No linear correlation between pelvic incidence and acetabular orientation: Retrospective observational study.

Links between sagittal spinal alignment and acetabular orientation attract considerable research attention with the goal of understanding “hip-spine syndrome.” However, whether pelvic incidence (PI) is related to acetabular orientation remains debatable. The purpose of the present study was to determine: 1. whether the correlation between PI and acetabular orientation is present in pelvises of young healthy adults, and 2. whether the correlation is present in subgroups of sex, or between the left and right pelvis. We analyzed 100 abdominopelvic computed tomography (CT) scans of skeletally healthy young adults. We measured PI and acetabular orientation with three-dimensional (3D) measurements. The orientation of 200 acetabula was measured using 3D reconstructed models of 100 pelvises in the coordinate system based on the anterior pelvic plane (APP). To quantify the acetabular orientation, the radiographic definitions of anteversion and inclination were used. To examine the correlation between acetabular orientation and PI, Pearson's correlation was used. The mean PI was 46.9° ± 10.2°, and the mean acetabular orientation 15.3° ± 5.7° anteverted and 37.5° ± 3.9° inclined. While no significant difference in the PI was observed, the average acetabular orientation of female pelvises (anteversion, 17.5° ± 5.6°; inclination, 36.7° ± 3.7°) was more anteverted and less inclined compared to that of male pelvises (anteversion, 13.2° ± 4.9°; inclination, 38.3° ± 3.9°, respectively; values < .05). The correlation between PI and acetabular orientation was statistically not significant. After division of study group by sex, the linear correlation between PI and acetabular orientation was not statistically supported. The asymmetry of the acetabular orientation between the left and right sides was not significant. The linear relationship between anatomical acetabular orientation and PI was not evident in the normal population. Our finding thus proves the absence of a linear relationship between the upper and lower articular orientation of the pelvic segment and deepens the understanding of the characteristics of acetabular orientation and PI.