Femoral nerve block was described by Labat in his textbook, Regional
Anesthesia, first published in 1923. Winnie modified the technique to
anesthetize the femoral, obturator and lateral femoral cutaneous nerves by a
single injection, calling his method "the 3-in-1 block."
The femoral nerve (L2-L4) arises from the lumbar plexus between the
psoas major and iliac muscles and is enveloped by the fascia iliaca. At
the groin, the psoas fascia separates the nerve from the femoral artery.
The femoral artery lies lateral to the artery and deep to the inguinal
ligament. About 2.5 cm below the inguinal ligament, the nerve divides
into muscular branches that extend to the muscles of the anterior thigh and
cutaneous branches comprised of the medial and intermediate cutaneous
nerves. The branches innervate the anterior thigh and the saphenous
nerve, also innervating the medial site of the leg to the middle of the medial
border of the foot.
The femoral nerve block can be used for superficial surgery of the
anterior aspect of the thigh, such as skin grafts, saphenous-vein stripping, or
harvesting of the saphenous vein. It is also recommended as part of a
multiple lower-extremity block for arthroscopic knee surgery, amputations and
ankle surgery. For the relief of postoperative knee pain, a catheter can
be placed on the femoral nerve for prolonged analgesia and healing of ischemic
ulcers on the medial aspect of the leg.
In developing this block, Winnie reasoned that because the lumbar plexus
is invested in the fascial compartment of the iliac and psoas muscles, drug
placed at the distal extension of that space will spread proximally, analogous
to the spread of local anesthetic agent in the brachial plexus sheath in the
upper extremity. Given this spread, the local anesthetic solution should
block the femoral, obturator and lateral femoral cutaneous nerves, hence the
name 3-in-1 block.
Position: The patient lies supine with the
thigh on a flat surface, slightly abducted from the midline.
Landmarks: The injection side is marked by
palpating the anterior superior iliac spine, the pubic symphysis, the pulsation
of the femoral artery, and the inguinal ligament. The point of entry is
one finger's breadth lateral to the femoral artery at the mid-inguinal point.
Procedure: After local infiltration at the
point of entry, the anesthesiologist's non-dominant hand is placed on the front
of the thigh with the middle finger on the femoral artery. The needle
(22-G, 3.8-cm) is placed on the femoral nerve, aided by the eliciting of
paresthesia or application of a nerve stimulator 1.5 cm lateral to the femoral
artery and s.5 cm distal to the inguinal ligament. The skin is pierced at
45? aiming cephalad and just lateral to the patient's umbilicus. As the
needle is advanced (approximately 2.5 cm), paresthesia of the femoral nerve is
elicited; if a nerve stimulator is used, patellar movement is sought. A
dose of 20-30 ml local anesthetic solution is injected when the needle is
confirmed to be on the nerve, putting pressure on the femoral artery and nerve
distal to the point of entry.
Once the position of the needle has been confirmed, the anesthesiologist
aspirates for blood and, if this test is negative, injects the calculated
volume of local anesthetic.
Signs of successful block include anesthesia of the anteromedial thigh
with motor loss of extension of the knee, some loss of flexion at the hip joint
and anesthesia of the medial aspect of the leg and medial border of the
proximal foot. While the initial report by Winnie suggested that a 100%
block of all three nerves is possible with 20 ml of solution, others have found
a 10% failure rate in blocking the obturator or lateral femoral cutaneous
nerves.
Infection at the site of entry, ulceration at the groin and tender
inguinal glands constitute contraindications for this technique.
Complications
Infection, hematoma, femoral neuritis and unduly prolonged block are
sometimes encountered.