Ilium

Published on March 14th 2022 by

What is the Ilium Bone

Ilium (plural: ilia), also known as the iliac bone, is one of the three bones that fuse to form the hip bone. The other two bones are:  ischium and pubis.  This largest and uppermost bone of the hip is an essential part of the pelvic girdle.

Where is the Ilium Bone Located

It is located on the uppermost portion of the hip bone. If you firmly press on your waists, you can feel the ilium, particularly the ilium crest.

Quick Facts

Type Flat bone
How many are there in the human body 2 (1 on each side)
Articulates with Sacrum

Functions

  • Form a part of the pelvis, thus protecting the reproductive organs, urinary bladder, and lower part of the digestive tract that lie within it.
  • Bear body weight, while resting or moving.

Ilium Anatomy: Parts and Bony Landmarks

This blade-shaped bone is consists of two main parts: the body and ala (wing).

Ilium

1. Body: The body is the smaller, inferior part of the bone that forms the upper boundary of the acetabulum. The internal surface of the body is part of the wall of the lesser pelvis, from where the some fibers of the obturator internus muscle originate. On the other hand, the external surface of the ilium is partly articular, partly non-articular. The articular segment forms part of the lunate surface of the acetabulum, whereas the non-articular portion contributes to the acetabular fossa. The lower portion of the body is continuous with the pelvic surfaces of the ischium and pubis. A faint line indicates the site of this union of these three bones.

2. Ala (Wing): It is the large, expanded portion of the bone, which laterally bounds the greater pelvis. The superior border of the wing is thickened, creating the iliac crest. It extends from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). On the posterior side, there is an indentation known as the greater sciatic notch. The inner surface of the wing has a concave shape, giving rise to the iliac fossa, the site of origin for the iliacus muscle. On the other hand, the external surface of the wing has a convex shape and provides attachments to the gluteal muscles.

Landmarks

The ilium has four major protruding areas or iliac spines, which are important bony landmarks.

Anterior Superior Iliac Spine (ASIS)

It is located at the anterior e nd of the iliac crest and serves as a point of attachment for the inguinal ligament.

Anterior Inferior Iliac Spine (AIIS)

It is located anterior to the supra-acetabular groove and acetabular margin. AIIS is separated from the ASIS by a short, vertical slope. It provides attachment points for the rectus femoris and proximal part of the iliofemoral ligament.

Posterior Superior Iliac Spine (PSIS)

This spine is located at the posterior end of the iliac crest and is laterally related to the iliac tuberosity and sacropelvic surface. It is commonly represented by a dimple above the medial gluteal region

Posterior Inferior Iliac Spine (PIIS)

It is located inferior to the PSIS.

Borders

The ilium features four distinctive borders; superior (iliac crest), anterior, posterior and medial.

1. Superior Border (Iliac crest)

The superior border of the ilium is known as the iliac crest. It is a rough, crescentic surface that starts at the PSIS posteriorly and arches forward, ending at the ASIS anteriorly. The iliac crest features inner and outer lips. The zone between these two lips is called the intermediate zone. The outer lip bears a prominence near the anterior end of the crest known as the iliac tubercle. The superior border serves as a site of attachment for several muscles and fascia of the abdominal wall, back, and lower limb.

2. Anterior Border

The anterior border of the ilium stretches from ASIS to the acetabulum. It bears AIIS just superior to its acetabular end. The part of the border that runs between these spines is concave anteriorly.

3.Posterior Border

The posterior border of ilium begins at the posterior superior iliac spine and extends to the posterior border of ischium. It features a posterior inferior iliac spine and contributes to the superior part of the greater sciatic notch. The course of this border is irregular; the part between the spines is concave posteriorly, while the part from the inferior spine to the ischial border first runs horizontally then posteroinferiorly to comprise the superior border of greater sciatic notch. This notch is completed inferiorly by posterior ischial border and ischial spine. The sacrospinous and sacrotuberous ligaments respectively enclose the notch superiorly and posteroinferiorly, converting it into the greater sciatic foramen.

