Femoral Nerve Block

Blockade of the femoral nerve provides sensory anesthesia of the anterior thigh, knee, and medial aspect of the calf, ankle and foot. The femoral nerve block should be distinguished from the "three-in-one" block, the technique of lumbar plexus anesthesia that achieves anesthesia of the lateral femoral cutaneous and obturator as well as the femoral nerves¹. Surgical anesthesia of the entire lower extremity can be obtained when the three-in-one block is combined with the sciatic block². This technique is used frequently at our institution as the primary anesthetic, or as the postoperative analgesic technique, for foot or ankle surgery.

Common Indications

In addition to foot and ankle surgery, femoral nerve block can be used to supply pre- or postoperative analgesia for femoral neck fractures³,4, or total hip arthroplasty⁵. When initiated preoperatively, it facilitates positioning for placement of neuraxial block⁶. Profound analgesia is obtained without the adverse effects associated with opioids or ketamine (i.e, respiratory depression, hemodynamic effects, or depression of consciousness). Other indications for femoral nerve block include providing pre- or postoperative analgesia for femoral shaft fractures⁷, surgical anesthesia for outpatient saphenous vein stripping⁸, anesthesia for outpatient knee arthroscopy⁹, postoperative analgesia for knee procedures or total knee arthroplasty¹⁰,11, and surgical anesthesia for anterior thigh muscle biopsies in children¹². More rapid recovery and lower incidence of urinary retention may result compared with spinal anesthesia when short-acting local anesthetics (i.e., 2-chloroprocaine) are used for outpatient procedures⁸,9.

Specific Contraindications

The presence of a prosthetic femoral artery graft is a relative contraindication to femoral nerve block. The procedure (when combined with sciatic block) is also relatively contraindicated in situations where a dense sensory block could mask the onset of lower extremity compartment syndrome (e.g., fresh fractures of the tibia and fibula, or especially traumatic and extensive elective orthopedic procedures of the tibia and fibula). This contraindication is not specific for the femoral nerve block but rather applies to regional anesthesia of the lower extremity in general. The surgeons should be consulted as to the likelihood of the development of compartment syndrome and their own preferences of postoperative analgesic technique when considering the risks and benefits of performing regional anesthesia.

Continuous Femoral Nerve Block: Indications

Postoperative analgesia can be continued for days with a local anesthetic infusion when a catheter is placed within the connective tissue "sheath" of the femoral nerve. This technique has been shown to significantly reduce systemic opioid requirements with a minimum of complications following knee procedures¹³,14, total knee arthroplasty¹¹,15, total hip arthroplasty⁴, and femoropopliteal bypass surgery¹⁶. Failure to obtain a opioid-sparing effect in certain knee procedures may be related to lack of blockade in sensory fibers from the sciatic and obturator nerves¹⁷.

Anatomy

The femoral nerve arises from lumbar plexus roots L2-L4 (Figure). The nerve travels through the substance of the psoas muscle, then passes anterior to the iliopsoas under the inguinal ligament before becoming more superficial in the anterior thigh. The nerve lies deep to the fascia lata and fascia iliacus (Figure). The fascia iliacus separates it from the vascular bundle containing the femoral artery and vein. It divides into numerous branches early in the proximal anterior thigh.

The Saphenous Nerve

The saphenous nerve is the terminal branch of the femoral nerve. It passes within the adductor canal beneath the sartorius muscle, then curves around the posteriomedial aspect of the knee to divide into branches along the anteriomedial aspect of the proximal tibia. This nerve supplies sensation to the medial aspect of the ankle and a variable portion of the medial foot. Several techniques for blockade of the saphenous nerve have been described.

Planning Neural Blockade

When performed preoperatively, the block can be used as the primary anesthetic technique. When bupivacaine is used, the "soak time" required to develop adequate surgical anesthesia can be considerable. Planning for early institution of the block for "first cases" of the day, or for performing the block for "to-follow" cases in the pre-operative holding area by a separate anesthesia team while the previous case is still in progress, may help avoid delays in OR turnover.

