Femoral Nerve Block

Quick Answer

Femoral nerve block targets the femoral nerve (L2-L4) as it passes beneath the inguinal ligament, lateral to the femoral artery. Coverage: Anterior thigh, femur, knee (anterior and medial), skin over medial lower leg (saphenous nerve). Indications: Anterior thigh surgery, knee surgery (arthroplasty, ACL repair, arthroscopy), femoral shaft fractures, knee analgesia, quadriceps tendon repair. Technique: Ultrasound-guided - visualize femoral artery, iliopsoas muscle, fascia iliaca; femoral nerve lateral to artery, deep to fascia iliaca; in-plane approach from lateral to medial; inject 15-20 mL LA between fascia iliaca and iliopsoas. Complications: LAST (vascular proximity), vascular puncture (femoral artery/vein), motor weakness (quadriceps), fall risk, infection. 3-in-1 block: Large volume (30 mL) with distal pressure can block lateral femoral cutaneous and obturator nerves (variable success). Catheter: Excellent for continuous infusion post-TKR (ropivacaine 0.2% 5-10 mL/hour). [1-10]

Anatomy

Femoral Nerve Anatomy

Origin and Course:

• Formed from: L2-L4 lumbar plexus (posterior divisions)
• Exits: Lateral border psoas muscle
• Passes: Beneath inguinal ligament
• Position: Lateral to femoral artery, deep to fascia iliaca, superficial to iliopsoas muscle

At Inguinal Crease:

• Femoral artery: Most medial, pulsatile
• Femoral vein: Medial to artery (often not visible in block position)
• Femoral nerve: Lateral to artery, deep to fascia iliaca
• Iliopsoas muscle: Deep to nerve
• Fascia iliaca: Superficial fascial layer covering nerve

Terminal Branches:

1. Muscular branches: Quadriceps, sartorius, pectineus
2. Saphenous nerve: Sensory to medial lower leg and foot
3. Articular branches: Hip and knee joints

Sensory Distribution:

• Anterior thigh: Skin and subcutaneous tissue
• Medial lower leg: Via saphenous nerve (medial ankle, foot)
• Knee joint: Anterior capsule

Motor Distribution:

• Quadriceps femoris: Knee extension
• Sartorius: Hip flexion, knee flexion
• Pectineus: Hip adduction

Fascial Layers

Fascia Iliaca:

• Attaches: To inguinal ligament
• Covers: Iliopsoas muscle, femoral nerve
• Continuous: With fascia lata distally

Fascia Lata:

• Superficial: Covers sartorius and anterior thigh
• Separate: From fascia iliaca by fat plane

Two "Pops":

• First pop: Through fascia lata
• Second pop: Through fascia iliaca (target)

Related Nerves

Lateral Femoral Cutaneous Nerve (LFCN):

• Origin: L2-L3
• Course: Medial to anterior superior iliac spine, beneath inguinal ligament
• Sensory: Lateral thigh
• Often blocked: With large volume femoral approach (3-in-1)

Obturator Nerve:

• Origin: L2-L4
• Course: Medial border psoas, through obturator canal
• Sensory: Medial thigh, hip/knee joints
• Variable block: With 3-in-1 technique (unreliable)

Indications and Contraindications

Indications:

Surgical:

• Total knee replacement (TKR): Primary indication, reduces opioid use
• Knee arthroscopy: Diagnostic and therapeutic
• ACL repair: Analgesia and muscle relaxation
• Femoral shaft fractures: Emergency department and perioperative
• Quadriceps tendon repair: Analgesia
• Anterior thigh surgery: Biopsy, soft tissue procedures
• Hip surgery: Limited role (obturator/lateral cutaneous often missed)

Medical:

• Chronic pain: CRPS, neuralgia
• Trauma: Femur fracture analgesia (avoids systemic opioids)

Contraindications:

Absolute:

• Infection at site: Cellulitis, abscess
• Patient refusal
• Allergy to local anaesthetics

Relative:

