Pelvis Lab 7B

Lab 7B: 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.

Short List of Structures - 7B

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.

The following section describes how you will make the cut.

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.

After studying the superficial aspect of the gluteus maximus:

Expose the superior or inferior (free) borders of the gluteus maximus m. by lifting up the muscle. Note: on many cadavers it is easier to find the correct plane starting at the superior border.
You may need to use scissor spreading and substantial force to free the inferior or superior border from the deep fascia
Once the border is freed, slide your hand deep to the gluteus maximus m. to separate the muscle from deeper muscles
Before any cuts are made, familiarize yourself with the location of the sacrotuberous ligament, this structure is often accidentally cut as it feels like bone and the gluteus maximus m. attaches to it.
Use a scalpel to cut the medial border of the gluteus maximus m. away from the sacrum. Similar to the reflection of the pectoralis major m., the attachment of the gluteus maximus m. is verty broad. 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.
Without cutting the sacrotuberous ligament, cut the gluteus maximus m. from the superficial surface of the sacrotuberous ligament.
Reflect the gluteus maximus m. laterally
Lift up the cut (medial) attachment of the gluteus maximus m.
Locate the gluteus medius m. and reflect it, using a similar method to what was used to reflect the gluteus maximus m., in order to reveal the gluteus minimus m.
Identify the structures listed in the following section:

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.

Nerves, vasculature, and muscles of the Gluteal Region

- Gluteus maximus m. - a large muscle, broadly attached to the ilium, sacrum, and coccyx.
- 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.
- Piriformis m. - most superior of the lateral rotators of the hip.
- Just inferior to the piriformis m. is a trio of small lateral rotators. The superior gemellus m. and the inferior gemellus m. run together, and sandwiched between them is the tendon of the obturator internus m.
- Inferior to the gemellus mm. is the quadratus femoris 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 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.

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.

Ischioanal Fossa and Pudendal Nerve

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

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.

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