Ischium

Recommended video: Bony pelvis [27:56]

Overview of the bony pelvis.

The ischium is a paired bone of the pelvis. Together with the ilium and pubis, they form the hip bone (or pelvic bone, or innominate bone or coxal bone).   

The ischium consists of two main parts:

• The body of ischium - the irregular-shaped part of the bone that forms the posterior one-third of the acetabulum. 
• The ramus (branch) of ischium - extends inferiorly from the body and unites with the inferior ramus of the pubis. The ramus participates in the formation of the obturator foramen. 

In addition, the ramus of the ischium has several important anatomical features that include: 

• The ischial tuberosity is a large posteroinferior protuberance for several muscle attachments (e.g. hamstrings, adductor magnus, inferior gemellus muscles).
• The ischial spine is a posteromedial projection near the merge of the ramus and the body. 
• 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 concavity located between the posterior inferior iliac spine and the ischial spine.