• Anticoagulation: Femoral vessels large, hematoma risk (ASRA guidelines)
• Previous femoral vascular surgery: Grafts, stents
• Significant quadriceps weakness pre-op: Risk falling
• Inability to use walking aids: Cannot compensate for motor block
• Local tumor: Interferes with landmarks
• Obesity: Deep nerve, difficult ultrasound

Caution:

• Compartment syndrome risk: Block masks pain (ensure monitoring if high risk)
• Pre-existing neuropathy: Document first

Technique

Ultrasound-Guided Approach

Equipment:

• Ultrasound: High-frequency linear probe (10-15 MHz)
• Needle: 50-80 mm, 22G, echogenic
• Local anaesthetic: 15-20 mL (0.5% ropivacaine or 0.375% bupivacaine)

Patient Position:

• Supine: Legs extended, slight external rotation helpful
• Pillow under knee: Slight flexion (optional, improves access)
• Expose: Groin and anterior thigh

Probe Position:

• Location: Inguinal crease/fold, perpendicular to vessels
• Orientation: Transverse, parallel to inguinal ligament
• Identify: Femoral artery first (landmark)

Sonographic Anatomy:

1. Femoral artery: Pulsatile, round, anechoic, non-compressible (most medial)
2. Femoral vein: Medial to artery (if seen), compressible
3. Femoral nerve: Lateral to artery
   • Triangular or oval hyperechoic structure
   • Sometimes appears as "honeycomb" or fascicular
   • Moves with probe pressure (different from artery)
4. Fascia iliaca: Hyperechoic line superficial to nerve
5. Iliopsoas muscle: Deep to nerve (hypoechoic with striations)
6. Fascia lata: Superficial layer

Needle Insertion:

• Approach: In-plane (lateral to medial)
  • Enter lateral to probe, aim medially toward nerve
  • Visualize entire needle shaft
  • Aim for beneath fascia iliaca, lateral to artery
• Alternative: Out-of-plane (cephalad to caudad)
  • More difficult
  • Higher risk of vascular puncture

Target:

• Location: Lateral to femoral artery, deep to fascia iliaca
• Goal: Local anesthetic spread circumferentially around nerve
• Technique:
  • Advance needle through fascia lata (first pop)
  • Advance through fascia iliaca (second pop)
  • Inject deep to fascia iliaca, around nerve

Injection:

• Aspiration: Before injection (vascular proximity)
• Test dose: 3-5 mL (watch for spread)
• Total volume: 15-20 mL
• End point: Circumferential spread around nerve, lifting it off iliopsoas

Saphenous Block (Distal):

• Adductor canal approach: For knee arthroscopy/TKR, preserves quadriceps
• Location: Mid-thigh, sartorius muscle, vastus medialis, adductor magnus triangle
• Benefit: Sensory block, minimal motor weakness

Fascia Iliaca Block (Alternative)

Difference:

• Lateral approach: Injection below inguinal ligament, lateral to femoral artery
• Higher volume: 30-40 mL
• Spread: Along fascia iliaca compartment
• Blocks: Femoral + lateral femoral cutaneous + obturator (variable)
• Blind technique: Historically done with "double pop" (now ultrasound preferred)

Advantages:

• Easier if ultrasound difficult
• No need to identify nerve specifically
• Blocks multiple nerves

Disadvantages:

• Variable spread
• Higher volume (LAST risk)
• Less reliable than targeted femoral

Landmark Technique (Historical)

Not recommended in modern practice

Fascia Iliaca Compartment Block:

• Landmark: 1 cm inferior to inguinal ligament, 1-2 cm lateral to femoral artery
• Technique: "Two pops" - fascia lata then fascia iliaca
• Volume: 30 mL
• Blind: High failure rate, vascular risk

Femoral Nerve Stimulator:

• Landmark: Lateral to femoral artery, 1-2 cm below inguinal ligament
• Stimulation: Quadriceps contraction (patellar twitch)
• Acceptable: 0.3-0.5 mA
• Rarely used: Ultrasound superior

