Purpose <p>To examine the factors associated with distal adding-on (DAO) in Lenke type 1, 2 adolescent idiopathic scoliosis (AIS), particularly concerning flexibility assessment using preoperative fulcrum side bending (FSB). In addition, a secondary aim of this study was to compare the effectiveness of Fulcrum Side Bending (FSB) and Active Side Bending (ASB) radiographs, obtained in standing and supine positions, in assessing the flexibility of the main curve.</p> Methods <p>We analysed 32 patients who underwent posterior corrective fixation for Lenke type 1, 2 AIS. Correction was performed using the all-pedicle screw method and the fixed caudal end was the last touching vertebra (LTV). Radiographic parameters, such as the standing Cobb angle (proximal thoracic, PT; main thoracic, MT; thoracolumbar, TL), were measured before surgery, after surgery and 2&#xa0;years postoperatively. The Cobb angle was also evaluated under stress conditions of ASB and FSB, and the difference from the standing Cobb angle was calculated as ΔASB and ΔFSB. Patients were grouped based on the occurrence of DAO.</p> Results <p>DAO occurred in 12 patients (37.5%); comparing the DAO and non-DAO groups, ΔFSB(PT) (11.2 ± 6.0 vs. 6.9 ± 5.8, p = 0.04) and ΔFSB(MT) (43.9 ± 13.1 vs. 35.9 ± 6.7, p = 0.02) and correction rate (MT) (81.2 ± 8.4 vs. 73.6 ± 9.8, p = 0.02) were significantly greater. Logistic regression analysis showed that the risk factors for DAO were ΔFSB (MT) (odds ratio 1.23 ± 0.098, p = 0.03) and correction rate (MT) (odds ratio 1.17 ± 0.083, p = 0.04).</p> Conclusions <p>ΔFSB and correction rate in MT were a risk factor for DAO. In cases with large MT corrections, the coronal plane was overbalanced in the TL, which could cause DAO during the postoperative course. Care should be taken in the setting of the lowest instrumented vertebra (LIV) to prevent DAO in patients with significant MT correction on preoperative FSB.</p>