Long-term radiographic outcomes of distal adding-on after posterior selective thoracic fusion in Lenke 1A adolescent idiopathic scoliosis: 5-year follow-up

Objective To clarify the onset timing, long-term radiographic evolution, and response to bracing of distal adding-on (DAO) following posterior selective thoracic fusion (PSTF) in Lenke 1A adolescent idiopathic scoliosis (AIS), with a minimum 5-year follow-up. Methods We retrospectively analyzed 119 patients (mean age 14.5 years; 27 males, 92 females) who underwent PSTF between 2012 and 2019. Radiographic parameters were assessed at immediate postoperative, DAO-occurrence, 1 year, 2 years, and final follow-up. Parameters included coronal balance (C7 plumb line to CSVL), distal Cobb angle, coronal angulation of the intervertebral space below the lowest instrumented vertebra (LIV), distance between the vertebra below LIV and the sacral midline (LIV+1-CSVL), shoulder height difference (SHD), T1 tilt angle, and clavicle angle. Changes during long-term follow-up after the onset of DAO were evaluated. Results DAO occurred in 22 patients (18.5%) postoperatively, comprising 5 males and 17 females. The mean postoperative follow-up duration was 64.7 months (range, 61–72 months). Twenty patients (90.9%) developed DAO within 6 months postoperatively. Significant differences were observed in shoulder balance parameters (SHD, T1 tilt angle, and clavicle angle) at the time of DAO occurrence compared to immediate postoperative values ( P < 0.05). When comparing the time of DAO occurrence with 1 year, 2 years, and final follow-up, no significant changes were noted in radiographic parameters ( P > 0.05). Three patients (13.6%) exhibited progression of DAO at the 1-year follow-up, all of whom improved after brace treatment. No revision surgeries were required, and no brace-related complications (e.g., skin breakdown, pulmonary restriction) were observed. Conclusion In Lenke 1A AIS following selective thoracic fusion, DAO predominantly appears within 6 months, warranting frequent early radiographic follow-up (e.g., at 3 and 6 months). Shoulder balance parameters at onset differ significantly from immediate postoperative values. Most DAO progresses slowly and remains stable long-term; progressive cases can be effectively managed with bracing.