FEMORAL NERVE BLOCK

 
 FEMORAL NERVE BLOCK

Historically this block was also known as the “3-in-1

block,” suggesting that the femoral, lateral femoral

cutaneous, and obturator nerves could be blocked

from a single paravascular injection at the femoral

crease. Studies have since demonstrated that the

femoral and lateral femoral cutaneous nerves

can be reliably blocked by a single injection, but

the obturator nerve is often missed. Therefore, a

posterior lumbar plexus block should be used when

all three nerves need to be anesthetized (although

this point remains controversial). The femoral

nerve block is an ideal block for surgeries of the

hip, knee, or anterior thigh and is often combined

with a sciatic nerve block for near complete lower

analgesia. Complete analgesia of the leg

can be achieved without lumbar plexus block by

extremity

combining a femoral nerve block with parasacral

sciatic nerve block (which blocks the obturator

over 90% of the time), or by adding an individual

obturator nerve block to the femoral nerve block.

ANATOMY

The femoral nerve, formed by the dorsal

divisions of the anterior rami of L2–L4, is the largest

terminal branch of the lumbar plexus. It travels

through the psoas muscle, leaving the psoas at its

lateral border. The nerve then descends caudally

into the thigh via the groove formed by the psoas

and iliacus muscles, entering the thigh beneath the

inguinal ligament . After emerging

from the ligament, the femoral nerve divides into

an anterior and posterior branch. At this level it is

located lateral and posterior to the femoral artery

. The anterior branch provides motor

innervation to the sartorius and pectineus muscles

and sensory innervation to the skin of the anterior

and medial thigh. The posterior branch provides

motor innervation to the quadriceps muscle (rectus

femoris, vastus intermedius, vastus lateralis, and

vastus medialis) and sensory innervation to the

medial aspect of the lower leg via the saphenous

nerve.

The anatomic location of the femoral nerve makes

this block one of the easiest to master because the

landmarks are usually simply identified (except in

cases of morbid obesity), the patient remains supine,

and the depth of the nerve is relatively superficial.

PROCEDURE

Landmarks.

anterior superior iliac spine and the pubic symphy

and draw a line between these two landmarks.

This line represents the inguinal ligament. The

Place the patient supine, identify thesis,

femoral nerve passes through the center of the line,

which makes this landmark useful for positioning

the needle in the inguinal crease, particularly in an

obese patient. Then palpate the femoral pulse and

mark it at the inguinal crease. Studies have dem

that the most successful point of needle

onstrated

entry is directly lateral (1–1.5 cm) to the artery in the

inguinal crease. At this location the femoral nerve is

wide and superficial, and the needle does not pass

through significant muscle mass. Direct the needle

cephalad toward the center of the inguinal ligament

line.

Needles

• 22-gauge, 5-cm insulated needle.

• 18-gauge, 5-cm insulated Tuohy needle for

catheter placement. The catheter is inserted 3 to 5

cm for the femoral block.

Stimulation.

The nerve stimulator is initially set

at 1.0 to 1.2 mA. The needle is directed cephalad at

approximately a 30° to 45° angle. A brisk “patellar

snap” with the current at 0.5 mA or less is indica

of successful localization of the needle near

tive

the femoral nerve. The nerve is usually superficial,

rarely beyond 3 cm from the skin .

Local Anesthetic.

In most adults, 20 to 40 mL of

local anesthetic will produce a successful femoral

block.

Teaching Points.

Studies have demonstrated

that the anterior branch of the femoral nerve is

usually encountered with the first needle pass,

which results in stimulation of the sartorius

muscle, often seen as contraction of the lower

medial thigh. If this occurs, advance the needle

tip until either the sartorius twitch is extin

or a patellar snap is elicited before redi

the needle. If the sartorious twitch is ex

without the patellar snap, withdraw

guishedrectingtinguished

the needle toward the skin (without exiting

the skin), and redirect it slightly lateral and

slightly deeper than the original needle pass.

The posterior branch of the femoral nerve is

typically lateral and deep to the anterior branch.

The anesthetist should resist the urge to use

the patient’s thigh as a hand rest while directing

the needle. Stimulation of the femoral nerve can

result in brisk vastus muscle twitching that can

disrupt needle positioning.

The femoral nerve block provides analgesia to

the anterior thigh, including the flexor muscles

of the hip and extensor muscles of the knee.