Femoral Nerve Block – Landmarks and Nerve Stimulator Technique

INTRODUCTION

The femoral nerve block is one of the most clinically applicable nerve block techniques that it is relatively simple to perform, carries a low risk of complications, and results in a high success rate.

Indications

Single-Injection Technique

Femoral nerve block is well suited for surgery on the anterior aspect of the thigh and for superficial surgery on the medial aspect of the leg below the knee. Some examples include repair of the quadriceps tendon or quadriceps muscle biopsy, long saphenous vein stripping, and postoperative pain management after femur and knee surgery. A perineural catheter can be placed to provide prolonged analgesia for patients with fractures on the femoral neck or shaft. Femoral nerve block provides effective analgesia following total knee arthroplasty. Femoral nerve block can also be used to supplement a sciatic or popliteal block to provide complete anesthesia of the lower leg and ankle.1–4

Continuous Technique

The primary indication of continuous femoral nerve block is pain management after major femur or knee surgery.5–21

In addition, when compared with a single-dose technique or placebo, continuous femoral nerve block significantly reduces postoperative morphine consumption in patients hav-ing total hip replacement.22,23

Continuous femoral nerve block provides excellent analgesia in patients with femoral shaft or femoral neck fractures.13,14,20,24 Its relative simplicity makes it uniquely suitable for use to pro-vide analgesia in the emergency room and facilitate physical and radiologic examinations as well as manipulations of the fractured femur or hip.After major knee surgery, continuous femoral nerve block provides better pain relief than parenteral administration of opioids (IV PCA, intramuscular)6,11,15,16,19 or intra-articular analgesia.17,25 For knee surgery, continuous femoral block is as effective as continuous lumbar plexus block26 or continuous epidural analgesia,11,19 but causes fewer complications.27

Contraindications

Relative contraindications for femoral nerve block include pre-vious ilioinguinal surgery (femoral vascular graft, kidney trans-plantation), large inguinal lymph nodes or tumor, local infection, peritoneal infection, and preexisting femoral neuropathy.

Anatomy

The femoral nerve is the largest branch of the lumbar plexus. It is formed by the dorsal divisions of the anterior rami of the L2, L3, and L4 spinal nerves. It emerges from the lateral bor-der of the psoas muscle, approximately at the junction of the middle and lower thirds of that muscle. Along its course to the thigh, it remains deep to the fascia iliaca. It enters the thigh posterior to the inguinal ligament, where it is positioned immediately lateral and slightly posterior to the femoral artery (Figure 82C–1). At this level, it is situated deep to both fascia lata and fascia iliaca (Figure 82C–2). As the nerve passes into the thigh, it divides into anterior and posterior branches (Figure 82C–3). Located above the fascia iliaca, the anterior branches innervate the sartorius and pectineus muscles (Figure 82C–4) and the skin of the anterior and medial aspects of the thigh.

FIGURE 82C–1. Anatomic relationship in the femoral triangle.

FIGURE 82C–2. Tissue sheaths and femoral nerve, artery and vein relationships.

FIGURE 82C–3. Composition of the femoral nerve at the level of blockade.

Clinical Pearls

  • In obese patients, the identification of the inguinal crease can be facilitated by asking an assistant to retract the lower abdomen laterally (see Figure 82C–7).

Equipment

A standard regional anesthesia tray is prepared with the follow-ing equipment:

  • Sterile towels and gauze packs
  • 20-mL syringe with local anesthetic
  • Sterile gloves, marking pen
  • One 25-gauge, 1.5-in. needle for skin infiltration
  • A 5-cm long, short-bevel, insulated stimulating needle
  • A peripheral nerve stimulator and a surface electrode
  • Injection pressure monitor

BLOCK TECHNIQUE

Patient position. The patient lies in the supine position. The ipsi-lateral extremity is abducted 10–20 degrees and slightly exter-nally rotated with the lateral side of the foot resting on the table.

Site of needle insertion. The site of needle insertion (Figure 82C–6) is located at the femoral crease but below the inguinal crease and immediately lateral (1 cm) to the pulse of the femoral artery.

Located under the fascia iliaca, the posterior branches inner-vate the quadriceps muscle and the knee joint and give off the saphenous nerve. The saphenous nerve supplies the skin of the medial aspect of the leg below the knee (Figure 82C–5).

Landmarks

The following landmarks are used to determine the site of needle insertion: inguinal ligament, inguinal crease, femoral artery (see Figure 82C–6).

