Pre-lab Exercise: Surface Anatomy of the Gluteal Region
Lab 14 Primary Lab Objectives:
Remove the skin on the buttocks. Identify the gluteus medius muscle and the iliotibial tract. Reflect the gluteus maximus muscle.
- Locate the piriformis muscle deep to the gluteus maximus. Examine the relationships of the superior and inferior gluteal nerves and vessels and the sciatic nerve to the piriformis muscle as they exit the greater sciatic foramen.
- Identify the gluteus medius muscle and transect the muscle to expose the underlying gluteus minimus muscle.
- Locate the deep muscles of the gluteal region: obturator internus, gemellus superior, gemellus inferior, quadratus femoris, and obturator externus. Trace the path of the pudendal nerve and the internal pudendal artery.
1. GLUTEAL REGION
- Remove the skin from the buttocks.
- Notice the cutaneous nerves you are cutting through as you skin the gluteal region.
- Remove the fat overlying the gluteus maximus muscle and note the muscle's lower edge crossing the fold under the buttock obliquely.
- Locate the specialized band of fascia of the thigh called the iliotibial tract. Its attachments are implied by its name.
The gluteus maximus muscle originates from 1) the back of the iliac crest, 2) the lower sacrum, and 3) all the dorsal ligaments binding the pelvis and sacrum together. It inserts into the iliotibial tract and the gluteal tuberosity of the femur. The gluteus maximus muscle lies directly behind the hip and acts primarily as an extensor of the hip joint. Notice that based on its attachments, it can also laterally rotate the thigh and tighten the iliotibial band. It is not active in walking since little active extension of the hip is involved but it comes into play in activities such as climbing stairs, rising from a squatting position, and running.
- Verify that the superior and superficial fibers of the gluteus maximus muscle insert into the iliotibial tract.
- The smaller tensor fasciae latae, like the gluteus maximus muscle, also attaches to the iliotibial tract.
- Reflect the gluteus maximus muscle laterally by cutting along the border of the sacrum and the sacrotuberous ligament.
NOTE: Cut the gluteus maximus carefully so that you don’t inadvertently take any fibers from the gluteus medius muscle below it. Also be careful for the blood vessels and nerves beneath the gluteus maximus. Attempt to keep all of these structures intact.
The head of the femur lies medial to the axis of the shaft. This produces bending stresses in the femur and tends to bow it outward laterally. This tendency is resisted by the functional importance of the iliotibial tract and the muscles pulling on it. The tract also improves the leverage of the gluteus maximus and tensor fasciae latae muscles in rotating the thigh medially and laterally.
2. PIRIFORMIS MUSCLE
a. With the gluteus maximus reflected, examine the sciatic nerve and the inferior and superior gluteal nerves.
b. Find the piriformis muscle emerging from the greater sciatic foramen to insert on the upper border of the greater trochanter of the femur. This is a major landmark in the gluteal region.
c. Below the piriformis muscle, find the inferior gluteal nerve and artery exiting through the foramen to supply the gluteus maximus muscle. The sciatic nerve also exits the greater sciatic foramen below the piriformis muscle.
NOTE: In 12-14 % o people, the sciatic nerve splits into the fibular nerve and tibial nerve at or near the piriformis muscle. When this happens, the fibular branch can run through the middle of the piriformis or exit the greater sciatic foramen above the piriformis muscle (see images about showing variation). See if your cadaver has one of these variations.
d. Above the piriformis muscle, the superior gluteal nerve and artery exit through the foramen to supply the gluteus medius & minimus and tensor fasciae latae muscles.
NOTE: Identification of the piriformis muscle will help to orient you in the gluteal region and find and identify the other rotators of the hip.
3. GLUTEUS MEDIUS AND MINIMUS
- Outline the gluteus medius muscle, noting its origin and insertion.
- Transect the gluteus medius muscle along its origin near the iliac crest. Reflect the gluteus medius inferiorly to expose the gluteus minimus muscle immediately deep to it.
NOTE: It is often difficult to separate the fibers of the gluteus medius from the gluteus minimus. Dissect carefully in this area.
- Trace the superior gluteal vessels and nerves running between the two muscles, supplying them and ending in the tensor fasciae latae muscle.
4. LATERAL ROTATORS
A group of small muscles including both elevators and depressors cross the hip joint dorsally to act as lateral rotators of the femur:
- Quadratus femoris
- Obturator internus
- Gemelli (L. ="little twins," diminutive of gemini)
- Obturator externus
- Separate the lateral rotators, beginning with the piriformis muscle.
- The piriformis muscle, which you located earlier, is just below the gluteus minimus muscle and exits the greater sciatic foramen.
- The next three muscles (in order from superior to inferior) are: the superior gemellus, obturator internus, and inferior gemellus muscles.
Hint: You will not see obturator externus yet. That will be visible from this aspect shortly when you transect quadratus femoris in a few steps.
- The quadratus femoris muscle is a square muscle that can be found inferior to the last three.
HINT: It is sometimes hard to see these small deep muscles. Here we provide a description of where these small muscles originate and insert to help orient you to this complex region. The piriformis muscle originates from the anterior surface of the sacrum and exits through the greater sciatic foramen to the greater trochanter. The quadratus femoris muscle runs from the ischial tuberosity to the back of the femur. The obturator internus muscle originates from the deep surface of the obturator membrane covering the obturator foramen and forms part of the lateral wall of the ischioanal fossa. Its tendon exits dorsally through the lesser sciatic foramen, which is the "door" to the ischioanal fossa. The gemelli muscles originate from the margins of the lesser sciatic foramen. These four muscles have separate little motor nerves from the lumbar plexus.
- Carefully transect the quadratus femoris muscle as shown by the dotted line in the diagram below to see the tendon of the obturator externus muscle wrapping around the hip joint posteriorly.
- Follow the pudendal nerve and accompanying vessels around the dorsal edge of the sacrospinous ligament into the ischioanal fossa.
The five lateral rotators can be assisted in their task by the much larger gluteus maximus muscle.
An intramuscular injection is used to deliver medication deep into the muscles. At this level, the medication can be absorbed quickly into the bloodstream (depending on the chemical properties of the drug). One way to determine where to give an intragluteal injection is to use surface anatomy. The sciatic nerve is approximately midway between the greater trochanter and the ischial tuberosity. Hence, it is best to give intragluteal injections in the upper lateral quadrant of the gluteal region to avoid any injuries by intramuscular injection. Typically, the common fibular nerve is more superficial and lateral as it branches off the sciatic, so it is commonly affected by these injections if they are positioned too far inferiorly.