If the primary purpose of the block is to provide postoperative analgesia, performing the block postoperatively rather than preoperatively has the obvious advantage of maximizing the duration of sensory analgesia. The block can be performed safely under general anesthesia at the end of the case or awake in the PACU following emergence. It is a common clinical impression that use of larger doses of local anesthetic produce longer-lasting analgesia. Thus a single injection technique which maximizes the dose of anesthetic may be the technique of choice when the block is performed for postoperative pain relief.

Even following injection of large doses of long-acting local anesthetic the block can be expected to dissipate in the middle of the night, unmasking severe pain. If no provision has been made for "rescue" postoperative analgesia, patients can become extremely dissatisfied with the technique no matter how well the block worked initially. At our institution, patients who receive peripheral nerve blocks are discharged from the PACU with intravenous opioids via PCA readily available. If systemic analgesia with opioids and NSAIDS proves inadequate, the block may be repeated.

Locating the Femoral Nerve

A number of techniques for locating the femoral nerve exist. Success rates are operator-dependent.

NERVE STIMULATOR

Use of a nerve stimulator has the advantages of continuous feedback and a definite endpoint for locating the nerve. This technique can be used with the patient under general anesthesia as long as pharmacological neuromuscular blockade has been reversed. Disadvantages include a requirement for additional equipment and presence of an assistant.

SEEKING PARESTHESIAE

Eliciting paresthesiae provides a definite endpoint for locating the nerve, but requires an awake and responsive patient. Paresthesiae elicited during axillary brachial plexus block have been associated with neural injury¹⁸, and this has raised concern about use of this technique in other peripheral nerve blocks.

LOSS OF RESISTANCE

The femoral nerve lies below two facial planes: the fascia lata and the fascia iliacus . Simply feeling two successive "pops" as a short bevel regional anesthesia needle passes through these fascial layers indicates placement of the needle in the perineural space¹⁹.

FIELD BLOCK

When landmarks are favorable, blind infiltration of local anesthetic using multiple injections to create a "fan" across the expected route of the femoral nerve is a simple and rapid technique. The risk of intravascular injection of a single large volume of local anesthetic is minimized by keeping the needle mobile during injection. Use of a constantly moving, small gauge needle may also minimize the risk of neural injury, since one possible cause neural injury is direct intraneural injection of a large volumes of anesthetic.

"Three-in-One" Block

The "three-in-one" block is indicated when anesthesia in the distributions of the obturator and lateral femoral cutaneous nerves as well as the femoral nerve is desired. This technique relies on a single injection of large volumes of local anesthetic within the neural "sheath" with the needle directed cephalad, and the subsequent spread of anesthetic proximally aided by pressure applied distal to the femoral nerve "sheath", to achieve anesthesia of the entire lumbar plexus. Dye injection studies in cadavers have cast doubt as to whether local anesthetic spreads to the parent trunk of the obturator nerve²⁰,21. Clinical studies also have demonstrated that failure to obtain anesthesia in the distribution of lateral femoral cutaneous and obturator nerves is common even with large volumes (40 ml) of local anesthetic²²,23.

Local Anesthetic

Typically, 10-15 ml of 0.375-0.5% bupivacaine with epinephrine 5 µg/ml is used. The addition of epinephrine will decrease plasma levels of bupivacaine in lower extremity blocks²⁴. Larger volumes of local anesthetic (30 ml or greater) are required to achieve the "three-in-one" block. Onset of surgical block requires 30-40 minutes. Postoperative analgesia typically lasts 12 hours, but can last as long as 24 hours.

The Nerve Stimulator Technique

THE LANDMARKS

The structures used to locate the femoral nerve are the inguinal ligament, and the femoral artery. A line representing the inguinal ligament is drawn between the anterior superior iliac spine and the pubic tubercle (Figure). The pulsations of the femoral artery are palpated and the artery is marked distal to the ligament (Figure). The femoral nerve lies lateral to the artery. Think of the mnemonic "NAVELS" to remember the relationship between the femoral nerve and the femoral artery (structures ordered laterally to medially: Nerve, Artery, Vein, Empty space, Lymphatics, pubic Symphysis) ( Figure). The point of needle insertion is marked 1.5 cm lateral and 1.5 cm distal to the intersection of the inguinal ligament and the femoral artery.

