It is essential for the clinician to understand how nerves and vessels enter and leave the pelvis and perineum. This information is used for administering injections and nerve blocks; performing catheterization; obtaining blood samples; designing surgical approaches; assessing pelvic trauma; etc. There are 4 principal "doorways" that are used.
The space bounded by the coxal bones is subdivided by muscular diaphragms. Several key ligaments define passages (doorways) for neurovascular structures to communicate with distinct compartments of the lower limb. Doorways and diaphragms can be an organizational tool for thinking about the pelvis in three dimensions.
Begin with the skeleton and key ligaments. (At any time you can rotate the image or add or subtract structures. Clicking the red hyperlink will bring you back to a starting point. Remember to use mouse rollovers to see structure labels. Also, by moving the planes in the 3D image, you can display the associated cross-section. Any coloration in the 3D image will be shown in the cross-sections.)
Observe the femoral artery, vein and nerve coursing inferior to the inguinal ligament. Now add the iliopsoas muscle and femur.
Which body compartments are connected by this doorway?
The abdomen and the anterior compartment of the thigh?
Many suspect a different compartment. Which one and why is it incorrect?
Right! The Pelvis. The psoas muscle and external iliac vessels follow the pelvic brim. The abdomen lies anterior to the plane defined by the pelvic brim and the pelvis lies posterior to it. This region of the abdomen is embraced by ala (wing) of the left and right ilium and is sometimes called the false pelvis.
The greater sciatic foramen transmits the piriformis muscle. Rotate this image to a frontal view. Now add the lumbosacral plexus forming the sciatic nerve. Rotate this image to a right posterolateral view and continue stepwise toward a posterior view observing the greater sciatic notch and piriformis muscle. Now add the pudendal nerve, superior gluteal artery, and inferior gluteal artery. For the last three additions, the sectional images are adjusted to place the new structure in the crosshairs (click the "move" botton and grab a sectional image with your mouse to highlight the crosshairs). Keep track of the previously labeled structures (click the "highlight" button and use mouse rollovers).
Which body compartments are connected by this doorway?
The pelvis and the gluteal region of the thigh.
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 a posterolateral view. 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 (use the INDEX tab to add the tendons of the obturator internus muscles). Rotate the image to a left posterolateral view. 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 them coursing on the deep surface of the muscle on the right side.
Which body compartments are connected by this doorway?
The gluteal region of the thigh and the ischiorectal fossa/perineum.
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 3D image to a left anterolateral view (use mouse rollovers to confirm your identification of the nerves and vessels). To see this doorway from the other side, rotate to a right anterolateral view (notice on the left side there is a common variation of the obturator artery!)
Which body compartments are connected by this doorway?
The pelvis and the adductor (medial) compartment of the thigh.
Now add 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° 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 a view just shy of directly posterior.
Which body compartments are separated by this boundary?
The pelvis lies superior and the ischiorectal fossa/perineum lies inferior.
Now add 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:
and some of its associated structures that attach to its superfical surface. These are the erectile tissue of the female:
Which body compartments are separated by this boundary?
The urogenital diaphragm separates the anterior recess of the ischiorectal fossa superiorly from the perineum. This region, anterior to the ischial tuberosities is known as the anterior, or urogenital triangle. The posterior, anal, triangle is not subdivided and is simply known as the ischiorectal fossa.