Lower limb blocks

10 min read Last updated April 9, 2026

Surgical approach (context: common lower-limb operations where blocks are used)

  • Hip surgery (THA/hip fracture fixation)
    • Incision lateral/posterolateral/anterior, deep dissection to capsule, reaming/implant, significant nociception from capsule and femur
    • Often elderly/frail, positioning lateral/supine, blood loss variable
  • Knee surgery (TKA/ACL/arthroscopy)
    • Midline/medial parapatellar approach for TKA, tourniquet common, high postoperative pain
    • ACL: arthroscopic portals + graft harvest, moderate–severe pain depending on graft site
  • Foot/ankle surgery (ORIF, bunion, Achilles repair)
    • Often requires lateral/medial incisions, may use thigh or calf tourniquet, pain can be severe (ankle ORIF/Achilles)

Anaesthetic management (typical patterns)

  • Type of anaesthesia
    • Regional alone: spinal for hip/knee, peripheral nerve blocks for foot/ankle, consider sedation
    • GA + regional: common for major surgery to improve analgesia and reduce opioids
  • Airway
    • If GA: SGA often suitable for short lower-limb cases, ETT for long cases, aspiration risk, prone/lateral positioning, or significant comorbidity
  • Duration (very approximate)
    • Hip fracture fixation: ~1–2 h, THA: ~1.5–3 h, TKA: ~1.5–3 h, ankle ORIF: ~1–2.5 h
  • How painful?
    • High: THA/TKA, ankle ORIF, Achilles repair (often severe first 24–48 h)
    • Moderate: arthroscopy/ACL (variable, higher if hamstring/patellar tendon harvest)
  • Analgesic strategy (overview)
    • Multimodal: paracetamol + NSAID (if appropriate) + regional + opioid rescue, consider ketamine for opioid-tolerant, consider dexamethasone for PONV/analgesia
    • VTE prophylaxis and timing of blocks/catheters: follow local policy and ASRA/ESAIC guidance for anticoagulants

Core principles

  • Define goals: surgical anaesthesia vs postoperative analgesia, motor-sparing where early mobilisation is required (e.g., TKA pathways)
  • Choose block based on dermatomes/osteotomes and tourniquet pain (femoral/obturator/sciatic contributions, saphenous for medial leg/ankle)
  • Safety: full monitoring, IV access, resuscitation drugs, intralipid, incremental injection, aspiration, ultrasound visualisation of needle tip and spread
  • Consent: benefits, failure rate, nerve injury, infection, bleeding/haematoma, LAST, motor weakness/falls, neuropathic symptoms

Relevant anatomy (exam-level)

  • Lumbar plexus (L1–L4): femoral (L2–L4), obturator (L2–L4), lateral femoral cutaneous (L2–L3)
  • Sacral plexus (L4–S3): sciatic (tibial + common peroneal), posterior femoral cutaneous, gluteal nerves
  • Femoral nerve: lateral to femoral artery under fascia iliaca, supplies anterior thigh, knee (via articular branches), sensory via saphenous to medial leg/ankle
  • Obturator nerve: medial thigh adductors, articular branches to hip/knee (variable but relevant for TKA pain)
  • Sciatic nerve: posterior thigh, divides into tibial and common peroneal (usually proximal to popliteal fossa), major contributor to below-knee/foot
  • Ankle/foot terminal nerves: tibial → medial/lateral plantar, deep peroneal (1st web space), superficial peroneal (dorsum), sural (lateral foot), saphenous (medial foot)

Block selection by surgery (pragmatic FRCA approach)

  • Hip fracture / THA analgesia
    • FICB or PENG for pre-op analgesia, consider spinal for surgery, consider catheter if severe pain/rehab needs
    • Lumbar plexus block provides dense analgesia but higher risk (retroperitoneal bleed, neuraxial spread), less common in UK pathways
  • TKA analgesia
    • Adductor canal block (ACB) + iPACK commonly used to spare quadriceps while covering posterior knee
    • Femoral nerve block gives excellent analgesia but increases quadriceps weakness and fall risk, reserve for selected patients
  • Below-knee / foot &amp, ankle surgery
    • Popliteal sciatic block ± saphenous (ACB or below-knee saphenous field block) for medial ankle/foot
    • Ankle block for forefoot procedures, avoid proximal motor block if early mobilisation required