Complications

Local Anaesthetic Systemic Toxicity (LAST)

Risk Factors:

• Vascular injection: Femoral artery/vein proximity
• High volume: Large amounts of LA
• Rapid injection: Bolus into vessel
• Location: Highly vascular area

Prevention:

• Aspiration: Before every injection
• Incremental injection: 5 mL aliquots
• Ultrasound: Avoid vessels
• Epinephrine marker: 1:400,000 (detects IV spread)

Management:

• Stop injection
• Intralipid 20%: 1.5 mL/kg bolus then infusion
• Supportive: Seizure control, airway, CPR if arrest

Vascular Complications

Femoral Artery Puncture:

• Incidence: 2-5%
• Consequence: Hematoma, pseudoaneurysm
• Prevention: Ultrasound, avoid medial approach
• Management: Pressure 10-15 minutes, monitor expansion

Femoral Vein Puncture:

• Less problematic than artery
• Still risk hematoma

Anticoagulation Considerations:

• ASRA guidelines: Similar to neuraxial
• Warfarin: INR <1.4
• Heparin: Timing from last dose
• DOACs: Per specific drug
• Aspirin: Generally safe

Motor Weakness and Falls

Quadriceps Weakness:

• Incidence: Nearly 100% with sufficient block
• Effect: Cannot extend knee, leg "gives way"
• Risk: Falls when ambulating

Prevention:

• Patient education: Warn before block
• Walking aids: Crutches or frame mandatory
• Assistance: Do not ambulate alone
• Low concentration: Ropivacaine 0.2% (less motor block than 0.5%)

Management:

• Protect limb: Until block resolves
• Knee immobilizer: Optional
• Bed rest: If severe weakness

Continuous Infusion and Falls:

• Risk continues with catheter
• Daily assessment of motor function
• Reduce rate or concentration if problematic

Neural Injury

Incidence:

• Temporary: 0.5-1%
• Permanent: <0.1%

Mechanism:

• Intraneural injection
• Needle trauma
• Compression (hematoma)

Prevention:

• Avoid intraneural: Do not inject if:
  • High pressure on injection
  • Severe paresthesia
  • Nerve swells on ultrasound
• Minimum current: If using stimulator, <0.5 mA
• Ultrasound: Avoid entering nerve fascicles

Treatment:

• Conservative (most resolve)
• Neurology referral if persistent >3 months
• Rehabilitation

Other Complications

Infection:

• Cellulitis, abscess: Rare with sterile technique
• Risk: Catheter left >3-4 days
• Management: Remove catheter, antibiotics if needed

Compartment Syndrome Masking:

• Risk: Block prevents pain sensation
• High-risk procedures:
  • TKR revision
  • Trauma with swelling
  • Vascular procedures
• Monitoring:
  • Serial compartment pressures if concern
  • Do not rely on pain
  • Check passive stretch pain (may be preserved)

Catheter Complications:

• Dislodgement: Secure well
• Kinking: Avoid sharp angles
• Leakage: Check connections
• Bacterial colonization: Remove at 3-5 days

Clinical Management

Preoperative

Assessment:

• Neurological: Baseline quadriceps function
• Vascular: Previous femoral catheterization, peripheral vascular disease
• Mobility: Ability to use crutches/walking frame
• Falls risk: History of falls, osteoporosis

Informed Consent:

• Expected: Numb anterior thigh, weak quadriceps (cannot straight-leg raise)
• Falls risk: Must use walking aids
• Duration: 12-24 hours (single shot), 2-5 days (catheter)

Contraindication Check:

• Anticoagulation status
• Infection at site
• Pre-existing neuropathy

Intraoperative

Single Shot:

• Positioning: Supine, groin exposed
• Sedation: Midazolam 1-2 mg, fentanyl 50-100 μg
• After block: Wait 20-30 minutes
• Test: Quadriceps weakness (cannot extend knee against gravity)

Catheter Insertion:

