1 Introduction

Subtalar arthroereisis is commonly performed on paediatric patients as a treatment for flexible planovalgus deformity. The primary aims of the operation in paediatric patients are pain relief and improvement of function by restoring the medial arch of the foot.¹ Caravaggi et al. performed a kinematic study using skin-markers to show that there is also greater hindfoot eversion and greater forefoot abduction and supination after subtalar arthroereisis.² Subtalar arthroereisis is increasingly being used in adults as an adjunct for the surgical management of flexible planovalgus.

Surgical intervention for flexible planovalgus is usually only offered following the failure of conservative measures, such as rest, physiotherapy and orthotics, when there is ongoing pain or functional impairment. Surgical options include modifying soft tissues, osteotomy, or arthrodesis.¹ Additional to these previously recognised surgical options, subtalar arthroereisis can be offered as an adjunct to a greater multi-step procedure. The principle of subtalar arthroereisis screw insertion into the sinus tarsi is to prevent subtalar joint hypermobilisation and pronation, thus stabilising the tendons around the joint and reducing planto-medial rotation of the talus.³ The advantage of subtalar arthroereisis are: short surgical time; minimally invasive; simple reversal with screw removal can be performed when complications arise; and patients are full weight-bearing post-operatively.¹ For correction of painful congenital flexible planovalgus in children, subtalar arthroereisis can be used as an adjunct to a multi-step procedure or as a stand-alone technique, whereas in adults subtalar arthroereisis is most often used as an adjunct to a multistep procedure.^3,4

In paediatric patients who undergo subtalar arthroereisis, the implant acts as an internal brace giving rise to tissue remodelling and maturation around the implant and is often only removed prematurely if it gives rise to sinus tarsi pain.⁵ A literature review by Metcalfe et al. investigated removal rates of subtalar arthroereisis implants in the paediatric population and found that for all implant types it is removed in 7.1% to 19.3% of patients.⁶

The hypothesised purpose in adults is to temporarily offload the medial soft tissue reconstruction to facilitate fibrosis and healing. In adults the most common complication is sinus tarsi pain, the cause of which is likely multifactoral and a combination of bone and soft tissue irritation as well as inadequate implant size and fixation.⁷ Some studies show that up to 50% of adults develop post-operative sinus tarsi pain, which often resolves after implant removal.¹ Due to such high rates of pain, some surgeons routinely remove the arthroereisis screw several months post-surgery. Needleman et al. analysed an adult population who underwent subtalar arthroereisis for treatment of flexible planovalgus and found that 11 of 28 feet (39%) required implant removal due to sinus tarsi pain.⁸ Stichnoth et al. compared subtalar arthroereisis, medialising calcaneal osteotomy and a combination of the two for adults with flexible planovalgus and found that metalwork was removed in 50% who underwent subtalar arthroereisis and 42% of those who had the combined procedure.⁹ A systematic review by Baryeh et al. reviewed outcomes of subtalar arthroereisis as management of adult acquired planovalgus¹⁰ and showed removed rate of implants from eight studies comprising 175 feet ranged from 6.7% to 58.3% with a mean of 22.9%.^11–17

This case series reviewed a local adult patient population who underwent subtalar arthroereisis screw insertion as an adjunct for correction of flexible planovalgus and monitored them for up to one year of follow-up to identify how many, if any, required removal of the implant and the reasons why. The series was compiled to compare with the limited previously published data, to attempt to identify reasons for removal and enable disclosure of the likelihood of requiring removal to patients prior to the procedure. The cohort underwent insertion of the Arthrex ProStop® arthroereisis screw, which is a non-bioabsorbable, conical-shaped implant and can be inserted using a minimally invasive technique.^18,19

2 Methods

This case series includes retrospective cases from a single surgeon across two operating sites. It reviewed the rate of removal of subtalar arthroereisis screws in patients aged 16 years and older who had a sinus tarsi implant inserted as part of operative management for flexible planovalgus. The surgical technique used was a combination of the following: (1) gastrocnemius release; (2) Hokes achilles lengthening; (3) calcaneal slide osteotomy; (4) Arthrex ProStop® arthroereisis implant insertion; (5) Tibialis posterior debridement and FDL tendon transfer with spring ligament double-breasted reefing supplemented with an internal brace; and (6) first ray opening wedge osteotomy.

