Sciatic Nerve Block

Blockade provides anaesthesia to:

Indications

Can be used:

  • Analgesia for
    • Knee surgery
  • Anaesthesia for:
    • Foot and ankle surgery
    • Knee surgery
      Requires concomitant femoral nerve block.

Contraindications

  • General contraindications:
    • Patient refusal
    • LA allergy
    • Local infection

Anatomy

Innervation of the lower extremities occurs via the lumbosacral plexus, which divides into the:

  • Sciatic nerve
  • Femoral nerve

Sciatic Nerve and Branches

The sciatic nerve:

  • Arises from L4-S3
  • Directly provides:
    • Motor supply to the posterior compartment of thigh
    • Sensory supply to posterior thigh
  • Divides at a variable location (but usually close to the popliteal fossa) into the:
    • Common peroneal nerve
      Divides at the knee into two branches which provides sensation to the posterolateral leg:
      • Superficial peroneal nerve
        Provides sensation to dorsum of foot and toes.
      • Deep peroneal nerve
        Provides sensation to the web space between the great and second toe.
    • Tibial nerve
      Innervates the posterior compartment and sole of the foot. Gives off the:
      • Sural nerve
        Provides sensation to the heel, back of the ankle, and the lateral malleolus.
      • Posterior tibial nerve

Femoral Nerve and Branches

The femoral nerve:

  • Arises from L2-4
    Largest branch.
  • Directly provides:
    • Motor supply to knee extensors
    • Sensory supply to anteromedial thigh
  • Divides into the:
    • Obturator nerve
      Formed from L2-4, and enters thigh through obturator foramen. Provides:
      • Motor supply to hip adductors
      • Sensory supply to a variable aspect of the medial thigh/knee
    • Lateral femoral cutaneous nerve
      Formed from L2-3, and runs on ventral surface of iliacus, beneath fascia iliaca. Provides:
      • Sensory supply to lateral aspect of thigh and knee
    • Saphenous nerve
      • Lies in the adductor canal, close to the femoral artery and vein
        Deep to sartorius and superficial to vastus medialis.
        • Typically not visible on ultrasound
          May be seen as a small, round, hyperechoic structure medial to the artery.
        • Typically 2-3cm deep
      • Entirely sensory nerve, providing sensation to medial lower leg and ankle.

Equipment

  • Standard nerve block tray
  • High-frequency linear probe or low-frequency curvilinear probe
  • 10-15cm needle
  • Local anaesthetic
    • 20-30ml of 0.75% ropivacaine

Technique

Indication determines level to be blocked:

  • Proximal
    Hip surgery.
  • Mid-thigh/bifurcation
    Knee/foot/ankle surgery.

Lateral Approach

  • Position
    Can be:
    • Semi-prone
      For all approaches.
    • Supine with hip and knee flexed
      For mid-distal approaches.
    • Oblique
      For mid-distal approaches.
  • Place probe on posterior thigh
  • Identify sciatic nerve
    • Hyperechoic
    • Large
    • Between gluteus maximus and adductor magnus
    • Posterior to femur
  • Follow course of nerve to confirm nerve identify and the intended level of block
  • Standard prep, drape, skin anaesthesia, etc
  • Perform block:
    • Whilst scanning the posterior thigh, insert the needle IP through the lateral thigh
    • Insert the needle anterior to the probe, such that the needle is heading directly towards the lateral edge of the nerve
      i.e. if the nerve is 5cm deep on ultrasound, insert the needle 5cm anterior to the probe position.
    • Aim to inject within the common epineurium of the sciatic nerve
      Separate blocks of the tibial and common peroneal nerves can be performed if required.
    • Aspirate and inject 1-2ml of LA to confirm location
      High pressure suggests intraneural injection - stop.
    • Inject remainder of LA

Anterior Approach

The anterior approach is:

  • Better for analgesia post TKR

  • Not well suited for catheter insertion
    Deep, and traverses several muscles.

  • Position patient supine

  • Use a curvilinear (low frequency) probe

  • Place probe ~8cm from inguinal crease on anteromedial thigh

  • Identify femoral vessels and femur

  • Identify sciatic nerve at the medial edge of the femur

    • Lies between the planes of adductor magnus and hamstrings
    • Probe may need to be angled medially

  • Perform block:
    OOP approach is typically easier than IP approach:

    • Identification of needle may be difficult
    • Walk needle off medial edge of femur
    • Aim to place needle adjacent to sciatic nerve in the plane between adductors and biceps femoris
    • Confirm needle position with:
      • Stimulator
      • Jiggling
      • Cautious aspiration and injection
        Don’t advance needle if injectate is not seen when OOP technique used.
    • Apirate and inject remainder of LA

Complications

  • General complications
    • Intravascular injection
    • LAST
    • Nerve injury
    • Infection
    • Failure
    • Allergy to LA

References

  1. Chuan A, Scott DM. Regional Anaesthesia: A Pocket Guide. 1st Ed. Oxford University Press
  2. NYSORA. Ultrasound Guided Popliteal Sciatic Block. Accessed September 2018.
  3. NYSORA. Sciatic Nerve Block: Anterior/Transgluteal/Subgluteal Approach. Accessed September 2018.