Ankle Block

Block of the 3 superficial and 2 deep nerves of the ankle:

Indications

  • Minor surgery below ankle
  • Avoids foot drop

Contraindications

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

Technique

The ankle can be blocked either at the midtarsal level (more reliable, but only covers the forefoot) or at the perimalleolar level (covers the entire foot).

For the perimalleolar approach:

The ring covers the three superficial nerves, blocking the saphenous nerve superior to the medial malleolus, the superficial peroneal nerve along the anterior intermalleolar line, and sural nerve posterior to the lateral malleolus.

  • Create a subcutaneous ring of 10-15mL of local anaesthetic:
    • From anterior to the achilles tendon at the level of the medial malleolus
    • Around the front of the ankle
    • To halfway between the lateral malleolus and the achilles tendon
  • Insert needle just lateral to the Extensor Hallucis Longus tendon, angled perpendicular to the tibia, advance until bone, withdraw slightly and inject 5mL of local anaesthetic to cover the deep peroneal nerve
  • Insert needle just posterior to the posterior tibial artery (or midway between the achilles tendon and the medial malleolus), angled 45° towards the tibia, advance until bone, withdraw and inject 2-5mL of local anaesthetic

Complications