Chapter 35. Femoral Nerve Block

The femoral nerve block is considered one of the basic nerve block techniques because it is relatively simple to perform, carries a low risk of complications, and results in a high success rate. When used alone, femoral nerve block is well suited for surgery on the anterior aspect of the thigh and for postoperative pain management after femur and knee surgery. However, when combined with a sciatic block, anesthesia of almost the entire lower limb from the mid-thigh level can be achieved.

Indications

Single-Dose Technique

When used alone, a 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. Femoral nerve block significantly improves postoperative analgesia after knee surgery during the first 8–12 hours postoperatively.1–5 However, when combined with a sciatic or popliteal block, femoral block provides anesthesia for entire lower leg or ankle surgery.

Continuous Technique

The primary indication of continuous femoral nerve block is pain management after major femur or knee surgery.6–22 In addition, when compared with a single dose technique or placebo, continuous femoral nerve block significantly reduces postoperative morphine consumption in patients after total hip replacement.23,24 For this application, the technique is as efficient as IV patient-controlled analgesia (PCA) with morphine or patient-controlled epidural analgesia, and it results in fewer technical problems and side effects.13

Continuous femoral nerve block provides excellent analgesia in patients with femoral shaft or femoral neck fractures.14,15,21,25 Its relative simplicity makes it uniquely suitable for 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)7,12,16,17,20 or intraarticular analgesia.18,26 For knee surgery, continuous femoral block is as effective as continuous lumbar plexus block27 or continuous epidural analgesia,12,20 with fewer risks of complications. Because this technique results in faster postoperative knee rehabilitation than IV PCA with morphine and fewer side effects than epidural analgesia, continuous femoral nerve block is probably the analgesic technique of choice in patients after total knee arthroplasty.7,12,20,28

Contraindications

Relative contraindications for femoral nerve block may include previous ilioinguinal surgery (femoral vascular graft, kidney transplantation), large inguinal lymph nodes or tumor, local infection, peritoneal infection, and preexisting femoral neuropathy.

Anatomy

The femoral nerve is the largest nerve 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 border 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 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 35–1). At this level, it is situated deep to fascia lata (Figure 35–2). As the nerve passes into the thigh, it divides into anterior and posterior branches (Figure 35–3). Located above the fascia iliaca, the anterior branches innervate the sartorius and pectineus muscles (Figure 35–4) and the skin of the anterior and medial aspects of the thigh. Located under the fascia iliaca, the posterior branches innervate 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 joint (Figure 35–5).

Landmarks

The following landmarks are used to determine the site of needle insertion: inguinal ligament, inguinal crease, femoral artery (Figure 35–6).

Equipment

A standard regional anesthesia tray is prepared with the following equipment:

• Sterile towers and gauze packs
• 20-mL Syringe with local anesthetic
• Sterile gloves, marking pen
• One 25-gauge, 1½-in. needle for skin infiltration
• A 5-cm long, short-bevel, insulated stimulating needle
• A peripheral nerve stimulator and a surface electrode

Patient position. The patient lies in the supine position. The ipsilateral extremity is abducted 10–20 degrees.

Site of needle insertion. The site of needle insertion (see Figure 35–6) is located at the femoral crease but below the inguinal crease and immediately lateral (1 cm) to the pulse of the femoral artery.

Single-Injection Technique

After skin disinfection, local anesthetic is infiltrated subcutaneously. In obese patients, the lower abdomen is retracted laterally to allow access to the inguinal area (Figure 35–7). The needle is connected to a nerve stimulator set at a current intensity of 1.0 mA (0.1 msec/2 Hz) and introduced at 45-degree angle to the skin in a cephalad direction (Figure 35–8).

The needle is advanced through the fascia lata (a loss of resistance is often perceived, but not relied upon) until quadriceps muscle contractions (ie, patellar twitch) are obtained. The current output is then gradually decreased while the needle is advanced. The position of the needle is judged adequate when patellar twitches are elicited with current output between 0.2 and 0.5 mA. After a negative aspiration test for blood, 15–20 mL of local anesthetic is injected. Some common responses to nerve stimulation and appropriate action to troubleshoot are featured in Table 35–1.

In an attempt to fasten the onset time and increase the safety of such block, a multiple injection technique, that is, elicitation of a vastus medialis, intermedius, and medialis twitch and separate injection of local anesthetic on each nerve branch, has been recently suggested.31,32 When compared with a single injection, the volume of local anesthetic required to block the nerve and the onset time of anesthesia were significantly reduced. However, 14% of patients reported paresthesia, and 28% reported discomfort during block performance.33 Therefore, more data and better injection monitoring techniques are necessary before this approach to femoral block can be widely recommended.

