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Portrait of Ischium

Anatomy · Bones

Ischium

The 'sitting bone' of the hip bone

Pelvic girdle · hip bone

The ischium is the posteroinferior part of the hip bone. Its roughened ischial tuberosity, the 'sit bone', bears the weight of the body when we sit and gives origin to the hamstrings. Its ramus joins the inferior ramus of the pubis to form the ischiopubic ramus, enclosing the obturator foramen.

How to spot them

  • ▸The lower, back part of the hip bone, forming the posterior and lower part of the acetabulum
  • ▸Large, roughened ischial tuberosity at its lower end, felt when you wiggle on a hard chair
  • ▸Ischial spine projecting between the greater and lesser sciatic notches
  • ▸A ramus running forward and up to fuse with the inferior ramus of the pubis around the obturator foramen

What to know

The posteroinferior part of the hip bone, which carries the body's weight when sitting

Ischium — the sitting bone.

You can feel two roughened prominences of this bone by wiggling your pelvis against a chair

Ischium — the ischial tuberosities.

The hamstring muscles arise from a roughened prominence at its lower end

Ischium — the hamstrings attach to the ischial tuberosity.

In labour, the level of the baby's head relative to its spines defines 'station zero'

Ischium — the ischial spines.

Bursitis from prolonged sitting on hard surfaces is called weaver's bottom

Ischium — ischial bursitis.

Its ramus meets the inferior ramus of the pubis to form a single bar that encloses the lower margin of the obturator foramen

Ischium — the ischiopubic ramus.

Its name derives from a Greek word for the hip or hip joint

Ischium — from Greek ischion.

Key ideas

  • The sacrotuberous ligament runs from the sacrum to the ischial tuberosity, and the sacrospinous ligament to the ischial spine; between them they convert the sciatic notches into foramina.
  • In Old World monkeys the ischial tuberosities carry hardened pads (ischial callosities) for sitting; humans have none.

Quick-fire facts

  • •It forms the posteroinferior part of the hip bone and, with the ilium and pubis, the acetabulum (about two-fifths of it).
  • •The ischial tuberosity supports the body in sitting and gives origin to the hamstrings.
  • •The ischial spine separates the greater and lesser sciatic notches; it is used as a landmark in obstetrics (station) and for pudendal nerve blocks.
  • •The ischial ramus joins the inferior pubic ramus to form the ischiopubic ramus.
  • •The obturator foramen lies between the ischium and pubis.
  • •Ischial bursitis is sometimes called 'weaver's bottom' or 'tailor's bottom'.
  • •The word ischium comes from Greek ischion, hip joint.
  • •Hamstring avulsion injuries can pull a fragment off the ischial tuberosity in young athletes.

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Sources