Pelvic Incidence is Not Fixed: Postoperative Changes in Thoracolumbar Scheuermann’s Kyphosis Compared With Thoracic Type

Purpose Pelvic incidence(PI) has traditionally been regarded as a fixed anatomical parameter. However, emerging evidence suggests that PI may change following spinal deformity correction. Comparative data on postoperative spinopelvic adaptations in thoracic(TSK) versus thoracolumbar(TLSK) Scheuermann kyphosis, and their deviation from healthy populations, remain limited. This study aimed to compare preoperative spinopelvic and sagittal alignment parameters between TSK and TLSK, evaluate their differences relative to healthy controls, and investigate postoperative changes and interrelationships among these parameters. Methods Fifty-two patients with Scheuermann kyphosis(30 TSK, 22 TLSK; mean follow-up 4.6 ± 2.2 years) were retrospectively analyzed and compared with 30 age-matched healthy controls. Radiographic parameters included thoracic kyphosis(TK), thoracolumbar kyphosis(TLK), lumbar lordosis(LL), cervical lordosis(CL), pelvic incidence(PI), sacral slope(SS), pelvic tilt(PT), sagittal vertical axis(SVA), and PI–LL mismatch. Measurements were obtained preoperatively, postoperatively, and at final follow-up. Group comparisons, correlation analyses, and multivariate regression were performed to identify predictors of PI. Results TLSK patients demonstrated significantly lower preoperative PI and SS compared with TSK and controls(p < 0.05). Both SK groups exhibited greater TK, TLK, LL, and CL compared with controls, while PT and SVA were similar. Following surgery, TLSK patients showed a significant increase in PI(p = 0.011) and SS, eliminating preoperative differences between groups. Preoperative disparities in SVA, TK, and TLK resolved by final follow-up, whereas CL remained significantly lower in TLSK(p = 0.012). Regression analysis identified LL, PI–LL mismatch, PT, and SS as significant predictors of PI(R²=0.945), with LL exerting the strongest influence. Conclusions Surgical correction achieves restoration of global sagittal balance in both TSK and TLSK. PI, long considered immutable, may increase postoperatively in TLSK, highlighting its dynamic behavior. Comprehensive preoperative assessment of sagittal alignment—particularly PI and LL—is crucial for optimal surgical planning in Scheuermann kyphosis.