Sacral orientation revisited

Study Design. A descriptive study of the sacral anatomic orientation (SAO) and its association with pelvic incidence (PI). Objective. To introduce the concept of SAO, establish a method for measuring it, and evaluate its association with pelvic orientation. Summary of Background Data. Pelvic orientation (PO) is considered a key factor in spinal shape and balance. Sacral slope (SS), PI, and pelvic tilt (PT) are the most frequently used parameters for evaluating PO. Nevertheless, the association between the anatomic orientation of the sacrum and these parameters has never been established. The aim of the present study is to define the anatomic orientation of the sacrum, to establish a reliable method for measuring it, and to examine its association with PI. Methods. SAO was defined as the angle created between the intersection of a line running parallel to the superior endplate surface of the sacrum and a line running between the anterior superior iliac spine (ASIS) and the anterior-superior edge of the symphysis pubis. Methods for measuring SAO and PI on both skeletal populations and living individuals are described. The study was carried out on 424 skeletons (articulated pelves) using a three-dimensional digitizer and on 20 adult individuals using CT three-dimensional images (volume-rendering method). Reliability (intratester and intertester) was assessed using intraclass correlation test. A regression analysis was carried out to evaluate the association between the two measurements. Results. The mean SAO and PI in the human skeletal population were found to be 48.46 degrees +/- 10.17 degrees and 54.08 degrees +/- 12.64 degrees, respectively and of the living individuals (CT) 52.76 degrees +/- 10.31 degrees and 57.14 degrees +/- 13.08 degrees, respectively. SAO and PI measurements were highly correlated (r = -0.824, and r = -0.828, P < 0.001 for skeletal material and living individuals, respectively). PI can be predicted via SAO, i.e., PI = [-0.971 x SAO] + 101.16 degrees. Conclusions. The newly suggested parameter (SAO) may be an important tool in defining the sagittal shape of the spine and understanding its association with spinal diseases.