Abstract
The sagittal alignment of the pelvis can be evaluated by the angle of pelvic incidence (PI), which is constant
for an arbitrary subject position and orientation and can be therefore compared among subjects in standing,
sitting or supine position. In this study, PI was measured from three-dimensional (3D) computed tomography
(CT) images of normal subjects that were acquired in supine position. A novel computerized method, based on
image processing techniques, was developed to automatically determine the anatomical references required to
measure PI, i.e. the centers of the femoral heads in 3D, and the center and inclination of the sacral endplate in
3D. Multiplanar image reformation was applied to obtain perfect sagittal views with all anatomical structures
completely in line with the hip axis, from which PI was calculated. The resulting PI (mean±standard deviation)
was equal to 46.6°±9.2° for male subjects (N = 189), 47.6°±10.7° for female subjects (N = 181), and
47.1°±10.0° for all subjects (N = 370). The obtained measurements of PI from 3D images were not biased by
acquisition projection or structure orientation, because all anatomical structures were completely in line with
the hip axis. The performed measurements in 3D therefore represent PI according to the actual geometrical
relationships among anatomical structures of the sacrum, pelvis and hips, as observed from the perfect sagittal
views.
