The Anatomy of the Ilium

The ilium makes up the upper portion of the hip bone and pelvis

In This Article

The largest and uppermost bone of the hip, the ilium, also known as the iliac bone, is an essential part of the pelvic girdle. In adults, this fan-shaped bone is fused with two other bones, the ischium and pubis, to make the hip bone (often referred to as the coxal bone). As such, the ilium serves a weight-bearing function and is part of the structure that ensures the spine is supported when the body is upright. It’s therefore essential as part of the apparatus that allows for locomotion.

Problems in the pelvis—most often seen in women—can impact this bone, as in cases of endometriosis (in which the uterine lining is found outside the uterus, leading to bleeding and other symptoms), pelvic inflammatory disease (formation of scar tissue that interferes with fertility), uterine fibroids (benign tumors in the uterus), and others. In addition, a fracture can occur here and the bone can be impacted by arthritis.

Anatomy

As part of the hip bone, the ilium, alongside the ischium and pubis, are fused to one another, and, via the sacroiliac ligaments, are attached to the sacrum (the tailbone). This juncture, which is largely immobile, is the sacroiliac joint.

Anatomically speaking, the ilium is broken down into two parts: the body and the wing.

The body of the ilium is its more central portion, and it forms a part of the acetabulum—the socket joint where the head of the femur (upper leg bone) rests—as well as the acetabular fossa, a deeper depression just above the joint.

The wing of the ilium, as the name implies, is the larger, expanded portion of the bone. On each side, it represents the outer, side-edge of the pelvis.

The ilium also has a number of important landmarks, including:

  • The iliac crest is the curved, upper margin of the ilium.
  • The anterior superior spine is a bony projection marking the limit of the iliac crest on the front.
  • The anterior inferior spine is a bony projection running beneath the anterior superior spine on the front side of the bone.
  • The posterior superior spine is the terminus of the iliac crest on the rear-facing side of the ilium.
  • The posterior inferior spine is below the posterior superior spine, and at the end of a larger, roughened region called the auricular surface.
  • The auricular surface connects with the sacrum via ligaments to form the sacroiliac joint.
  • The iliac fossa is a shallow depression on the internal surface of the upper part of the bone.
  • The arcuate line is a ridge that forms the bottom border of the ilium, created by the change in curvature between the upper and lower portions of the bone.
  • The greater sciatic arch is the larger U-shaped indentation at the rear margin of the lower ilium.

Anatomical Variations

In general, differences in the shape of the pelvis—and by extension, the ilium—are seen between men and women. Basically, women’s pelvises are wider and display a greater distance between the anterior superior iliac spines, whereas men’s pelvises tend to be deeper and have stronger and thicker bones to support their (typically) heavier upper bodies.

It’s recognized that there are four variations: android, gynecoid, anthropoid, and platypelloid, which are differentiated by the shape of the pelvic inlet, weight, subpubic angle, and other characteristic elements.

Function

As indicated above, the primary purpose of the ilium is to serve as part of the pelvis and assist in both supporting the upper body and facilitating locomotion and walking. A number of muscles and nerves connect to the ilium, helping determine this bone’s function. Relevant muscles here include:

  • The sartorius muscle, which attaches to the anterior superior iliac spine, is associated with hip and knee motion.
  • The rectus femoris is one of the quadriceps of the thigh and arises at the anterior superior iliac spine.
  • The piriformis helps the hip rotate, letting the leg and foot move outward just under the posterior inferior iliac spine.
  • The gluteus maximus, medius, and minimus—the primary muscles of the buttocks—also emerge from the ilium.
  • The iliacus muscle emerges from the iliac fossa and provides flexion in the thigh.
  • The tensor fascia latae muscle, which originates from the anterior and dorsal sides of the iliac crest, is involved in maintaining balance during standing or walking.
  • The quadratus lumborum, one of the deepest abdominal muscles, terminates at the ilium.
  • The internal and external oblique muscles of the abdominal muscles enter the iliac crest.

A number of ligaments also connect to the ilium, and these are often associated with the stabilizing functions of this bone. From the anterior superior iliac spine arise the inguinal and iliofemoral ligaments, which connect to the pubic bone and the femur, respectively. The sacrotuberous ligament supports the sacrum and prevents it from moving; it attaches to the iliac tuberosity.

Finally, several other supportive ligaments—the dorsal, interosseous, and ventral sacroiliac ligaments, as well as the iliolumbar ligaments—also connect to the iliac tuberosity.

Associated Conditions

The ilium can be the site of problems as part of diseases that affect the pelvis. Primarily, these affect women.

For instance, scar tissue can arise in this area as a result of a number of diseases and can lead to pain and infertility. In addition, the ilium can be implicated in cases of endometriosis, a disorder in which uterine tissue grows outside of the uterus, leading to pelvic pain, painful menstruation, severe cramps, infertility, among a number of other symptoms.

Pelvic inflammatory disease leads to scar tissue formation in and around the ilium and can lead to difficult fertilization in women. An infection that arises in the uterus, this disease, as the name implies, leads to a severe inflammatory response throughout the area.

In addition, the formation of uterine fibroids—benign tumors—can lead to pain in and around the pelvis. These lead to increased urinary frequency, constipation, and other symptoms.

The ilium can also become fractured due to falls or other trauma. Symptoms of this include sharp pain, swelling and bruising, as well as an inability to put weight on the hip.

Genetic deformities can also arise in the shape of the ilium and pelvis, which can also lead to a range of problems.

Finally, inflammation of the sacroiliac joint—a condition called sacroiliitis—can lead to pain symptoms as well as fever and stiffness. This often arises due to arthritis in the hip.

Rehabilitation

Treatment for issues surrounding the ilium and pelvis varies depending on the severity of the condition.

Endometriosis, for instance, can be treated using hormonal treatment, the use of pain medications, as well as surgery to remove the damaged tissues. For pelvic inflammatory disease, treatment ranges from antibiotics to minor surgery as well. If fibroids grow too large, they also may need to be removed surgically.

Rehabilitation for fractured ilium depends on the scope of injury. More minor cases may require little more than bed-rest as well as pain-relieving and anti-inflammatory medication. Physical therapy and the use of crutches may also be required and, in extreme cases, surgery may be required to repair the area.

With sacroiliitis or other symptoms of hip arthritis, treatment ranges from the administration of pain medicine and anti-inflammatory pills like naproxen, acetaminophen, and others. Stretches and exercises can also help, but if the damage is too extensive hip resurfacing or replacement surgery may be required. In the former, a part of the hip is replaced by a metal prosthesis, whereas, in latter cases, a part of the hip is recreated using both the head of the femur and the socket are replaced.

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Article Sources

  1. Rice University. The pelvic girdle and pelvis. In Anatomy and Physiology. Houston, TX. Pressbooks. Published 2019.

  2. Wobser A, Wobser R. Anatomy, Abdomen and Pelvis, Bones (Ilium, Ischium, and Pubis). Published 2018.

  3. Jarvis M, Batta N. Ilium: Radiology Reference Article. Radiopaedia.org. Published 2018.

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