4.Medial border

The medial border is present between the iliac fossa and the sacropelvic surface. This border features the arcuate line, which is a smooth and rounded line, running anteroinferiorly from the auricular surface to the acetabulum. It marks the transition between the body and wing of the ilium.

Surfaces

The four borders of the ilium bound its three bony surfaces; gluteal, sacropelvic and iliac (internal).

1.Gluteal Surface

The gluteal surface faces posterolaterally and provides various points of attachment for gluteal and thigh muscles. It is bounded superiorly by the iliac crest, inferiorly by the upper part of the acetabular margin and by the anterior and posterior borders. This surface features three curved ridges known as the anterior, posterior and inferior gluteal lines. The anterior gluteal line travels obliquely along the gluteal surface from the tubercle of the iliac crest towards the greater sciatic notch. The posterior gluteal line descends from the external lip of the crest and ends anterior to the posterior inferior iliac spine. The inferior gluteal line runs superior to the acetabular margin. Between the inferior gluteal line and the acetabular margin is the supraacetabular groove, which is the attachment point for the reflected head of the rectus femoris muscle.

2. Sacropelvic Surface

On the medial aspect of the ilium, posterior to the iliac fossa is the sacropelvic surface, which is composed of the iliac tuberosity, auricular and pelvic surfaces. The auricular surface is an ear-shaped surface that articulates with the sacrum to form the sacroiliac joint. The iliac tuberosity is the posterior portion of the sacropelvic surface; it is a roughened, elevated area that provides a point of attachment for ligaments of the sacroiliac joint. The narrow pelvic surface is found anteroinferior to the auricular surface and contributes to the lateral wall of the lesser pelvis.

3. Iliac (internal) Surface

The iliac fossa, is the large concave anteromedial surface of the wing of the ilium. This fossa forms the smooth posterolateral wall of the greater pelvis and is bounded superiorly by the inner lip of the iliac crest, inferiorly by the arcuate line, anteriorly by the anterior border of the ilium and posteriorly by the medial border which separates it from the sacropelvic surface.

Articulations

Sacroiliac joint: It is a synovial joint formed between the ilium and the sacrum.

Muscle and Ligament Attachments

Several important muscles and ligaments originate or insert onto the ilium.

Muscle Attachments

Muscles that originate from ilium:

  1. Sartorius muscle: At the anterior superior iliac spine.
  2. Rectus femoris muscle: From the anterior inferior iliac spine, the reflected head of this muscle originates from the supra-acetabular region of the ilium.
  3. Gluteus maximus, medius and minimus muscles: From the gluteal surface.
  4. Iliacus muscle: From upper two-thirds of the iliac fossa.
  5. Tensor fascia lata: From the anterior and dorsal aspect of the iliac crest.

Muscles that insert onto the ilium:

  1. Quadratus lumborum muscle
  2. External oblique
  3. Internal oblique
  4. Transversus abdominis muscles
  5. Latissimus dorsi

All of these muscles insert at the iliac crest.

Ligament Attachments

  1. Inguinal ligament: At the anterior superior iliac spine.
  2. Iliofemoral ligament: At the anterior inferior iliac spine.
  3. Sacrotuberous ligament: At the posterior inferior iliac spine.
  4. Posterior sacroiliac ligament: At the iliac tuberosity.
  5. Interosseous sacroiliac ligament and ventral sacroiliac ligament: At the auricular surface of iliac tuberosity.
  6. Iliolumbar ligament: At the anterior side of iliac tuberosity.

References

    1. Ilium – Radiopaedia.org
    2. The Hip Bone – Teachmeanatomy.info
    3. Ilium – Med.libretexts.org
    4. Hip bone – Kenhub.com
    5. Ilium (Bone) – Sciencedirect.com

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