Return to Sport After Subtalar Arthroereisis in Pediatric Flexible Flatfoot: Radiographic Correction and Residual Pain Are Not Independent Predictors

Background/Objectives: Subtalar arthroereisis (STA) is widely used for symptomatic pediatric flexible flatfoot and provides consistent radiographic correction. However, return to sport (RTS) after STA is less well defined, and the relative role of early postoperative pain versus radiographic correction remains unclear. Methods: We performed a retrospective observational cohort study of consecutive skeletally immature patients treated with STA using a non-absorbable endosinotarsal screw at a single tertiary center. Inclusion criteria were symptomatic flexible flatfoot refractory to >6 months of conservative treatment, complete pre-/postoperative weight-bearing radiographs, complete functional data, and minimum follow-up of 6 months. The primary endpoint was RTS to the pre-symptom primary sport at final follow-up. Secondary outcomes were UCLA Activity Score, FAAM Sport subscale, and postoperative VAS. Radiographic correction was quantified as delta change (Δ) in Meary, Costa–Bertani, and Kite angles. Group comparisons used nonparametric tests. A parsimonious multivariable logistic regression model (EPV-constrained) included UCLA at 3 months, VAS pain, and ΔMeary. Results: Fifty patients were included (mean follow-up 9.2 ± 2.5 months; range 6–14); 27/50 (54%) resumed their primary sport. Baseline characteristics were comparable between Returners and Non-Returners. Returners showed higher early postoperative UCLA scores than Non-Returners (8.0 [7.0–8.5] vs. 6.0 [5.0–7.0], p = 0.005). FAAM Sport and VAS pain did not differ significantly between groups (p = 0.224 and p = 0.493, respectively). Radiographic correction magnitude was similar between groups (ΔMeary p = 0.938; ΔCosta–Bertani p = 0.984; ΔKite p = 0.108). In multivariable analysis, UCLA at 3 months was the only independent correlate of RTS (OR 2.65 per point, 95% CI 1.34–6.15; p = 0.009), whereas VAS pain (OR 0.98, 95% CI 0.76–1.25; p = 0.892) and ΔMeary (OR 1.01, 95% CI 0.91–1.13; p = 0.875) were not significant. Conclusions: In this cohort, STA achieved substantial radiographic correction, but neither correction magnitude nor early postoperative pain independently correlated with RTS at short-term follow-up. Early postoperative activity level was the strongest independent correlate of sport resumption, supporting a function-centered postoperative assessment beyond radiographic alignment alone.