• Same technique: Plus catheter 3-5 cm beyond needle
• Test dose: 3-5 mL through catheter
• Securing: Adhesive, clear dressing, written date
• Infusion: Start in PACU

Combination with General:

• Common for TKR (GA + femoral block ± sciatic)
• Reduced opioid requirements
• Faster recovery

Postoperative

Single Shot:

• Analgesia: Excellent for 12-24 hours
• Supplement: Paracetamol, NSAIDs, weak opioids
• Monitoring: Motor function recovery
• DVT prophylaxis: Continue as per protocol (block does not contraindicate)

Catheter Management:

• Infusion: Ropivacaine 0.2% 5-10 mL/hour
• Bolus option: 2-4 mL q30-60 min PRN
• Monitoring: Daily motor assessment, infusion site
• Complications: Infection, dislodgement, breakthrough pain
• Removal: When oral analgesia adequate (usually POD 2-3)

Falls Prevention:

• Walking aids: Mandatory
• Assistance: For all ambulation
• Bed alarm: If confused/risk
• Education: Patient and family
• Low threshold: X-ray if fall (fracture risk)

Specific Procedures

Total Knee Replacement:

• Gold standard: Femoral catheter (± sciatic if severe pain)
• Benefits: Reduced opioid use, earlier mobilization, higher satisfaction
• Adductor canal: Alternative (spares quadriceps, better for mobilization)

ACL Repair:

• Single shot: Often adequate
• Volume: 20 mL
• Duration: 18-24 hours typically

Femoral Fracture:

• Emergency department: Block for analgesia and positioning
• Facilitates: Traction, imaging, preoperative preparation
• Technique: Single shot, 15-20 mL

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Access Issues:

• Regional centers: Orthopaedic surgery may require travel
• Trauma: Higher rates in some communities

Postoperative Care:

• Falls risk: Important in elderly, those with renal disease
• Communication: Clear instructions on walking aids
• Follow-up: Ensure access to catheter management if sent to remote area

Māori Health Considerations

Health Needs:

• Access to orthopaedic and regional anaesthesia services
• Falls prevention education
• Family support during recovery

ANZCA Final Exam Focus

Key Points

1. Anatomy: Femoral nerve lateral to femoral artery, deep to fascia iliaca, over iliopsoas
2. Coverage: Anterior thigh, knee (anterior/medial), medial lower leg (saphenous)
3. Indications: TKR, knee arthroscopy, femoral fractures, quadriceps surgery
4. Motor effect: Quadriceps weakness (100%) - fall risk, need walking aids
5. Complications: LAST (vascular), vascular puncture, falls, neural injury
6. Volume: 15-20 mL LA
7. Catheter: Excellent for TKR continuous infusion
8. Alternative: Fascia iliaca block (lateral approach, higher volume, multiple nerves)
9. Saphenous block: Adductor canal approach for knee (spares quadriceps)
10. Contraindications: Infection, anticoagulation (relative), inability to use walking aids

References

1. Mariano ER et al. Ultrasound-guided femoral nerve block. In: Hadzic's Peripheral Nerve Blocks. 2nd ed. McGraw-Hill; 2011:203-214.
2. Fabian-Guajardo CB et al. Continuous femoral nerve block. Curr Opin Anaesthesiol. 2020;33(5):658-664.
3. Barrington MJ et al. Ultrasound-guided regional anesthesia. Reg Anesth Pain Med. 2014;39(6):508-521.
4. Neal JM et al. Upper and lower extremity blocks. Reg Anesth Pain Med. 2020;45(11):911-935.
5. Winnie AP et al. Does the 3-in-1 block work? Anesthesiology. 1973;39(5):568-569.
6. Mariano ER et al. A randomized comparison of 0.2% ropivacaine. Anesth Analg. 2009;108(4):1327-1333.
7. Bauer M et al. Femoral nerve block. BJA Educ. 2016;16(6):213-217.
8. ASRA. Regional anesthesia in the patient receiving antithrombotic therapy. Reg Anesth Pain Med. 2018;43(3):263-309.