Ala of ilium

The ala of ilium refers to the upper, fan-shaped (or wing-shaped) portion of the ilium. In contrast, the ‘body’ of the ilium refers to its lower section, which is more compact and articulates with the ischial and pubic components of the hip bone within the acetabulum.

The ala of ilium features two surfaces: an inner (medial) and an outer (gluteal) surface:

• The medial surface of the ilium contains an oblique bony ridge called the arcuate line. Antero-inferiorly, it continues as the pectineal line (a.k.a. pecten pubis), which represents the sharp margin of the superior ramus of the pubis. Postero-superiorly, it extends along the ala and promontory of the sacrum. Collectively, they form the linea terminalis, contributing to the pelvic inlet or pelvic brim.

  The arcuate line (or linea terminalis or pelvic inlet) divides the inner aspect of the ilium into anterosuperior and posteroinferior regions. The antero-superior part constitutes a concave area known as the iliac fossa, which forms part of the false pelvis. The posteroinferior part is referred to as the sacropelvic surface, consisting of three sub-parts: 1) an upper, thickened iliac tuberosity, 2) an auricular surface situated anterior and inferior to the iliac tuberosity (which articulates with the sacrum at the sacroiliac joint), and 3) a pelvic surface. It’s important to note that the anterosuperior region (iliac fossa) and the iliac tuberosity and auricular surfaces constitute the ala of ilium, while the pelvic surface, contributing to the true pelvis, corresponds to the body of the ilium.

• The outer surface of the ala of ilium forms part of the gluteal surface of the hip bone. It displays three distinct gluteal lines. From anterior to posterior, these include the inferior, anterior, and posterior gluteal lines. The external surface provides origins for three major gluteal muscles: the gluteus maximus, medius, and minimus.

For a comprehensive description of the ilium, please refer to the following link: ilium.