Key blocks: indications, sonoanatomy, technique, complications

  • Fascia iliaca compartment block (FICB)
    • Indications: hip fracture analgesia, femoral shaft, aims to block femoral + lateral femoral cutaneous (obturator unreliable)
    • Ultrasound: femoral artery/nerve, fascia lata and fascia iliaca as two hyperechoic lines, inject under fascia iliaca lateral to nerve with spread
    • Risks: LAST (large volumes), femoral weakness/falls, intravascular injection, haematoma, infection
  • PENG block (Pericapsular Nerve Group)
    • Indications: hip pain (fracture/arthroplasty analgesia), targets articular branches (femoral, obturator, accessory obturator) to anterior hip capsule
    • Ultrasound: identify AIIS, iliopubic eminence, psoas tendon, inject in plane to deposit local between psoas tendon and pubic ramus/iliopubic eminence
    • Cautions: variable motor-sparing, potential femoral weakness if spread, avoid intrapelvic/vascular puncture
  • Femoral nerve block (FNB)
    • Indications: anterior thigh/knee analgesia, femoral shaft, rescue analgesia, less favoured for TKA pathways due to quadriceps weakness
    • Ultrasound: femoral nerve lateral to artery under fascia iliaca, inject around nerve to achieve circumferential spread
    • Complications: falls, vascular puncture, LAST, nerve injury
  • Adductor canal block (ACB) / saphenous nerve block
    • Indications: medial leg/ankle, TKA analgesia with motor-sparing (primarily sensory), often combined with iPACK
    • Ultrasound (mid-thigh): femoral artery under sartorius, canal bounded by sartorius (roof), vastus medialis (anterolateral), adductor longus/magnus (posteromedial), inject around artery beneath sartorius
    • Note: local may spread to nerve to vastus medialis and occasionally femoral nerve branches → some quadriceps weakness still possible
  • Obturator nerve block
    • Indications: adductor spasm (urology/orthopaedics), adjunct for knee/hip analgesia (variable benefit in TKA)
    • Ultrasound: identify adductor muscles, anterior and posterior branches between adductor longus/brevis and brevis/magnus, inject small volumes at each plane
    • Risks: vascular puncture (obturator vessels), incomplete block, nerve injury
  • Sciatic nerve block (popliteal approach)
    • Indications: surgery below knee (except medial leg/foot unless add saphenous), excellent for foot/ankle analgesia
    • Ultrasound: popliteal artery/vein, tibial nerve superficial/lateral then track proximally to sciatic before bifurcation, inject around sciatic or around tibial + common peroneal separately
    • Complications: foot drop (expected motor block), nerve injury, vascular puncture, LAST, beware tight casts/compartment syndrome masking (controversial—ensure monitoring and education)
  • iPACK (Infiltration between Popliteal Artery and Capsule of the Knee)
    • Indications: posterior knee pain after TKA while sparing tibial/common peroneal motor function (analgesic field block)
    • Ultrasound: identify femoral condyles/popliteal artery at distal femur, inject in plane into tissue plane between artery and posterior capsule
    • Risks: vascular puncture, spread to tibial nerve causing weakness, local anaesthetic dose stacking with ACB/LIA
  • Ankle block (5-nerve block)
    • Indications: forefoot surgery, toe procedures, can avoid proximal motor block, often performed as field blocks at ankle level
    • Targets: tibial (posterior to medial malleolus), deep peroneal (either side of dorsalis pedis), superficial peroneal (subcutaneous across dorsum), sural (between lateral malleolus and Achilles), saphenous (anterior to medial malleolus/subcutaneous)
    • Risks: intravascular injection (dorsalis pedis/posterior tibial), incomplete coverage, local infection at injection sites

Local anaesthetic dosing (exam essentials)

  • Always calculate maximum safe dose (mg/kg) and account for multiple injections (ACB + iPACK + LIA is common in TKA)
  • Use the lowest effective concentration/volume, ultrasound allows dose reduction but does not eliminate LAST risk
  • Consider adjuncts (per local policy): dexamethasone (perineural vs IV), clonidine, document off-label use and risks

Complications and management

  • LAST: CNS symptoms (tinnitus, metallic taste, agitation, seizures) → cardiovascular collapse, treat with airway/oxygen, seizure control, intralipid, ALS modifications
    • Intralipid 20% typical regimen: bolus 1.5 mL/kg then infusion 0.25 mL/kg/min, repeat bolus if unstable, increase infusion, observe and avoid large adrenaline doses
  • Nerve injury: minimise by avoiding intraneural injection (high opening pressure/pain/paresthesia), visualise needle tip, stop if pain, document neuro exam when appropriate
  • Bleeding/haematoma: higher risk with anticoagulation, compressible sites preferred, follow anticoagulant timing guidance, consider ultrasound-guided compression
  • Falls: femoral/FICB can weaken quadriceps, implement mobilisation precautions, physiotherapy awareness, knee brace if needed
  • Compartment syndrome: regional analgesia does not reliably mask it if monitored, ensure patient/staff education, frequent neurovascular checks, escalating pain despite block is concerning

Test yourself…

Describe the sensory innervation relevant to knee surgery and how this informs your choice of blocks for TKA.

Aim: cover anterior/medial knee pain while preserving quadriceps, and address posterior capsule pain.

  • Anterior knee: mainly femoral nerve articular branches, medial leg via saphenous (terminal sensory branch of femoral)
  • Medial knee: contributions from obturator articular branches (variable but can be clinically relevant)
  • Posterior knee: branches from tibial nerve (sciatic) and posterior obturator, hence ACB alone often insufficient
  • Plan: ACB (motor-sparing) + iPACK (posterior capsule field block) ± surgeon LIA, avoid routine femoral block if early mobilisation pathway
Talk me through an ultrasound-guided adductor canal block: probe position, anatomy, needle path and endpoint.

