Introduction: Postoperative shoulder imbalance (PSI) is an important cosmetic and psychological complication following corrective surgery for adolescent idiopathic scoliosis (AIS). Although PSI has been widely studied in relation to thoracic curve patterns, little is known about its occurrence in Lenke type 5 curves. This study aimed to identify radiographic predictors of PSI in Lenke type 5 AIS patients, with a focus on preoperative fulcrum side-bending (FSB) radiographs as a measure of curve flexibility.

Methods: A retrospective analysis was performed on 21 patients (mean age 14.6 ± 2.6 years) with Lenke 5C AIS who underwent posterior spinal fusion with all-pedicle screw constructs. Pre- and postoperative radiographic parameters, including Cobb angles of the proximal thoracic, main thoracic (MT) and thoracolumbar/lumbar (TL) curves, were evaluated. Flexibility was assessed using active side-bending (ASB) and FSB radiographs. ΔASB and ΔFSB were calculated as the difference from standing radiographs. PSI was defined as a coracoid height difference of at least 9 mm or a clavicle angle of at least 2°.

Results: PSI occurred in seven patients (33.3%). Compared with the non-PSI group, the PSI group demonstrated a significantly greater ΔFSB in the TL curve (41.7° ± 5.7 vs. 35.3° ± 2.2, p = 0.01), a smaller ΔFSB in the MT curve (p = 0.04) and a higher TL correction rate (p = 0.03). Logistic regression identified ΔFSB in the TL curve (OR 1.21, p = 0.03) and TL correction rate (OR 1.15, p = 0.04) as independent predictors of PSI.

Conclusions: Preoperative FSB radiographs provide critical information about both curve flexibility and PSI risk in Lenke type 5 AIS. Surgeons should pay attention to the relative flexibility between the TL and MT curves. When MT flexibility is limited, excessive TL correction may increase the risk of PSI, so intraoperative moderation of TL correction should be considered.