Pelvis Lab 8A

Lab 8A: Gluteal region and ischioanal fossa

Lab Overview

Dissect and reflect the gluteus maximus muscle to reveal the deeper gluteal muscles (gluteus medius and minimus) and the small lateral rotators of the thigh.
Identify the vessels and nerves structures that pass through the greater and lesser sciatic foraminae
Find the pudendal nerve in the ischioanal fossa.

Gluteal Region

Several vessels, nerves, and muscles pass from the inside of the pelvis, where you’ve been working, to the outside of the pelvis; into the gluteal region, the perineum, and the lower limb. You will now reflect the gluteus maximus muscle and dissect the gluteal region to find those arteries and nerves. This will allow you to demonstrate the continuity of structures as they pass from inside the pelvis to outside of the pelvis. We will consider the muscles of the gluteal region, and their action on the lower limb, in our future dissection of the lower limb.

Figure 4.1

Dissection:

Turn the lower limb so that the skin of the buttocks is facing you. Using a scalpel, remove the skin of the gluteal region.

Try to get as much of the fat as possible off with the skin (there will be a lot of it) while minimizing damage to the underlying muscle.

Identify the gluteus maximus on both sides. Study its attachments along the iliac crest, sacrum, sacrotuberous ligament, and coccyx (this muscle also attaches to the iliotibial tract) and femur.

Figure 4.2

HELPFUL HINTS

The gluteus maximus is thick. Also it will not be as easy to separate the the maximus from the underlying muscles as shown in figure 4.2. You may have to scissor spread some fascia inferior to the muscle or even use a scalpel but take your time and try not to damage the underlying nerves that you will look for next.

Gluteal Region

After studying the superficial aspect of the gluteus maximus:

Expose the inferior (free) border of the gluteus maximus m. by lifting up the inferior border
You may need to use scissor spreading and substantial force to free the inferior border from the deep fascia
Once the inferior border is freed, slide your hand deep to the gluteus maximus m. to separate the muscle from deeper muscles
Use a scalpel to cut the medial border of the gluteus maximus m. away from the sacrum. The sacral attachment of the gluteus maximus m. is angled, such that the most superficial part of the attachment is medial to the deepest part of the attachment. Work your way along the sacral attachment with the scalpel at an angle.
Cut the gluteus maximus m. from the superficial surface of the sacrotuberous ligament, without cutting that ligament.
Reflect the gluteus maximus m. laterally only as far as necessary to uncover the sciatic n. and piriformis m.
Lift up the cut (medial) attachment of the gluteus maximus m. to reveal and identify the following:

Figure 4.3

HELPFUL HINTS

At this time we’re interested in tracking structures from inside to outside the pelvis. We’ll look at the muscles of this region in more detail as part of the lower limb dissection.

- Gluteus medius m. - you can see the supero-lateral part of the gluteus medius m. before reflecting the gluteus maximus m.
- Gluteus minimus m. - deep to the gluteus medius m. Detach the gluteus medius from its superior attachments on the ileum and reflect it inferiorly to reveal the gluteus minimus m.
- Sacrotuberous ligament - deep and parallel to the inferior border of the gluteus maximus m.; a firm band that may feel as stiff as bone
- Palpate the inferior (lateral) attachment of the ligament to the ischial tuberosity
- Palpate the superior (medial) attachment of the ligament to the sacrum; clean the entire length of the ligament with forceps by scraping the surface and edges of the ligament
- Deep to the ligament is a neurovascular bundle; use forceps to reveal and identify the following:
- Pudendal n. and internal internal pudendal a. and v. — emerge inferior to the piriformis m., via the greater sciatic foramen. The pudendal nerve passes between the sacrospinous and sacrotuberous ligaments in very medial location; it’s tucked in there pretty tightly.
- Superficial to the ligament, and attached to the deep surface of the gluteus maximus m., is part of the neurovascular supply to that muscle. Use forceps to dissect the inferior gluteal n., a., and v.
- Ischial spine - attachment for the sacrospinous ligament, which is deep to the sacrotuberous ligament
- Use your finger to probe around the pudendal n. and internal pudendal a. and v. to locate a pointed bone — the ischial spine, to which is attached the sacrospinous ligament
- The pudendal n. and internal pudendal a. and v. course between the sacrospinous and sacrotuberous ligaments (through the lesser sciatic foramen)
- Sacrospinous ligament — attached to the sacrum and ischial spine
- Piriformis m. - attached to the sacrum and greater trochanter of the femur; the sciatic n., pudendal n., internal pudendal a., and inferior gluteal n., a., and v. exit the pelvis inferior to the piriformis m. via the greater sciatic foramen.
- Sciatic n. - formed from ventral rami of spinal nerves L4-S3; also emerges inferior to the piriformis m. via the greater sciatic foramen
- The inferior gluteal bundle of nerves and vessels was likely cut as you reflected the gluteus maximus.

