How to Locate the Posterior Superior Iliac Spines (PSIS)
The posterior superior iliac spines are best located by palpating for bony prominences on either side of the lower back, typically at the level of the S2 vertebral body, positioned approximately 2-3 cm lateral to the midline where visible or palpable dimples often appear on the skin surface.
Palpation Technique
The PSIS can be identified through systematic palpation:
- Position the patient standing or prone with relaxed muscles
- Locate the dimples that are often visible on the lower back, approximately 2-3 cm lateral to the midline on each side - these surface landmarks typically correspond to the PSIS location 1
- Palpate for bony prominences at these locations, feeling for the posterior-most projection of the iliac crest
- The PSIS sits at the junction where the iliac crest curves posteriorly and medially
Anatomical Relationships
Understanding the vertebral level helps confirm proper identification:
- The PSIS corresponds to the S2 spinous process in 81% of individuals 2, making it a more reliable landmark than the iliac crest (which corresponds to L4 in only 59% of cases)
- A horizontal line connecting both PSIS typically intersects the S1 vertebral body in 62% of cases, though it can range from L5 to S3 3
- The PSIS is located more consistently at S2 compared to the iliac crest at L4 (distance measures show PSIS is closer to S2 versus iliac crest to L4, p<0.01) 2
Important Clinical Considerations
Morphological variations exist that affect palpation reliability:
- Four distinct PSIS morphologies have been identified: V-shape (36.8%), U-shape (42.0%), W-shape (12.5%), and ossification-shape (8.7%) 4
- Females tend to have larger distances and widths of surrounding landmarks compared to males 4
- Interexaminer reliability for PSIS palpation is poor (mean kappa = 0.27), though intraexaminer reliability is better 5
Practical Tips
- Use both visual and tactile cues: Look for skin dimples first, then confirm with palpation
- The PSIS is typically at the level where you would place your hands on your hips, but slightly more posterior and medial
- Bilateral comparison helps ensure accurate identification - both PSIS should be at approximately the same level
- In patients with higher body mass index, palpation may be more challenging and less reliable 1
Common Pitfalls to Avoid
- Do not confuse the PSIS with the iliac crest - the iliac crest is the superior rim of the pelvis, while the PSIS is a distinct posterior projection
- Avoid relying solely on palpation for precise spinal level identification in clinical procedures requiring exact localization, as reliability is limited 5
- Be aware that in approximately 19% of cases, the PSIS may not correspond to S2, ranging from L5 to S3 2