Ilium- posterior inferior iliac spine

Medial view (deep surface), right side

Medial view (deep surface), left side

The posterior inferior iliac spine serves as part of the attachment for the sacrotuberous ligament. This strong and relatively inflexible ligament extends from the posterior inferior iliac spine, the lateral part of the sacrum and coccyx to the ischial tuberosity. Because of the nature of this ligament, it forms part of the perimeter of the pelvic outlet. Functionally it of great significance because with the sacrospinous ligament it prevents the sacrum from rotating and breaking away from the os coxae. Because of the force from the lumbar vertebrae pushing inferiorly on the sacral promontory, it would rotate forward and inferiorly if it were not for these two ligaments.