Ischium

Gregory R. Waryasz, MD

The Common Vein Copyright 2010

Definition

The ischium of the musculoskeletal system is characterized by its large ischial tuberosity which supports the body’s weight during sitting.

It is part of the pelvis making up the posteroinferior aspect of the acetabulum. It consists of bone tissue once mature.

Its unique structural features include the body of the ischium, the ramus of the ischium, the ischiopubic ramus, the greater sciatic notch, the ischial spine, the lesser sciatic notch, and the ischial tuberosity.

The superior body of ischium fuses with the pubis and ilium to form the acetabulum.

Along with the inferior ramus, the ramus of the ischium forms a bar of bone known as the ischiopubic ramus. This is the site of the inferomedial border of the obturator foramen. The obturator foramen contains the obturator membrane and the obturator canal. The obturator canal allows for passage of the obturator nerve, vein, and artery.

The greater sciatic notch is an indentation located on the posterior ischium.

The ischial spine is a site of ligamentous attachment and the origin of the superior gemelli muscle.

The lesser sciatic notch is smaller than the greater sciatic notch in depth that functions similar to a pulley to help with mechanical advantage muscles.

The ischial tuberosity is the site where the body rests during sitting.  This tuberosity is the muscular origin for the inferior gemelli, quadrates femoris, semimebranosis, semitendiosis, and the long head of the biceps femoris.

The ischium undergoes primary ossification starting and appearing at age 4 month and undergoing fusion at 15 years.  The ischium, ilium, and pubis fuse 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 ischial tuberosity is a site of secondary ossification starting and appearing at 16 years and fusing at 30 years. The ischium as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in embryo.

The function of the support and protect the internal organs, allow for muscle attachments, and make up part of the acetabulum or the hip socket. The ilium, 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 fracture, pressure sores, and bursitis.

 Ischial bursitis occurs when there is repetitive microtrauma to the bursa and inflammation.

Pressure sores often develop on the skin overlying the ischial tuberosity in patients who are debilitated due to being under high stress during sitting.

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. Bursitis is treated with NSAIDs and steroid injections.  Pressure sores are treated with wound care techniques including dressing changes, frequent posture changes, and sometimes a wound vac.

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.