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Definition
The ilium of the musculoskeletal system is characterized by thick medial columns for weightbearing and wing-like alae that serve as attachments for muscles. The biiliac width measurement is important for obstetrics to determine if the pelvis is wide enough to allow for passage of an infant or if there needs to be a cesarean section (c-section). This bililiac wide is approximately 28 cm in the average female when measured with calipers.
It is part of the pelvis, making up the superior portion of the acetabulum, and forms ½ of the sacroiliac joint. It consistsof bone tissue once mature.
Its unique structural features include the anterior superior iliac spine (ASIS), anterior inferior iliac spine (AIIS), ala or wing of ilium, posterior superior iliac spine (PSIS), posterior inferior iliac spine (PIIS), greater sciatic notch, the gluteal lines, and iliac fossa.
The ASIS provides the origin for the sartorius, transverse abdominus, and internal abdominus muscles. The ASIS is a bony prominence that can be palpated at the lateral edge of the inguinal ligament.
The AIIS provides the origin for the rectus femoris muscle. It is difficult to palpate the AIIS.
The PSIS is approximately 4-5cm lateral to the S2 spinous process, it can form a skin dimple. Bone grafts can be taken from this area.
The anterior, posterior, and inferior gluteal lines serve as areas for the origins of the gluteal muscles.
The ala has a large bony depression known as the iliac fossa where the iliac muscle attaches.
The ilium undergoes primary ossification starting and appearing at age 2 months and undergoing fusion at age 15. The iliac crest is a site of secondary ossification starting at age 16 and fusing at age 25. The anterior inferior iliac spine is a site of secondary ossification that starts at age 16 and fuses by age 25. The ilium fuses with the pubis and ischium at the triradiate cartilage at the acetabular dome to form the innominate bone at maturity around age 15 to form the bony acetabulum. The two innominate bones and the sacrum make up the pelvis. The ilium as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in embryo.
The function of the ilium is to support and protect the internal organs, allow for muscle attachements, help form the sacroiliac joint, and make up part of the acetabulum or the hip socket. The ilium along with the ischium and pubis provide vital protection to veins, arteries, and nerves in the pelvis. Fractures to the pelvis can injury the vascular, neurologic, gastrointestinal, and genitourinary system.
Common diseases include fractures and sacroiliac joint arthritis. Sacroiliac joint arthritis can be causes by osteoarthritis or a seronegative spondyloarthropathy. The biiliac measurement is important in obstetrics for determining if there is adequate space for an infant to pass through the birth canal.
Hip pointer injuries occur when there is bruising to the muscles that have origins or insertions at the iliac crest.
Avulsion fractures can occur at the ASIS.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, MRI, bone scan and CT scan.
It is usually treated with internal or external surgical fixation or non-operative approaches for fractures. Arthritis can be treated with NSAIDs, steroid joint injections, or DMARDs. Hip pointer is treated with rest, NSAIDs, and physical therapy.
References
Elstrom J, Virkus W, Pankovich (eds), Handbook of Fractures (3rd edition), McGraw Hill, New York, NY, 2006.
Koval K, Zuckerman J (eds), Handbook of Fractures (3rd edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless’ Textbook of Orthopaedics: Crescent Frx (http://www.wheelessonline.com/ortho/crescent_frx_sacroiliac_fracture)