The posterior inferior iliac spine (PIIS) is the attachment site for the posterior sacroiliac ligamentous complex and the sacrotuberous ligament, functioning as a bony anchor rather than a ligament itself, with the surrounding fibrous attachments forming the posterior pelvic ring constraint.
Provides the bony anchor for the posterior SIJ ligamentous complex at the PIIS, serves as the attachment point for the long posterior sacroiliac ligament superiorly, and is the bone at risk in avulsion injuries involving the sacrotuberous or posterior SIJ ligaments.
PIIS avulsion fracture (less common than ASIS avulsion) can occur from hamstring or sacrotuberous ligament traction. The PIIS is the landmark for identification of the sciatic notch in posterior approaches to the hip and pelvis.
The posterior inferior iliac spine identifies the superior boundary of the greater sciatic notch in posterior hip approaches and pelvic osteotomies, guiding safe exposure of the sciatic nerve.
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