Adolescent Idiopathic Scoliosis in the Adult Patient: New Classification with a Treatment-Oriented Guideline

Background/Objectives: Adolescent Idiopathic Scoliosis persisting into adulthood (AAIS) presents progressive stiffening and degenerative changes that are not fully captured by existing classifications. This heterogeneity complicates clinical decision-making and surgical planning. The aim of this study was to propose a novel, treatment-oriented classification system for AAIS. Methods: A retrospective review was performed on patients with AAIS who underwent surgical correction between 2018 and 2022. Pre- and postoperative radiographs, CT scans, and MRI were analyzed to define curve characteristics and evaluate surgical outcomes. Subgroups were identified according to age and deformity features, and corresponding surgical strategies were outlined. Results: AAIS was stratified into Young Adult Idiopathic Scoliosis (YAdIS, 19–30 years) and Adult Idiopathic Scoliosis (AdIS, >30 years). YAdIS was divided into mild, flexible curves (YAdIS 1) and severe/stiff curves (YAdIS 2). AdIS was classified into three categories: AdIS 1 (isolated coronal deformity), AdIS 2 (combined coronal and sagittal deformity), and AdIS 3 (revision cases). Within AdIS 1, additional refinement by age (30–45, 45–60, >60 years) reflected increasing stiffness and degenerative changes. Tailored surgical strategies included selective fusions, posterior releases, high-density constructs, three-column osteotomies, and combined anterior–posterior approaches, depending on curve type and age group. Conclusions: This classification provides a comprehensive, treatment-oriented framework to support surgical decision-making in AAIS, enabling optimized planning and improved outcomes for adult patients with scoliosis of adolescent onset.