Femoral Nerve Block

Published on 06/02/2015 by admin

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Last modified 06/02/2015

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40 Femoral Nerve Block

The femoral nerve is the largest branch of the lumbar plexus. It provides motor innervation to the quadriceps, sartorius, and pectineus muscles. The sensory branches include the anterior cutaneous nerve of the thigh, the infrapatellar nerve, and the saphenous nerve. These nerves innervate the anterior thigh, the patella, and the medial leg and foot, respectively.

The normal femoral nerve is oval or triangular in cross section, with dimensions of about 3 mm in anteroposterior diameter and 10 mm in mediolateral diameter in the inguinal region.¹ The femoral nerve usually lies lateral to the femoral artery but can contact or lie underneath the femoral artery in 15% of cases. In about one third of individuals, the femoral nerve is triangular in the suprainguinal region on short-axis scans.

The femoral nerve is covered by echogenic subcutaneous tissue and fascia. The nerve lies on the hypoechoic iliopsoas muscle, which has a characteristic mediolateral inclination. This incline positions the lateral side of the nerve slightly closer to the skin surface. At this interface of bright fascia and dark muscle, the nerve can be difficult to visualize. The best nerve visibility is proximal to the inguinal crease before the femoral nerve and femoral artery divide into smaller branches distally. In this proximal location, the femoral nerve puts a small dent in the surface of the iliopsoas muscle. This occurs because the femoral nerve usually lies in the groove between the iliacus (lateral) and psoas (medial) components of the muscle. The tilt of the transducer strongly influences femoral nerve visibility owing to anisotropic effects.² With ultrasound guidance it may be possible to perform more proximal drug injections and therefore more complete resultant femoral nerve blocks.

Suggested Technique

Femoral nerve block is performed in the supine position with the nerve in short-axis view. The lateral corner of the nerve is targeted to avoid the femoral vessels and to make the injection closest to the skin surface. Both out-of-plane (from distal to proximal) and in-plane (from lateral to medial) approaches can be used. It is critical that the needle tip be positioned between the fascia iliaca and iliopsoas muscle.³ It is not important to position the needle tip immediately adjacent to the nerve.

The needle tip should be placed in the layer under the femoral nerve so that the injection lifts the nerve toward the surface. This is especially important when catheters are placed. Successful injections not only surround the femoral nerve but also track along its small distal branches. If there is any question that the injection has not surrounded the femoral nerve, the distal tracking of local anesthetic can be verified by sliding the transducer along the nerve path. If local anesthetic appears to only layer over the femoral nerve, be concerned the fascia iliaca is still intact.

Dull block needles can be used to detect tissue layers, both by visual inspection of the tent and recoil with needle advancement and by tactile sense. If sharp needles are used, the operator’s hand motion and needle tip position will be more closely correlated. It may be necessary to combine ultrasound and nerve stimulation for femoral nerve blocks in morbidly obese patients. In these patients the femoral artery can be imaged to give an approximate location for nerve stimulation.

Posterior acoustic enhancement occurs deep to the femoral artery.⁴ This artifact should not be confused with the femoral nerve. The femoral nerve and iliopsoas tendon can be distinguished because the iliopsoas tendon normally lies deep within the iliopsoas muscle.

The lateral to medial in-plane approach to femoral block has potential disadvantages because of the longer needle path, which can be near the lateral femoral cutaneous nerve. In addition, the needle has a tendency to skim along the fascia iliaca, deforming it rather than puncturing it. This can be particularly frustrating as the needle tip approaches the femoral artery.

Many institutions have found the out-of-plane approach to be very safe and effective.⁵ However, the operator must be cognizant of the unimaged needle path. Branches of the femoral artery may lie within the unimaged needle path short of the scan plane.⁶ Scanning before needle placement can be advantageous.

The incidence of vascular puncture rates with nerve stimulation–guided femoral nerve catheters is about 6%.⁷ Ultrasound guidance likely reduces that incidence. Puncture of the inguinal vessels can result in blood tracking into the retroperitoneum, even if the puncture occurs distal to the inguinal ligament.⁸

Key Points

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Femoral Nerve Block	The Essentials
Anatomy	The FN lies lateral to the FA on the surface of the iliopsoas muscle.
	The FN lies in the groove between the iliacus and psoas muscles.
	The LCFA can lie over the FN.
	The FN is about 3 by 10 mm in diameter.
Image orientation	The iliopsoas muscle has an inclined surface from lateral to medial.

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Atlas of Ultrasound-Guided Regional Anesthesia 2e