Tan Guan Wei, SY Lee, S Chandirasegaran, CK Chiu, CYW Chan, MK Kwan

Universiti Malaya, Kuala Lumpur, Malaysia

Adult Idiopathic Scoliosis (AdIS) is a complex three-dimensional spinal deformity in which accurate characterization of curve morphology and global spinal alignment is essential for surgical planning. The AdIS classification system proposed by James D. Lin incorporates coronal curve pattern, lumbosacral structural status, and global alignment modifiers, providing a comprehensive framework for adult spinal deformity assessment. This retrospective study evaluated the distribution and applicability of the AdIS classification system in surgically treated adult idiopathic scoliosis over a 3-year period at a single tertiary institution. Consecutive patients aged 18 years and above who underwent posterior spinal fusion between 2023 and 2025 were included. Erect anteroposterior, lateral and supine anteroposterior whole-spine radiographs were reviewed. Curves were classified according to AdIS curve type (Types 1–6), lumbosacral modifier (NS/S), and global alignment modifier (Aligned, Sagittal, Coronal, or Combined Malalignment). A total of 215 patients were included, of which 205 (95.3%) were classifiable according to the AdIS system, while 10 (4.7%) demonstrated reverse curve patterns and were unclassifiable. Among unclassifiable cases, Type 2 and Type 5–related patterns were most frequent (30.0% each), followed by Type 6 (20.0%). For the classifiable cases, Type 1 was the most prevalent curve type (34.6%), followed by Type 2 (17.1%) and Type 6 (16.1%). Lumbosacral non-structural (NS) modifier predominated (88.8%), while structural (S) involvement was present in 11.2% of patients. Most patients demonstrated preserved global alignment, with 98.0% classified as Aligned. Sagittal and coronal malalignment were observed in 0.5% and 1.5% of cases respectively, while no patients demonstrated combined malalignment. Complete subclassification revealed that Type 1 NS Aligned was the most common overall pattern (33.2%). In conclusion, the AdIS classification system demonstrates high applicability in adult idiopathic scoliosis and may facilitate more individualized surgical planning through comprehensive assessment of curve morphology and global alignment. However, a subset of reverse curve patterns remains unclassifiable, highlighting potential limitations of the current framework and the need for further refinement.