Accurate assessment of curve flexibility is critical for surgical planning and classification in Adolescent Idiopathic Scoliosis (AIS). Supine side-bending radiographs are the most used method for evaluating spinal flexibility and play a central role in determining curve structurality within the Lenke classification system. Traditionally, these radiographs are obtained under direct physician supervision to ensure maximal patient effort; however, this approach is limited by operator dependence, variability in patient cooperation, and increased clinical workload. A standardized radiolucent support frame combined with a radiographer-led protocol represents a potential alternative by providing consistent mechanical assistance and standardized positioning. We hypothesized that supine side-bending (SSB) radiographs acquired using a support frame would provide curve flexibility measurements comparable to those obtained under physician supervision. This retrospective study included AIS patients who underwent posterior spinal fusion and preoperative SSB radiographs between 2017 and 2025. Patients imaged using the support frame were propensity scores–matched to those who underwent physician-supervised side-bending radiographs based on demographic characteristics, skeletal maturity, curve type, apical vertebral level, and preoperative Cobb angles, resulting in 23 matched patient pairs. Proximal thoracic, main thoracic, and thoracolumbar/lumbar curve flexibility were calculated from preoperative erect and side-bending Cobb angles and compared between groups using t-tests. After matching, baseline characteristics and preoperative curve magnitudes were well balanced between groups. No significant differences were observed in proximal thoracic flexibility (41.7% vs 43.7%, p = 0.708), main thoracic flexibility (61.8% vs 58.0%, p = 0.829), or thoracolumbar/lumbar flexibility (99.4% vs 96.7%, p = 0.765) between the support frame and physician-supervised groups. In conclusion, supine side-bending radiographs performed using a standardized support frame with a radiographer-led protocol provide results equivalent to those obtained under direct physician supervision and may improve workflow efficiency while maintaining diagnostic accuracy in the assessment of spinal flexibility in AIS.