Blocks of the lower extremity - Questions
43.
What are the four major nerves of the lower extremity?
44.
What nerves form the sciatic nerve, and what are its approximate
dimensions in the pelvis?
45.
What area is anesthetized by sciatic nerve blockade and what
additional blocks are usually necessary to provide lower extremity anesthesia?
46.
How is sciatic nerve blockade achieved? What volume of local
anesthetic should be deposited for sciatic nerve blockade?
47.
What nerves form the femoral nerve? How does it reach the thigh?
What is the terminal branch of the femoral nerve?
48.
For what procedure is femoral nerve blockade a definitive anesthetic?
49.
How is femoral nerve blockade achieved? What volume of local
anesthetic should be deposited for femoral nerve blockade?
50.
What nerve forms the saphenous nerve?
51.
How is blockade of the saphenous nerve performed at the level of the
thigh?
52.
Where along the course of the sciatic nerve is the popliteal nerve
block performed? What surgical sites are best covered by this block?
Blocks of the lower extremity - Answers
43.
The four major nerves of the lower extremity are the sciatic, femoral,
lateral femoral cutaneous, and obturator nerves. (293)
44.
The sacral plexus (L4-5, S1-3) gives rise to the sciatic nerve, which is
nearly 2cm wide as it leaves the pelvis. (294-296)
45.
Sciatic nerve blockade provides nearly complete anesthesia of the foot
and lower part of the leg. More often, a sciatic nerve block is combined with a
femoral nerve block to provide more extensive anesthesia of the lower extremity.
(294-296)
46.
The classic approach to sciatic nerve blockade is with the patient lying
on the side opposite the nerve to be blocked (Figure 15-14). A line is drawn between
the posterior superior iliac spine and the greater trochanter of the femur. Using a
peripheral nerve stimulator, the needle is inserted about 5cm caudad from the
midpoint of this line. Foot movement evoked by nerve stimulation is a satisfactory
end point for needle placement before the injection of local anesthetic solution
(about 25 to 30mL is typically used). (294-296)
47.
The femoral nerve is derived from L2, L3, and L4. The femoral nerve
reaches the thigh by passing underneath the inguinal ligament just lateral to the
femoral artery and vein, and separated from them by the iliopectineal ligament. The
femoral nerve lies deep to the fascia iliaca, which invests the iliopsoas muscle. Along
its course, the femoral nerve divides into multiple anterior cutaneous branches of
the thigh and gives rise to the saphenous nerve of the medial aspect of the leg. (293294)
48.
Femoral nerve blockade is most often used together with other nerve
blocks for surgery in the leg. Alone, femoral nerve blockade provides anesthesia of
the anterior thigh and may be used for muscle biopsies in that area. (293)
49.
Femoral nerve blockade is performed with the patient in the supine
position and the thigh slightly abducted and externally rotated to improve access.
The femoral nerve is appreciated on sonograms as a flattened bundle of fascicles
lying between the hypoechoic subcutaneous tissue and the hyperechoic iliopsoas
muscle (Figure 15-12). The block needle is advanced within the plane of imaging
from lateral to medial until it punctures the fascia iliaca, with a distinct pop. The
needle position is optimized to create the circumferential spread of 20 to 30mL of
local anesthetic around the femoral nerve. (294)
50.
The saphenous nerve is a branch of the femoral nerve that contributes
to innervation below the knee. (294)
51.
The saphenous nerve is blocked at the midthigh level, where it lies
anterior to the femoral artery (Figure 15-13). The femoral artery is a landmark for
the saphenous nerve, both of which lie just deep to the sartorius muscle (see Figure
15-13). The block needle is advanced within the plane of imaging and 5 to 10mL of
local anesthetic is deposited deep to the sartorius muscle, adjacent to the femoral
artery. (294-295)
52.
Popliteal nerve blockade provides sciatic nerve anesthesia near the
point where the sciatic nerve divides into its common peroneal and tibial nerve
components in the popliteal fossa. It is most commonly used for foot and ankle
surgery. (296)