The ilium is the broad upper part of each hip bone that helps form the pelvis, carries body weight, and anchors major muscles.
If you’ve searched for “What Is The Ilium Bone?” you’re asking about the wide, flared part of the hip bone on each side of the pelvis. It’s the part many people picture when they think of the “hip bone,” and it works while you stand, walk, sit, bend, and turn.
Each hip bone has three joined parts: the ilium, the ischium, and the pubis. The ilium is the largest. Its upper rim forms the iliac crest, the curved ridge you can often feel above your waist. Lower down, it also helps form the socket of the hip joint, where the femur meets the pelvis.
That mix of size, shape, and position is why the ilium matters. It links the spine to the lower limbs through the pelvis, gives muscles a broad place to attach, and helps shield organs in the lower belly.
What Is The Ilium Bone? Main Parts And Landmarks
The ilium has two broad parts. The first is the body, which joins the other pelvic bones and helps make the acetabulum, the cup-shaped socket of the hip joint. The second is the ala, often called the wing. That wing fans upward and outward, giving the pelvis its broad sidewall.
Several landmarks on the ilium show up in anatomy classes, scan reports, and clinic visits:
- Iliac crest: the top rim of the ilium, easy to feel through the skin.
- Anterior superior iliac spine: the front point of the crest, often called the ASIS.
- Posterior superior iliac spine: the back point of the crest, often called the PSIS.
- Iliac fossa: the smooth inner hollow where the iliacus muscle lies.
- Auricular surface: the ear-shaped area that meets the sacrum at the sacroiliac joint.
- Arcuate line: part of the pelvic brim on the inner surface of the ilium.
Those names make more sense once you match them to what the bone does. The crest is a ridge for attachment. The spines are anchor points and surface markers. The inner surface shapes the pelvis. The area beside the sacrum helps carry load from the spine.
The ilium’s job in daily movement
The ilium is not just a shell around other structures. It helps pass weight from the upper body into the pelvis and then down toward the legs. When you stand on one leg, climb stairs, or push out of a chair, the ilium is part of the bony ring taking that force.
It also gives muscles room to attach. The abdominal wall, iliacus, gluteal muscles, tensor fasciae latae, and other soft tissues all connect in or around the ilium. That helps explain why soreness near the iliac crest can come from more than the bone itself. Muscles, tendons, ligaments, and the sacroiliac joint can all create pain in the same neighborhood.
How the ilium fits with the rest of the pelvis
Each ilium joins the sacrum at the sacroiliac joint in back. In front, the pelvis meets at the pubic symphysis. Lower down, the ilium joins the ischium and pubis to help make the hip socket. Together, those pieces form a strong ring that balances movement with stability.
In childhood, the ilium, ischium, and pubis start as separate bones. They later fuse around the acetabulum, so a young person’s pelvis does not look the same as an adult pelvis.
| Landmark | Where It Is | Why It Matters |
|---|---|---|
| Iliac crest | Top curved rim of the ilium | Easy to feel; attachment line for muscles and fascia |
| ASIS | Front tip of the iliac crest | Surface marker used on exams and posture checks |
| AIIS | Just below the ASIS | Attachment area for rectus femoris |
| PSIS | Back tip of the iliac crest | Common landmark near the “dimples” of the low back |
| PIIS | Just below the PSIS | Helps frame the greater sciatic notch |
| Iliac fossa | Inner smooth hollow of the wing | Home surface for the iliacus muscle |
| Auricular surface | Inner back part of the ilium | Forms the sacroiliac joint with the sacrum |
| Arcuate line | Inner border running forward | Part of the pelvic brim |
Ilium Bone Anatomy In Plain Terms
A good way to picture the ilium is as a broad wing wrapped around the side of the pelvis. The outer side faces the gluteal region. The inner side faces the pelvic cavity. The top is the crest. The lower part narrows toward the hip socket.
