On either side of the body of the base of the sacrum is a large triangular surface, which supports the Psoas major and the lumbosacral trunk, and in the articulated pelvis is continuous with the iliac fossa. This is called the ala.
It is slightly concave from side to side, convex from before backward, and gives attachment to a few of the fibers of the iliacus.
The posterior fourth of the ala represents the transverse process, and its anterior three-fourths the costal process of the first sacral segment.
It serves as part of the border of the pelvic brim. Additionally ala also forms the base of the lumbosacral triangle.
Structures Attached to Ala:
Iliolumbar ligament at the lateral portion of tuberosity
Lumbosacral ligament at the medial portion of the rough portion
Origin of a few fibres of the illiacus from the smooth part
Structures in relation:
Iliolumbar artery (branch of posterior division of internal iliac artery)