The ischium (Latin: os ischii) is a paired bone of the pelvis that forms the lower and back part of the hip bone, as well as the posterior and inferior boundary of the obturator foramen.

Parts of ischium

The ischium consists of two main parts:

  • body of the ischium, and
  • ramus of the ischium.

The body of the ischium is the portion that forms the posterior one-third of the acetabulum.

Ramus of ischium

The ramus of the ischium is the part of the bone that extends downwards from the body, then turns anteriorly and unites with the inferior ramus of the pubic bone. The ramus of the ischium presents the following landmarks:

  • ischial tuberosity,
  • ischial spine,
  • lesser sciatic notch,
  • greater sciatic notch.

The ischial tuberosity is a bony process on the ramus of the ischium at the lower end of the lesser sciatic notch.

The ischial spine is a bony prominence found on the ramus of the ischium between the greater and lesser sciatic notches.

The lesser sciatic notch is a notch on the ramus between the ischial spine and the ischial tuberosity.

The greater sciatic notch is a larger notch located between the posterior inferior iliac spine and the ischial spine.