The posterior superior iliac spine is the posterior projection of the iliac crest, producing the two dimples of Venus visible on the lower back of lean individuals. It is the posterior attachment of the posterior sacroiliac ligaments and the origin of the multifidus and erector spinae at this level. The PSIS is used clinically to assess sacroiliac joint position and movement asymmetry.
The PSIS is the primary clinical landmark for assessing sacroiliac joint dysfunction — standing flexion test and prone knee flexion test compare PSIS movement between sides to identify asymmetric sacroiliac motion. It is also the primary bony landmark for percutaneous iliac crest bone graft harvest, where a trocar can be inserted 2 cm anterior to the PSIS to access the posterior iliac crest marrow cavity.
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