Untitled Document

Compartmentation of the Pelvis

Goals: To examine:

- How ligaments and the obturator canal form doorways between internal compartments and the compartments of the lower limb.

- How the pelvic diaphragm (levator ani) separates the pelvis from the perineum/ischiorectal fossa.

- The division of the perineum by the urogenital diaphragm.

Let's begin with the skeleton and key ligaments. (At any time you can rotate the image or add or subtract structures. Clicking the link will always bring you back to a starting point. Remember to use rollovers to see the labels for the various structures. Also, by moving the axial plane to any point in the image on the right, you can see the cross-section on the lower left. Any coloration in the right image will be shown in the cross-sections.)

- Rotate this image to 75 degrees to see the doorway between the coxal bones and the inguinal ligament. This doorway connects the abdomen with the anterior compartment of the thigh.

- Rotate this image to 105 degrees to see how the sacrospinous and sacrotuberous ligaments form the lesser sciatic foramen and how the greater sciatic foramen lies superior to the sacrospinous ligament. It may help to hold the shift key while you click on the sacrospinous ligament. When you roll off the ligament, its highlight will turn off and you can see it better against the sacrotuberous ligament.

Let's look at some of the things that go thru the doorway under the inguinal ligament. Here are the femoral artery, vein and nerve. Now add in the iliopsoas muscle and femur.

Let's look at the greater sciatic foramen. We've added in the piriformis muscle. Rotate this image to 0 degrees. Now we add in the lumbosacral plexus forming the sciatic nerve. Rotate this image to 130 degrees and then 160 degrees.

Watch the pudendal nerve exit the greater sciatic foramen and enter the lesser sciatic foramen. When they are superficial to the sacrospinous ligament, they are in the gluteal region. To see the origin of the pudendal nerve rotate the image to 135 degrees. Superior to the sacrospinous ligament the pudendal nerve is in the pelvis; inferior to the ligament, it is in the perineum/ischiorectal fossa.

Notice how the obturator internus covers the obturator foramen. Its tendon exits the lesser sciatic foramen to attach on the greater trochanter. To see this rotate the image to 225 degrees. Notice on the left side the pudendals are not seen inferior to the sacrospinous ligament. They are on the deep side of the muscle. Notice how in the right side you see them coursing on the deep surface. On the right side, there is a tiny gap in the obturator internus, near the superior pubic ramus. This is a doorway between the pelvis and the medial (adductor compartment) of the thigh. Let's demonstrate that by adding in the obturator neurovascular bundle . For a better view, rotate the image to 320 degrees (use rollovers to confirm the identification of the nerves and vessels). To see this doorway from the other side, rotate to 40 degrees (notice on the left side there is a common variation of the obturator artery!)

Now we will add in the levator ani and coccygeus muscles. These form a basket that is hard to appreciate in this image. You will need to slowly rotate the image trough 360 degrees to appreciate it. First, notice that the pudendal nerve is seen only superior to the supraspinous ligament. That is because the levator ani divides the region into pelvis above and perineum/ischiorectal fossa below. Confirm this by rotating the image to 170 degrees.

Now let's add in the urogenital diaphragm, which divides the perineum. It is not well labeled in this program, but we can get a sense of it by highlighting some of its parts:

external sphincter
transverse perineal muscle
perineal body (note the levator ani is funnel shaped and extends inferiorly to the perineal body, but that feature is not labeled in this program.)

and some of its associated structures that attach to its superfical surface. These are the erectile tissue of the female:

ischiocavernosus (The crus of the clitoris, or penis in the male.)
bulbospongiosus (In the male this is not split along the midline, because there is no vaginal opening to accomodate)