Vertebral Level of the Posterior Superior Iliac Spine
The posterior superior iliac spine (PSIS) corresponds to the S2 vertebral level in the majority of individuals.
Primary Anatomical Relationship
The PSIS most consistently aligns with the S2 spinous process, with this relationship demonstrated in 81% of subjects in radiological studies 1. This landmark shows significantly greater anatomical consistency compared to other pelvic reference points 1.
Supporting Evidence and Reliability
High consistency: The PSIS-to-S2 relationship is more reliable than the commonly cited iliac crest-to-L4 relationship, with the PSIS showing closer proximity and more frequent alignment with S2 than the iliac crest does with L4 1.
Clinical validation: Multiple imaging studies confirm that the PSIS identifies the S2 spinous process in 51% of patients and S1 in 44% of patients, with the majority clustering around the S1-S2 region 2.
Surgical applications: The PSIS serves as a critical anatomical landmark for S2-alar-iliac screw placement, with the starting point typically positioned 15 mm cephalad to the PSIS 3.
Range of Variation
While S2 is the most common level, the PSIS can correspond to a range from the L5-S1 interspace to the S2 spinous process depending on individual anatomical variation 4. However, the S2 level remains the most consistent and reliable reference point for clinical and surgical applications 1.
Clinical Implications
Palpation reliability: The PSIS demonstrates significantly greater interexaminer reliability for spinal level identification compared to the iliac crest 4.
Surgical planning: The PSIS is used as a key landmark for posterior pelvic fixation and sacral bar insertion, with measurements confirming safe bony purchase at this level 5.