). In 2012 (13), Graham and Jawrani further divided the self-locking wedge type into three types: type IA, type IB, and type II. The difference among these three types lies in the geometric design and the specific location of implantation for the implants. Type IA is designed in a cylindrical geometry, while type IB is characterized by a conical shape. Notably, both of these structures are inserted into the tarsal sinus in a lateral to medial orientation. Type II is featured with a medial-cylindrical and lateral-conical geometry, and inserted in an orientation from anterior-lateral-distal to posterior-medial-proximal. The representative implants include MBA, Talar-Fit, and HyproCure, which are widely recognized and routinely employed in clinical practice at present.

However, STA still remains controversial (14). Using the Talar-Fit as an example, the provided guidelines recommend inserting the screws into the tarsal sinus; however, many researchers prefer to insert implants into the sinus tarsal canal along the longitudinal axis of the sinus tarsal canal, anchoring them in the sinus tarsal canal (15). As a consequence, there is no uniform standard regarding the position and orientation of implants following modification. In a study conducted by Wang et al. (16), a comparison was made between two different types of devices, with two different insert positions, in order to assess the ability to correct the flatfoot deformity. The study utilized a cadaveric flatfoot model and found that HyproCure exhibited greater correction ability in both the transverse and sagittal planes when compared with Talar-Fit. Furthermore, he proposed that insertion into the canalis portion would be more beneficial compared with insertion into the sinus portion. Husain (17) studied the biomechanics of STA using screws of different sizes, finding a positive correlation between the size of the screw and the degree of restriction on the subtalar joint, with 6, 8, 9, 10, and 12 mm screws reducing the range of motion by 32.0%, 44.8%, 59.0%, 65.5%, and 76.8%, respectively. Based on the aforementioned information, it can be argued that the selection of implants should take into account the position and size of these implants.

Therefore, the aims of this research are as follows: to present the mid-term outcomes of STA with Talar-Fit implant for treating pediatric flexible flatfoot, to compare the radiographic and clinical outcomes associated with different sizes of Talar-Fit implant, and to analyze the optimal position of the inserted implants.

In recent years, podiatrists have increasingly utilized subtalar arthroereisis in treating pediatric FFF (24). As aforementioned, the concept was first introduced by Chambers in 1946. He inserted autologous bone graft into the tarsal sinus to correct the flatfoot by restricting the excessive eversion of the subtalar joint. In 1952, Grice modified the procedure. A cortical bone graft was used to hold the sinus tarsi open and create extra articular subtalar arthrodesis (8). However, this surgery was no longer adopted due to high incidence of complications such as secondary osteoarthritis. Thus, most of the podiatrists continue to endorse the concept proposed by Chambers, arguing that it is fundamental to restrict the excessive eversion of the subtalar joint without resorting to arthrodesis (23). This provides a direction for the treatment of pediatric FFF. Nowadays, the progressive advancement of technology and materials has facilitated the gradual maturation of subtalar arthroereisis, resulting in the emergence of numerous implant options available in the market. Nevertheless, the use of subtalar arthroereisis continues to be a topic of controversy, particularly with regard to product selection, size choice, and implants positioning.

In recent years, there are lots of reports on subtalar arthroereisis, including biomechanical assessment and clinical follow-up. The majority of scholars believe that STA is beneficial to FFF, which is also confirmed by a retrospective study conducted by Graham et al. (25). By conducting a retrospective analysis of clinical and radiographic data from a sample of 83 adult cases who underwent STA with the implantation of a self-locking wedge type, it was found that most of the patients were satisfied with the functional outcomes of their feet, which exhibited significant changes in relevant angles. Specifically, the mean angle of talar declination and talar second metatarsal decreased by 5.7° and 19°, respectively, whereas the mean angle of calcaneal pitch increased by 0.8°. De Pellegrin et al. (26) investigated the clinical and radiographic outcomes of 485 FFF children treated with the calcaneo-stop implant within 4.5 years, and the result demonstrated that 93.7% of the patients were satisfied, and no complications were reported during the course of the study. The result was also consistent with that of our study, even when the Talar-Fit implant was utilized. In this study, 90.0% of the patients were satisfied with the provided services. The AOFAS ankle–hindfoot score was significantly improved from 58.6 ± 10.9 to 85.2 ± 8.6 during the last follow-up (P < 0.05), with related angles changed at different levels.

