Femoral Nerve Block

Overview
  • Indications: Anterior thigh and knee surgery
  • Landmarks: Femoral (inguinal) crease, femoral artery pulse
  • Nerve Stimulation: Twitch of the patella (quadriceps) at 0.2-0.5 mA current
  • Local anesthetic: 20 mL
  • Complexity level: Basic


General considerations

A femoral nerve block is a basic nerve block technique that is easy to master, carries a low risk of complications, and has a significant clinical applicability for surgical anesthesia and post-operative pain management. This block is well suited for surgery such on the anterior thigh, knee, quadriceps tendon repair, and postoperative pain management after femur and knee surgery. When combined with a block of the sciatic nerve, anesthesia of almost the entire lower extremity from the mid-thigh level can be achieved. The success rate of this block for surgery is very high, nearing 95%.
Femoral nerve block for quadriceps muscle biopsy on the anterior thigh.

Regional anesthesia anatomy


The femoral nerve is the largest branch of the lumbar plexus. It arises from the second, third, and fourth lumbar nerves. The nerve descends through the fibers of the psoas muscle, emerging from the psoas at the lower part of its border, and passes down between the psoas and the iliacus. Eventually, the femoral nerve passes underneath the inguinal ligament into the thigh, where it assumes a more flattened shape. As the femoral nerve passes underneath the inguinal ligament, it is positioned immediately lateral and slightly deeper than the femoral artery.

At the femoral crease, the nerve it is covered by the fascia iliaca and separated from the femoral artery and vein by a portion of the psoas muscle and the ligamentum ileopectineum. This physical separation of the femoral nerve from the vascular fascia explains the lack of the spread of a "blind paravascular" injection of local anesthetics toward the femoral nerve.

TIP:
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 the vessels in the femoral triangle.

The relationship of the femoral artery, femoral nerve, and sartorius muscle. The motor branches to the sartorius muscle depart from the anteromedial aspect of the femoral nerve toward the sartorius muscle. Because the anesthesiologist can never be sure whether stimulation of the sartorius muscle is obtained in the sheath of the femoral nerve or outside of it, this should always be confirmed by obtaining quadriceps stimulation before injecting local anesthetic.
  • ASIS - anterior-superior iliac spine
  • FN - Femoral nerve
  • FA - Femoral artery
  • SM - Sartorius muscle
  • PT - pubic tubercle
The femoral nerve supplies muscular branches of the iliacus and pectineus, and the muscles on the anterior thigh, except the tensor fascie femoris. The nerve also provides cutaneous filaments to the front and inner side of the thigh and to the leg and foot (saphenous nerve), as well as the articular branches to the hip and knee joints.

Femoral Nerve Branches
  • Anterior division:
    • Middle cutaneous
    • Medial cutaneous
    • Muscular (sartorius)
  • Posterior division:
    • Saphenous nerve (most medial)
    • Muscular (individual heads of the quadriceps muscle)
    • Articular branches (hip and knee)
Distribution of anesthesia
A femoral block results in anesthesia of the entire anterior thigh and most of the femur and knee joint. The block also confers anesthesia of the skin on the medial aspect of the leg below the knee joint (saphenous nerve - a superficial terminal extension of the femoral nerve).

Patient positioning
The patient is in the supine position with both legs extended. In obese patients, a pillow placed underneath patient's hips may facilitate palpation of the femoral artery and block performance.

Equipment

A standard regional anesthesia tray is prepared with the following equipment:
  • Sterile towels and 4"x4" gauze packs
  • 20 mL syringes with local anesthetic
  • Sterile gloves, marking pen, and surface electrode
  • One 1˝" 25-gauge needle for skin infiltration
  • A 5-cm long, short bevel, insulated stimulating needle
  • Surface electrode
  • Peripheral nerve stimulator

Landmarks

Surface Landmarks

The following surface anatomy landmarks are used to determine the insertion point for the needle:
  1. Femoral crease
  2. Femoral artery
Anatomic Landmarks

Landmarks for the femoral nerve block are easily recognizable in all patients and include:
  1. Femoral crease
  2. Femoral artery pulse

Needle insertion site is labeled immediately lateral to the pulse of the femoral artery. All landmarks should be outlined with a marking pen.

TIPS:
  • Note that this technique differs from common descriptions of the femoral nerve block, where the needle is inserted at the level of the inguinal ligament. Instead, in this technique the needle is inserted at the level of the femoral crease, a naturally occurring, oblique skin fold positioned a few centimeters below the inguinal ligament.
  • The femoral crease can be accentuated in obese patients by asking an assistant to retract the lower abdomen laterally. The retraction of the abdomen should be maintained throughout the procedure to facilitate palpation of the femoral artery and block performance.
Technique


After a thorough cleaning with an antiseptic solution, local anesthetic is infiltrated subcutaneously at the estimated site of needle insertion. The injection for the skin anesthesia should be shallow and in a line extending laterally to allow for more lateral needle reinsertion when necessary.

The anesthetist is standing on the side of the patient with the pal-pating hand on the femoral artery. The needle is introduced immediately at the lateral border of the artery and advanced in the saggital and slightly cephalad plane.

TIP:
The nerve stimulator is initially set to deliver 1.0 mA (2 Hz, 100 µsec). With proper needle position, advancement of the needle should not result in any local twitches; the first response is usually that of the femoral nerve.

Goal


The femoral nerve innervates a number of muscle groups. A visible or palpable twitch of the quadriceps muscle (patella twitch) at 0.2-0.5 mA current is the optimal response.