 

Clinical Pearls

  • It is useful to think of the mnemonic VAN (vein, artery, nerve) going from medial to lateral when recalling the relationship of the femoral nerve to vessels at the ingui-nal crease.
  • The femoral nerve is approached at the femoral crease, rather than at the inguinal ligament.


FIGURE 82C–4. Motor branches of the femoral nerve.

 

FIGURE 82C–5. Sensory innervation of the femoral nerve and distribution of anesthesia with femoral nerve block.

Single-Injection Technique

In obese patients, the lower abdomen is retracted laterally to allow access to the inguinal area (Figure 82C–7). The needle is connected to a nerve stimulator set at a current intensity of 1 mA (0.1 msec/2 Hz) and introduced at a 30- to 45-degree angle to the skin in a cephalad direction (Figure 82C–8).

FIGURE 82C–7. The abdomen is retracted laterally to facilitate exposure of the anatomy during femoral nerve blockade.

FIGURE 82C–8. The needle is connected to a nerve stimulator set at 1 mA current intensity and introduced at a 30- to 45-degree angle to the skin in a cephalad direction.

The needle is advanced through the fascia lata and iliaca, often associated with a certain feeling of a “pop” as the needle pierces the fasciae.As the quadriceps muscle contractions (i.e., patellar twitch) are obtained, the current is gradually decreased while the nee-dle is advanced. The position of the needle is adequate when patellar twitches are elicited with current output between 0.3 and 0.5 mA. After negative aspiration; 15–20 mL of local anes-thetic is injected. Some common responses to nerve stimula-tion and appropriate action to troubleshoot are featured in Table 82C–1.

TABLE 82C–1.Common responses to nerve stimulation and action to obtain femoral nerve twitch.

Response ObtainedInterpretationProblemAction
No responseThe needle is inserted either too medially or to laterallyFemoral artery not properly localizedFollow the systematic lateral angulation and reinsertion of the needle as described in the technique
Bone contactThe needle contacts hip or superior ramus of the pubic boneThe needle is inserted too deepWithdraw to the level of the skin and reinsert in another direction
Local twitchDirect stimulation of the illiopsoas or pectineus muscleToo deep insertionWithdraw to the level of the skin and re-insert in another direction
Twitch of the sartorius muscleSartorius muscle twitchThe needle tip is slightly anterior and medial to the main trunk of the femoral nerveRedirect the needle laterally and advance deeper 1–3 mm
Vascular punctureNeedle placement into the femoral or femoral circumflex artery, less commonly - femoral veinToo medial needle placementWithdraw and reinsert laterally 1 cm
Patella twitchStimulation of the main trunk of the femoral nerveNoneAccept and inject local anesthetic

Multiple injection techniques have also been described, where vastus lateralis, intermedius, and medialis twitches are individually identified and separate injections of local anes-thetic made at each nerve branch.30,31 When compared with a single injection, the total volume of local anesthetic required and the block onset time were significantly reduced. However, 14% of patients reported paresthesia, and 28% reported discomfort during block performance.32 Conse-quently, this technique has been largely abandoned as being unnecessary.

Clinical Pearls

  • The needle tip must be positioned below fascia iliaca to obtain a complete femoral nerve block.
  • Larger volume than 15-20 mL is not necessary because it is not associated with a better success rate.33

FIGURE 82C–6. Anatomic landmarks for femoral nerve block. The needle insertion site (X) is located just below the inguinal crease, 1–2 cm lateral to the pulse of the femoral artery.

Continuous Femoral Nerve Block

The continuous technique is similar to the single-injection technique. After passage through both fascia lata and iliaca, the needle is advanced to elicit a patellar twitch using a current output between 0.3 and 0.5 mA (0.1 msec) (Figure 82C–9).

The catheter is then inserted 5 cm beyond the tip of the needle and secured in place. After a negative aspiration test for blood, a bolus dose of 10 mL of local anesthetic is injected and fol-lowed by a continuous infusion of dilute local anesthetic and/or intermittent boluses of 5 ml hourly. (Figure 82C–10).

  • Catheter insertion under the fascia iliaca should be without resistance. When this is not the case, the needle is probably not under the fascia iliaca. The needle should be withdrawn to the skin and reinserted.
FIGURE 82C–9. Continuous femoral nerve block: Needle insertion.

