Femoral Nerve Block

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1248 times

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

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.
Positioning Supine with leg slightly abducted
Some pannus retraction may be necessary
Operator Standing on the side of the patient
Display Across the table
Transducer High-frequency linear, 38- to 50-mm footprint
Initial depth setting 25 to 30 mm
Needle 20 to 21 gauge, 70 mm in length
Anatomic location Begin by scanning with the probe along the inguinal crease.
Slide proximally until the common FA and FN are seen in SAX view.
Best FN imaging is usually 1 to 2 cm proximal to the inguinal crease.
Approach SAX view of the FN, in-plane from the lateral side
Place the needle tip through the fascia iliaca at lateral corner of FN.
Inject underneath the FN (between the FN and iliopsoas muscle).
Sonographic assessment The injection should track along the FN branches (SAX slide).
Anatomic variation Distance of FN from FA is variable
Different nerve morphologies (commonly oval or triangular shape)

FA, Femoral artery; FN, femoral nerve; LCFA, lateral circumflex femoral artery; SAX, short axis.

Clinical Pearls

• Some clinicians advocate an oblique out-of-plane approach to femoral nerve block.9 This approach from the lateral aspect of the thigh allows needle tip placement under the posterior aspect of the nerve. The needle tip punctures the fascia iliaca at the lateral corner of the femoral nerve.

• The iliopsoas tendon lies under the femoral artery and should not be mistaken for the femoral nerve.10

• Transducer location is of paramount importance to femoral nerve visibility. If the transducer is too distal, the femoral nerve and artery will divide.11 This compromises nerve visibility because it is difficult to image small femoral nerve branches, particularly those that branch to the surface, within the echobright subcutaneous tissue (only after injection of anechoic fluid can these branches be seen). If the transducer is too proximal, the nerve and artery will dive away from the transducer on the surface on the iliacus muscle.

image

FIGURE 40-1 Course of the femoral nerve.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Churchill Livingstone; 2004.)

image

FIGURE 40-2 External photograph showing approaches to femoral nerve block in the inguinal region. An in-plane approach from the lateral aspect of the thigh is shown (A). An out-of-plane approach is also shown (B).

image

FIGURE 40-3 Short-axis views of the femoral nerve in the inguinal region. The inclination of the transducer is critical to visibility of the femoral nerve (A and B). In this example the echoes from a triangular femoral nerve disappear when the transducer is tilted (the property of anisotropy).

image

FIGURE 40-4 Morphologies of the femoral nerve in the inguinal region. Short-axis views are shown with foil (A), triangle (B), crescent (C), oval (D), and indented (E) shapes of the femoral nerve. Ultrasound visibility of the femoral nerve is complicated by the fact that the nerve lies at the interface of hyperechoic subcutaneous tissue and hypoechoic muscle. In many cases the femoral nerve is only visible as an indentation in the surface of the iliopsoas muscle.

image

FIGURE 40-5 Posterior acoustic enhancement artifact occurs deep to the femoral artery and can resemble the femoral nerve.

image

FIGURE 40-6 The femoral artery is seen in short-axis view with the lateral circumflex femoral artery (LCFA) in long-axis view (A). A duplex image is shown with power Doppler verifying color encoding of the vessels (B).

image

FIGURE 40-7 Short-axis in-plane approach to femoral nerve block. The needle tip approaches the femoral nerve from the lateral side to cross the fascia iliaca at the lateral border of the nerve.

image

FIGURE 40-8 Femoral nerve visibility after injection of local anesthetic. The nerve visibility is improved by posterior acoustic enhancement and better definition of the nerve borders. Local anesthetic is seen to completely surround the nerve.

image

FIGURE 40-9 Femoral nerve in short-axis view before (A) and after (B) injection of local anesthetic. In this example a triangular femoral nerve is observed to lie in the groove between the iliacus and psoas muscles. After injection the nerve fascicles are more clearly seen.

image

FIGURE 40-10 Femoral nerve in short-axis (A) and long-axis (B) views after injection of local anesthetic. This tracking pattern of local anesthetic verifies successful block.

image

FIGURE 40-11 Femoral nerve in short-axis view after injection of local anesthetic. In this example, local anesthetic has separated the femoral nerve from the iliopectineal arch, resulting in a “paisley” shape to the femoral nerve.

image

FIGURE 40-12 Inguinal lymph nodes are commonly observed during femoral nerve block. Lymph nodes can be recognized by their oval shape and echogenic hilum.

image

FIGURE 40-13 Femoral artery plaque observed in short-axis (A) and long-axis (B) views during femoral nerve block. This pathology is a common incidental finding that may warrant further medical workup.

image

FIGURE 40-14 Femoral nerve branches observed after injection of local anesthetic. In this example the transducer slides distally with the nerve viewed in short axis. Local anesthetic is observed to track around and along the small nerve branches, confirming successful distribution for regional block.

image

FIGURE 40-15 Intraneural injection observed during femoral nerve block. In this example, local anesthetic was inadvertently injected into the femoral nerve, resulting in small cystic collection of fluid inside the nerve. There were no neurologic sequelae.

References

1 Gruber H, Peer S, Kovacs P, et al. The ultrasonographic appearance of the femoral nerve and cases of iatrogenic impairment. J Ultrasound Med. 2003;22:163–172.

2 Soong J, Schafhalter-Zoppoth I, Gray AT. The importance of transducer angle to ultrasound visibility of the femoral nerve. Reg Anesth Pain Med. 2005;30:505.

3 Dalens B, Vanneuville G, Tanguy A. Comparison of the fascia iliaca compartment block with the 3-in-1 block in children. Anesth Analg. 1989;69:705–713.

4 Filly RA, Sommer FG, Minton MJ. Characterization of biological fluids by ultrasound and computed tomography. Radiology. 1980;134:167–171.

5 Sites BD, Spence BC, Gallagher JD, et al. Characterizing novice behavior associated with learning ultrasound-guided peripheral regional anesthesia. Reg Anesth Pain Med. 2007;32:107–115.

6 Orebaugh SL. The femoral nerve and its relationship to the lateral circumflex femoral artery. Anesth Analg. 2006;102:1859–1862.

7 Wiegel M, Gottschaldt U, Hennebach R, et al. Complications and adverse effects associated with continuous peripheral nerve blocks in orthopedic patients. Anesth Analg. 2007;104:1578–1582.

8 Spies JB, Berlin L. Complications of femoral artery puncture. AJR Am J Roentgenol. 1998;170:9–11.

9 Fredrickson M. “Oblique” needle-probe alignment to facilitate ultrasound-guided femoral catheter placement. Reg Anesth Pain Med. 2008;33:383–384.

10 Mulroy RD. The iliopsoas muscle complex: iliacus muscle, psoas tendon release. Clin Orthop. 1965;38:81–85.

11 Muhly WT, Orebaugh SL. Ultrasound evaluation of the anatomy of the vessels in relation to the femoral nerve at the femoral crease. Surg Radiol Anat. 2011;33(6):491–494.

Related posts:

Anisotropy Default ThumbnailSelf-Assessment Questions: Text Anterior Sciatic Nerve Block Mental Nerve Block Fascia Iliaca Block Advanced Self-Assessment Questions: Images