Thoracic kyphosis is associated with somatosensory dependence and postural control in adolescent idiopathic scoliosis

BACKGROUND: The contribution of sagittal spinal alignment to sensorimotor function in adolescent idiopathic scoliosis (AIS) remains unclear. We examined whether thoracic kyphosis (TK) is associated with sensory integration and postural balance in patients with AIS, independent of coronal structural misalignment. METHODS: Forty-five AIS patients (Cobb angle ≥ 10°) were consecutively recruited from the outpatient rehabilitation clinic of a university-affiliated hospital (median age 11.0 years, IQR 10.0-13.0; 33 female). There was no selection criterion related to kyphosis. All participants were newly screened and had not yet initiated scoliosis-specific exercise, physiotherapy, or brace treatment. Participants completed the Sensory Organization Test (SOT) and full-spine radiography. Five sensory ratios and four postural control indices were derived from SOT condition-level equilibrium scores (ES). The C4/C1 ratio reflects the ability to maintain postural stability when somatosensory input is disrupted by a sway-referenced support surface (C4) relative to a fixed surface (C1). Associations were examined with Spearman and partial Spearman correlations (adjusting for Cobb angle, age, and BMI) and tertile comparisons with false discovery rate (FDR) correction. FINDINGS: Forty-two patients had complete data for primary analyses (median Cobb angle 15.3°, IQR 11.2°-20.0°). Greater TK was associated with a higher C4/C1 ratio (Spearman ρ = 0.416, p = 0.006), and this association remained significant after adjustment for Cobb angle, age, and BMI (partial Spearman ρ = 0.475, p = 0.002). Cobb angle was not associated with any sensory ratio (all p > 0.1) but was positively associated with firm-surface equilibrium scores (C1: ρ = 0.525, p < 0.001). The C4/C1 ratio increased monotonically across TK tertiles (Low 0.701 → Mid 0.832 → High 0.861; Kruskal-Wallis H = 8.58, p = 0.014) and was associated with better postural control across all domains (composite ES: ρ = 0.453; variability: ρ = -0.410; adaptation: ρ = 0.433; all p < 0.01). INTERPRETATION: TK and Cobb angle showed differential associations with postural control: TK was associated with the C4/C1 ratio, whereas Cobb angle was associated with firm-surface stability. The C4/C1 ratio statistically carried the association between TK and postural control via an indirect-only pathway. These findings suggest that sagittal and coronal spinal parameters are differentially associated with postural control, which may inform future research on phenotype-specific assessment strategies.