THE BONE & JOINT JOURNAL 254 J. P. Y. Cheung, P. W. H. Cheung From The University of Hong Kong, Hong Kong SAR, China Correspondence should be sent to J. P. Y. Cheung; email: cheungjp@hku.hk © 2020 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.102B2. BJJ-2019-0916.R1 $2.00 Bone Joint J 2020;102-B(2):254–260.  Children’s orthopaediCs Supine fexibility predicts curve progression for patients with adolescent idiopathic scoliosis undergoing underarm bracing aims The aim of this study was to assess whether supine fexibility predicts the likelihood of curve progression in patients with adolescent idiopathic scoliosis (AIS) undergoing brace treatment. Methods This was a retrospective analysis of patients with AIS prescribed with an underarm brace between September 2008 to April 2013 and followed up until 18 years of age or required surgery. Patients with structural proximal curves that preclude underarm bracing, those who were lost to follow-up, and those who had poor compliance to bracing (<16 hours a day) were excluded. The major curve Cobb angle, curve type, and location were measured on the pre-brace standing posteroanterior (PA) radiograph, supine whole spine radiograph, initial in-brace standing PA radiograph, and the post-brace weaning standing PA radio- graph. Validation of the previous in-brace Cobb angle regression model was performed. The outcome of curve progression post-bracing was tested using a logistic regression model. The supine fexibility cut-off for curve progression was analyzed with receiver oper- ating characteristic curve. Results A total of 586 patients with mean age of 12.6 years (SD 1.2) remained for analysis after ex- clusion. The baseline Cobb angle was similar for thoracic major curves (31.6° (SD 3.8°)) and lumbar major curves (30.3° (SD 3.7°)). Curve progression was more common in the thoracic curves than lumbar curves with mean fnal Cobb angles of 40.5° (SD 12.5°) and 31.8° (SD 9.8°) respectively. This dataset matched the prediction model for in-brace Cobb angle with less mean absolute error in thoracic curves (0.61) as compared to lumbar curves (1.04). Re- duced age and Risser stage, thoracic curves, increased pre-brace Cobb angle, and reduced correction and fexibility rates predicted increased likelihood of curve progression. Flexibili- ty rate of more than 28% has likelihood of preventing curve progression with bracing. Conclusion Supine radiographs provide satisfactory prediction for in-brace correction and post-bracing curve magnitude. The fexibility of the curve is a guide to determine the likelihood for brace success. Cite this article: Bone Joint J 2020;102-B(2):254–260. Introduction The aim of treating patients with adolescent idio- pathic scoliosis (AIS) who are skeletally immature is to prevent curve progression during growth, 1 and bracing is the most commonly used treatment during adolescent growth to reduce curve progres- sion. 1-3 The appropriate timing for brace treatment is near the peak height velocity where the risk of curve progression is at its highest. 4,5 However, the reported success rate of bracing is not perfect. Weinstein et al 2 suggested that the likelihood of preventing curve progression is dependent on good brace compliance. However, only 75% of patients experience treatment success. Hence, some patients do not respond well even with good brace compliance. 6