In the intervening years since Recaredo Álvarez proposed the original subtalar arthroereisis technique in 1972, many different variations have been reported. In this study we refer only to variations in the extra-articular (SESA) technique, leaving the discussion on techniques inside the sinus tarsi and comparisons to other authors.
The surgical indications for FFF have to be rigorous, especially as the distinction between a physiological and a pathological situation is not always clear-cut. Several authors [11–13] have reported that the physiological valgus of the child’s foot spontaneously evolves to the shape of the adult’s foot at around 10 years of age and that thereafter there are no important changes of the foot’s longitudinal arch and its global shape. Based on these studies, we therefore recommend waiting to see whether the foot develops normally—until the child is at least 10 years old—before proceeding with a surgical correction.
Numerous surgical procedures for the correction of FFF have been proposed. These can be categorised as soft tissue plications, tendon lengthening and transfers, osseous excisions, osteotomies, arthrodesis of one or more joints and the interposition of bone or synthetic implants into the sinus tarsi [14, 25–34].
In 1946, Chambers [35, 36] described for the first time the concept of arthroereisis for pathologic pronation of the foot. He believed that the excessive eversion would be limited by elevating the sinus tarsi floor with an autogenous bone graft under the leading edge of the calcaneus posterior facet. In 1970, LeLièvre  employed autogenous bone grafts within the sinus tarsi to limit pronation, using a free-floating bone graft obtained from the base of the proximal phalanx, which was then resected as part of the hallux valgus repair. This author was the first to introduce the term “lateral arthroereisis” . In 1983, Smith and Millar  first described the subtalar arthroereisis-peg procedure, using a device made of ultrahigh molecular-weight polyethylene. Other devices which have been used include a threaded polyethylene plug inserted into the sinus tarsi, described by Giannini in 1985 , and a champagne glass-shaped silicon implant, described by Viladot in 1992 .
In the 1980s, Pisani et al. [24, 39] introduced into Italy the technique suggested by Recaredo Álvarez in Spain in 1970, subsequently published by Burutaran in 1979 . This technique, also known as “calcaneo-stop”, is an extra-articular arthroereisis of the subtalar joint and therefore performed outside the sinus tarsi. Its general application quickly spread throughout Italy [20, 41–45] and, more recently, into other European countries [20, 46–48]. Although many different variations of the original technique have appeared since its introduction, the principles of the correction are still the same [41, 49].
Magnan et al.  compared the original technique and Castaman’s modified technique  and found that the mechanical component of the technique was more important than the type of screw implanted. In an Italian study , 306 patients affected by flatfeet (480 feet) underwent surgery with the either Alvarez technique or Castaman technique (in which the screw is implanted in the talus instead of the calcaneus). The screws used were AO screws (cortical or cancellous). In the Alvarez arm of this study, both types of screw were used, while in the Castaman arm, only the cancellous screws were implanted. In those patients treated with the Alvarez technique, the authors observed no significant differences in the results and complications (loosening, osteolysis, rupture of the screws) between the cancellous (diameter 4.5/6.5 mm) and cortical (diameter 3.0/4.5 mm) screws. In contrast, in those patients treated with the Castaman technique, rupture of the cancellous screw implanted in the talus was reported in 6.3 % of cases. These results imply that the position and mechanical action of the implant is fundamental to the success of the technique—and not the type of implant itself. However, the action of the screw in the original technique is presumed to be more than just mechanical, as suggested in our study where there were only a few cases of loosening of the screw and osteolysis of the talus lateral process (in long term follow-up) was absent in the great majority of patients. Moreover, in a few cases of bilateral involvement, we observed a spontaneous correction of the non-operated foot prior to surgery on the contra-lateral. The mechanism underlying this correction is assumed to be the proprioceptive one.
How does the screw work? First, we know it has a mechanical effect because the result is immediate: in the younger children enrolled in our study the correction decreased with growth and in eight cases reported here there was a protrusion of the screw head into the talus, where contact is the greatest. Second, it is known that joint stability is constituted by static and dynamic elements, with the former depending on the anatomical congruity of joint surfaces and on ligamentous restraints which limit joint translations. In contrast, the dynamic joint stability implies a proprioceptive control of the compressive and directional muscular forces which act on the joint . Proprioception plays a critical role in ankle joint stability; in particular, the subtalar joint has a critical function in adapting the foot to the ground .
The role of proprioception post-SESA has been suggested in previous studies [24, 41]. Based on the analysis of our 23-year data set, we agree with this theory as we encountered a number of interesting aspects in our patients. During our clinical experience, we have also performed SESA in patients with osteogenesis imperfecta [20, 52], who have a lower bone resistance, and found no signs of screw protrusion in these patients. Another aspect we considered is that the screw becomes shorter during foot growth, as seen at removal surgery; however, as the correction is persistent in most cases, another type of correction rather than the mechanical one is implied. Furthermore, in 14 patients we noted a peroneal contracture, which is a reaction to pain and to stimulation of the sinus tarsi mechanoreceptors, as described in the following text.
