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Topic

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Ilium bone

The ilium is the largest and most superior portion of the hip bone, forming the superolateral aspect of the bony pelvis. It articulates with the sacrum at the sacroiliac joint posteriorly and contributes to the formation of the acetabulum laterally, which articulates with the femoral head to form the hip joint.

The ilium is divided into:

  • Body of ilium: contributes to the acetabulum.

  • Ala (wing) of ilium: broad, expanded portion forming the iliac fossa on the medial surface and the iliac crest superiorly.

The iliac crest forms the superior border of the ilium and terminates anteriorly at the anterior superior iliac spine (ASIS) and posteriorly at the posterior superior iliac spine (PSIS). Inferior spines (AIIS and PIIS) are located below these landmarks. The medial surface shows the iliac fossa, which provides origin for the iliacus muscle, and the auricular surface, which articulates with the sacrum.

The ilium is clinically important for muscle attachment, bone marrow biopsy, orthopedic surgery, fracture evaluation, and pelvic landmarking. It is also a key site in pelvic fractures, metastatic disease, and bone graft harvesting.

Synonyms

  • Iliac bone

  • Wing of ilium

  • Ala ossis ilii

Function

  • Provides structural support for the pelvis and weight-bearing skeleton

  • Forms part of the acetabulum, enabling hip joint articulation

  • Provides extensive muscle attachment sites for trunk, pelvic, and thigh muscles

  • Protects pelvic organs and supports abdominal wall

  • Serves as a landmark for surgical and radiological procedures

Muscle Attachments

  • Iliacus muscle (iliac fossa)

  • Gluteus maximus, medius, minimus (gluteal surface of ala)

  • Tensor fasciae latae (near ASIS)

  • Sartorius (ASIS)

  • Rectus femoris (straight head) (AIIS)

  • Abdominal muscles (external oblique, internal oblique, transversus abdominis) (iliac crest)

  • Quadratus lumborum and erector spinae (posterior iliac crest)

Nerve Supply (related attachments)

  • Gluteal muscles: superior and inferior gluteal nerves

  • Iliacus: femoral nerve (L2–L4)

  • Abdominal wall muscles: thoracoabdominal and subcostal nerves

Arterial Supply

  • Iliolumbar artery (branch of internal iliac)

  • Superior gluteal artery (branch of posterior division of internal iliac)

  • Deep circumflex iliac artery (branch of external iliac)

  • Obturator artery contributes to adjacent areas

Venous Drainage

  • Iliac veins (internal and external) via iliolumbar, obturator, and gluteal veins

  • Connects to systemic venous circulation

MRI Appearance

T1-weighted images:

  • Bone cortex: uniformly hypointense (dark)

  • Bone marrow: intermediate signal (fatty marrow brighter than red marrow)

  • Clearly depicts marrow replacement, infiltration, or fractures

T2-weighted images:

  • Cortex: hypointense rim

  • Marrow: variable — fatty marrow appears hyperintense compared to muscle, edema appears brighter

  • Useful in detecting subchondral cysts, fractures, and marrow edema

PD Fat-Saturated (Proton Density FS):

  • Cortex: hypointense

  • Marrow edema: appears bright hyperintense

  • Sensitive for stress fractures, bone marrow edema, and soft tissue enthesopathy

STIR:

  • Cortex: remains hypointense

  • Marrow edema, trauma, tumors, and inflammation: hyperintense

  • Excellent for early stress reactions, bone bruises, or infiltrative lesions

T1 Post-Gadolinium (with fat saturation):

  • Cortex: non-enhancing

  • Bone marrow: enhances if inflamed, vascularized, or infiltrated by tumor

  • Highlights osteomyelitis, tumors, and inflammatory enthesitis

3D T2-weighted Imaging:

  • Cortex: sharply hypointense

  • Provides multiplanar reconstructions of iliac crest, spines, and acetabulum

  • Useful in pre-surgical planning and pelvic morphology assessment

CT Appearance

Non-contrast CT:

  • Cortex: hyperdense, sharply defined

  • Bone marrow: hypodense compared to cortex

  • Gold standard for fractures, dysplasia, sacroiliac joint evaluation, and cortical lesions

CT Post-Contrast:

  • Bone: unchanged

  • Soft tissue/vascular structures: enhance with contrast

  • Useful for osteomyelitis, tumors, and periprosthetic evaluations

  • 3D reconstructions aid in trauma and orthopedic surgical planning

Clinical Significance

  • Fractures: Iliac wing fractures, avulsion fractures at ASIS/AIIS, part of complex pelvic fractures

  • Bone marrow biopsy site: Commonly performed at posterior iliac crest

  • Bone graft harvesting site: Widely used in reconstructive surgery

  • Tumors/metastasis: Common location for skeletal metastases, plasmacytomas, and pelvic bone tumors

  • Inflammatory diseases: Sacroiliitis and pelvic enthesitis can involve iliac bone

  • Landmark: Guides surface anatomy, imaging planes, and surgical approaches (e.g., Smith-Petersen approach to hip)

CT VRT 3D image

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MRI image

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