Femoral nerve block

12 min read Last updated April 9, 2026

Surgical approach (where this block is used)

  • Not an operation itself, commonly used as analgesia/anaesthesia for surgery involving the anterior thigh and knee.
  • Typical operations where femoral nerve block (FNB) may be requested/used
    • Femoral shaft fracture fixation / traction / splintage (analgesia in ED/trauma)
    • Knee surgery: arthroscopy, ACL reconstruction (often combined with other blocks), patella surgery
    • Total knee arthroplasty (increasingly adductor canal block preferred for motor-sparing, FNB still relevant)
    • Skin grafting / wound care to anterior thigh

Anaesthetic management (typical context)

  • Type of anaesthesia
    • Regional analgesia adjunct to GA for knee/thigh surgery, or standalone block for analgesia (e.g., femoral fracture) with/without light sedation.
    • If used as sole anaesthetic for surgery: usually requires additional blocks (e.g., sciatic/obturator/lateral femoral cutaneous) depending on procedure.
  • Airway
    • If GA: SGA often suitable for short knee arthroscopy, ETT for longer/complex surgery, aspiration risk, positioning, or need for paralysis.
    • If block + sedation: maintain spontaneous ventilation, be prepared to convert to GA if inadequate block or agitation/pain.
  • Duration
    • Block performance: typically 5–15 min, onset 10–30 min (agent dependent).
    • Analgesia duration: ~6–18 h (single-shot, depends on LA choice/adjuncts). Continuous catheter can provide days.
  • How painful is the surgery?
    • Femoral fracture and knee surgery can be very painful, multimodal analgesia usually required.
    • FNB covers anterior knee/anterior thigh well but does not reliably cover posterior knee pain (often sciatic contribution).
  • Key perioperative considerations
    • Motor weakness (quadriceps) → falls risk, mobilise with caution and document advice.
    • Anticoagulation status and infection risk (particularly for catheter techniques).
    • Local anaesthetic systemic toxicity (LAST) risk: calculate maximum dose, incremental injection with aspiration, consider ultrasound guidance.

Overview

  • Femoral nerve block provides analgesia/anaesthesia to the anterior thigh and much of the anterior knee, also contributes to medial leg/foot via saphenous nerve (terminal sensory branch).
  • Common uses: analgesia for femoral fractures, perioperative analgesia for knee surgery, can be part of combined blocks for more extensive lower limb surgery.

Applied anatomy (high yield)

  • Origin and course
    • Lumbar plexus: posterior divisions of L2–L4 (sometimes L1 contribution).
    • Emerges lateral to psoas major, runs between psoas and iliacus, passes under inguinal ligament into femoral triangle.
  • Relations in femoral triangle (classic exam anatomy)
    • Lateral to femoral artery, outside femoral sheath (artery/vein within sheath, nerve outside).
    • Order lateral → medial: femoral Nerve, Artery, Vein, (empty space), Lymphatics (&quot,NAVeL&quot,).
    • Often lies beneath fascia iliaca, fascia lata is more superficial.
  • Branches and sensory/motor supply (what it covers)
    • Motor: quadriceps femoris (knee extension), sartorius, pectineus (variable).
    • Sensory: anterior thigh (anterior cutaneous branches), medial leg/ankle/foot via saphenous nerve, articular branches to hip and knee.
    • Does NOT reliably cover: lateral thigh (lateral femoral cutaneous nerve), posterior knee (sciatic), medial thigh (obturator).

Indications

  • Analgesia for femoral shaft/neck fractures (often as part of fascia iliaca compartment block pathway, FNB is a targeted alternative).
  • Perioperative analgesia for knee surgery (arthroscopy, ligament reconstruction, patella surgery, TKA—though adductor canal block often preferred for mobilisation).
  • Analgesia for anterior thigh procedures (skin graft donor site).
  • Diagnostic/therapeutic pain interventions (less common in FRCA context).

Contraindications

  • Absolute
    • Patient refusal / lack of capacity without appropriate best-interest decision.
    • Allergy to local anaesthetic (true IgE-mediated rare, clarify reaction).
    • Infection at injection site.
  • Relative / caution
    • Anticoagulation/bleeding diathesis: peripheral nerve blocks generally lower risk than neuraxial, but femoral region is non-compressible deep to fascia, consider haematoma risk and local policy/ASRA/RA-UK guidance.
    • Pre-existing femoral neuropathy or significant peripheral neuropathy (risk of attribution/worsening).
    • Severe sepsis/haemodynamic instability (risk–benefit, may still be appropriate for analgesia but ensure resuscitation and monitoring).
    • Inability to cooperate/position safely (consider sedation/GA or alternative technique).

