Defining Sitting Spinal Alignment through Pelvic Incidence and Vertebropelvic Angles

Introduction: Vertebropelvic angles (VPAs) have shown strong correlations with pelvic incidence (PI) in standing radiographs of asymptomatic patients, yet their relationship with sitting alignment remains underexplored. This study aimed to (1) determine whether lumbar lordosis (LL) correlates with PI in the sitting position and (2) define thoracolumbar alignment in the sitting position using VPAs. We hypothesized that spinal alignment in the sitting position could be characterized by PI-based formulas for thoracolumbar VPAs.

Methods: We conducted a retrospective analysis of lateral sitting XRs from patients being considered for hip replacement surgery. Patients with prior spinal fusion or scoliosis (max Cobb angle ≥20°) were excluded. Radiographical measurements, including PI, PT, L1-S1 lordosis (LL), Vertebral Tilts (VT), and VPAs from T10 to L5, were collected. Linear regression models were applied to determine correlations between PI, LL, and VPAs and describe normal variance as a function of radiographical parameters.

Results: The study included 1428 patients (764 females and 664 males), with a mean age of 62.9 (±11.29) years, a mean BMI of 28.89 (±5.71) kg/m², and a mean sitting PI° of 55.94 (±11.16). Sitting VPAs were as follows: L5PA 8° (6–10), L4PA 15° (12–18), L3PA 18° (13–23), L2PA 19° (13–24), L1PA 19° (13–25), T12PA 20° (13–26), T11PA 20° (14–26), and T10PA 21° (14–27.25). Vertebral tilt values in the sitting position are: L5VT -25.5° (-32.4, -18.1), L4VT -14.1° (-23.0, -4.9), L3VT 10° (6, 13), L2VT 4° (1, 7), L1VT 1° (-1, 3), T12VT 1° (-1, 3), T11VT 1° (-1, 2), and T10VT 1° (0, 3). Sitting PT (32.8° ±11.72) was strongly positively correlated with sitting PI (rs=.644, p<.001). On linear regression, PI was more strongly correlated with T10 and L1PAs (R2=0.41 and R2=0.53, both p< 0.001) than LL (R2=0.1, p< 0.001). The following formulas could be used to describe T10PA and L1PA, respectively, based on sitting PI: T10𝑃𝐴=0.558⨯PI–10.46 and 𝐿1𝑃𝐴=0.562⨯PI–12.25

Conclusion : Similar to standing alignment, PI in the sitting position correlates more strongly with thoracolumbar VPAs than with LL. Our study describes PI-based formulas for T10PA and L1PA in the sitting position.