Rein et al.  analysed the pattern and types of mechanoreceptors (Ruffini endings, Pacini corpuscles, Golgi-like endings, free nerve endings and unclassifiable corpuscles) in the different anatomical complexes of ankle ligaments using designated immunohistochemical markers. The free nerve endings were the predominant mechanoreptor type, followed by Ruffini endings, indicating that nociception and joint position are greatly important in terms of ankle proprioception. In a following study, Rein et al.  showed that the lateral root of the inferior extensor retinaculum at the entrance of the sinus tarsi was richly innervated with free nerve endings, as compared to the deeper situated canalis tarsi ligament. Based these observations, it may be assumed that the pain of the sinus tarsi syndrome mainly originates at the entrance of this structure. Other studies have shown that patients with functional ankle instability and pain near the sinus tarsi have a prolonged peroneal reaction time (PRT) . This prolonged PRT suggests a proprioceptive role of the sensory nerve endings at the sinus tarsi in regulating the activities of the gamma motor neurons of the peroneal muscles, which in turn may cause the symptoms of functional ankle instability and prolonged PRT. These studies by Rein et al. [53, 54] provide the basis to explain how the screw works at the level of the lateral subtalar joint, below the talar lateral process, by explaining the proprioceptive effect of the screw on one hand and possibly elucidating those cases of peroneal muscle contracture which have no identifiable failure of the surgical technique on the other hand. An interesting research question in the framework of comparing inside and outside sinus tarsi devices would be to examine how a device implanted inside the canalis tarsi can stimulate the receptors (both in a mechanical and proprioceptive manner).
Among the patients with FFF treated with SESA in our study, the clinical and X-ray studies during the follow-up period show good outcomes in approximately 94 % of patients, even after screw removal. Our X-ray measurements show a greater improvement of the Costa-Bartani and talar inclination angles than of the calcaneal pitch angle, probably due to the site of correction, i.e. the subtalar joint. The average follow-up was 4.5 years, and, at the time of evaluation all our patients had reached complete foot skeletal maturity and had no recurrence of the deformity. We evaluated the outcome in 121 feet after screw removal, which occurred on average 2.9 years post-SESA. The data collected after screw removal show similar pre- and post-operative values of the Costa-Bartani and talar inclination angles, which, however, are improved with respect to the measurements post-SESA. Interestingly, the calcaneal pitch angle at screw removal, in contrast to the immediate post-SESA period, did improve, although in a statistically nonsignificant manner. This result may be explained by the calcaneus progressive correction after the improvement of the talo-calcaneal relationship at the subtalar joint and demonstrates that the correction obtained with SESA is effective, progressive and maintained.
In our population there was never the need to perform a gastrocnemuis recession in idiopathic FFF, although Achilles tendon retraction in FFF was an indication for surgery; physiotherapy has proved to be an effective treatment after SESA. The term “flexible flatfoot” implies a deformity which can actively be corrected in all planes when the patient is on tiptoes and manually during the examination. The concomitant presence of an Achilles tendon retraction before surgery does not limit the “flexibility” of the deformity, but it may determine the absence of a future spontaneous correction. However, in our patients, if the retraction was present it did not modify our surgical procedure.
Approximately 15 % of our patients had an in-toeing gait and a foot in the supination position for the first 3 months, which we did not consider as complications. The actual complication rate among our patients was 6.3 % and includes patients with ankle joint effusion or haemarthrosis, contracture of the peroneal muscles due to an antalgic position in pronation and stress fractures of the fourth metatarsal bone due to an abnormal gait with excessive weight-bearing on the fourth to fifth rays. Clearly fracture of the fourth metatarsal occurred, as the fifth ray is physiologically more mobile. These patients were all treated accordingly and symptom resolution occurred in most cases.
One limitation of this study was the lack of a patient’s satisfaction survey and a validated evaluation score after surgery. However, during the long follow-up period, nearly all patients were clinically and functionally satisfied with the outcomes. The indication for surgery was given after the patient had reached an age of 10 years, following which, as reported in the literature, there will be no further spontaneous improvement in the natural history of FFF occurs. Therefore, another limitation to our study was the lack of a control group of children with FFF with surgical indication who were aged >10 years and had not undergone surgery.
In approximately 50 % of our patients there was a monolateral involvement. Interestingly, in the majority of these patients the clinical findings were markedly “monolateral”. The definition of “pathologic” is much clearer in these cases with respect to patients with bilateral involvement, where the concept of “physiological variant” may arise.