Pelvic Incidence: Computed Tomography Study Evaluating Correlation with Sagittal Sacropelvic Parameters

Normal values for spinal alignment are often based on the pelvic incidence (PI), defined as the angle subtended by a line from the bicoxofemoral axis to the midpoint of the sacral endplate and a line perpendicular to the midpoint. Despite widespread use, determinants of its values remain obscure. The aim of this study was to determine correlation of sacropelvic parameters with the PI on computed tomography (CT). CT scans performed for trauma were identified over a 1-year period. Patients aged over 16 were included. PI, sacral anatomic orientation, sacral table angle (STA), sacral kyphosis (SK), pelvic thickness (PTH), femorosacral pelvic angle, pelvisacral angle, and sacropelvic angle were measured. Additional novel measures including crest-to-pubis distance, crest-to-sacrum distance (CSD), inlet distance, outlet distance, and inlet-outlet angle were taken. One hundred and seventy-seven scans were analyzed. Mean age 44.3 years; 62% male. The mean PI was 50.1 (SD 10.8; range 29-87). SK (r = 0.769), inlet-outlet angle (r = -0.533), PTH (r = -0.370), CSD (r = 0.290), and STA (r = -0.276) significantly correlated with PI. Multivariate analysis developed a predictive equation of: PI = 101.45 - (0.52 x STA) + (0.67 x SK) - (0.34 x inlet-outlet angle), with an adjusted R-2 0.734 (P < 0.001). Measures that represent the sacral morphology, particularly SK, and the position of the sacrum in space correlated strongly with the PI and contributed strongly to a predictive equation. These findings may direct further efforts to explore how the PI is determined and therefore how it may be modified. (C) 2019 Wiley Periodicals, Inc.