Pelvis part 1

Last topic for ANHB2212!

Introduction to the pelvis (what is the point of a pelvis?)

There are lots of reasons why pelvises are important. They provide places where muscles can attach and they contain the pelvic organs. Stuff (babies, urine, poo etc.) can also pass through the pelvis.

The parts of the pelvis 

The main parts of the pelvis are the sacrum and the os coxa (hipbones). The hipbones are actually made up of 3 main bones fused together. (There are many others when we are babies, but they fuse up pretty quickly). The three bones that make up the pelvis are the ilium (the wide flarey bits), pubis (the bit in front) and ischium. The ischium and pubis both have a body and a superior and inferior ramus. The three bones are initially joined by triradiate cartilage, which fuses during childhood.

The pelvis can also be divided into the "true pelvis" and the "false pelvis." These parts are separated by the pelvic brim, which is basically the bits of bone around the edge of the main "hole" in the pelvis. More specifically, the pelvic brim is the promontory and alae of the sacrum (the front part of the body and the wingy bits), the pubic symphysis and the iliopectineal lines joining the two (made up of arcuate line, iliopubic eminence and pecten pubis, in that order from back to front). Everything above the pelvic brim is the false (or greater) pelvis, and everything below the pelvic brim is the true (or lesser) pelvis.

The pelvic brim is also known as the pelvic inlet. As you might guess, there is also a pelvic outlet. This is the inferior opening of the pelvic cavity, surrounded by the ischial tuberosities, ischiopubic rami and sacrotuberous ligaments.

Joints of the pelvis
Ligaments of the pelvis

The most important joints of the pelvis are the lumbosacral joints (between L5 and S1), the sacroiliac joints (between sacrum and ilium) and the pubic symphysis. All of these are heavily reinforced by ligaments.

We'll start with the pubic symphysis, because it's the joint that you're probably the most familiar with. As you can guess from its name, it is a symphysis. (If you've forgotten what a symphysis is, see here.) It is reinforced by the arcuate ligament and the superior pubic ligament. It is rare for the pubic symphysis to become fused. In fact, in women who have had children, joint cavities can develop here.

The sacroiliac joint is the joint between the sacrum and ilium. It is a synovial joint that is reinforced by a bunch of ligaments, most notably the dorsal interosseous sacroiliac ligament, which is very strong. (There is also a ventral sacroiliac ligament, don't you worry.) The articular surfaces are not flat, they're rough, and the degree of roughness is one of the things that forensic anthropologists look at when trying to work out how old a person was when they died. The roughening up of the articular surface may also change according to activity (horse-riding etc.).

Biomechanics of the pelvis

The centre of gravity is actually a bit in front of the sacroiliac joint, so there is a tendency for the lumbar vertebrae to slide forwards. Usually, this is adequately resisted by ligaments such as the iliolumbar ligament, which attaches between lumbar transverse processes and the iliac crest. The articular processes also interlock to prevent slippage. However, some people have a weakness in the pars articularis. The pars articularis of the lumbar vertebra is the bony bit that connects the inferior articular processes and spinous processes to the vertebral body and superior articular processes. If this is weak, the articular processes do not provide enough support, leaving the person prone to spondylolisthesis (slipping forward of the lumbar vertebrae).

There is also a tendency for the sacrum to slide down between the ilia, a phenomenon known as sacroiliac subluxation. This is usually resisted by the iliolumbar ligament as well as all of the ligaments reinforcing the sacroiliac joint. These ligaments also prevent any possible rotation of the sacrum from the lumbar vertebrae pressing down more on the anterior than posterior side. All of these issues are even more pertinent if you're pregnant. (Remind me to never become pregnant.)

Male and female pelvis

As you're probably well aware, female pelvises are usually wider to allow babies to pass through. However, it's a bit more complicated than that: male pelvises are actually normally larger externally to provide more points of attachment for big, manly muscles. Female pelvises are wider internally (i.e. wider pelvic inlet/outlet) to allow babies to pass through.

There is still a wide variation though. While having a nice, feminine gynecoid pelvis would be optimal for a baby's head, not every female has the luxury of having a gynecoid pelvis. In fact some have anthropoid pelvises, which are like typical male pelvises in that the inlet and outlet are smaller. Some people have android pelvises, which have a wider inlet than outlet which makes it hard for a baby's head to get out, while some really unfortunate people have platypelloid pelvises which have a wider outlet than inlet and make it hard for the baby's head to even get into the true pelvis in the first place.

Usually, a baby's head will turn as it passes through the pelvis. At the pelvic inlet, the baby will face sideways as the pelvic inlet is wider transversely there. As it moves further down, it will turn its head so that it is facing posteriorly at the pelvic outlet, again for optimal movement.

And I think that's it for part 1!