Pelvic Incidence Is Associated With Sacral Curvature, Sacroiliac Joint Angulation, and Sacral Ala Width

Study Design. Cross-sectional anatomical study in dry cadaveric specimens. Objective. This study evaluates how the sacroiliac (SI) joint angulation, midsagittal sacral curvature, and sacral ala width relate to pelvic incidence (PI). Summary of Background Data. Numerous spinopelvic pathologies have been linked with pelvic incidence; however, the manner in which sacral morphology fits into this association is largely unknown. Methods. A total of 120 cadaveric sacra and corresponding innominate bones were obtained from an osteological collection. Pelvic incidence was measured in a previously validated method. To gauge ala width, calipers were used to measure the distance between the midpoint of the sacral endplate and the anterolateral aspect of the right and left ala. Sacral curvature was evaluated by measuring the difference of the direct distance and the curved bony distance from the sacral promontory to the inferior aspect of the fourth intervertebral foramen. Three separate angle measurements were made to quantify the angulation of the SI joint, which we divided into an upper and lower limb. Angle 1 approximated the relationship between the upper limb and the sacral endplate; angle 2 between the upper and lower limbs; angle 3 between the lower limb and a tangential line to the fourth sacral foramen. Results. Average age at death for all specimens was 31.9 +/- 6.3 years with 63% representation of males and an equal distribution of Caucasian and African Americans. The mean PI of our study population was 45.1 +/- 12.6 degrees. Results from our regression showed statistically significant associations between our measured angles evaluating SI joint angulation (angles 1, 2, 3) and PI. Standardized beta for angle 1 was -0.421, angle 2 was 0.419, and angle 3 was -0.439 (all P < 0.001). Additionally, the average endplate-ala distance (beta = -0.254) and average difference between tape-caliper measurements, our measurement for sacral arc, (beta = 0.178) were significantly associated with PI (all P < 0.05). Conclusion. Our data supports the theory that increased PI is associated with a highly angulated and curved sacrum, with corresponding changes in the SI joint, and narrowed sacral alae.