Dynamic Adaptations of the Thoracolumbar Junction: A Cross-Sectional Analysis of Vertebral Pelvic Angle Changes from Standing to Sitting.

STUDY DESIGN: Cross-sectional imaging analysis. OBJECTIVE: To quantify changes in VPAs between standing and sitting positions and to evaluate the impact of pelvic incidence (PI), age, body mass index (BMI), gender, and osteoarthritis (OA) on these changes. SUMMARY OF BACKGROUND DATA: Spinal alignment changes greatly between the sitting and standing positions. Vertebropelvic angles (VPAs) are increasingly used as metrics to describe spinal sagittal alignment. However, VPA changes in the sitting position and their relationship with patient-specific factors have not been described. METHODS: We included a total of 1,197 adult patients (632 females, 565 males; mean age 62.2±11.1 years, BMI 28.8±5.6 kg/m², PI 52.5°±11.8) without prior spinal surgery, spondylolisthesis, or a maximum coronal Cobb angle exceeding 20°. VPAs were assessed on radiographs up to the highest visible vertebral level (T10). The main outcome of this study was changes in VPAs (ΔVPAs), defined as the difference between standing and sitting VPAs (a more negative value indicates a larger increase when sitting). Group comparisons were conducted by age quartiles and gender. Multiple linear regression was used to evaluate associations of ΔVPAs with PI, age, BMI, and gender. RESULTS: All VPAs demonstrated significant changes between the two positions (P<0.001). Greater positional changes were observed in the thoracic spine compared to the lumbar spine: ΔL5PA (-0.24°) to ΔT10PA (-13.72°). PI was associated with smaller ΔVPAs only at L4 and L5 (P<0.001). Older age was associated with smaller ΔVPAs across all levels (P<0.001). Higher BMI was associated with greater pelvic tilt excursion (P=0.01) but with smaller thoracolumbar changes, particularly at T12-L4 (all P<0.001). Males demonstrated greater pelvic excursion (P=0.03) with localized increases in ΔT12PA and ΔL1PA (P=0.01 and 0.03, respectively). CONCLUSION: VPAs increase in the sitting position, particularly in the lower thoracic and proximal lumbar spine. The T10PA-L1PA relationship reverses across postures: T10PA is lower than L1PA in standing but equal to or greater in sitting, highlighting TLJ as the key transition. These adaptations are strongly influenced by age-related stiffness and BMI-related compensatory patterns. LEVEL OF EVIDENCE: III.