Ultrasound-guided Femoral Nerve Block
- Dr Arul James, MD, FRCA, FFPMRCA test
- 11 hours ago
- 6 min read
Femoral Nerve Block - ARiRA Pro Tips
Indications
The femoral nerve block is a high-yield regional anaesthesia technique for providing effective analgesia to the hip, anterior thigh, knee, and medial leg. It is widely used in:
Hip fracture management and preoperative pain relief in emergency settings
Total hip arthroplasty and femoral shaft fracture fixation
Quadriceps or patellar tendon repair
Anterior cruciate ligament (ACL) reconstruction
Saphenous vein stripping and anterior thigh skin grafts
Postoperative analgesia as part of a multimodal approach to reduce opioid use
This block targets the femoral nerve (L2–L4) before it divides into terminal branches, providing broad motor and sensory coverage of the anterior compartment of the thigh. It is a key Plan A Block technique in the ARiRA regional anaesthesia and nerve block course.
Anatomy
The femoral nerve arises from the lumbar plexus (L2–L4) and descends between the psoas major and iliacus muscles, passing under the inguinal ligament into the femoral triangle.
In the femoral triangle, the nerve lies:
Lateral to the femoral artery
Deep to the fascia iliaca
Superficial to the iliopsoas muscle
Understanding these anatomical relationships is key for precise needle placement and avoiding vascular puncture.
Patient Positioning
Position the patient supine, with the hip in neutral position
Slight external rotation and abduction of the leg on the side to be blocked improves exposure of the inguinal region
A pillow under the knee may be used to slightly flex the hip, helping to relax thigh muscles and facilitate landmark identification
Ensure the inguinal area is fully exposed for optimal ultrasound access and needle entry
Probe Placement
Use a high-frequency linear probe (8–15 MHz)
Place transversely just below the inguinal crease
Identify the femoral artery first as a pulsatile, anechoic circle
The femoral nerve is usually seen as a hyperechoic, triangular or oval structure lateral to the artery
Landmarks
Femoral artery: Pulsatile, medial reference point
Femoral vein: Compressible and lies medial to the artery
Femoral nerve: Bright, speckled structure lateral to artery, deep to fascia iliaca
Iliopsoas muscle: Hypoechoic, lies deep to the femoral nerve
Needle Insertion
The in-plane lateral-to-medial approach is standard.
Advance the needle under direct vision towards the lateral border of the femoral nerve.
Use hydrodissection with saline to confirm the correct plane below the fascia iliaca
Deposit local anaesthetic around the nerve, observing for spread
Injection Volume
Adults: 15–20 mL of local anaesthetic
0.25% levobupivacaine or 0.2% ropivacaine is commonly used
Always aspirate and use incremental dosing
Maximising Block Success
Confirm the correct plane by visualising nerve separation from the fascia iliaca during injection.
Use colour Doppler to avoid vascular structures
Use echogenic needles for precise needle tracking
Minimising Complications
Femoral nerve block is generally safe when performed with real-time ultrasound guidance. However, key precautions include:
Intravascular injection: Use incremental doses and aspirate frequently
Local Anaesthetic Systemic Toxicity (LAST): Monitor for early signs, calculate total dose carefully
Quadriceps weakness: Delay mobilisation or fall risk in elderly — consider adductor canal block alternative for pure sensory analgesia.
Haematoma or vascular injury: Especially in anticoagulated patients — visualize femoral vessels clearly
Explore ARiRA’s Ultrasound-Guided Regional Anaesthesia Resources
At ARiRA.co.uk, we bring you the latest in immersive, interactive regional anaesthesia education, including:
Ultrasound simulation with real-time probe control
3D cadaveric visualisation of neural anatomy
Femoral Nerve Block – Frequently Asked Questions
Q: What is a femoral nerve block?
A: A femoral nerve block is a regional anaesthetic technique involving the injection of local anaesthetic near the femoral nerve as it passes through the femoral triangle. It provides effective pain relief to the hip, anterior thigh, knee, and medial aspect of the lower leg, making it ideal for lower limb surgeries and acute pain management.
Q: How long does the block last?
A: The duration varies depending on the choice of local anaesthetic and whether any adjuvants are used. A single injection typically provides analgesia for 8 to 12 hours. For extended relief, a continuous catheter technique may be used, allowing infusions over several days.
Q: Will it affect the ability to walk?
A: Yes. Since the femoral nerve innervates the quadriceps muscle, blocking it may cause temporary weakness or paralysis of that muscle group. Patients may need walking aids and supervision to prevent falls until motor function returns.
Q: Are there any risks or side effects?
A: Although safe when performed under ultrasound guidance, potential risks include bleeding, haematoma, infection, nerve damage, and local anaesthetic systemic toxicity. These risks can be minimised using meticulous technique and appropriate training, as taught in ARiRA regional anaesthesia course.
Q: Is the procedure painful?
