Ilium

The ilium constitutes the uppermost portion of the hip bone, with the other two being the ischium and pubis.

The ilium exhibits four distinct borders: superior, anterior, posterior, and medial. It possesses two surfaces, namely the outer gluteal surface and inner sacropelvic surface. The upper segment of the bone fans out to form the ‘ala (or wing)’ of the ilium, while the lower segment, known as the body, articulates with the ischium and pubis within the acetabulum.

The medial border runs along the inner aspect of the ilium. It features an oblique bony ridge referred to as 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 connects with the ala and promontory of the sacrum. Together, the arcuate and pectineal lines are collectively known as the iliopectineal line. When combined with other structures such as the sacral promontory, ala of the sacrum, pubic crest, and pubic symphysis, they form the linea terminalis. The linea terminalis of the right and left sides together constitute the pelvic brim or pelvic inlet.

The medial border divides the inner aspect of the ilium into two regions: antero-superior and postero-inferior. The antero-superior region forms a concave area known as the iliac fossa, which contributes to the false pelvis. The posteroinferior region is referred to as the sacropelvic surface and consists of three sub-parts: 1) an upper thickened iliac tuberosity, 2) an auricular surface located anterior and inferior to the iliac tuberosity, which articulates with the sacrum at the sacroiliac joint, and 3) a pelvic surface. It is important to note that the anterosuperior region (iliac fossa) and the iliac tuberosity and auricular surfaces form part of the ala of ilium, while the pelvic surface, which contributes to the true pelvis, corresponds to the body of ilium.

The superior border of the ilium thickens to form the iliac crest, which extends from the anterior superior iliac spine to the posterior superior iliac spine. It serves as the attachment site for various muscles. Near its anterior end, there is a prominent bony tubercle known as the iliac tubercle, projecting from the outer lip of the iliac crest.

The anterior border extends from the anterior superior iliac spine to the anterior inferior iliac spine and continues downwards to the acetabulum. The posterior border spans from the posterior superior iliac spine to the posterior inferior iliac spine and extends downward to form the upper part of the greater sciatic notch.

The outer surface of (ala of) ilium, referred to as the gluteal surface, exhibits three distinct gluteal lines: inferior, anterior, and posterior, arranged in an anterior-to-posterior direction. The external surface provides origins for three large gluteal muscles: the gluteus maximus, medius, and minimus.