A Quantitative CT-Based Analysis of Vertebral Rotational Asymmetry and Pulmonary Function in Scoliosis

Background/Objectives: Scoliosis is a three-dimensional deformity involving coronal curvature, axial rotation, and sagittal imbalance, which may alter thoracic geometry and reduce ventilatory capacity. Traditional two-dimensional radiographic measures incompletely represent the complexity of axial rotation, and the apical vertebra is not always the most rotated vertebra. We aimed to determine whether computed tomography (CT)-derived three-dimensional vertebral rotation indices, particularly global rotational asymmetry between main and compensatory curves, are associated with pulmonary function impairment in a large heterogeneous scoliosis cohort. Methods: We retrospectively reviewed 250 patients with scoliosis who underwent full-spine CT and spirometry within a 1-year interval (2013-2023). Vertebral rotation was measured using the Aaro-Dahlborn method. Rotation indices included apical rotation (R(Apex)), averaged apical rotation across the apical vertebra and adjacent levels (R(Avg)), and rotational asymmetry defined as the absolute difference between rotation in the main and compensatory curves (ΔR(M-C)). Pulmonary function outcomes were FVC (L), FEV1 (L), FVC% and FEV1%. Pearson correlation and multivariate linear regression, adjusted for age, sex, height, and weight, were performed; sensitivity analyses, additionally adjusted for upright Cobb angle and thoracic kyphosis (TK) to evaluate whether ΔR(M-C) provided independent explanatory information, and subgroup analyses by etiology were performed. Results: The cohort had a mean age of 15.6 ± 5.7 years; 49.6% were female. All rotation indices showed significant negative correlations with pulmonary function in the overall cohort. ΔR(M-C) showed the strongest associations with FVC% (r = -0.66) and FEV1% (r = -0.64), as well as with absolute volumes (FVC, r = -0.59; FEV1, r = -0.58). In adjusted multivariate analyses, models incorporating ΔR(M-C) consistently demonstrated the highest explanatory performance compared with models based on R(Apex) or R(Avg). Subgroup analysis revealed the strongest associations in neurofibromatosis-related scoliosis (r = -0.87) and congenital scoliosis (r = -0.71). Associations were attenuated in neuromuscular subtypes and did not reach statistical significance in SMA. In sensitivity analyses adjusting for Cobb angle and thoracic kyphosis, ΔR(M-C) retained a robust independent association with FVC% (unstandardized B = -0.82 percentage points per 1°, 95% CI -0.98 to -0.66; p < 0.001; partial F = 103, p < 0.001), while neither Cobb angle nor TK remained statistically significant after adjustment for ΔR(M-C); comparable patterns were observed across FEV1%, FVC, and FEV1. Conclusions: CT-derived global rotational asymmetry between the main and compensatory curves is strongly associated with pulmonary function impairment in scoliosis and demonstrates superior explanatory performance to single-level rotation indices and retains independent explanatory value after adjustment for conventional 2D radiographic parameters (Cobb angle and thoracic kyphosis). Incorporating a CT-derived metric may complement traditional two-dimensional assessments for functional risk stratification.