The ilium (plural ilia) is among the three bones of the innominate bone: ilium, ischium, and pubis. These are individual bones in the youth and unite to form one bone in adults, the principal union being in the acetabulum.
The ilium is not to be confused with the ileum: see here.
The ilium consists of a flared expanded upper part forming the iliac crests, the iliac spines, and a small inferior part forming two-fifths of the acetabulum.
It has four borders - superior, anterior, posterior and medial borders; and three surfaces - gluteal, iliac fossa, and sacropelvic.
The iliac crest is the superior border of the ilium extending from the anterior superior iliac spine to the posterior superior iliac spine. It has a longer ventral segment forming two-thirds of the crest and a smaller dorsal segment. The ventral segment has outer and inner lips and midway across it is the tubercle of the iliac crest. The summit of the crest is a little behind its midpoint, at the level of the third or fourth lumbar spine.
The anterior border of the ilium descends to the acetabulum, and just short of it forms the rough anterior inferior iliac spine. The posterior border of the crest curves into a small notch with a small projection, called the posterior inferior iliac spine, below which the greater sciatic notch is formed at the junction of ilium and ischium. Beyond this it meets the medial border to form the sacropelvic surface. The medial border of the ilium forms the iliopectineal or iliopubic line, which exhibits an iliopectineal or iliopubic eminence and meets the superior pubic arch.
The gluteal surface faces dorsolaterally, shows three long and prominent ridges called the posterior, anterior and inferior gluteal lines which roughly demarcate the attachments of the bulky gluteal muscles. The iliac fossa gives attachment to the iliacus muscle. The sacropelvic surface exhibits a large iliac tuberosity with an auricular surface or ear-like area articulating with the lateral mass of the sacrum and pelvic surface contributing to the lateral wall of the true pelvis.
The sacropelvic surface of ilium shows an ear-like auricular surface which articulates with the sacrum (sacroiliac joint), however this surface is irregular and rough.
Muscles that originate from ilium:
- sartorius muscle at the anterior superior iliac spine
- rectus femoris from the anterior inferior iliac spine, the reflected head of this muscle originates from the supraacetabular region of ilium
- gluteus maximus, medius and minimus from the gluteal surface divided by bony impression or lines, the posterior, anterior and inferior gluteal lines
- iliacus originates from upper two-thirds of the iliac fossa
- tensor fascia lata origins from the anterior and dorsal aspect of the iliac crest
Muscles that insert onto the ilium:
- quadratus lumborum at the iliac crest and onto the iliolumbar ligament
- external, internal oblique muscles and transversus abdominis of the abdominal musculature as well as latissimus dorsi insert at the iliac crest
- inguinal ligament: anterior superior iliac spine
- iliofemoral ligament: anterior inferior iliac spine
- sacrotuberous ligament: posterior inferior iliac spine
- dorsal sacroiliac ligament: iliac tuberosity
- interosseous sacroiliac ligament and ventral sacroiliac ligament: just posteroinferior and ventral to the auricular surface of iliac tuberosity
- iliolumbar ligament: anterior aspect of iliac tuberosity
A branch of the iliolumbar artery enters the nutrient foramen at the medial border of the ilium.
Single primary center appears at eighth week of prenatal life. At birth the whole iliac crest, acetabular floor and acetabular cup (Y shaped) are cartilaginous. Two secondary centers for the iliac crest appear at puberty and join between 15-25 years of age. Two centers of the acetabular cartilage fuse to form substantial part of the articular surface of acetabulum, and the anterior inferior iliac spine may be ossified from the triradiate center or from a separate ossific nucleus.
Please refer to the article on anatomical variants of the Ilium for a specific discussion.