Femoral Nerve Block

ANATOMY

The femoral nerve is one of the major branches of the lumbar plexus. The femoral nerve is consistently lateral to the femoral artery, deep to the fascia iliaca and superficial to the iliopsoas muscle. The anterior approach to block the femoral nerve at the groin (inguinal region) is most commonly performed for knee surgery.

FA = femoral artery   FN = femoral nerve   FV = femoral vein   Picture taken from Colour Atlas of Anatomy: A photographic study of the human body (3rd edition)

FI = fascia iliaca   FL = fascia lata   FS = femoral sheath   IPM = iliopsoas muscle   PECT = pectineus muscle

SCANNING TECHNIQUE

• Position the patient supine with the leg in the neutral position.
• Expose the groin and mark the inguinal crease.
• After skin and transducer preparation, place a transducer with the appropriate frequency range (10-12 MHz) along the inguinal crease. If the femoral artery and nerve are deep (> 4 cm, use a 7 MHZ transducer.
• Optimize machine imaging capability; select appropriate depth of field (usually within 1-3 cm), focus range and gain.

	Transducer over right inguinal crease

ANATOMICAL CORRELATION

Transverse Scan of the Inguinal Region

Box = scanned area   FA = femoral artery   FN = femoral nerve   FV = femoral vein   IPM = iliopsoas muscle

NERVE LOCALIZATION

• Perform a systematic anatomical survey from medial to lateral and superficial to deep.
• The femoral nerve is generally easy to locate in this region.
• First, identify the femoral artery. If the image shows more than 1 artery, scan more proximally (cephalad) to visualize the artery before the profunda femoris artery branches off.
• The femoral vein is medial to the artery. The vein may not be visible until the transducer pressure on the skin is lessened.
• Deep to the femoral vessels is the iliopsoas muscle bulk.
• The femoral nerve is often found within a triangular hyperechoic region, lateral to the femoral artery and superficial to the iliopsoas muscle.
• The femoral nerve may be quite thin and flat in this region as the nerve fans out into multiple branches.
• Note the fascia iliaca (a hyperechoic line) superficial to the femoral nerve and its branches.
• Inguinal lymph nodes also appear hyperechoic and may be confused with the nerve in the short axis view. To distinguish the two, scan proximally and distally in this region. A nerve is a continuous structure that can be traced while a lymph node is not and can be seen only in a discrete location.

OUT OF PLANE NEEDLE INSERTION APPROACH

• Ultrasound guided femoral nerve block is considered a BASIC skill level block because this is a superficial block.
• Insert a 5 cm 22 G insulated needle perpendicular to the transducer and the ultrasound beam. In this case, only the cross section of the needle shaft (a white dot) may be observed during needle advancement.

It can be technically challenging to track the location of the needle tip during needle insertion without an echogenic tip design. Move the needle tip slightly from side to side or in and out to bring the tip into view.

• Injection of a small amount of fluid e.g., D5W will expand the femoral triangle and the hypoechoic fluid collection can bring the hyperechoic nerve and the fascia iliaca into view.
• Identity the femoral nerve branches by electrical stimulation. Aim to evoke patellar contraction.
• Injection of D5W (1-5 mL) will also intensify the motor response to nerve stimulation.
• The posterior division of the femoral nerve which innervates the quadriceps muscles is most commonly located on the lateral aspect of the femoral triangle. It is therefore recommended to first point the needle towards the lateral aspect of the femoral triangle under ultrasound guidance.

FA = femoral artery   FN = femoral nerve   FV = femoral vein   LA = local anesthetic   NT = needle tip

LOCAL ANESTHETIC INJECTION

• Once satisfied with needle placement, inject 20-30 mL of local anesthetic for surgical anesthesia or postoperative analgesia. Observe “sheath” distention and a hypoechoic ring of local anesthetic solution around the hyperechoic nerve structures.
• Scan proximally and distally to assess the extent of local anesthetic spread.

Local Anesthetic Spread Within the Femoral Nerve Compartment

Pre-injection   FA = femoral artery   FI = fascia iliaca   FV = femoral vein

	Post-injection   Arrowheads = branches of the femoral nerve   FI = fascia iliaca   LA = local anesthetic

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