FIGURE 82C–10. Spread of the injectate underneath the fascia iliaca after injection through the femoral catheter. (Used with permission from Dr. Philippe Gauiter, Bruxelles, BE.)

Block Assessment

Sensory blockade is assessed by cold or pin prick test on the anterior and medial aspect of the thigh (femoral nerve) and on the medial aspect of the lower leg (saphenous nerve). Motor blockade is evaluated by asking the patient to extend the knee (e.g., to elevate the foot from the table).

CHOICE OF LOCAL ANESTHETIC

Single-Injection Technique

For surgical anesthesia, mepivacaine or lidocaine 1.5–2.0% or ropivacaine 0.5%–0.75% are frequently used, depending on the expected duration of surgery. For postoperative analgesia alone, a more dilute concentration of long acting local anes-thetic (eg ropivacaine or bupivacaine 0.2%–0.25%) is appro-priate. Onset times and mean duration of both anesthesia and analgesia with different types and concentrations of local anes-thetic solution are presented in Table 82C–2.

TABLE 82C–2.Onset and duration of 20 ml of local anesthetic in femoral block.

 Onset (min)Anesthesia (h)Analgesia (h)
3% 2-Chloroprocaine10–1512
3% 2-Chloroprocaine (+ HCO3 + epi)10–151.5–22–3
1.5% Mepivacaine15–202–33–5
1.5% Mepivacaine (+ HCO3 + epi)15–202–53–8
2% Lidocaine10–202–53–8
0.5% Ropivacaine15–304–85–12
0.75% Ropivacaine10–155–106–24
0.5 Bupivacaine15–305–158–30

Continuous Technique

The initial bolus of 10–15 mL is followed by an infusion of dilute concentration (e.g., ropivacaine 0.2%). A typical infu-sion regimen is 5 mL/h basal infusion with a 5 mL/q60min patient-controlled bolus.

PERIOPERATIVE MANAGEMENT OF FEMORAL NERVE BLOCKS

The performance of femoral nerve block is associated with minor patient discomfort because the needle passes only through the skin and adipose of the inguinal region. Femoral nerve block is associated with weakness of the quadriceps muscle, leading to the decrease in its use in some practices, particularly where ultrasound is available for adductor canal blocks. This is because knee extension and weight bearing on the blocked side are impaired with femoral nerve block, which must be clearly explained to the patient to reduce the risk of falls. The use of a knee immobilizer for ambulation following femoral nerve block has been shown to reduce the risk of falls, particularly following total knee arthroplasty.34,35

Complications and How to Avoid Them

Complications of femoral nerve block include13 vascular punc-ture,33 femoral nerve compression by a hematoma,36 diffusion of the local anesthetic solution into the epidural space with resul-tant epidural block,37 catheter shearing,38 and femoral nerve injury (incidence of 0.25%).12,39,40 With regard to continuous catheters, bacterial contamination of the catheters commonly occurs after 48 hours.32,39,40 However, local or systemic infection remains rare, with the estimated risk of 0.13%41, Table 82C-3.

TABLE 82C–3.Femoral nerve block: complications.

Hematoma• When the femoral artery or vein is punctured, the procedure should be stopped and pressure applied over the puncture site for 2–3 min
Vascular puncture• Maintain a palpating finger on the femoral pulse and insert the needle just lateral and parallel to the pulse.
• The needle should never be directed medially.
Nerve injury• Use a nerve stimulator avoid injection when motor response is present at <0.3 mA (0.1 msec).
• Do not seek paresthesia as method of localization of FN because paresthesia is rarely elicited with femoral nerve block and should not be sought or relied on to indicate an intraneural injection. However, should severe pain on injection be reported, abort the injection.
• Do not inject when high pressures on injection are encountered.
• Use the minimal efficient volume and concentration of local anesthetic (15-20 mL).
Catheter infection• Use strict aseptic technique during catheter insertion.
• Sterile drapes should be used with continuous techniques.
• Remove the catheter after 48–72 h (risk of infection increases with time).
Prevention of Falls• Instruct patient on the inability to bear weight on the blocked extremity.

SUMMARY

emoral nerve block is easy to perform and associated with a low risk of complications. It is suitable for catheter insertion. When used alone, it is effective for surgery on the anterior thigh and for postoperative pain management after femur and knee surgery. When combined with sciatic and/or obturator nerve blocks, anesthesia of almost the entire lower limb from the midthigh level can be achieved.

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Jerry D. Vloka, Admir Hadzic, and Philippe Gautier



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