Accurate assessment of curve flexibility is essential for surgical planning in adult idiopathic scoliosis (AdIS). Supine side-bending radiographs are commonly used to evaluate coronal flexibility but are traditionally performed under direct physician supervision, which may limit workflow efficiency and introduce operator-dependent variability. A standardized radiolucent support frame combined with a radiographer-led protocol may offer a reliable alternative by providing consistent mechanical assistance and standardized positioning. We hypothesized that supine side-bending radiographs acquired using a support frame would provide curve flexibility measurements comparable to those obtained under physician supervision. This retrospective study included 26 AdIS patients who underwent preoperative supine side-bending radiographs between October and December 2025. The 26 AdIS patients were propensity score–matched to 189 AdIS patients who underwent physician-supervised supine side-bending radiographs based on demographic characteristics, skeletal maturity, AdIS classification, apical vertebral level, lumbosacral curve and preoperative supine Cobb angles, resulting in 20 matched patient pairs. Proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar (TL) curve flexibility were calculated from the formula: Flexibility (%): (Preoperative Cobb angle – Preoperative SB Cobb angle)/Preoperative Cobb angle x 100. Comparison between groups was conducted using paired statistical analyses. After matching, baseline characteristics and preoperative supine Cobb angles were well balanced between groups. No significant differences were observed in PT flexibility (36.5% vs 31.3%, p = 0.586), MT flexibility (47.6% vs 51.5%, p = 0.281), or TL flexibility (97.1% vs 100.0%, p = 0.709) 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 yield flexibility measurements comparable to those obtained under direct physician supervision and represent a reliable and efficient alternative for preoperative flexibility assessment in AdIS.