Pelvic incidence correlates more strongly with proximal than distal lordosis and most strongly with L1-pelvic angle.

Abstract

Pelvic incidence (PI) is a critical morphologic parameter influencing sagittal spinal alignment. While the PI-lumbar lordosis (LL) mismatch is well studied, the extent to which PI correlates with proximal versus distal LL remains unclear. Additionally, the L1-pelvic angle (L1PA) is emerging as a potential metric for sagittal alignment assessment. This study aimed to evaluate the associations between PI, segmental LL, and L1PA to establish goal sagittal alignment benchmarks in a symptomatic non-deformity cohort. A retrospective analysis (STUDY-17-00660) was conducted on a large consecutive series of adults undergoing full-body biplanar radiographs. Patients with spinal deformity or prior spinal surgery were excluded. Measurements included PI, proximal LL (L1-L4), distal LL (L4-S1), global LL (L1-S1), and L1PA. Multivariable linear regression models were adjusted for age, sex, and body mass index (BMI). Correlation coefficients and Fisher's z tests were used to compare the strength of associations between PI and each alignment measure. A total of 507 patients (mean age 48.4 ± 17.2 years; 50.9% female) were included. Multivariate regression demonstrates a 10° increase in PI was associated with increases of 3.7° in proximal LL, 1.9° in distal LL, 5.6° in global LL, and 5.0° in L1PA (all p < 0.001). PI showed a stronger correlation with proximal LL (r = 0.50) than distal LL (r = 0.23) and the strongest correlation with L1PA (r = 0.79). PI correlates more strongly with proximal than distal lumbar lordosis and most strongly with L1PA. These findings support greater emphasis on proximal lordosis and the utility of L1PA in the restoration of sagittal alignment in adult spinal deformity.