Anatomy

What structures are most commonly injured?

The lower part of the spine ends in the sacrum - a group of vertebral bones fused together at birth. On each side of the sacrum is a sacroiliac joint that joins the ilium of the pelvis to the sacrum. The ilium is the large blade of bone which arches from the spine at the back to the hip joint at the side.

There are strong ligaments joining the sacrum to the ilium. In front is the anterior sacroiliac ligament and behind is the posterior sacroiliac ligament. Together the sacrum, the sacroiliac joint with its ligaments, and the adjacent ilium can be called the sacroiliac complex. Because the pelvis is a ring, when the front is broken it is common for the sacroiliac complex to be disrupted partially or completely. If the line of injury goes through the ligament(s) this can be difficult to diagnose.

The ilium is fused to two other bones, the ischium and the pubis. Together these three structures form the socket of the hip joint called the acetabulum. The pubis is a C-shaped bone which projects inwards from the hip socket as the superior pubic ramus in towards the mid line at the front of the abdomen. There it meets up with the pubis of the other side forming the pubic symphysis. The pubis then passes down and outward again to form the inferior pubic ramus which meets up with the ischium. Together the pubis and ischium form a ring at the front of the pelvis. The hole in the ring is called the obturator foramen.

The ischium is also C-shaped. It projects backwards and downwards from the hip joint then forwards and inwards to meet up with the pubis. The bone is thickened at its lowest point, also called the ischial tuberosity. This takes much of the weight of the upper body when you are sitting and also forms the point of attachment of the hamstring muscles of the thigh.

You can feel quite a number of bony landmarks on the pelvis. When your mother placed her hands on her hips in an authoritarian manner, she was actually pressing her hands onto the crest of the ilium on both sides. The hip joint itself is too deep in the root of the thigh to be felt. If you feel forward on the crest of the pelvis you find that it ends as the anterior superior iliac spine; this is a landmark for measuring the length of the legs. If you sit on your fingertips you will feel the ischial tuberosities. The pubic symphysis can be felt in men at the root of the penis and just above the labia in women.

The pelvis itself does not move very much although the spine and legs are very mobile. There is very little movement at either the sacroiliac joints or the pubic symphysis. However, there is some flexibility of the ring of the pelvis so it can distort a certain amount without breaking. Any ring structure is more likely to break in two places rather than one; this means that if there is a fracture at the front of the pelvis one must look carefully for another injury at the back or on the other side.

The pelvis is also the site of attachment of important muscles of the thigh and abdomen. The oblique abdominal muscles take origin from the wing of the ilium and the rectus abdominis muscle attaches to the pubic bones. The muscles that move the thigh, swinging it forwards, backwards, outwards or inwards mostly take origin from the pelvis. Some of these muscles are strong enough to pull off pieces of their bony attachment. This is called an avulsion fracture).

Many important blood vessels pass close to the pelvis and may be injured when the pelvis is fractured. The internal and external iliac arteries are large vessels that supply most of the blood to the legs. Damage to even a branch of these vessels results in dangerous bleeding; a major pelvic fracture may be associated with a dangerous or even fatal amount of blood loss. The nerves to the legs, rectum, bladder, and genitalia pass inside the pelvis over the sacrum and sacroiliac joints. They may be injured when this part of the pelvic ring is disrupted.

The vagina, the bladder, and the urethra are close to the pelvis and may be injured in a pelvic fracture. Other abdominal organs at risk include the colon and rectum. The pregnant uterus is also very vulnerable to injury in a major pelvic fracture, placing both mother and fetus at risk. Because a pelvic fracture is often associated with major force it is common to see a wide spectrum of other injuries as well - head injuries, chest and abdominal injuries, and multiple long bone fractures.