A: Discomfort is typically minimal. You might feel a sharp pinch during the skin injection and some pressure as the needle advances, but most patients tolerate the procedure well, especially under ultrasound guidance.
Q: What happens if the block doesn't work?
A: Occasionally, the block may be incomplete due to anatomical variation or technical difficulty. In such cases, a top-up block, additional nerve blocks, or systemic analgesia may be used.
Q: What anatomical landmarks are essential for performing a femoral nerve block under ultrasound guidance?
A: The femoral nerve lies lateral to the femoral artery, deep to the fascia iliaca, and superficial to the iliopsoas muscle within the femoral triangle. On ultrasound, identify the femoral artery first — a pulsatile, anechoic circle. The femoral vein lies medial and compresses with pressure. The femoral nerve appears hyperechoic, oval or triangular, lateral to the artery. Needle advancement should occur in-plane beneath the fascia iliaca to ensure accurate local anaesthetic deposition. These sonoanatomical skills are taught in detail during the ARiRA regional anaesthesia course.
Q: How do you confirm correct needle tip placement during a femoral nerve block?
A: Correct placement is confirmed by real-time ultrasound visualisation of the needle tip deep to the fascia iliaca, adjacent to the femoral nerve. Hydrodissection with saline should visibly lift the fascia iliaca, creating a space around the nerve. Local anaesthetics should appear as a hypoechoic halo or crescent encasing the nerve upon injection. Maintaining continuous needle visualisation throughout is critical for safety and effectiveness.
Q: What are the advantages of a femoral nerve block compared to systemic opioids in lower limb surgery?
A: Femoral nerve block offers superior pain control, reduces opioid requirements, and decreases side effects like nausea, vomiting, sedation, and constipation. It promotes earlier mobilisation, facilitates physiotherapy, and improves overall patient satisfaction. In elderly or frail patients, it may also reduce the risk of delirium. These benefits are central to modern enhanced recovery protocols and are reinforced in the ARiRA nerve block course.
Q: What are the common causes of block failure in femoral nerve blocks?
A: Causes of failure include:
Incorrect identification of the nerve (e.g., injecting too medial or superficial)
Inadequate hydrodissection resulting in poor spread
Insufficient volume of local anaesthetic
Anatomical variation, such as early nerve branching
Failure to penetrate beneath the fascia iliaca. Using ultrasound guidance and following a structured block technique, as emphasised in expert-led training, significantly reduces the likelihood of failure.
Q: What is the difference between a femoral nerve block and a fascia iliaca block?
A: A femoral nerve block specifically targets the femoral nerve, making it ideal for knee and anterior thigh procedures. In contrast, a fascia iliaca block aims to deposit anaesthetic more laterally in the fascia iliaca compartment, with the goal of spreading to the femoral, lateral femoral cutaneous, and sometimes the obturator nerve. Fascia iliaca block provides broader coverage and is often used in trauma or prehospital settings.
Q: How do you minimise motor block while preserving analgesia for knee surgery?
A: To preserve quadriceps strength, consider using lower concentrations of local anaesthetic (such as 0.1–0.2% ropivacaine). Alternatively, the adductor canal block targets the saphenous nerve (a sensory branch of the femoral nerve), providing effective analgesia while sparing motor function. This makes it preferable for day-case surgery and enhanced recovery after knee arthroplasty.
Q: What complications should clinicians monitor for after a femoral nerve block?
A: Clinicians should watch for:
Quadriceps weakness, increasing fall risk
Local anaesthetic systemic toxicity (LAST) from inadvertent intravascular injection
Haematoma, particularly in anticoagulated patients
Nerve injury, which is rare but serious
Infection at the injection site—maintain strict asepsis. Proper documentation, observation of motor function, and regular site checks are essential. These principles are embedded in ARiRA’s block safety framework.
Q: When is a continuous femoral nerve catheter indicated, and how is it managed?
A: Continuous catheters are used for major orthopaedic surgery (e.g., total knee replacement), femoral fractures, or prolonged postoperative pain. Under ultrasound, a Tuohy needle is placed adjacent to the femoral nerve, and the catheter is advanced into the fascial plane. Infusions of low-concentration local anaesthetic (e.g., 0.1–0.2% ropivacaine at 5–10 mL/hr) maintain analgesia. Monitor for sensory and motor effects, infection, and catheter dislodgement.
Q: How does patient obesity affect the femoral nerve block technique?
A: In obese patients, the femoral nerve may lie deeper than usual (>4 cm). This necessitates using longer needles, steeper angles, and optimised ultrasound settings (increased depth, lower frequency). A head-down tilt can help bring the target structures closer to the probe. Visualising the needle tip becomes more challenging, so echogenic needles and meticulous scanning are particularly important.
This content has been specifically tailored for healthcare professionals, focusing on unique strategies to optimise safety and efficacy during rectus sheath block procedures. Always apply clinical judgment tailored to individual patient needs.