Influence of Anatomical Pelvic Tilt and Anatomical Sacral Slope on Pelvic Orientation and Lumbar Lordosis Distribution in Subjects with High Pelvic Incidence

Background/Objectives: Pelvic incidence (PI) is a key anatomical parameter that influences sagittal spinal alignment and hip–spine mechanics. Anatomically, PI is composed of anatomical pelvic tilt (aPT) and anatomical sacral slope (aSS). However, whether variation in these anatomical components is associated with lumbar lordosis distribution in subjects with high PI remains unclear. This study aimed to investigate the association between aPT and distal lumbar lordosis distribution in subjects with high PI. Methods: This retrospective cross-sectional study included 320 subjects, of whom 68 had high PI (PI ≥ 60°). Sacral slope (SS), pelvic tilt (PT), anterior pelvic plane angle (APPA), PI, aPT, aSS, thoracic kyphosis (TK), lumbar lordosis (LL), lower arc lordosis (L4–S1 lordosis), and the L4–S1/LL ratio were measured using standing lateral radiographs. In subjects with high PI, associations of aPT as a continuous variable with lower arc lordosis and the L4–S1/LL ratio were evaluated using correlation and multiple linear regression analyses adjusted for age and body mass index (BMI). Results: Sixty-eight subjects (21.3%) met the definition of high PI. Greater aPT was modestly associated with greater lower arc lordosis in unadjusted analysis (Pearson r = 0.258, p = 0.034), whereas the association with the L4–S1/LL ratio did not reach statistical significance (r = 0.219, p = 0.072). After adjustment for age and BMI, the association between aPT and lower arc lordosis was attenuated and did not reach statistical significance (standardized β = 0.217, p = 0.063). The adjusted association between aPT and the L4–S1/LL ratio was also not significant (standardized β = 0.191, p = 0.113). Conclusions: Among subjects with high PI, greater aPT showed a modest positive association with distal lumbar lordosis in unadjusted analysis; however, this association was attenuated after adjustment for age and BMI. These findings suggest that age may partly contribute to variation in distal lumbar lordosis among individuals with high PI and that the anatomical composition of PI should be interpreted with consideration of potential confounding factors.