Segmental relationships between thoracic asymmetry and spinal deformity in mild-to-moderate idiopathic scoliosis patients at first presentation.

PURPOSE: The structural characteristics underlying the early development of idiopathic scoliosis (IS) remain incompletely understood. In particular, the relationship between segmental thoracic asymmetry, vertebral rotation, and coronal curve magnitude in untreated mild-to-moderate IS has not been fully investigated. This study examined these relationships in children with untreated mild-to-moderate IS before the initiation of conservative treatment. METHODS: In this retrospective cross-sectional study, 249 children with untreated idiopathic scoliosis (Cobb angle 10-40°) were included. The primary outcome was the Segmental Rib Index (SRI), measured at each thoracic vertebral level (T1-T12). Associations between SRI and spinal deformity parameters, including Cobb angle and apical vertebral rotation (AVR), were evaluated using sequential linear mixed-effects models with interaction and post hoc simple-slope analyses. RESULTS: The global Rib Index (RI_max DRCS) was not significantly correlated with either Cobb angle (r = 0.032, p = 0.616) or AVR (r = - 0.009, p = 0.888). In contrast, linear mixed-effects modelling demonstrated a significant thoracic vertebral level × AVR interaction (F = 4.003, p < 0.001), whereas no significant interaction was observed for scoliosis severity (p = 0.270). Post hoc analyses identified significant positive associations between AVR and SRI at T9-T11 after Holm-Bonferroni correction. Sequential mixed-effects modelling showed that AVR remained significantly associated with SRI in the unadjusted and partially adjusted models, whereas the initial Cobb angle was not. In the fully adjusted model, the AVR-SRI association was attenuated and no longer reached statistical significance (β = 0.0035, 95% CI - 0.0002 to 0.0071; p = 0.060). CONCLUSION: RI measurements provide clinically valuable three-dimensional information on thoracic deformity in IS. SRI is closely associated with AVR, and this relationship varies across thoracic levels and according to curve severity. These findings support the use of SRI as a complementary measure to the Cobb angle for a more comprehensive assessment of transverse-plane deformity.