BACKGROUND CONTEXT: Menarche is widely recognized as one of the prognostic factors for curve progression in patients with adolescent idiopathic scoliosis (AIS). However, few studies focus on the relationship between small AIS curves without brace treatment and menarche, presenting a challenge to building further evidence. PURPOSE: This study aims to investigate the chronological changes in curve progression and risk of final brace initiation around menarche in small AIS curves under 25°. STUDY DESIGN: This was a retrospective cohort study. PATIENT SAMPLE: We longitudinally examined 1,090 AIS patients with a curve of less than 25° at the initial visit. OUTCOME MEASURES: Patients were followed up until they achieved skeletal maturity or initiated brace treatment. METHODS: Curve progression based on time from menarche was analyzed using a t-test. Receiver operating characteristic curve analysis was performed based on the time from menarche, with curve magnitude as the independent variable and the final initiation of brace treatment as the dependent variable. RESULTS: Overall, 1,090 female patients were included, with a mean initial visit age of 12.9 years (standard deviation [SD]: 1.5) and a mean coronal Cobb angle of 17.5° (SD: 4.3). Curve progression was significantly decreased between 0-1 and 1-2 years post-menarche (0-1 year post-menarche: 2.9°/year vs. 1-2 years post-menarche: 1.3°/year
p=.03). After 2 years from menarche, the mean curve progression was less than 0.4°/year. The cut-off value of the curve magnitude for the final initiation of brace treatment at the timing of menarche was 20.5° (area under the curve: 0.89, p<
.001, 95% confidence interval: 0.86-0.91). CONCLUSIONS: This study highlights that in small AIS curves under 25°, minimal curve progression was observed after 2 years post-menarche, aiding follow-up strategies for AIS conservative treatment.