Discordance in spinopelvic alignment between weight-bearing and supine positions on three-dimensional computed tomography.

Given the functional role of spinal curves supporting dynamic motion, imaging in the supine position does not adequately reflect the biomechanical state under physiological loading. This study aims to evaluate changes in spinopelvic parameters, particularly pelvic incidence (PI), between weight-bearing and non-weight-bearing computed tomography (CT), and to identify contributing clinical and anatomical factors. Fifty elderly patients (mean age: 76.84 years) with degenerative lumbar disorders underwent both weight-bearing CT (WBCT) and non-weight-bearing CT (NWBCT). A 3D analysis framework was used to measure PI, pelvic tilt (PT), sacral slope (SS), and the S1-femur coronal angle (S1FCA). Paired t-tests, correlation, and regression analyses were conducted. PI was significantly higher in the weight-bearing condition (WBCT PI: 52.04 ± 10.14° vs. NWBCT PI: 49.35 ± 9.90°, p < 0.001). PT also increased and SS decreased under weight-bearing. S1FCA change was moderately correlated with ΔPI (r = 0.346, p = 0.014) and served as an independent predictor in regression analysis. Patients with prior spinal fusion showed significantly smaller PI changes (-1.55° vs. + 2.96°, p = 0.021), suggesting restricted sacral mobility. Furthermore, PI values differed significantly between WBCT and standing X-ray (WBXR) even in the same posture (p = 0.031), possibly due to projectional distortion. PI is a dynamic, posture- and modality-dependent parameter affected by coronal rotation and fusion status. These findings challenge its conventional interpretation and support the use of weight-bearing 3D imaging and coronal parameters such as S1FCA for accurate evaluation.