Dynamic remodeling of pelvic sagittal and hip coronal parameters after posterior spinal fusion in adolescent idiopathic scoliosis

Background Adolescent idiopathic scoliosis (AIS) correction may influence pelvic sagittal alignment and hip coronal morphology, but growth-stage trajectories remain unclear. Methods We retrospectively included 138 female AIS patients who underwent posterior spinal fusion (PSIF). Standing full-length spine–pelvis radiographs were obtained preoperatively and at 6 months, 1 year, and 2 years postoperatively. Parameters included Cobb angle, lateral center–edge angle (LCEA), Tönnis angle, iliac obliquity (IO), sacral obliquity (SO), and pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS). Analyses were stratified by age (10–14 vs 15–18 years) and Lenke type (1 vs 5). Within-subject comparisons used repeated-measures ANOVA or Friedman tests with Bonferroni-adjusted pairwise tests; between-group tests and correlations used standard parametric/non-parametric procedures. Results Age- and Lenke-specific remodeling patterns were evident. In 10–14-year-olds with Lenke 1, Tönnis decreased (right − 2.25° at 6 months; left − 2.72° at 2 years) and SS increased at 2 years (+ 3.27°). In 10–14-year-olds with Lenke 5, right LCEA increased at 2 years (+ 3.26°) while both Tönnis angles decreased (− 3.54° and − 3.31°). Changes were small in 15–18-year-olds; only Lenke 5 showed a slight increase in left Tönnis at 1 year (+ 0.95°). Conclusions Pelvis–hip parameters remodel within two years after PSIF, with greater improvements in acetabular coverage and sagittal adaptation among younger adolescents (10–14 years), especially Lenke 1/5, suggesting higher biomechanical plasticity before skeletal maturity. Routine postoperative assessment should incorporate pelvis–hip parameters with age/Lenke context to inform long-term management.