Purpose: This study aimed to identify factors associated with distal adding-on (DAO) in adolescent idiopathic scoliosis (AIS) with Lenke type 1 and 2 curves, with particular focus on preoperative flexibility assessment using fulcrum side bending (FSB) radiographs. A secondary objective was to compare the utility of FSB and active side bending (ASB) radiographs—obtained in standing and supine positions—for evaluating main thoracic (MT) curve flexibility.

Methods: We analyzed 32 consecutive patients who underwent posterior corrective fusion for Lenke type 1 or 2 AIS. All surgeries were performed using an all–pedicle screw construct, and the caudal fusion level was set at the last touching vertebra (LTV). Radiographic parameters, including standing Cobb angles of the proximal thoracic (PT), main thoracic (MT), and thoracolumbar (TL) curves, were evaluated preoperatively, postoperatively, and at 2-year follow-up. Curve flexibility was assessed using ASB and FSB radiographs, and flexibility indices (ΔASB, ΔFSB) were calculated as the difference from the standing Cobb angles. Patients were categorized according to the presence or absence of postoperative DAO. Between-group comparisons and logistic regression analyses were used to identify independent predictors of DAO.

Results: DAO occurred in 12 patients (37.5%). Compared with the non-DAO group, patients with DAO demonstrated significantly greater ΔFSB(PT) (11.2 ± 6.0 vs. 6.9 ± 5.8°, p = 0.04), ΔFSB(MT) (43.9 ± 13.1 vs. 35.9 ± 6.7°, p = 0.02), and MT correction rate (81.2 ± 8.4% vs. 73.6 ± 9.8%, p = 0.02). Logistic regression identified ΔFSB(MT) (odds ratio [OR] 1.23 ± 0.098, p = 0.03) and MT correction rate (OR 1.17 ± 0.083, p = 0.04) as independent risk factors for DAO. ASB-derived flexibility parameters showed no significant association with DAO.

Conclusions: Greater MT flexibility on FSB radiographs (ΔFSB) and higher MT correction rates were identified as significant predictors of DAO. Excessive MT correction may induce coronal imbalance at the thoracolumbar level during the postoperative course, contributing to DAO. In patients demonstrating substantial MT flexibility on preoperative FSB, careful selection of the lowest instrumented vertebra (LIV) is warranted to minimize the risk of DAO.