Pelvic girdle

The pelvic girdle comprises three separate bones, the two innominate bones and the sacrum, the ring of bone so formed uniting the trunk and the lower limbs. The innominate is a large and irregular bone with two expanded triangular blades twisted 90° with respect to each other in the region of the acetabulum. Each triangular blade is also twisted within itself. The innominate bone is formed from three separate bones, the ilium, ischium and pubis, which come together and fuse in the region of the acetabulum so that in the adult the innominate appears as a single bone. The sacrum is made up of five fused vertebrae and is roughly triangular. The coccyx, which is the remnant of our tail, consists of four fused coccygeal vertebrae. Each innominate bone articulates with the sacrum posteriorly by joints which are synovial(anteriorly) and fibrous(posteriorly). The innominate bones also articulate with each other anteriorly at the pubic symphysis, by a secondary cartilaginous joint.

The pelvis has several functions:

1. it supports and protects the pelvic viscera;
2. it supports the weight of the body transmitted through the vertebrae, thence though the sacrum, across the sacroiliac joints to the innominate bones and then to the femora in the standing position, or to the ischial tuberosites when sitting;
3. during walking the pelvis swings from side-to-side by a rotatory movement at the lumbosacral articulation which occurs together with similar movements of the lumbar intervertebral joints; even if the hip joints are fused, this swing of the pelvis enables a patient to walk reasonably well;
4. as with all but a few small bones in the hand and foot, the pelvis provides attachments for muscles;
5. in the female it provides bony support for the birth canal.

The pelvic girdle transmits the weight from the vertebral column to the lower limbs, its bony and ligamentous components reflecting this function. There are some differences in the pelvis between females and males. In the female, these are due to adaptation for childbearing and the transmission of the relatively large fetal head during childbirth. The pelvis is essentially a basin, the upper part being known as the greater or false pelvis containing abdominal viscera. That part below the brim or pelvic inlet is the lesser or true pelvis. In the anatomical position, the pelvic inlet forms an angle of about 60° with the horizontal. The acetabulum is directed outward and downward and the acetabular notch should point directly downward. The anterior superior iliac spines and the pubic tubercles lie in the same vertical frontal plane. The lowest part of the sacrum lies above the level of the symphysis pubis.

The anterior superior iliac spines can be easily palpated in the living subject, particularly in females, where they tend to be further apart than in males. The iliac crest can also be palpated about 10cm above the greater trochanter of the femur. In the sitting position, the ischial tuberosites can be felt, the weight of the body resting on them. The body of the right and left pubic bones can also be palpated; they separate the anterior abdominal wall from the genitalia.