Ala ossis sacri

On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the sacroiliac joints.

The alae support the psoas major muscles and the lumbosacral trunk which connects the lumbar plexus with the sacral plexus.

In the articulated pelvis the alae are continuous with the iliac fossa.

Each ala is slightly concave from side to side, and convex from the back and gives attachment to a few of the fibers of the iliacus muscle.

The posterior quarter of the ala represents the transverse process, and its anterior three-quarters the costal process of the first sacral segment.

Each ala also serves as part of the border of the pelvic brim.

The alae also form the base of the lumbosacral triangle.

The iliolumbar ligament and lumbosacral ligaments are attached to the ala.