The pelvis is a really important area to consider given its importance with biomechanics, with
ambulation, all the different muscles that attach to the pelvis from the lower extremities to the
back. You have also the important organs within the pelvis that could be affected with biomechanical
restrictions and somatic dysfunction of the pelvis. So let's take a closer look at the pelvis.
So the pelvis itself is also called the innominate. It consists of 3 separate bones at birth.
You have the ilium which is more superior, you have the ischium which is a little bit more
posterior/inferior and the pubic bone is anterior. So the 3 bones come together and form the
acetabulum. The acetabulum is the socket where the femoral head articulates with the innominate.
So, the SI joint is a joint where the sacrum articulates with the innominate. This joint has a
little bit of play to allow for mobility. In males, the SI joint is actually a little bit more well
developed, the ligaments are a little bit stronger, the pelvis is actually a little bit tighter. In
females, the pelvis is a little bit wider, the ligaments are a little bit more lax. This is to assist
with the changes in motion and movements that have to occur during child birth. So the motion
in general becomes more and more limited as one ages. The SI joint is usually L or C-shaped and
is contoured with a shorter upper arm and a longer lower arm. The main axis of the junction
between the sacrum and the pelvis lies at S2. The pubic symphysis is the joint where the innominates
meet anteriorly. The 2 pubic bones will meet anteriorly forming the circle along with the sacrum
and there is a fibrocartilage disc in between the two. There are many ligaments surrounding
the sacrum and pelvis. The sacral pelvic ligaments kind of help to form a suspensory bridge for
the sacrum to be supported there. There is not a lot of muscle that keeps the sacrum in place
with the innominates, in fact it's these ligaments that help to support the sacrum. So the sacrum is
supported by 3 true ligaments and 3 accessory ligaments. The true ligaments are the anterior
iliac ligament, the interosseous sacroiliac ligaments and the posterior sacroiliac ligaments.
There are some accessory ligaments that includes sacrotuberous, sacrospinous and the iliolumbar
ligament. All these ligaments kind of combine and help to suspend the sacrum bearing the front
of all the forces of the rest of the spine from above and also the forces from the lower extremities
and impact from ambulation and walking.