Femoral Nerve Block

When performing femoral nerve blockade, the ultrasound probe is placed in the inguinal crease. This flexion line is closest to the horizontal plane, but the medial edge of the probe is more caudad than the lateral edge (Fig. 21.9a). A slight caudad inclination of the probe may provide the best short-axis view of the femoral nerve, artery, and vein (Fig. 21.9b). Pulsatile femoral artery is the easiest structure to identify, and the collapsible femoral vein is positioned medial to the artery. The profunda femoris artery usually arises from the lateral side of the common femoral artery from 2.5 to 5 cm below the inguinal ligament, so it may sometimes be seen on ultrasound at the inguinal crease. Lateral to the femoral artery, the curved surface of the iliacus muscle (fascia iliaca) can be identified inclining posteromedially, and the femoral nerve appears as a hyperechoic, ovoid structure with the same posteromedial inclination as the iliacus muscle (Fig. 21.9b). The femoral nerve is apposed to the anteromedial surface of the iliacus, just deep to its fascia iliacus. Further medial and posteriorly, the tendon of the iliopsoas complex can be identified and is also hyperechoic and fibrillar in appearance. Medial to the femoral vein are superficial muscular layers consisting of the pectineus laterally and the adductor longus medially. Posterior (deep) to both superficial muscle layers is the adductor brevis muscle, and the anterior and posterior branches of the obturator nerve lie on the anterior and posterior surface of this muscle at the level of the inguinal crease. The lateral femoral cutaneous nerve enters the thigh just medial to the anterior superior iliac spine and is a purely sensory nerve. The obturator nerve enters through the superomedial aspect ofthe obturator foramen, enters the medial side of the thigh and divides into an anterior branch, which lies between the short adductor, external obturator, long adductor, and pectineus muscles and a posterior branch, which pierces the external obturator muscle, lies above the great and short adductor muscles.

Femoral nerve block, or the so-called 3-in-1 block, is a regional anesthetic technique used to block the femoral, lateral femoral cutaneous, and obturator nerves by a single injection

Figure 21.9a Ultrasound probe orientation for femoral nerve blockade. Note medial-lateral orientation of probe to optimize cross section of femoral anatomy below. The needle orientation in the photo is showing an out-of-plane technique, but an alternative would be an in-plane technique with the block needle parallel to the ultrasound probe in a lateral-medial orientation.

Figure 21.9a Ultrasound probe orientation for femoral nerve blockade. Note medial-lateral orientation of probe to optimize cross section of femoral anatomy below. The needle orientation in the photo is showing an out-of-plane technique, but an alternative would be an in-plane technique with the block needle parallel to the ultrasound probe in a lateral-medial orientation.

Figure 21.9b Ultrasound image of femoral anatomy. FA femoral artery, FV femoral vein, N (with arrow) identifying the femoral nerve, Med medial, Lat lateral, top red line fascia latta, curved (lower) red line fascia iliacus.

of local anesthetic lateral to the femoral vessels and caudal to the inguinal ligament. This block concept remains controversial because all three nerves are rarely anesthetized with this single injection technique (blockade of the obturator nerve is often spared). With the use of ultrasound, a 7.5 MHz linear array transducer made it possible to visualize the femoral nerve and the relevant adjacent structures (vessels, muscles, ligaments) and to inject local anesthetic under control. Results of this described ultrasound guided femoral nerve blockade may now permit real-time visualization of local anesthetic spread, resulting in blockade of the femoral nerve as well as local anesthetic spread lateral and slightly medial to the femoral nerve for blockade of the lateral femoral cutaneous and the anterior branch of the obturator nerve (Fig. 21.9c). A significantly faster sensory block (onset time reduced) and improved quality of anesthesia of all three nerves could be achieved in comparison to the traditional nerve stimulation method.

Indications for an ultrasound femoral block include surgical procedures in the sensory distributions of the femoral, lateral femoral cutaneous, and anterior branch of the obturator nerve. A femoral nerve block together with a sciatic nerve block (described below) may provide anesthesia/analgesia for surgical procedures of the majority of lower extremity surgeries. The ultrasound-guided femoral block is a simple technique, but good anatomical knowledge is necessary for optimal performance of this block. In many surgical indications, only a femoral nerve block is necessary and since the femoral nerve is a superficial structure just distal to the inguinal ligament, a high-frequency linear ultrasound probe (>10 MHz) is adequate for optimal visualization.

The sonographic view of the femoral nerve is different from all other nerve structures because it has divided into a number of terminal branches at the level of the distal inguinal ligament. Therefore, the typical ultrasound appearance represents several distal sensory and motor branches of the femoral nerve (Fig. 21.9d). Femoral nerve block needle orientation can be performed in the long-axis or short-axis (Fig. 21.9a) orientation to the probe, but is most easily performed in the short axis using a 50-mm short bevel needle. Once the fascia iliaca

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