Body mass index is positively associated with flatfoot and inversely associated with hindfoot valgus in school-aged children: a cross-sectional analysis of 3,292 screening records.

BACKGROUND: A higher body mass index (BMI) has been associated with pediatric flatfoot, but it is unclear whether BMI relates similarly to arch morphology, hallux alignment, and hindfoot alignment when these measures are collected within the same school-screening workflow. Herein, we examined phenotype-specific associations between BMI and three scanner-derived foot measures. METHODS: This secondary cross-sectional analysis linked project-based school foot-screening records to on-site anthropometric measurements. After linkage and quality-control exclusions, 3,292 records from six primary schools were analyzed. Record-level outcomes were moderate-to-severe flatfoot (arch index ≥ 0.28), mild hallux valgus (15° to <30°), and moderate-to-severe hindfoot valgus (≥4°), with positivity defined by either foot. BMI was analyzed in sample-specific quartiles and per 1 internal SD. Multivariable binary logistic regression adjusted for age, sex, and school; threshold and duplicate-identifier sensitivity analyses were performed. RESULTS: The 3,292 records represented 3,266 unique school-student identifiers; median age was 10 years (IQR: 9-12), and There were 1,616 female records (49.1%). Screening prevalence was 64.3% for flatfoot, 11.5% for mild hallux valgus, and 52.8% for hindfoot valgus. Each 1-SD higher BMI (4.73 kg/m2) was associated with higher odds of flatfoot [odds ratio (OR): 1.37, 95% CI: 1.26-1.49; P < 0.001] and lower odds of hindfoot valgus (OR: 0.82, 95% CI: 0.76-0.88; P < 0.001), while mild hallux valgus showed no statistically significant association (OR: 0.93, 95% CI: 0.82-1.05; P = 0.244). More stringent threshold analyses and analyses retaining one record per identifier produced materially similar BMI associations. CONCLUSIONS: BMI was positively associated with flatfoot and inversely associated with hindfoot valgus, while no significant association was observed for mild hallux valgus. These cross-sectional associations describe operational scanner-derived screening phenotypes and should not be interpreted as causal effects or clinical diagnoses. Prospective studies with documented anthropometric quality control and validated clinical or radiographic measures are needed.