Key is identifying femoral artery under sartorius at mid-thigh and depositing LA in the canal.

  • Position: supine, leg slightly externally rotated, linear probe mid-thigh anteromedial
  • Sonoanatomy: sartorius superficial/medial (roof), vastus medialis anterolateral, adductor longus/magnus posteromedial, femoral artery within canal
  • Needle: in-plane lateral-to-medial (or medial-to-lateral) aiming adjacent to artery beneath sartorius, avoid vascular puncture
  • Endpoint: spread around artery in the canal, aspiration/incremental injection, document sensory distribution expectations (medial leg) and possible partial quadriceps weakness
Fascia iliaca block: what nerves does it reliably block, and why might it fail to cover hip fracture pain?

Hip pain is multifactorial, FICB is not a complete hip block.

  • Reliable: femoral nerve, lateral femoral cutaneous often, obturator is unreliable (especially with infra-inguinal approach)
  • Hip capsule innervation includes obturator and accessory obturator articular branches, posterior capsule also has sciatic contributions
  • Therefore analgesia may be incomplete, consider PENG (anterior capsule) or additional strategies (systemic analgesia, spinal for surgery)
Describe the PENG block: target, sonoanatomy, and a key complication to mention in consent.

Designed to cover anterior hip capsule articular branches with relative motor-sparing, but motor weakness can still occur.

  • Target: plane between psoas tendon and pubic ramus/iliopubic eminence to block articular branches (femoral/obturator/accessory obturator)
  • Sonoanatomy: identify AIIS and iliopubic eminence, visualise psoas tendon, needle in-plane to deposit LA deep to tendon in target plane
  • Complication: unintended femoral nerve block/quadriceps weakness → falls risk, also vascular puncture/LAST
Popliteal sciatic block: how do you ensure medial ankle surgery is covered?

Sciatic does not cover the saphenous territory.

  • Popliteal sciatic covers tibial + common peroneal distributions (most of foot/ankle)
  • Medial leg/ankle/foot is saphenous (femoral) → add ACB (saphenous) or a below-knee saphenous field block
Explain tourniquet pain and which blocks help.

Tourniquet pain is complex and may break through despite dense distal block.

  • Mechanisms: deep tissue ischaemia, C-fibre transmission, and proximal nerve compression, often occurs after 30–60 minutes
  • Thigh tourniquet: requires femoral + sciatic coverage (and sometimes obturator), spinal/epidural reliably covers tourniquet pain
  • Calf tourniquet: sciatic + saphenous may suffice depending on site
Local anaesthetic systemic toxicity (LAST): give a structured management plan.

FRCA expects a rehearsed algorithm including lipid therapy.

  • Stop injection, call for help, ABC: airway control, 100% oxygen, ventilate to avoid acidosis, treat seizures (benzodiazepine, avoid large propofol doses if unstable)
  • Start intralipid 20%: bolus 1.5 mL/kg then infusion 0.25 mL/kg/min, repeat bolus if persistent instability, continue until stable (respect max dose per guidelines)
  • Cardiac arrest: ALS with modifications—small adrenaline doses, avoid vasopressin, avoid lidocaine, consider amiodarone, prolonged resuscitation may be required
  • Post-event: ICU monitoring, document, report, counsel patient
A patient has persistent numbness and weakness after a femoral nerve block. How do you assess and manage this?

Differentiate expected block duration from nerve injury/haematoma/compartment issues.

  • Immediate assessment: time since injection, drug/dose, progression, full neuro exam (motor/sensory), pain out of proportion, vascular status, check for back pain if neuraxial spread possible
  • Exclude reversible causes: tight dressings/cast, haematoma (especially anticoagulated), infection, consider ultrasound/CT if concern for bleed
  • Escalate early: inform surgical team, acute pain/regional lead, consider neurology review if persistent &gt,48–72 h or progressive deficit, document clearly
Discuss regional anaesthesia in an anticoagulated patient needing a lower limb block.

Principles: balance bleeding risk vs benefit, choose compressible sites, follow published guidance and local policy.

  • Assess anticoagulant type, timing of last dose, renal function, and bleeding history, check platelet count/coagulation where relevant
  • Prefer superficial/compressible blocks (e.g., ACB, popliteal) over deep non-compressible (lumbar plexus) when bleeding risk is significant
  • Catheters: consider insertion and removal timing as separate anticoagulation risk points
  • If in doubt: discuss with senior/regional lead and document risk-benefit and plan
How would you provide analgesia for a frail elderly patient with a hip fracture in ED awaiting theatre?

Focus on early analgesia, minimal delirium/respiratory depression, and facilitation of positioning for spinal.

  • Perform FICB or PENG early with full monitoring and resus readiness, use incremental dosing and consider lower concentrations in frail patients
  • Add paracetamol, cautious opioids (small titrated doses), consider avoiding NSAIDs if renal impairment/bleeding risk
  • Reassess pain and function, plan for spinal in theatre, communicate block performed, drug/dose/time, and expected motor effects

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