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Curve magnitude and skeletal maturity are important factors in determining the efficacy of bracing for the treatment of adolescent idiopathic scoliosis, but curve morphology may also affect brace success. The purpose of this study was to determine the influence of curve morphology on the response to bracing with a thoracolumbosacral orthosis (TLSO).A retrospective review of patients managed with an orthosis for the treatment of adolescent idiopathic scoliosis who were prospectively enrolled at the initiation of brace wear and followed through completion of bracing or surgery was performed. Inclusion criteria were main curves of 25° to 45° and a Risser stage of 0, 1, or 2 at the time of brace prescription. Compliance with bracing was measured with Maxim Integrated Thermochrons. Radiographs made at brace initiation, brace cessation, and final follow-up were used to retrospectively categorize curves with use of the modified Lenke (mLenke) classification system and more broadly to categorize them as main thoracic or main lumbar. The effect of morphology on outcome was evaluated using chi-square and Fisher exact tests.One hundred and sixty-eight patients were included. There was no difference in curve magnitude at the time of brace initiation (p = 0.798) or in average hours of daily brace wear (p = 0.146) between groups. The rate of surgery or progression of the curve to ≥50° was 34.5% (29 of 84) in mLenke-I curves, 54.5% (6 of 11) in mLenke-II curves, 29.4% (10 of 34) in mLenke-III curves, 17.6% (3 of 17) in mLenke-V curves, and 13.6% (3 of 22) in mLenke-VI curves. There were no mLenke-IV curves at the time of brace initiation. The rate of surgery or progression to ≥50° was 34.1% (44 of 129) in the combined thoracic group and 15.4% (6 of 39) in the combined lumbar group (p = 0.0277). In brace-compliant patients (>12.9 hours/day), the rate of surgery or progression to ≥50° was 30.3% (20 of 66) in main thoracic curves and 5.3% (1 of 19) in main lumbar curves (p = 0.0239). One-tenth of curves changed morphology during bracing. The rate of surgery or progression to ≥50° was 35.8% (43 of 120) in persistent main thoracic curves, 20.0% (6 of 30) in persistent main lumbar curves, 12.5% (1 of 8) in main thoracic curves that became main lumbar curves, and 0% (0 of 9) in main lumbar curves that became main thoracic curves (p = 0.0383).Thoracic curves are at greater risk for brace failure than lumbar curves are despite similar initial curve magnitudes and average amount of daily brace wear. A change in curve pattern may imply flexibility and is associated with brace success. Patients with thoracic curves should be counseled accordingly.Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.