Femoral Nerve Block

The femoral nerve block (FNB) was once the preferred peripheral nerve block for patients undergoing total knee arthroplasty (TKA).  However, the FNB results in quadriceps weakness which impairs early ambulation and increases the potential for postoperative falls making other blocks, such as the adductor canal block, more desirable in this patient population.  This block is a good choice for surgeries involving the anterior thigh and patella when early mobilization is not desirable or possible.  It is an easy to master as the femoral nerve is large and the associated landmarks are readily identifiable.

Indications: Surgical procedures of the anterior thigh, knee, femur an cutaneous procedures of the medial lower leg

Patient Position: Supine with slight external rotation of the extremity

Transducer: High-frequency linear array transducer

Transducer Position: Transverse orientation; perpendicular to the skin and parallel to the iliac crease

Needle: 5 cm, 22 gauge stimulating needle

Local Anesthetic: Up to 20 mL of local anesthetic

Keys To A Successful Block: The needle must pass through the fascia lata and iliaca; observation of circumferential spread around the nerve

Functional Anatomy:

The femoral nerve arises primarily from the ventral roots of the 2nd, 3rd, and 4th nerves of the lumbar plexus.  It courses distally within the psoas major and iliac muscles running under the inguinal ligament. It courses distally beneath the fascia lata and iliaca, lateral to the femoral artery and superior to the iliopsoas muscle .  At this level, the femoral nerve remains consolidated and is the desired area for placing local anesthetic.  As the nerve continues caudad, it divides into anterior (sensory) and posterior (motor) branches, increasing the potential for an incomplete block.

 Sonoanatomy:

The image below demonstrates the sonoanatomy for a FNB.  The femoral nerve is located beneath fascia lata(yellow) and fascia iliaca (red), lateral to the femoral artery, and superior to the iliopsoas muscle.

 Pre-procedure Scan:

The video below shows the femoral anatomy beginning at the inguinal crease, the optimal location for performing the femoral block.  The common femoral artery and vein are  visible as are the fascia lata and iliaca.  Moving caudad, the lateral circumflex artery arises from the femoral artery, coursing laterally through the femoral nerve.  Continuing distally, the femoral artery divides into the superficial femoral and profunda femoris arteries. Performing a block at this level would most likely result in a “patchy” block.   The transducer is then moved cephalad to its original position.

Technique:

The patient is placed in the supine position, with the operative leg in a neutral, or slightly externally rotated position.  A high-frequency transducer is used, however a low-frequency transducer may be required in patients with a large body habitus.  The transducer is placed at the inguinal crease and the major anatomical landmarks are identified.  The image is optimized so that the nerve appears in the center of the screen.  Slight rotation of the transducer will often make the fascia lata and iliaca more visible.  The needle is passed using an in-plane or out-of-plane approach through the two fascia layers, just lateral to the femoral artery.  When performing the in-plane approach, the anesthetist should note the depth of the nerve to determine the approximate angle of needle insertion.  A 5cm, 22 gauge B-bevel block needle can be used for a single injection, while an 18 gauge Tuohy-style needle is used if a catheter is placed.  The needle is inserted from lateral to medial.  The out-of-plane approach is more difficult, as only the tip of the needle or a small portion of the needle shaft is observed at any one time.  Hydro-location can be used to help determine the location of the needle tip.  The posterior division of the femoral nerve innervates the quadriceps muscles.  It is generally found on the lateral aspect of the femoral triangle and the block needle should be directed there.  For new practitioners, a peripheral nerve stimulation (PNS) can be used in conjunction with US to verify proper needle placement prior to injection of local anesthetic.  Contraction of the sartorius muscle in the medial thigh is a common initial contraction.  Redirecting the needle more laterally and deeper will cause contraction of the quadriceps muscle and elicit a “patellar snap”.  20 to 30 mls of local anesthetic is injected, with circumferential spread of local anesthetic most desired.

If the femoral artery is bifurcated (below left), it is likely that the femoral nerve has divided as well, increasing the chance of an incomplete block.  Slide the transducer proximal, or cephalad, until the common femoral artery is identified.

The video below shows an ultrasound-guided femoral block used in conjunction with nerve stimulation.  The needle is inserted in-plane from lateral to medial.  Initially, a “sartorius”response was elicited with nerve stimulation, so the needle is redirected until a “patellar snap” was attained.  The video ends with the initial injection of local anesthetic that separates the nerve from the iliopsoas muscle and pushing it superficially.


References

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:115-123.

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:50-53.

Butterworth JF, Mackey DC, Wasnick JD. Morgan&Mikhail’s Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill Lange; 2013:1001-1006.

Hadzic A. ed. Hadzic’s Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia. 1st. ed. New York, NY: McGraw-Hill Medical; 2012: 397-404.

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