Background: The indication and optimal timing of surgery for adult residual adolescent idiopathic scoliosis (AdIS) remain controversial because its clinical characteristics and outcomes are not fully characterized. We hypothesized that clinical presentation and surgical outcomes in AdIS differ according to the location of the major curve (thoracic-major vs lumbar-major).

Methods: This study was a retrospective analysis of prospectively collected data. We included 59 AdIS patients who had been diagnosed as AIS Lenke type 1 or type 5 in adolescence and underwent posterior correction and fusion surgery between 20 and 40 years of age. The major-curve Cobb angle ranged from 40° to 60°. Patients were divided into two groups based on the adolescent Lenke classification (type 1 vs type 5). Preoperative and postoperative radiographic parameters and SRS-22 scores at final follow-up were compared between groups.

Results: Mean age at surgery was 24.7±5.6 years in the type 1 group and 27.4±7.4 years in the type 5 group. Preoperative major-curve Cobb angle was 55.4±9.6° (type 1) and 53.0±3.4° (type 5), with similar postoperative Cobb angles (16.1±3.7° vs 16.9±1.1°). Preoperatively, SRS-22 function and pain scores were lower in the type 5 group (function: 4.7±0.3 vs 4.1±0.1; pain: 4.3±0.5 vs 3.7±0.1), and mental health was also lower in type 5 (3.8±0.6 vs 3.3±0.1), whereas self-image was similarly low in both groups (2.7±0.1 vs 2.7±0.6). Postoperatively, SRS-22 domain scores were comparable between groups, and satisfaction was high in both (4.5±0.2 vs 4.5±0.2). Self-image improved in both groups (+1.6±0.6 vs +1.4±0.1), while function and pain improved notably in type 5 (+0.7±0.2 and +0.7±0.2, respectively).

Discussion: Adult residual AIS patients with lumbar-major curves showed greater preoperative pain and functional impairment than those with thoracic-major curves, despite similarly low self-image scores. Posterior correction and fusion achieved comparable radiographic correction and high postoperative satisfaction in both curve types, with meaningful improvements in self-image across groups and additional gains in pain and function particularly in lumbar-major curves. These findings support curve-location–based counseling regarding symptom profiles and expected postoperative benefits.