Preparation and monitoring (OSCE/viva structure)

  • Consent: purpose, expected benefits, alternatives (systemic opioids, fascia iliaca block, adductor canal block), and specific risks.
  • Monitoring: standard AAGBI monitoring, IV access, resuscitation equipment and intralipid immediately available.
  • Asepsis: full barrier precautions for catheter, at least sterile gloves, skin prep, sterile probe cover/gel for ultrasound.
  • Local anaesthetic safety: calculate maximum safe dose (consider lean body weight, age, frailty), use incremental injection with frequent aspiration, consider using lower concentration/volume when appropriate.

Techniques

  • Approaches
    • Ultrasound-guided (preferred): direct visualisation of nerve, artery, fascia, lower vascular puncture and potentially lower LA volume.
    • Nerve stimulator (landmark-based): relies on motor response (quadriceps/patellar twitch).
    • Landmark/paresthesia techniques: less favoured due to higher failure/complication risk.
  • Ultrasound anatomy (transverse at inguinal crease)
    • Femoral artery: pulsatile anechoic circle, femoral vein medial and compressible.
    • Femoral nerve: hyperechoic, triangular/oval structure lateral to artery, under fascia iliaca, superficial to iliopsoas.
    • Fascia lata (superficial) and fascia iliaca (deeper) appear as hyperechoic lines, aim to deposit LA under fascia iliaca around nerve.
  • Ultrasound-guided steps (in-plane lateral-to-medial typical)
    • Position: supine, leg slightly abducted and externally rotated, expose groin, identify landmarks and scan.
    • Needle: in-plane, advance towards lateral aspect of nerve, avoid intraneural injection (high resistance, nerve swelling, pain/paresthesia).
    • Hydrodissection with small aliquots to confirm correct plane under fascia iliaca, then inject incrementally to surround nerve (circumferential spread desirable but not always necessary).
    • Aspirate every 3–5 mL, observe spread, reassess needle tip frequently.
  • Nerve stimulator technique (key points)
    • Needle inserted 1–2 cm lateral to femoral artery at inguinal crease, seek quadriceps contraction/patellar twitch.
    • Typical stimulator settings: start 1.0 mA, 0.1 ms, 2 Hz, acceptable response at ~0.2–0.5 mA suggests close proximity (avoid &lt,0.2 mA with strong response—possible intraneural).
    • Inject after negative aspiration, stop if severe pain/paresthesia or high resistance.
  • Catheter technique (continuous femoral nerve block)
    • Indications: major knee surgery (e.g., TKA) where prolonged analgesia desired, consider motor weakness vs adductor canal catheter alternative.
    • Thread catheter 3–5 cm beyond needle tip under ultrasound, secure well, label catheter, prescribe infusion with clear limits and monitoring plan.

Local anaesthetic choice and dosing (typical ranges, tailor to patient)

  • Single-shot volumes commonly 10–20 mL with ultrasound (higher volumes may be used with landmark techniques but increase LAST risk).
  • Agents
    • Lidocaine 1–2%: faster onset, shorter duration.
    • Bupivacaine 0.25–0.5% or levobupivacaine 0.25–0.5%: longer duration.
    • Ropivacaine 0.2–0.5%: long-acting with relatively less motor block at lower concentrations (still causes quadriceps weakness with FNB).
  • Adjuncts (departmental policy dependent)
    • Dexamethasone (perineural or IV) may prolong duration, consider infection risk, hyperglycaemia, and local governance.
    • Clonidine may prolong block but can cause hypotension/sedation.

Block assessment and expected effects

  • Sensory: reduced cold/pinprick anterior thigh and medial leg (saphenous).
  • Motor: reduced knee extension (quadriceps) and reduced patellar reflex.
  • Analgesic limitations: persistent posterior knee pain suggests sciatic contribution, lateral thigh pain suggests lateral femoral cutaneous nerve, medial thigh pain suggests obturator.

