Determining the minimal clinically important difference of angle of trunk rotation for monitoring conservatively treated adolescent idiopathic scoliosis

Purpose: The angle of trunk rotation (ATR) is widely used in the clinical follow-up of adolescent idiopathic scoliosis (AIS); however, the minimal clinically important difference (MCID) has not been clearly defined. This study aimed to determine treatment-specific MCID values for ATR in conservatively treated AIS patients. Methods: This retrospective cohort study included 316 AIS patients treated with brace plus exercise or exercise alone. Radiographic outcomes at 6 months were classified as improvement (ΔCobb ≤ − 5°), stabilization (− 5° < ΔCobb < + 5°), or worsening (ΔCobb ≥ + 5°). Receiver operating characteristic (ROC) analyses were performed to determine optimal ΔATR cut-off values for clinically meaningful improvement and worsening in the overall cohort and separately by treatment group. Results: In the overall cohort, ΔATR showed good discriminative ability for improvement (AUC = 0.778) and worsening (AUC = 0.777). The optimal cut-offs were ΔATR ≤ − 1.5° for improvement and ΔATR ≥ + 0.5° for worsening. In subgroup analyses, the improvement threshold was ΔATR ≤ − 1.5° in the exercise-only group and ≤ − 4.5° in the brace-plus-exercise group. For radiographic worsening, the optimal thresholds were ΔATR ≥ + 0.5° in the exercise-only group and ≥ − 2.5° in the brace-plus-exercise group. Conclusions: Treatment-specific ΔATR thresholds provide clinically meaningful benchmarks for monitoring both improvement and worsening, supporting the use of ATR as a practical radiation-free follow-up parameter in AIS management.