Ischium

The ischium is one of the three fused bones that form the hip bone (os coxae). It lies posteroinferiorly and contributes to the acetabulum, the socket of the hip joint, as well as to the posterior part of the pelvic floor. The ischium consists of a body and two rami. The body forms the posteroinferior portion of the acetabulum, while the ramus ascends and anteriorly joins the inferior ramus of the pubis to form the ischiopubic ramus.

Prominent landmarks include the ischial spine, which lies between the greater and lesser sciatic notches. The greater sciatic notch is above the spine, and the lesser sciatic notch is below it. Ligaments such as the sacrospinous and sacrotuberous convert these notches into the greater and lesser sciatic foramina, through which nerves and vessels pass.

The inferior aspect of the ischium bears the ischial tuberosity, a large rough projection known as the sitting bone. The tuberosity serves as the major weight-bearing point when seated and as an attachment site for several muscles, including the hamstrings (semitendinosus, semimembranosus, and the long head of the biceps femoris) and portions of the adductor magnus.

Articulations and relations: the ischium forms part of the acetabulum with the ilium and pubis and contributes to the obturator foramen with the pubis. Clinically, injuries to the ischium can result from falls or trauma and may involve fractures of the tuberosity or chronic ischiopubic region pain.