The surgeon decided on the size of the Arthrex ProStop® arthroereisis implant based on radiological parameters and on table inspection of heel alignment. The image intensifier parameters were as follows: the implant engages the lateral half of the sinus tarsi on the antero-posterior x-ray view; the implant should not be medial to the midline of the talar neck.¹⁸ The further parameter used was the degree to which Meary's angle was reconstituted. The on-table inspection involved examining the plantar view of the heel in relation to the tibia to bring rotational alignment to neutral.

Patients were identified by the surgical department administration lead at each operative site, searching for the patients who had had a subtalar arthroereisis implant ordered prior to their procedure. Each hospital number was inputted into the local operating programme, Bluespier at one operating site and IMS Maxims at the other, to identify the relevant patient cohort. Each patient had the following data recorded: operative procedure; date of operation; date of birth; age; sex; implant size; procedural steps; follow-up plan. All data was stored in a password-encrypted file. The Arthrex ProStop® arthroereisis screw was the only screw used by the surgeon. The retrospective search identified patients that underwent operations occurring between August 2022 and May 2024. Once the patient cohort was identified, each of their follow-up clinic appointment letters were reviewed to assess their post-operative prognosis. All patients were followed up with for one year, unless they were discharged earlier.

The inclusion criteria used to select patients for the study was: patients aged 16 years and older; operated on between August 2022 and May 2024; had an Arthrex ProStop® arthroereisis implant inserted for surgical correction of flexible planovalgus; followed up in local clinic; access available to all clinic notes for review. Exclusion criteria: patients aged under 16 years; surgical correction of flexible planovalgus without the insertion of an arthroereisis implant. No bias was identified in selecting patients for the project as all patients that met the inclusion criteria throughout the time period of the search were included. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Kettering General Hospital on August 08, 2025, with the need for written informed consent waived. There was no consideration of co-morbidities or pre-operative deformity severity in patient selection. No standardised outcome measure was used to follow-up the patients. Due to the use of a small sample size, Clopper-Pearson exact confidence intervals were used.

3 Results

Eleven patients were identified as having undergone insertion of a subtalar arthroereisis screw for surgical flexible planovalgus correction. One patient was excluded from the study as they were 14 years old at the time of surgery. The demographics of the patients included in the study are as per Table 1 and of the ten patients included three were male and seven female, with an average age of 53.8 (range, 17-82) years. The implant used in each patient was the Arthrex ProStop® arthroereisis screw. ProStop® arthroereisis subtalar implants for correction of flexible flatfoot deformities come in the following sizes (width x length): 7 mm × 12 mm; 8 mm × 14 mm; 9 mm × 14 mm; 10 mm × 14 mm; 11 mm × 16; 12 mm × 16 mm.¹⁸ As seen in Table 2, sizes (width) 7 mm, 8 mm and 10 mm were used in this cohort of patients. Every patient underwent flexible planovalgus correction and the insertion of the ProStop® arthroereisis implant was one step in a multi-step operation. All patients have been followed up within clinic post-operatively for up to one year or until discharge (if prior to one year).

After one year of follow-up, one patient required removal of the arthroereisis implant. Upon review in clinic six months post-procedure, the patient complained of pain at the implant insertion site. Radiographs on the foot and ankle showed the implant was backing out of the sinus tarsi, which was causing their symptoms. For this patient a 7 mm Arthrex ProStop® arthroereisis implant was used. Implant removal was carried out two months later through the anterolateral scar, which resulted in complete resolution of symptoms and they were subsequently discharged.

As for the remaining nine patients, six have been discharged without complications, one had no complications and has been listed for surgical correction of the contralateral side and one is awaiting final clinic follow-up having yet to have any complications. One patient is developing navicular cuneiform joint arthritis (unrelated to the arthroereisis implant) and is awaiting a diagnostic steroid injection with no plan to remove the subtalar arthroereisis implant. Therefore, subtalar arthroereisis implants were removed in 1 of 10 patients (10%, 95% CI 0.3 – 44.5%; Clopper-Pearson exact).

4 Discussion

After 12 months of follow-up, one patient (10%, 95% CI 0.3 – 44.5%; Clopper-Pearson exact) required removal of the arthroereisis implant. This was identified during post-operative review in clinic with radiographs showing that the implant was in an unsatisfactory position and was backing out of the sinus tarsi. After removal of the implant, all symptoms resolved. The wide confidence intervals reflect the small size of the cohort in the case series.

