Femoral Nerve Block anatomy

Relevant Anatomy

Femoral nerve (posterior divisions of L2-4) is formed in the psoas major muscle, runs between psoas and iliacus muscles and enters the thigh under the inguinal ligament lateral to the femoral artery, at which point it divides into multiple terminal branches (usually classified as anterior [mostly cutaneous] and posterior [mostly motor])

Landmarks and Surrounding Structures

Important landmarks include the femoral crease, ASIS, pubic tubercle, femoral artery (palpable) and veins (not palpable), both located medially

Cutaneous Innervation

The femoral nerve blocks the anterior thigh, as well as the medial lower leg (from the saphenous nerve). Note that it misses a portion of the medial thigh innervated by the obturator nerve (which also innervates the medially-located obturator externus, adductors [brevis, longus, and magnus], and gracilis muscles)

Muscular Innervation

Major muscles supplied by the femoral nerve include the anterior compartment muscles (quadriceps femoris, sartorius, and pectineus muscles), as well as the more proximal iliacus and psoas major muscles

Distributions Missed

Medial thigh (obturator nerve)

===================================

Updated definition 2020

Femoral Nerve Structure and Branches:

The femoral nerve, the largest branch of the lumbar plexus, arises from the ventral rami of L2-4, and emerges from the lateral border of the psoas muscle. It gives off the nerve to the pectineus immediately above the inguinal ligament, which passes behind the femoral sheath to enter the anterior surface of the pectineus muscle. The femoral nerve enters the thigh posterior to the inguinal ligament, where it lies lateral and slightly posterior to the femoral artery. It splits into an anterior division (supplying the sartorius muscle) and a posterior division (supplying the rectus femoris muscle, the three vasti muscles, and the articularis genus muscle). The anterior division gives off anterior cutaneous branches (the intermediate femoral cutaneous nerve and the medial femoral cutaneous nerve), while the posterior division gives off the saphenous nerve.

The hip joint is supplied by nerve to the rectus femoris, while the knee joint is supplied by the nerves to the three vasti, the largest being the vastus medialis (which contains proprioceptive fibers from the knee). The femoral nerve also supplies branches to the femoral artery and its branches.

Femoral Nerve Function:

The femoral nerve supplies motor innervation to the sartorius muscle, the rectus femoris muscle, the vastus medialis muscle, the vastus lateralis muscle, the vastus intermedius muscle, and the articularis genus muscle. It supplies sensation to the anterior thigh and knee as well as the medial lower extremity below the knee (via the saphenous nerve).

Femoral Nerve Block Indications:

The femoral nerve block may be utilized as the primary anesthetic for anterior thigh surgery, combined with the sciatic nerve block for complete lower extremity coverage below the knee, and combined with both the sciatic and obturator nerve block to provide complete lower extremity anesthesia. The femoral nerve block also provides valuable analgesia in femoral neck fractures, femur fractures, and patellar injuries, either alone or in combination with a multimodal analgesia regimen.

Femoral Nerve Block Contraindications:

Absolute contraindications include patient refusal, inability to cooperate, or known/severe local anesthetic allergies. Relative contraindications include previous ilioinguinal surgery, large inguinal lymph nodes or tumor, peritoneal infection, infection around the proposed injection site, anticoagulated patients, or patients with bleeding disorders. Patients with preexisting femoral neuropathy damage (severe diabetes, nerve trauma, concurrent neurotoxic medication administration) should be cautioned about their increased susceptibility to further nerve injury

Landmark-Guided Femoral Nerve Block and Relevant Anatomy:

Position the patient in the supine position, abduct the appropriate extremity 10-20 degrees, and externally rotate it slightly. Insert the needle at the femoral crease and immediately lateral to the femoral arterial pulse (approximately 1 cm). At this level, the posterior branches lie under the fascia iliaca.

Nerve stimulation may be utilized by connecting the needle to a nerve stimulator set at 1 mA and inserting the needle at a 30-45 degree angle to the skin in a cephalad direction. As the needle advances through the fascia lata and iliaca, a “popping” sensation is often appreciated. Once patellar twitches are appreciated (signaling quadriceps muscle contraction), decrease the current while advancing the needle. Needle position is deemed adequate once patellar twitches are elicited with current output between 0.3 and 0.5 mA. After negative aspiration, 15-20 mL of local anesthetic should be injected.

Multiple choice questions frequently test the anatomy of the thigh by asking for the appropriate action if certain phenomena are encountered during femoral nerve blocks utilizing nerve stimulation. If local twitching occurs around the injection site, the needle is likely too deep and stimulating the iliopsoas or pectineus muscles; it should be withdrawn and reinserted in another direction. Sartorius muscle twitch indicates that the needle tip is too anterior and medial to the femoral nerve trunk; the needle should be redirected laterally and advanced deeper. Vascular puncture indicates that the needle is too medial; the needle should be withdrawn and reinserted laterally 1-2 cm.

Ultrasound-Guided Femoral Nerve Block and Relevant Anatomy:

Position the patient in the supine position and place a linear transducer (8-18 MHz) transversely on the femoral crease, over the pulse of the femoral artery. Identify the femoral artery; if the femoral artery and deep artery of the thigh are both seen, move the transducer proximally until only the femoral artery is seen. The hyperechoic, triangular-shaped femoral nerve is located lateral to the femoral artery. The nerve is enveloped within two layers of the fascia iliaca, usually at a depth of 2-4 cm.

Once the femoral nerve is identified, a skin wheal of local anesthetic is made 1 cm away from the lateral edge of the transducer. The needle is inserted in-plane, lateral to medial, and advanced toward the femoral nerve. 10-15 mL of local anesthetic is adequate for a successful block. Circumferential spread is not necessary.