Ischium

The ischium is the posteroinferior, or lower rear, part of each hip bone. It is one of the three bones that form the hip bone, together with the ilium and pubis. These three bones are separate during early development but later fuse, meeting in the region of the acetabulum, the socket of the hip joint .¹

The ischium consists of a body and a ramus. Its body contributes to the acetabulum, while the ramus extends forward and joins the inferior ramus of the pubis. Together, the ischium and pubis form much of the boundary of the obturator foramen, a large opening in the hip bone .¹

Several prominent anatomical landmarks occur on the ischium. The ischial spine projects from its posterior border and lies between the greater and lesser sciatic notches. Below it is the ischial tuberosity, a thickened prominence that bears substantial body weight during sitting. The ischial tuberosity also provides attachment for structures including several hamstring muscles and the sacrotuberous ligament .¹

The ischium should not be confused with the entire hip bone or with the ischial tuberosity alone. The ischium is one of the three major components of the hip bone, whereas the ischial tuberosity is a specific bony prominence on the ischium .¹