Complications and management

  • Block failure / incomplete block
    • Causes: wrong plane (above fascia iliaca), inadequate volume, anatomical variation, intravascular uptake, time insufficient for onset.
    • Management: reassess clinically and with ultrasound, consider top-up/repeat with safe dosing, supplement with additional blocks or systemic analgesia, convert to GA if required.
  • Vascular puncture / haematoma
    • Avoid: ultrasound identification of artery/vein, in-plane visualisation, aspiration, gentle technique.
    • Manage: direct pressure, reassess anticoagulation, document, consider imaging/surgical review if expanding haematoma or neurovascular compromise.
  • Local anaesthetic systemic toxicity (LAST)
    • Early features: tinnitus, metallic taste, circumoral numbness, agitation, seizures, later: arrhythmias, cardiovascular collapse.
    • Immediate management: stop injection, call for help, airway/oxygen/ventilation, treat seizures (benzodiazepine), start intralipid per guidelines, manage arrhythmias (avoid large doses of propofol in unstable patient, avoid vasopressin, use reduced-dose adrenaline).
  • Nerve injury
    • Mechanisms: intraneural injection, needle trauma, ischaemia/haematoma, neurotoxicity, stretch/compression.
    • Prevention: avoid paresthesia/pain on injection, avoid high opening pressure, keep needle tip in view, use minimal effective dose, avoid deep sedation during injection so patient can report symptoms.
    • If suspected: stop, document, examine and follow local nerve injury pathway, early neurology referral if severe/progressive deficit.
  • Infection (esp. catheter)
    • Asepsis, secure dressing, daily review, remove catheter if signs of infection or unexplained sepsis.
  • Falls due to quadriceps weakness
    • Warn patient and staff, physiotherapy precautions, consider knee brace if mobilising, consider adductor canal block as alternative for motor-sparing knee analgesia.
  • Femoral nerve block vs fascia iliaca compartment block (FICB)
    • FNB: targeted block of femoral nerve, reliable quadriceps weakness, good anterior thigh/knee analgesia.
    • FICB: aims to block femoral + lateral femoral cutaneous ± obturator by high-volume spread under fascia iliaca, often used in ED/trauma pathways, may be less reliable for obturator.
  • Femoral nerve block vs adductor canal block (ACB)
    • ACB targets saphenous nerve/nerve to vastus medialis in adductor canal → better motor preservation for mobilisation after knee surgery, but may provide less analgesia than FNB for some procedures.

Documentation (often examined in OSCE)

  • Record: indication, side, technique (US/NS), asepsis, LA drug/concentration/volume, adjuncts, needle/catheter details, complications, block assessment, advice re falls, and post-block monitoring plan.

Test yourself…

Describe the anatomy relevant to a femoral nerve block at the inguinal crease.

Aim for relations, fascial planes, and what structures are in the femoral sheath.

  • Femoral nerve arises from posterior divisions of L2–L4 and enters the femoral triangle under the inguinal ligament.
  • In the femoral triangle, the nerve lies lateral to the femoral artery and outside the femoral sheath.
  • Femoral sheath contains artery and vein (and lymphatics medially) but not the nerve.
  • Fascial layers: fascia lata superficial, fascia iliaca deeper—femoral nerve lies beneath fascia iliaca on iliopsoas.
What areas are anaesthetised by a femoral nerve block, and what areas are not covered?

Examiners want clear dermatomal/territorial coverage and limitations for knee surgery.

  • Covers: anterior thigh (anterior cutaneous branches), anterior knee (articular branches), medial leg/ankle/foot via saphenous nerve.
  • Motor: quadriceps weakness (knee extension) and reduced patellar reflex.
  • Does not reliably cover: posterior knee (sciatic), lateral thigh (lateral femoral cutaneous), medial thigh/adductor region (obturator).
Talk me through how you would perform an ultrasound-guided femoral nerve block safely.

Structure: preparation → scanning → needle approach → injection safety → post-block care.

  • Preparation: consent, check anticoagulation/infection, IV access, monitoring, intralipid available, full asepsis and sterile ultrasound setup.
  • Scan at inguinal crease: identify femoral artery, vein (medial), and femoral nerve (hyperechoic lateral) under fascia iliaca on iliopsoas.
  • Needle in-plane (often lateral-to-medial) with tip visualised at all times, aim to place tip just under fascia iliaca adjacent to nerve.
  • Inject incrementally with aspiration every 3–5 mL, observe spread around nerve, stop if pain/paresthesia, high resistance, or nerve swelling.
  • Post-block: assess sensory/motor effect, document, and give falls precautions due to quadriceps weakness.
How would you use a nerve stimulator to perform a femoral nerve block? What response are you looking for and at what current?