Continuous Technique

The continuous technique is similar to the single-injection technique. After passage through the fascia lata, the needle is advanced to elicit a patellar twitch using a current output between 0.2 and 0.5 mA (0.1 msec) (Figure 35–9). The catheter is then inserted 5–10 cm beyond the tip of the needle or introducer. It is secured in place with a stitch tunnelling and/or a dressing. After a negative aspiration test for blood, a bolus dose of 20 mL of local anesthetic is injected and followed by a continuous infusion of dilute local anesthetic (Figure 35–10).

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 leg (saphenous nerve). Motor blockade is evaluated by asking the patient to extend the knee (eg, to elevate the foot from the table).

Single-Injection Technique

The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or for pain management. Examples of onset times and mean duration of both anesthesia and analgesia with different types and concentrations of local anesthetic solution are presented in Table 35–2.

In our institution, when the technique is used to provide surgical anesthesia, 1% mepivacaine or 0.5% ropivacaine or levobupivacaine with epinephrine 1:300,000 is administered. When it is used only for postoperative analgesia, 0.25 ropivacaine or levobupivacaine with epinephrine 1:300,000 is injected.

Continuous Technique

Initial Bolus Dose

The choice of local anesthetic depends on the duration of surgery and whether the catheter is planned for surgical anesthesia or for postoperative analgesia alone. In our institution, when the technique is used to provide surgical anesthesia, 1% mepivacaine or 0.5% ropivacaine or levobupivacaine with epinephrine 1:300,000 is administered. When it is used only for postoperative analgesia, 0.2–0.25% ropivacaine or levobupivacaine with epinephrine 1:300,000 is injected. The spread of the local anesthetic after a bolus of 20 mL is shown in Figure 35–11.

Maintenance

The most suitable local anesthetic solution to maintain continuous femoral nerve block has not yet been determined. Most authors advocate the use of a dilute solution of long-acting local anesthetic such as 0.2% ropivacaine, 0.125% bupivacaine, or 0.125% levobupivacaine.6–10,12,13,17,20,27,29 In our institution, we use 0.2% plain ropivacaine. Addition of clonidine to the local anesthetic infusion is not recommended because it delays recovery of motor function without prolongation of analgesia.35

Several regimens of infusion through the femoral catheter have been recommended. Femoral perineural catheter can be managed by continuous infusion of a dilute local anesthetic solution (eg, 0.125% levobupivacaine, 0.2% ropivacaine) at the rate of approximately 8 mL/min.6,7 A background infusion (eg, 5 mL/h) of 0.125% bupivacaine with small-volume (eg, 2.5 mL/30 min) PCA boluses provides excellent pain relief and a 32% reduction in the local anesthetic consumption.8,36,37 The latter regimen also allows reinforcement of the block before a knee physiotherapy session. PCA boluses alone (eg, 10 mL/h–1) achieved comparable results with a more significant reduction (58%) in the bupivacaine consumption. However, this regimen requires additional effort to educate the patients. Comparable results have been recently obtained when ropivacaine is used,9 and also after total hip arthroplasty.9 Thus, in most patients, a low basal infusion rate (eg 5 mL/h) associated with PCA boluses (eg, 2.5 mL–lockout: 30 minutes) could be recommended as the infusion regimen of choice to maintain continuous femoral nerve block.

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. Regardless, patients should be premedicated to the level according to their own preferences. Femoral nerve block is associated with weakness of the quadriceps for the duration of the blockade muscle. Knee extension and thus weight bearing on the blocked side are impaired. This must be clearly explained to the patient to prevent the risk of falls.

Complications & How to Avoid Them

Complications of femoral nerve block are relatively rare and may include13 vascular puncture,34, femoral nerve compression by a hematoma,38 diffusion of the local anesthetic solution into the epidural space with resultant epidural block,39 catheter shearing,40 nerve injury and femoral dysesthesia (incidence of 0.25%).13,41,42 With regard to continuous catheters, bacterial contamination of the catheter is common after 48 hours.31,41,42 However, local or systemic infection remains rare, with the estimated risk of 0.13%.43 Suggestions on how to decrease the risk of such complications is presented in Table 35–3.

Femoral nerve block is easy to perform and associated with a low risk of complications. It is particularly suitable for catheter insertion. When used alone, it is well suited for surgery on the anterior aspect of the 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 mid-thigh level can be achieved.