Atypical-laterality curve patterns in adult idiopathic scoliosis: refining the AdIS classification

PURPOSE: The objective of this study was to analyze the distribution of curve patterns in adult idiopathic scoliosis (AdIS) patients according to the AdIS classification by Lin et al. We also aim to identify the prevalence of atypical curve patterns that are not represented within the existing framework and propose a laterality modifier as an extension of the AdIS classification.

METHODS: Adult idiopathic scoliosis patients aged above 18 years who underwent posterior spinal fusion (PSF) between 2023 and 2025 were retrospectively reviewed. Radiographic parameters were measured using pre-operative whole spine radiographs. Curves were classified according to AdIS classification by Lin et. al. Reversed curve patterns not represented by the current AdIS classification were recorded as atypical-laterality variants with an added laterality modifier, denoted "R" (reversed). Descriptive statistical analysis was performed.

RESULTS: A total of 215 patients were included. The existing AdIS classification successfully classified 92.6% of patients, whereas 7.4% exhibited atypical-laterality curve patterns. AdIS type 1 curves were the most prevalent (34.7%), followed by type 2 (17.6%) and type 6 (15.1%). The majority of curves demonstrated non-structural lumbosacral modifiers (89.4%) and preserved global alignment (98.0%). Among the atypical-laterality cases, AdIS 6-R and 5-R curves were the most numerically frequent subtypes, accounting for five (31.3%) and four (25.0%) cases respectively.

CONCLUSION: The AdIS classification demonstrated excellent applicability, successfully classifying 92.6% of patients in our study cohort. Atypical-laterality patterns were identified in 16 patients (7.4%), with AdIS 6-R being the commonest pattern encountered.