A retrospective analysis of clinical data was conducted on 102 cases (102 feet) of patients with CLAI with SCF who were admitted from November 2016 to November 2022. All patients at least underwent arthroscopic modified Broström procedure as a minimum surgery intervention. Group assignment was determined retrospectively on the basis of the performed surgery: Patients receiving arthroscopic modified Broström procedure alone comprised the control group, while those undergoing arthroscopic modified Broström procedure with minimally invasive calcaneal osteotomy comprised the experimental group. The decision to perform osteotomy intraoperatively was influenced by severity of SCF and surgeon assessment of persistent instability after ligament repair.

The inclusion criteria were as follows: (1) unilateral CLAI; (2) failing to respond to 6 months of conservative treatment preoperatively, still experiencing pain in the affected ankle and discomfort after walking; (3) physical examination revealing a positive drawer test, increased arch height, hindfoot varus, a positive peek-a-boo heel sign, a positive Coleman block test, and normal lower limb muscle strength; (4) preoperative magnetic resonance imaging (MRI) indicating that the diameter of the lateral ankle ligaments is ≥ 1 mm, with irregular shape or increased thickness; (4) normal electromyography results; and (5) complete follow-up data available.

The exclusion criteria were as follows: (1) MRI findings showing absence of lateral ankle ligaments or diameter < 1 mm, necessitating ligament reconstruction surgery; (2) undergoing adjunctive procedures, such as first metatarsal elevation, gastrocnemius slide, or muscle balance surgery; (3) severe peripheral vascular disease, autoimmune disease, or poorly controlled diabetes; (4) severe rheumatoid arthritis, gouty arthritis; (5) severe heart or lung function impairment; (6) inability to cooperate owing to mental illness; and (7) history of trauma or surgery to the affected foot.

For the detection of SCF, we assessed using radiographic measures (Fig. 1). The calcaneal pitch angle, Meary’s angle, and arch height were assessed using foot weight-bearing lateral view X-ray. The calcaneal pitch angle has a normal range of 20–30° and is at 30° for SCF. Meary’s angle has a normal range of 0–5°. It is greater than 5° or negative in SCF. The arch height was the measurement from first to fifth metatarsal, which is greater than 14 mm in SCF. Subsequently, calcaneus valgus angle was determined using hindfoot long axial view X-ray. It is defined as the angle between the median line of the long axis of the tibia and the calcaneus. A positive angulation is observed on the fibular side of the tibial axis, while a negative angulation is noted on the tibial side. The normal range for this measurement is 0–5°, which can be used to assess the improvement in the line of force through the foot [8].

Imaging indicators for patients with chronic lateral ankle instability with subtle cavus foot. A Calcaneal pitch angle; B Meary’s angle; C arch height from first to fifth metatarsal; and D calcaneus valgus angle

Interobserver reliability of radiographic measurements was assessed by two orthopedic surgeons who independently evaluated 30 randomly selected cases. The intraclass correlation coefficient (ICC) demonstrated excellent agreement: 0.92 for calcaneal pitch angle, 0.89 for Meary’s angle, 0.91 for arch height, and 0.88 for calcaneus valgus angle (all ICC > 0.75).

For patients undergoing minimally invasive calcaneal osteotomy, a 1.5–2 cm oblique incision was made at the inferior lateral aspect of the calcaneus. The periosteum of the lateral wall of the calcaneus was dissected using a periosteal elevator along the incision line. A thin bone knife, measuring 1 cm in width and 1 mm in thickness, was employed to perform an osteotomy approximately 2 cm posterior to the posterior facet of the subtalar joint, perpendicular to the long axis of the calcaneus. Then, the calcaneal tuberosity fragment was laterally displaced by 5–7 mm and temporarily secured with a K-wire. Fluoroscopic confirmation of the foot alignment was performed, followed by fixation with two 6.5 mm hollow screws.

For patients undergoing arthroscopic modified Broström procedure, a modified anteromedial approach was used for the anterior tibialis tendon laterally in a neutral position of the ankle joint, serving as both the observation and working portals for the arthroscopy. The medial and lateral ankle joint spaces were explored, and the proliferative tissue was excised. The anterior talofibular ligament was examined for its morphology and integrity, and the bifurcate ligament was identified. The area beneath the anterior talofibular ligament footprint was located, and two 2.9 mm suture anchor screws (Osteoraptor, Stryker, USA) were inserted. A 50 ml needle with nylon suture was passed through the ligament to retrieve the anchor suture externally, maintaining the ankle joint in a neutral and inverted position. A knot was tied to secure the anterior talofibular ligament along with the joint capsule to the bone surface (Fig. 2).

Minimally invasive calcaneal osteotomy and arthroscopic modified Broström procedure. A An incision was made at the inferior lateral aspect of the calcaneus, and an external calcaneus osteotomy was performed with a bone knife; B intraoperative imaging showing the satisfactory position of two hollow screws; C the arthroscopic modified Broström procedure was performed; D the anterior talofibular ligament footprint was located by arthroscopic examination, and two suture anchor screws were inserted

General data, such as imaging indicators, clinical indicators, and complications, were recorded. Imaging indicators included calcaneal pitch angle, Meary’s angle, and arch height (first to fifth metatarsal) on foot weight-bearing lateral view, and calcaneus valgus angle on hindfoot alignment view.

Clinical evaluation indicators included the American Orthopedic Foot and Ankle Society (AOFAS) Ankle-Hindfoot Score and Visual Analogue Scale (VAS) score, which were used to assess the enhancement of ankle functionality and pain levels.

SPSS 26.0 statistical software (IBM, Armonk, NY, USA) was used for data analysis. The normality of the data was assessed using the Shapiro–Wilk test. Normally distributed data were presented as mean ± standard deviation (\(\overline}}\)± SD), and between-group comparisons were made using t-tests. Subsequent pairwise comparisons were performed using the least significant difference (LSD) t-test. Non-normally distributed data were described using median (M (Q1, Q3)). The Friedman test was used for comparisons at different time points within the group, and subsequently, Bonferroni correction was used for pairwise comparisons. The Mann–Whitney U test was used for intergroup comparisons, and the Hodges–Lehmann test was used to estimate the median difference and its 95% confidence interval (CI). In this study, P < 0.05 was considered statistically significant.

The study was approved by an ethics committee. The confidentiality of patient information was protected. The study was conducted in compliance with the Declaration of Helsinki.