Lower limb blocks

10 min read Last updated April 8, 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 & 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
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 >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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