Sex-Equitable Machine Learning Prediction of Ankle Reinjury in Individuals with Chronic Ankle Instability: A 2-Year Prospective Cohort Study with Fairness Audit

BACKGROUND: Patients with chronic ankle instability (CAI) face a 2-year reinjury rate approaching 50%, but individualized prediction models that quantify which baseline impairments predict subsequent reinjury within an established CAI cohort remain scarce, and existing prediction models for ankle injury have not assessed sex-related fairness. The aim was to develop and internally validate sex-equitable machine learning (ML) models predicting 2-year ankle reinjury in individuals with CAI, identify the most influential baseline prognostic markers, and audit sex-stratified model performance. METHODS: One hundred physically active individuals (50 male, 50 female) with CAI (mean age 21.8 ± 2.3 years) were recruited from a university population between February 2017 and February 2018 and prospectively followed for 2 years; 96 (49 male, 47 female) with at least one follow-up response were retained for analysis (41 reinjury events, 43% reinjury rate). Fourteen baseline predictors spanning demographic, anthropometric, injury-history, patient-reported, biomechanical, postural-control, and functional-performance domains were collected. Six algorithms were trained with nested cross-validation: an unpenalized logistic regression baseline and five ML algorithms (Elastic Net, support vector machine, random forest, XGBoost, LightGBM). Predictive contribution of each feature was quantified using Shapley additive explanations and partial-dependence plots. A pre-specified fairness audit compared single global and sex-specific Youden thresholds derived out-of-fold using the equalized-odds gap and disproportionate-impact ratio. RESULTS: All algorithms achieved area under the curve (AUC) values above 0.70; Elastic Net was selected as the primary model (AUC = 0.807, 95% CI 0.714-0.893; calibration slope = 1.015). Sex, arch height index, previous sprains, dorsiflexion range of motion, and Biodex Overall Stability Index consistently emerged as the most influential prognostic markers. Within-sex discrimination was comparable between males and females, but a single global threshold produced an equalized-odds gap of 0.384 that was reduced by 57% (to 0.164) through sex-specific thresholding without retraining the model. CONCLUSIONS: ML models with a fairness-audited threshold strategy predicted 2-year ankle reinjury in individuals with CAI, reducing the equalized-odds gap by 57%, though the disproportionate-impact ratio (0.683) remained below the 0.80 benchmark. External validation is required before clinical deployment.