Comparing the anatomical consistency of the posterior superior iliac spine to the iliac crest as reference landmarks for the lumbopelvic spine: a retrospective radiological study.

A palpation reference line coursing between the superior-most aspect of the iliac crests has been reported to cross the L4 spinous process or L4/L5 intervertebral space in approximately 80% of the population. Comparable data have not been defined for the line coursing between the posterior superior iliac spines (PSIS). The purpose of this study was to compare the anatomical consistency of the PSIS to the iliac crest as landmarks used for spinal palpation. One hundred computerized tomographic images were reviewed in a three-dimensional setting. Two horizontal lines were constructed on each image: Line 1 representing the superior-most aspect of the iliac crest and Line 2 representing the inferior margin of the PSIS. The vertical distance between each horizontal line and the inferior edge of its respective spinous process were measured. The PSIS corresponded to the S2 spinous process in 81% of subjects and the iliac crest to the L4 spinous process in 59% of subjects. Distance measures suggest that the PSIS was closer to S2 versus the iliac crest to L4 (t = 6.998; P < 0.01). The PSIS crossed S2 more frequently than the iliac crest crossed L4 (chi(2) = 12.719, P < or = 0.01). The study findings support the relationship between the PSIS, and the spinous process of S2 is more consistent when compared to the iliac crest and the spinous process of L4. The PSIS reference line may be used to find S2 as a reference standard in validity and reliability palpation studies in the lower lumbar spine.