Pre-lab Exercise: Surface Anatomy of the Gluteal Region
Lab 14 Primary Lab Objectives:
Remove the skin on the posterior surface of the lower limb down past the knee, noting the presence of the cutaneous nerves and veins.
- Note the attachments of the tensor fascia latae and gluteus maximus muscles to 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 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.
- Locate the sciatic nerve.
- Identify and trace the hamstring muscles from their common origin to their insertions.
1. GLUTEAL REGION
The gluteal muscles are innervated by nerves that leave the pelvis through the greater sciatic foramen.
- Skin the gluteal region and the posterior thigh down to the knee.
- Notice the cutaneous nerves you are cutting through as you skin the gluteal region.
- You can identify these as cutaneous branches of dorsal rami innervating the fat-laden skin of the buttocks.
- Remove the extensive 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.
- Trace the tensor fasciae latae and part of the gluteus maximus muscle to their insertions into the iliotibial band.
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.
2. ILIOTIBIAL TRACT
- 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 toward the greater trochanter of the femur.
- Note the bursa that lies between this muscle and the greater trochanter.
NOTE: Cut the gluteus maximus carefully so that you don’t inadvertently take any fibers from the gluteus medius muscle below it. Some fibers of the the gluteus maximus attach to the iliotibial tract, which is a tight band of fascia on the lateral part of the thigh. Also be careful for the blood vessels and nerves beneath the gluteus maximus. Attempt to keeep 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.
3. PIRIFORMIS MUSCLE
a. With the gluteus maximus reflected, work lateral to the sciatic nerve to avoid damaging the inferior gluteal nerve.
b. Define the medial edge of the greater sciatic foramen.
c. The little piriformis muscle emerges from this foramen.
d. Below the piriformis muscle, the inferior gluteal nerve and artery exit through the foramen to the gluteus maximus muscle. The sciatic nerve also exits the greater sciatic foramen below the piriformis muscle.
NOTE: Sometimes the sciatic nerve splits into the fibular nerve and tibial nerve early and the tibial branch can run through the middle of the piriformis or exit the greater sciatic foramen above the piriformis muscle. See if your cadaver has one of these variations.
e. Above the piriformis muscle, the superior gluteal nerve and artery exit through the foramen to 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.
4. 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 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.
5. “LATERAL ROTATORS”: PIRIFORMIS, SUPERIOR AND INFERIOR GEMELLUS, OBTURATOR INTERNUS, QUADRATUS FEMORIS, AND OBTURATOR EXTERNUS.
A group of small muscles including both elevators and depressors cross the hip joint dorsally to act as lateral rotators of the femur:
- Piriformis: the only elevator in this group.
- 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.
- Trace the obturator internus muscle from its origin to its insertion noting the bursa lying between its tendon and the body of the ischium.
The five lateral rotators can be assisted in their task by the much larger gluteus maximus muscle.
6. SCIATIC NERVE
- With the cadaver positioned to allow a view of the interior of the pelvis, trace the sciatic nerve from the lumbosacral plexus to the greater sciatic foramen.
- Turn the cadaver over and look for this nerve as it runs down the thigh under cover of the hamstrings.
The sciatic nerve contains both anterior and posterior divisions of the lumbosacral plexus. The anterior-division fibers run in the medial side of the nerve and become the tibial nerve. The posterior-division fibers run in the lateral side and become the common fibular [=peroneal] nerve.
7. HIP EXTENSORS
- Identify the hip extensors (hamstrings): the biceps femoris, semitendinosus, and semimembranosus muscles.
The hamstrings are depressor musculature (roughly corresponding to the biceps brachii muscle in the arm). They receive motor branches from the medial (tibial) edge of the nerve. An exception is the short head of biceps femoris, which is not really a hamstring, but an isolated elevator muscle that inserts with the long head of biceps femoris. The short head is therefore innervated by the posterior-division fibers from the lateral side of the nerve. The hamstrings can flex the knee and extend the hip joint. In walking, they contract extending the hip joint as the heel touches the ground, and flexing the knee as the toes leave the ground.
- Trace the hamstrings from their common origin on the ischial tuberosity to their insertions.
- Then transect the hamstrings at different levels and reflect them (to help identify them later), noting and cutting their motor nerves.
- Look for the bursa underlying the insertion of the semimembranosus muscle.
- Leave the short head of the biceps femoris muscle and the sciatic nerve intact.
NOTE: the semimembranosus and semitendinosus can be distinguished by the fact that the semitendinosus has a long thin tendon distally which lies on top of the semimembranosus 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.
Position of Femoral Artery to an Anatomical Landmark
The femoral artery can be palpated near the mid-inguinal point, halfway between the pubic symphysis and anterior superior iliac spine. Beyond taking the pulse of this vessel, the ability to compress the common femoral artery against the head of the femur has significance in clinical situations. For example, radiologists frequently use the common femoral artery as a site to gain entry into the arterial system. At the end of a procedure, when the catheter is withdrawn, bleeding from the common femoral artery may be controlled by compressing it against the femoral head.