Key points: insertion point, motor response, safe current threshold, and what to avoid.

  • Insert needle 1–2 cm lateral to femoral artery at inguinal crease, advance until quadriceps contraction/patellar twitch seen.
  • Start around 1.0 mA, 0.1 ms, 2 Hz, reduce current—acceptable response typically at ~0.2–0.5 mA.
  • Avoid injecting if response persists at very low current (&lt,0.2 mA) with strong twitch (possible intraneural) or if injection is painful/high resistance.
A patient still has severe posterior knee pain after a femoral nerve block for knee surgery. Why, and what would you do?

This is a common FRCA viva scenario: recognise incomplete coverage and propose a safe plan.

  • Reason: posterior knee pain is commonly supplied by sciatic nerve branches, femoral block mainly covers anterior knee.
  • Assess: confirm block success (anterior thigh/knee sensation and quadriceps weakness), check surgical site and tourniquet pain, consider opioid requirement.
  • Management options: supplement analgesia (paracetamol/NSAID if appropriate, opioids), consider additional regional technique (e.g., sciatic block or IPACK depending on setting/skills), or convert to GA if intraoperative pain.
List the complications of femoral nerve block and how you would minimise them.

Examiners want: vascular puncture, LAST, nerve injury, infection (catheter), and falls.

  • Vascular puncture/haematoma: ultrasound identification, in-plane technique, aspiration, avoid multiple passes, apply pressure if puncture occurs.
  • LAST: dose calculation, incremental injection, aspiration, ultrasound guidance, immediate recognition and intralipid-based resuscitation.
  • Nerve injury: avoid intraneural injection, avoid high pressure/pain on injection, keep patient responsive enough to report symptoms, visualise needle tip.
  • Infection (catheter): strict asepsis, secure dressing, daily review, timely removal.
  • Falls: warn patient/staff, physiotherapy precautions, consider motor-sparing alternatives (adductor canal block) when appropriate.
Discuss local anaesthetic choice and volume for femoral nerve block, and how you ensure safe dosing.

They want principles rather than a single recipe: minimum effective volume, patient factors, and total dose across multiple blocks.

  • Choose LA based on desired onset/duration: lidocaine for rapid onset, bupivacaine/levobupivacaine/ropivacaine for prolonged analgesia.
  • Typical ultrasound-guided volume 10–20 mL, use the minimum effective volume to reduce LAST risk.
  • Calculate maximum safe dose considering weight, age, frailty, comorbidities, and total dose if combining blocks, inject incrementally with aspiration and monitoring.
How does femoral nerve block compare with fascia iliaca compartment block for hip/femoral fracture analgesia?

A frequent FRCA theme: compare targets, reliability, and practicalities in ED/trauma.

  • FNB: targeted femoral nerve block, reliable for femoral nerve territory, requires accurate placement near nerve, causes quadriceps weakness.
  • FICB: high-volume injection under fascia iliaca aiming to block femoral + lateral femoral cutaneous ± obturator, often used in ED pathways, obturator block is variable.
  • Both reduce opioid requirements and improve comfort for positioning/spinal anaesthesia, choice depends on local expertise, equipment, and desired nerve coverage.
A patient becomes agitated and then has a seizure shortly after you inject local anaesthetic for a femoral nerve block. What is your diagnosis and immediate management?

This is a classic crisis viva: treat as LAST until proven otherwise.

  • Diagnosis: local anaesthetic systemic toxicity (LAST).
  • Immediate actions: stop injection, call for help, maintain airway and give 100% oxygen, support ventilation to avoid acidosis.
  • Treat seizure: benzodiazepine (e.g., midazolam), avoid large propofol doses if cardiovascular instability.
  • Start lipid emulsion therapy per guideline, manage arrhythmias/cardiovascular collapse with modified ACLS (small adrenaline doses, avoid vasopressin).
  • Post-event: ICU/HDU monitoring, document, report, and counsel patient.
What advice would you give regarding mobilisation after a femoral nerve block?

This often appears as a safety/quality question.

  • Warn that quadriceps weakness can occur and increases falls risk, patient should mobilise only with assistance until strength returns.
  • Communicate with nursing/physio staff, consider mobility aids or knee brace if appropriate, document advice clearly.

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