The ischium 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. It consists of two main parts:

Body of the ischium - the part that forms the posterior one-third of the acetabulum;
Ramus of the ischium (sometimes distinguishing a superior and inferior ramus) - the portion of the ischium that extends downwards from the body, then turns anteriorly and unites with the inferior ramus of the pubic bone.

The ischium presents the following landmarks:

Ischial tuberosity - a bony process at the lower end of the lesser sciatic notch;
Ischial spine - a bony prominence between the greater and lesser sciatic notches;
Lesser sciatic notch - a notch between the ischial spine and the ischial tuberosity;
Greater sciatic notch - a larger notch between the posterior inferior iliac spine and the ischial spine.

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Ischium