TECHNIQUE

The groin is prepped and draped sterilely.

In the awake patient, cutaneous anesthesia at the point of needle insertion is obtained with a skin wheal of local anesthetic .

A 2 inch 22-gauge short-bevel teflon-coated nerve stimulator needle is inserted through the skin at a 45 degree angle to the skin and directed cephalad and slightly medially toward the umbilicus (Figure).

Once the needle is through the skin the nerve stimulator output is adjusted to 1.5-2.0 mA with a frequency of 1.0 Hz.

An evoked response of the rectus femoris is sought as the needle is carefully advanced. Movement of the patella indicates stimulation of the femoral nerve (Figure, and Movie: QuickTime, 309K). If this motion is not found, the needle is brought back to the skin and advanced after redirecting the needle in a more lateral or medial direction. The needle should be systematically redirected in a "fan" across the expected path of the femoral nerve if the nerve is not easily located.

Once the nerve is located the needle position optimized and the stimulus intensity is adjusted downward until a patellar twitch remains present at a output of 0.3 to 0.4 mA.

Aspiration is performed to rule-out intravascular placement as the needle is held immobile. An epinephrine test dose is also administered to rule-out intravascular placement. Injection of a small amount of local anesthetic should abolish the evoked motor response. Persistence of the twitch may indicate that the nerve is separated from the needle by a thin tissue membrane that conducts current easily but prevents local anesthetic from reaching the nerve. The needle should be advanced further and the test dose repeated.

Following a negative test dose, local anesthetic is injected in divided doses while watching for signs of systemic local anesthetic toxicity (Figure).

The Continuous Catheter Technique

The nerve stimulator technique described above can be modified for placement of an indwelling catheter for continuous femoral nerve block²⁵. Continuous techniques have the advantage of providing high quality postoperative analgesia and minimizing systemic opioid requirements, without the disadvantages of epidural analgesia such as urinary retention, orthostatic hypotension, and impaired ambulation.

A femoral catheter can be placed using common supplies. A 4 inch 22-gauge short-bevel teflon-coated nerve stimulator needle is inserted into a plastic cannula taken from a long 18-gauge intravenous catheter. The femoral nerve is located using the nerve stimulator as described previously. A skin nick is made around the needle with a number 15 blade and the plastic cannula is advanced through the skin over the nerve stimulator needle into the "sheath" of the femoral nerve. The initial dose of local anesthetic can be injected through the cannula to establish the block and facilitate advancement of the catheter. A 20-gauge (epidural-type) nylon catheter is threaded through the cannula into the femoral "sheath". The cannula is then removed. The catheter is sutured into place and dressed sterilely. Bupivacaine 0.25-0.125% at 0.14 ml/kg/hr is infused. Use of 0.125% bupivacaine may reduce the amount of local anesthetic used without significantly changing morphine requirements or quality of postoperative analgesia²⁶.

Specific Complications

Persistent quadriceps weakness postoperatively suggests neural injury. The mechanism of nerve injury following peripheral nerve block includes direct nerve trauma from the needle, injury from intraneural injection, and compressive-ischemic injury caused by local hematoma formation²⁷. The differential diagnosis of postoperative peripheral nerve injury also includes compressive injury from improper patient positioning, ischemic injury from prolonged tourniquet use, and direct surgical trauma. Peripheral nerve injury related to regional anesthesia can be permanent but usually resolves over weeks. Efforts to avoid this complication include:

1. Avoidance of eliciting paresthesiae (intentionally or unintentionally) when locating the nerve.

2. Immediately stopping injection if a paresthesia is elicited during injection. Painful paresthesiae and resistance to injection suggests that the needle tip lies inside the nerve and that the solution is being injected intraneurally. If this occurs the needle should be pulled back slightly.

3. Avoiding injection when evoked motor responses occur at stimulus intensities <0.3 mA. Strong motor responses at such low intensities may indicate placement within the substance of the nerve.

4. Limiting the total volume of local anesthetic injected.

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