Distal Adding-On as a Natural Shoulder Rebalancing Mechanism in Lenke Type 2A AIS with Right Sacral Slanting.

Background/Objectives: Distal adding-on (DA) is a common postoperative phenomenon in Lenke type 2A adolescent idiopathic scoliosis (AIS). Postoperative shoulder imbalance (PSI) is a clinically significant issue following AIS correction, as it may lead to aesthetic dissatisfaction, functional impairment, and reduced quality of life. This study investigated radiographic changes in DA and shoulder balance in Lenke type 2A AIS, particularly focusing on distal wedge angle (DWA) and radiologic shoulder height (RSH) in patients with right sacral slanting (RSS). Methods: We retrospectively analyzed 120 patients with Lenke type 2A AIS who underwent posterior spinal fusion. Patients were grouped by sacral slanting: right (RSS), left (LSS), or none (NS). Radiographic parameters including proximal thoracic curve angle, main thoracic curve angle, DWA, RSH were assessed at multiple time points. Univariate and multivariate linear regression analyses were used to identify factors associated with DA. Results: The RSS group consistently showed the highest DWA and the greatest incidence of DA. RSH initially exceeded the PSI threshold in all groups but decreased to approximately 10 mm by final follow-up. In the RSS group, the inverse relationship between increasing DWA and decreasing RSH was most pronounced. Univariate regression identified postoperative RSH and sacral slanting angle as significant predictors of DWA, though not in the final multivariate model. Conclusions: In Lenke type 2A AIS with RSS, an increasing DWA and decreasing RSH over time suggest that DA may serve as a compensatory mechanism for PSI. Sacral slanting and postoperative RSH may be relevant predictors of this dynamic alignment change.