The ischium (latin: os ischii) is a paired bone forming the lower and back part of the hip bone, as well as the posterior and inferior boundary of the obturator foramen.
The ischium consists of two main parts:
- body of ischium,
- ramus of ischium.
The body of the ischium is the part that forms the posterior one-third of the acetabulum.
The ramus of the ischium is the portion of the ischium that extends downwards from the body of the ischium, 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 on the ramus of the ischium between the greater and lesser sciatic notches.
The lesser sciatic notch is a notch on the ramus of the ischium between the ischial spine and the ischial tuberosity.
The greater sciatic notch is a larger notch on the ramus of the ischium between the posterior inferior iliac spine and the ischial spine.