The aforementioned systematic review by Baryeh et al. identified an average implant removal of 22.9% across all studies, however five of the nine studies had removal rates between 6.7% and 11.5%.^11–15 Of these studies, Ozen et al.¹³ had the highest removal rate and saw three of 26 implants removed, however only two due to sinus tarsi pain and one due to loss of fixation. These results would suggest that, contrary to previous opinion, keeping subtalar arthroereisis implants in-situ in the adult population could be justified if they are not symptomatic and, by doing so, would remove the risks associated with further surgery.

When comparing findings in this case series with the literature, Saxena et al.²⁰ performed a prospective study analysing the removal rates of subtalar arthroereisis implants in adults over 18 years old. They followed up a total of 100 adults (104 procedures) for a minimum of two years and found that 22.1% required removal, with implant size being the biggest risk factor (11 mm (width) implants most commonly removed, p = 0.02). In this study by Saxena et al., only the removal of 11 mm implants was statistically significant. They did not find any statistical significance between the mean ages of patients that required removal of the implant and those that did not, nor between the different implant manufacture companies. There were 79 procedures performed where an implant of 10 mm or smaller was inserted and 16.5% of these required removal. For a comparison of the same manufacturer, the Arthrex ProStop® implant was used in 55% of procedures in their study and 20.7% required removal.

Whilst this case series has a much smaller patient cohort, the lower removal rate could be explained due to the use of smaller arthroereisis implants, with 10 mm being the largest used. As stated by Arthrex, because the ProStop® implant is a soft tissue screw and not one that is inserted into bone, the mantra of “larger screw, better fixation” is often not valid for this procedure.¹⁸ The only patient requiring implant removal was one that had a 7 mm Arthrex ProStop® implant inserted. There are, however, a number of factors that could explain this finding aside from the size of the implant alone, such as the patient's anatomy, surgical technique, undersizing of the sinus tarsi during insertion and reliability of the patient and their cooperation with post-operative instructions. It could be argued that the removal rate seen in this study would begin to align with that seen in the study by Saxena et al. if follow-up were to be extended to two years.

It should be noted that much of the literature states that after subtalar arthroereisis screw removal prognosis is positive. Needleman et al.⁸ showed that of those that developed pain after arthroereisis implant insertion, all patients without pre-existing subtalar joint pathology experienced a reduction or resolution of sinus tarsi pain after implant removal. They also showed that after removal of the implant, flatfoot correction was maintained.

There are a number of limitations identified for this case series. The sample size is small, meaning comparing the findings with the literature is difficult. Another limitation to consider is that all patients have been operated on by one surgeon, which could mean that extrapolating the results to all surgeons is less reliable. The process used for patient selection may introduce bias and there were no validated scoring systems used for pain, function or radiographic measurements for pre-surgery or follow-up. Reliance on clinic letters introduces the risk of human error and reporting bias. Finally, it should be acknowledged that follow-up of a longer duration could yield different results.

5 Conclusion

This case series focusing on the removal of subtalar arthroereisis implants in a local adult patient cohort identified a 10% implant removal rate after one year of follow-up. The series compiled data on arthroereisis implant sizes of 10 mm or less. If planovalgus correction is deemed successful with an implant within this size range, post-operative complications of further surgery could be avoided whilst achieving the intended outcome of the procedure. Due to the limitations of the small cohort used in this study, a larger multi-centre, multi-surgeon study evaluating the rate of removal of adjunct subtalar arthroereisis implants with sub-analysis of each screw according to size would be of benefit. The aim would be to establish whether it could be recommended against the routine removal of certain sized subtalar arthroereisis implants in the management of flexible planovalgus in the adult population, unless patients are symptomatic.

Patient consent

The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Kettering General Hospital on August 08, 2025, with the need for written informed consent waived.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Ethical approval

The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Kettering General Hospital on August 08, 2025, with the need for written informed consent waived.

Credit author statement

1. Daniel Carter

a. Methodology; formal analysis; investigation; data curation, writing – original draft; visualisation; project administration.

2. Thomas Ward

a. Validation; data curation; writing – review and editing; supervision; project administration.

3. Sayyied Kirmani

a. Conceptualization; methodology; writing – review and editing; supervision.

Funding statement

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