An NCBI Bookshelf anatomy review describes the ilium as the largest pelvic bone, made of a wing-shaped ala and an inferior body, with the iliac crest running along the top. A Cleveland Clinic pelvis anatomy page also notes that the ilium is the broad, concave part of the pelvic girdle that wraps around the sides of the hips.
Those source descriptions match what you can feel on your own body. Put your hands on the upper sides of your hips and you’re close to the iliac crests. Move forward to the front bony points and you’re near the ASIS. Move back toward the low back dimples and you’re near the PSIS.
Why doctors pay attention to the ilium
The ilium shows up in medicine for three common reasons. First, it is a landmark. Clinicians use its ridges and spines to judge alignment, locate joints, and read scans. Second, it can be a source of pain. A fall, a stress injury, sacroiliac strain, muscle pull, or inflammation near the crest can all hurt here. Third, it can be a procedure site.
The back of the iliac crest is a common place to collect bone marrow. MedlinePlus bone marrow tests states that most marrow samples are taken from the back of the hip at the iliac crest. The bone is easy to reach and gives access to marrow without entering the chest or abdomen.
What pain near the ilium can mean
Pain “over the hip bone” does not always mean the bone is damaged. The area is crowded. Gluteal muscles attach on the outer side. The iliacus lines the inner side. Ligaments cross near the sacroiliac joint. The crest can also feel sore after overuse, a direct blow, or long-distance running.
Still, bone problems do happen. These include bruises, avulsion injuries where a tendon pulls on a bony point, pelvic fractures, stress reactions, and less common bone disease. The spot of pain matters, though the pattern matters just as much.
Clues that need a medical visit
- Pain after a fall, crash, or hard twist
- Trouble bearing weight
- Fever, night pain, or swelling
- Numbness, weakness, or pain running down the leg
- Pain that keeps building instead of settling
| Term You May Hear | What It Means | Where It Connects To The Ilium |
|---|---|---|
| Sacroiliac joint | Joint between sacrum and ilium | Back inner surface of the bone |
| Acetabulum | Hip socket | Lower part of the ilium helps form it |
| Iliac crest | Top rim of the bone | Most palpable part |
| ASIS | Front bony point | Front end of the crest |
| PSIS | Back bony point | Back end of the crest |
| Iliac fossa | Inner hollow | Surface for the iliacus muscle |
How To Spot The Ilium On A Diagram Or Scan
Start with the biggest clue: width. On a front view, the ilium is the broad upper flare on each side of the pelvis. On a side view, it sits above the hip socket. On the inner surface, it has the iliac fossa. On the top, it has the crest. Once you find those shapes, the rest falls into place.
On an X-ray or CT, the ilium frames the upper part of the hip bone and stretches back toward the sacrum. If a report mentions the iliac wing, iliac crest, iliac fossa, or sacroiliac joint, it is still talking about this same bone. Once you know the landmarks, the map gets easier to read.
The Ilium In One Clear Picture
The ilium is the large upper part of each hip bone. It forms the broad sides of the pelvis, helps create the hip socket, meets the sacrum at the sacroiliac joint, and carries force between the spine and legs. Its crest and spines are easy surface landmarks, so they show up often in anatomy, sports medicine, orthopedics, and bone marrow procedures.
If you’re trying to identify it on your own body, start with the curved ridge above your waist. If you’re trying to identify it on a scan, start with the wing-shaped upper pelvis. Once you know that cue, the ilium becomes easy to place.
References & Sources
- NCBI Bookshelf.“Anatomy, Bony Pelvis and Lower Limb: Pelvis Bones.”Describes the ilium as the largest pelvic bone and outlines the ala, iliac crest, spines, and sacroiliac articulation.
- Cleveland Clinic.“Pelvis: What It Is, Where It Is, Types & Anatomy.”Explains how the ilium fits into the pelvic girdle and summarizes the pelvis’s load-bearing and joint anatomy.
- MedlinePlus.“Bone Marrow Tests.”States that most bone marrow samples are taken from the back of the hip at the iliac crest.