Distal Femoral Triangle/Adductor Canal Block

This is a block of the femoral nerve in the mid-thigh, which aims to achieve analgesia to the anterior knee and medial lower leg, whilst sparing the nerve to vastus medialis and therefore motor function of the quadriceps.

Indications

Anaesthesia for lower leg surgery * May facilitate superficial procedures * Usually supplements a sciatic block when performing foot and ankle procedures

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
  • 5-10cm 22G short-bevel needle
  • 5-10ml local anaesthetic

Technique

  • Position supine
  • Place US probe over sartorius in the mid-thigh
  • Identify sartorious
    Triangular, ‘hull-shaped’ muscle, with femoral artery running along the ‘keel’. Saphenous nerve runs with the artery in this canal.
  • Identify femoral artery
    Can scan caudally from groin if artery is not immediately identifiable.
  • Standard prep, drape, skin anaesthesia, etc
  • Perform block IP or OOP:
    • Aim to place needle tip just medial to femoral artery, deep to sartorius
    • Ensure good deposition of LA below sartorious, forming ring around artery

Complications

  • General complications
    • Intravascular injection
    • LAST
    • Nerve injury
    • Infection
    • Failure
    • Allergy to LA
  • Specific complications
    • Motor block to vastus medialis
      Avoid unsupported ambulation immediately post.

References

  1. Chuan A, Scott DM. Regional Anaesthesia: A Pocket Guide. 1st Ed. Oxford University Press.
  2. NYSORA. Ultrasound Guided Saphenous Block. Accessed September 2018.