Pelvis and Perineum

Osseous Anatomy

Natisha Busick, B.S., Eric Brandser, M.D., Alexander Sandra, Ph.D.
Peer Review Status: Internally Peer Reviewed

The pelvic girdle consists of two coxal bones. As seen on an anterior-posterior radiograph of the pelvis, these two bones unite anteriorly at the symphysis pubis and posteriorly at the sacrum. 3-D views of the pelvis can be made from a CT scan by electronically "erasing" the soft tissues and then stacking the slices. The sacrum, coccyx, and the two coxal bones form the pelvis. The pelvis is divided into the greater, false, pelvis and the lesser, true, pelvis. The greater pelvis lies superior of the pelvic brim. It consists laterally of the ilium and posteriorly of the superior portion of the sacrum. The anterior aspect has no osseous components, it is formed by the abdominal wall. The lesser pelvis lies inferior and posterior to the pelvic brim. It is formed by the lower aspect of the ilium, the pubis, the lower aspect of the sacrum, and the coccyx. The superior opening of the lesser pelvis is known as the pelvic inlet, while the inferior opening is the pelvic outlet.

The two coxal bones grow from three separate bones, the ilium, ischium, and pubis. The area where the three fuse is a deep, lateral fossa called the acetabulum, which is more clearly shown on an oblique view. The acetabulum is also the socket for the head of the femur.

The largest of the three divisions of the coxal bone is the ilium. The superior border of the ilium is known as the iliac crest, which is best seen in a lateral view. It ends anteriorly at the anterior superior iliac spine and posteriorly at the posterior superior iliac spine. The anterior inferior iliac and posterior inferior iliac spines are located just inferior to their respective superior spines. These structures of the ilium are important because they are the points of attachment for muscles of the abdominal wall. Inferior to the posterior inferior iliac spine is the greater sciatic notch. The medial internal surface of the ilium is concave. It is called the iliac fossa and is the point of attachment for the iliacus muscles.

The inferior, posterior portion of the coxal bone is the ischium. It's most prominent features are an ischial spine, a lesser sciatic notch, which lies just below the spine, and an ischial tuberosity, which is more fully displayed by rotating the pelvis to an oblique view. The remainder of the ischium, joins with the pubis to form the obturator foramen.

The pubis is the anterior and inferior part of the coxal bone. It consists of the superior ramus, the inferior ramus, and the body. The symphysis pubis is the joint that connects the two coxal bones at this area.

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