Nerves and Vessels of the Gluteal Region

Veins (vena comitantes) are not shown in figure 2.4, but accompany each artery.
Turn the lower limb prone; use forceps to reveal and identify the following by using blunt dissection:
Superior gluteal a. and superior gluteal n. — exit the pelvis through the greater sciatic foramen, superior to the piriformis m. (accompanied by the superior gluteal v.)
Inferior gluteal a. and inferior gluteal n. — exit the pelvis through the greater sciatic foramen, inferior to the piriformis m. (accompanied by the inferior gluteal v.)
Posterior cutaneous n. of the thigh — exits the pelvis with the sciatic and inferior gluteal nn.
Sciatic n. - the largest nerve in the body; exits the pelvis with the inferior gluteal n., inferior to the piriformis m.; descends between the greater trochanter of the femur and ischial tuberosity, between the hamstring muscles

Figure 4.4

Exiting the Pelvis

Follow the major nerves as they exit the pelvis.

Nerves to follow: Superior gluteal, inferior gluteal, sciatic, posterior femoral cutaneous, pudendal (see next page for detail), obturator, femoral, genitofemoral, lateral femoral cutaneous.

Figure 3.15 Anterior View

Figure 3.16 Posterior View

Figure 3.17 Right Posterior View

Ischioanal - (Ischiorectal) Fossa

Use blunt dissection (not scissors or scalpels) to peel and scrape away the fat in both ischioanal fossae to try to follow the pudendal n., a., and v., and attempt to view some of their branches (they are sometimes hard to find!).
The pudendal n. brings sensation back from the perineal region (skin of the external genitalia, and the skin around the anus and perineum). It also supplies motor innervation to several muscles of the pelvis, including the external urethral and anal sphincters, the levator ani, and the bulbospongiousis and ischiocavernosus mm. After passing between the sacrotuberous and sacrospinous ligaments it passes into the ischioanal fossa, a region of fatty tissue that lies between the rectum/pelvic diaphragm and the bony margins of the pelvis.

HELPFUL HINTS

Dissection of the ischioanal fossa allows you to follow the complex path of the pudendal nerve, an important nerve of the perineum.
Use this opportunity to follow all of the vessels and nerves from inside to outside of the pelvis, noting their course relative to the piriformis muscle and the greater and lesser sciatic foramina

Figure 4.5

Exiting the Pelvis- Pudendal Nerve

Follow the pudendal nerve from its exit out the greater sciatic foramen, around the sacrospinous ligament, under the sacrotuberous ligament, and anteriorly through the pudendal canal (sometimes called Alcock’s canal) to where it becomes the nerves of the external genitalia. You can do this in two ways: (1) you can approach from the back and remove the sacrotuberous ligament where it attaches medially to the ischial tuberosity. You can then use a blunt dissection tool to find the pudendal canal. Alternatively, (2) you can remove or reflect (if possible) the organs of the pelvis on one side and dissect through the levator ani muscles to find the canal medial to the tuberosity. This dissection allows you to follow the branches of the pudendal nerve further into the external genitalia.

Figure 3.18

Figure 3.19

Figure 3.20

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