TIPS
  • The most common response to nerve stimulation with this technique is twitch of the sartorius muscle. This results in a band-like contraction across the thigh without movement of the patella.
  • It should be kept in mind that sartorius muscle twitch is not reliable because the branches to the sartorius muscle may be outside the femoral sheath. When the sartorius muscle twitch occurs, the needle is simply redirected laterally and advanced several mm deeper.
Failure to localize the femoral nerve

When stimulation of the quadriceps muscle is not obtained on the first needle pass, the palpating hand should not be moved from its position. First, visualize the needle plane in which the stimulation was not obtained and follow this algorithm:
  • Ensure that the nerve stimulator is properly connected and functional.
  • Withdraw the needle to the skin, redirect 10-15o laterally, and repeat the needle advancement.

When the above procedure fails to produce a twitch, the needle is withdrawn from the skin, reinserted 1 cm laterally, and the above steps are repeated with progressively more lateral needle insertion. After the initial stimulation of the femoral nerve is obtained, the stimulating current is gradually decreased until twitches are still seen or felt at 0.2 - 0.4 mA current. This typically occurs at a depth of 2-3 cm. After an aspiration that is negative for blood, 20-25 mL of local anesthetic is slowly injected.
TIP: Again, it is essential to keep the palpating finger in the same position throughout the procedure. This helps prevent repeatedly reinserting the needle in the same plane.
Troubleshooting

Response Obtained Interpretation Problem Action
No response The needle is inserted either too medially or too laterally Femoral artery not properly localized or the palpating hand moved during the procedure Follow the systematic lateral angulation and reinsertion of the needle as described in the technique
Bone contact The needle contacts hip or superior ramus of the pubic bone The needle is inserted too deep Withdraw to the level of the skin and reinsert in another direction
Local twitch Direct stimulation of the illiopsoas or pectineus muscle Too deep insertion Withdraw to the level of the skin and reinsert in another direction
Twitch of the sartorius muscle Sartorius muscle twitch The needle tip is slightly anterior and medial to the main trunk of the femoral nerve Redirect the needle laterally and advance deeper 1-3 mm
Vascular puncture Blood in the syringe invariably indicates placement into the femoral artery Too medial needle placement Withdraw and reinsert laterally 1 cm
Patella twitch Stimulation of the main trunk of the femoral nerve None Accept and inject local anesthetic

Choice of local anesthetic

A femoral block can be accomplished with as little as 10 mL of local anesthetic. However, we often use larger volumes of local anesthetic (e.g., 20-25 mL), because the local anesthetic often disperses underneath fascia iliaca laterally and results also in block of the lateral femoral cutaneous nerve of thigh. The block of lateral cutaneous nerve of the thigh, in return, confers anesthesia to the lateral aspect of the thigh, which nicely complements the femoral nerve block.

The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or pain management. Long-acting local anesthetic should be avoided in ambulatory patients undergoing relatively minor procedures as ambulation is affected by prolonged motor block of the quadriceps muscle.

The onset times and duration of anesthesia with different types and concentrations of local anesthetics and the addition of vasoconstrictor.

Onset
(min)
Anesthesia (hrs) Analgesia (hrs)
3% 2-chloroprocaine (+ HCO3) 10-15 1 2
3% 2-chloroprocaine (+ HCO3 + epi) 10-15 1.5-2 2-3
1.5% Mepivacaine (+ HCO3) 15-20 2-3 3-5
1.5% Mepivacaine (+ HCO3 + epi) 15-20 2-5 3-8
2% lidocaine (+ HCO3 + epi) 10-20 2-5 3-8
0.5% ropivacaine 15-30 4-8 5-12
0.75% ropivacaine 10-15 5-10 6-24
0.5 Bupivacaine (or l-bupivacaine) 15-30 5-15 6-30

Block Dynamics and Perioperative Management

This technique is associated with minimal patient discomfort, because the needle passes only through the skin and adipose of the femoral inguinal region. However, many patients feel uncomfortable being exposed during palpation of the femoral artery and appropriate sedation is necessary for the patient's comfort and acceptance. Midazolam 1-2 mg after patient is positioned and alfentanil 250-500µg just before the local infiltrations suffices for most patients. A typical onset time for this block is 10-15 minutes, depending on the type, concentration, and volume of local anesthetic used. The first sign of onset of blockade is the loss of sensation of the skin over the medial aspect of the leg below the knee (saphenous nerve). Weight bearing on the blocked side is impaired and this should be clearly explained the patient to prevent the risk of falls.

Complications and How to Avoid Them


Infection - Just like with any other procedure, use a strict aseptic technique
- Catheters at his location are difficult to keep sterile and should probably removed after 48 hours
Hematoma - Avoid advancement of the needle when the patient reports pain; this may indicate insertion of the needle through the illiopsoas or pectineus muscles
- When the femoral artery or vein are punctured, the procedure should be stopped and a firm and constant pressure applied over the femoral artery for 2-3 minutes before proceeding with the blockade.
- In a patient with difficult anatomy or severe peripheral vascular disease, use a single-shot smaller gauge needle to localize the femoral nerve before proceeding with a larger gauge needle for the continuous technique.
Vascular Puncture - Never redirect the needle medially!
- The needle is first inserted just lateral to the femoral artery and the consequent insertions and redirections should all be progressively more lateral
Nerve Injury - Use nerve stimulation and slow needle advancement
- Distinct paresthesia is almost never elicited with femoral nerve block and should not be sought
- Do not inject when the patient complains of pain or when high pressures on injection are met
Other Instruct the patient on the inability to bear weight on the blocked extremity

Bibliography
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with permission from Admir Hadzic, M.D., Ph.D.