Regional Block

Femoral Nerve Block (FNB)

The femoral nerve (L2, L3, L4) is the largest branch of the lumbar plexus. It exits the pelvis beneath the inguinal ligament, lateral to the femoral artery and vein (NAVY: Nerve, Artery, Vein, Y-fronts from lateral to medial). The nerve lies within the femoral triangle, beneath the fascia lata and fascia iliaca, and lateral to the femoral artery. It divides into anterior and posterior divisions below the inguinal ligament. The saphenous nerve (its terminal branch) provides sensation to the medial lower leg and foot. It is the primary sensory supply to the anterior thigh and motor supply to the quadriceps.

Onset10–20 minutes sensory block

Duration8–24 hours (bupivacaine/levobupivacaine 0.5%)

CoverageAnterior and medial thigh (femoral nerve proper)

Clinical Note

Critical safety point: document quadriceps motor block and communicate to nursing staff. Patient must not be permitted to weight-bear without fall risk assessment. Ensure appropriate mobility aids and bed positioning.

Indications

Femoral shaft fracture (analgesia)
Supracondylar femur fracture
Patella fracture
Anterior knee pain and quadriceps haematoma
Hip fracture (adjunct — provides femoral but not obturator coverage)
Knee arthroscopy supplemental analgesia
Saphenous nerve block for medial lower leg procedures
As a component of knee arthroplasty anaesthesia
Skin lesion removal on anterior thigh

Contraindications

Patient refusal
Infection/cellulitis over femoral triangle
Previous ipsilateral femoral artery bypass or significant femoral vascular surgery
Significant coagulopathy (femoral haematoma risk — femoral artery in close proximity)
Known allergy to local anaesthetic
Femoral arteriovenous fistula or graft
Pre-existing femoral neuropathy (relative contraindication)

Technique

Patient supine with leg slightly abducted and externally rotated
Linear high-frequency ultrasound probe placed in inguinal crease, perpendicular to femoral artery
Identify femoral artery (pulsatile, compressible on Doppler), femoral vein (medial, compressible), femoral nerve (hyperechoic triangular structure lateral to artery)
Observe fascial planes: fascia lata and fascia iliaca overlying the nerve
In-plane needle from lateral to medial; target deep to fascia iliaca, adjacent to nerve
AVOID INTRANEURAL INJECTION — nerve should be seen to move away from needle; do not inject against resistance
Hydrodissect with 1 mL saline; inject 10–20 mL local anaesthetic with continuous visualisation
Nerve should be surrounded ("donut sign") by LA — visualise spread
Avoid circumferential injection as this may cause compressive injury

Drug Doses

Agent	Concentration	Volume	Total Dose	Notes
Ropivacaine	0.5%	15–20 mL	75–100 mg	Good sensory/motor differentiation; preferred for ambulatory patients. Max 3 mg/kg.
Bupivacaine	0.25–0.5%	15–20 mL	37.5–100 mg	Longer duration at 0.5%. Max 2 mg/kg; do not exceed 150 mg.
Levobupivacaine	0.25–0.5%	15–20 mL	37.5–100 mg	Reduced cardiotoxicity versus bupivacaine. Max 2 mg/kg.
Lignocaine (adjunct/short procedure)	1–2%	10–15 mL	100–300 mg	Short duration (1–2h). Can add adrenaline 1:200,000 to extend. Max 3 mg/kg (7 mg/kg with adrenaline).

Onset

10–20 minutes sensory block; motor block within 15–30 minutes

Duration

8–24 hours (bupivacaine/levobupivacaine 0.5%); 2–4 hours with lignocaine

Complications

Quadriceps Weakness / Fall Risk

Motor blockade of the quadriceps is an expected side effect. Patients must not bear weight without assessment and support. Clear fall risk documentation required.

Femoral Vessel Injury / Haematoma

Femoral artery puncture can cause significant haematoma. Ultrasound guidance essential. Apply direct pressure for arterial puncture. Risk of retroperitoneal haematoma if proximal tracking.

LAST

Highly vascular region with femoral vessels in close proximity. Strict aspiration before and during injection. Maximum dose must be respected.

Nerve Injury

Peripheral neuropathy is rare (<0.1%). Risk factors: intraneural injection, tourniquet, surgical traction. Do not inject against resistance; use nerve stimulator or ultrasound confirmation.

Block Failure

Ultrasound-guided success rate >95%. Failure most often due to proximal division of nerve or inadequate volume. Consider repeat injection or adductor canal approach for distal coverage.

Landmarks

Femoral artery (pulsatile, medial to nerve)
Inguinal ligament (superior boundary of femoral triangle)
Femoral vein (medial to artery)
Femoral nerve (lateral to femoral artery, beneath fascia iliaca)
Iliopsoas muscle (deep to nerve)
Femoral triangle: boundaries — inguinal ligament (superior), sartorius (lateral), adductor longus (medial)

Sensory Coverage

Anterior and medial thigh (femoral nerve proper). Medial lower leg and foot (saphenous nerve — terminal branch). Quadriceps motor supply. DOES NOT cover: posterior thigh (sciatic/posterior femoral cutaneous nerve), lateral thigh (lateral femoral cutaneous nerve), or medial thigh below adductor hiatus (obturator nerve).

Clinical Pearls

Confirm needle tip position before injection — never inject against resistance

For purely sensory block (e.g., postoperative, avoiding quadriceps weakness), use low-concentration ropivacaine 0.2%

Adductor canal block (distal femoral nerve block) provides saphenous coverage with less quadriceps weakness

FICB provides similar analgesia to FNB for hip fracture but also covers LFCN and obturator — consider for hip surgery

Document and communicate to ward: patient has a femoral nerve block and must have fall precautions

Nerve stimulator endpoint: quadriceps twitch at 0.3–0.5 mA before LA injection if using nerve stimulation technique

Background & Evidence

The femoral nerve block remains a widely used lower limb block, particularly for femoral shaft and knee surgery. In the ED context, it is useful for femoral shaft fractures. However, for proximal femur fractures, the FICB is generally preferred as it provides broader coverage (femoral, LFCN, and variable obturator). The adductor canal block (a distal femoral nerve block) is increasingly favoured over the classic FNB for knee arthroplasty as it provides similar analgesia with significantly less motor block.

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