The selection of the implant size is crucial to this procedure. Insufficient correction of deformity may occur if the implant size is smaller. However, if the implant size is larger, it raises an additional concern regarding the potential for overcorrected deformities. In this regard, different scholars hold different views. Yang pointed out that the principle entails selecting the minimal size that is capable of effectively correcting the deformity and can be stably maintained in the tarsal sinus during the movement of the patients (23). However, according to the study by Wang et al. (15), the selection of implant size should be different. They suggested that selecting the larger option would be the optimal choice, provided that both the smaller and larger options yielded satisfactory outcomes. He insisted that selecting the larger one could decrease the incidence of postoperative sinus tarsi pain. Cook et al. (22) conducted a prospective study, identifying the size of implants as a risk factor of postoperative complications, and also suggested that if both small and large sizes yield satisfactory results, choosing larger sizes may potentially reduce postoperative pain. In addition, another biomechanical assessment in a cadaveric study conducted by Husain (17) reported that the ability of correction increased with the size of implants. In our study, some interesting results were found. It is clear that there were no significant differences observed in the Meary's angle and the AOFAS scores as the size increased. However, the calcaneal pitch angle, the talar declination angle, and the medial longitudinal arch angle were significantly different. As for deeper analysis of pairwise comparisons across groups, the differences were not significant. It was observed that STA not only restricted excessive eversion of the subtalar joint but also raised the head of the caput tali when considering these differences (9). The calcaneus was elevated concurrently with the elevation of the caput tali. Consequently, a bigger size of implants could lead to more obvious elevation of the caput tali. It was recommended to choose the larger option if both sizes were sufficient.

Another key point of this surgery is the positioning of the inserted implants. However, the specific normative standards for Talar-Fit have not yet been definitively established. In addition, there are limited reports available to help guide the management of this surgery. Some researchers believe that implants should be inserted into the tarsal sinus in the direction of the tunnel, with the head of the implant not crossing the midline of the talus neck (23). This study investigated the exact position of Talar-Fit inserted. It was found that the longitudinal axis of the implant was perpendicular to the midline of the talus neck, forming a nearly 90° angle. The angle might be a judgement standard for orientation of the inserted implant. Upon closer observation, the overall depth of the implant was found to be inconsistent with previous reports. As the result showed, the average depth was 0.9 mm ± 2.1 mm. Moreover, the average depths in different groups were 1.38 mm ± 1.70 mm, 1.35 mm ± 2.1 mm, 1.00 mm ± 2.91 mm, 0.76 mm ± 1.23 mm, and −0.03 mm ± 1.5 mm, respectively. A primary observation of depths revealed that the depth decreased as the size increased. However, the results of the statistical analysis did not reveal any significant difference between the groups. The result indicated that each size of Talar-Fit might exceed the midline of the talus neck. Thus, it was hypothesized that the medial edge of the implant might exceed the midline of the talus neck. Further research is still required to determine the extent to which depth can be exceeded, as some researchers consider depth as a risk factor for developing sinus tarsi pain (14, 15, 27, 28).

While this study provides valuable insights, it has some limitations. First, the presence of information bias and selection bias should be acknowledged due to the retrospective design of this study. Second, there was no comparison made between the evaluation of STA and the patients treated with conservation treatment. Third, the sample size was small, resulting in smaller sample sizes across subgroups. A prospective and randomized controlled study should be conducted in the future to confirm our results.

In conclusion, the findings of our study indicated that subtalar arthroereisis with Talar-Fit implant proved to be an effective treatment for pediatric flexible flatfoot. Furthermore, it was demonstrated that a larger implant size could generate a superior effect compared with the small one. In addition, the present study determined the exact position of Talar-Fit inserted, and indicated that the medial edge of the implant might be able to exceed the midline of the talus neck. Future research can seek to increase the sample size to provide long-term outcomes, and RCTs must be conducted to improve the quality of such studies.