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Figure 1. Cutaneous innervation of the foot (a) plantar surface (b) dorsal surface.

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Figure 2. The relationship of the tibial nerve to the tendons at the level of the medial malleolus.

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Figure 3. Origin and course of (a) tibial and sural nerve and (b) superficial and deep peroneal nerve.

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Procedure

The following paragraphs provide details of the performance of the 5 nerves required for a complete ankle block. Clinical pearls are summarized at the end of article.

Tibial nerve (L4, L5, S1-3)

The patient is semi-recumbent or supine, and the ipsilateral hip is externally rotated, as shown (Figure 4a). A linear transducer is placed transversely, obtaining a scan of the tibial nerve (hyperechoic oval) proximal to medial malleolus, showing the tendons of the tibialis anterior and flexor digitorum profundus anterior to the nerve and gastrocnemius-soleus posterior to it (Figure 4b). The posterior tibial artery and the tendon of flexor hallucis longus lie deep to the nerve, often with two small veins on either side of the artery. The needle is introduced in-plane (or out-of-plane), proximal to the medial malleolus to approach near the tibial nerve (Figure 4c). Entry into the fascial sheath containing the nerve and subsequent local anesthetic injection (5-10 ml 0.5% ropivacaine) produces a characteristic circumferential spread around the tibial nerve (Figure 4d).  

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Figure 4. (a) Patient position for tibial nerve scanning at the medial malleolus; (b) Ultrasound scan of the structures slightly proximal to the medial malleolus; (c) Transverse placement of transducer and in-plane needling proximal to medial malleolus; (d) Ultrasound appearance of the tibial nerve post-injection. (TA - tibialis anterior, FDP - flexor digitorum profundus, FHL - flexor hallucis longus. Yellow arrowhead denotes tibial nerve; red arrowhead denotes posterior tibial artery; white arrowhead denotes local anesthetic spread around the nerve.)

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Deep peroneal nerve (deep fibular nerve) (L4, L5, S1)

The leg is extended and placed on a bolster in a neutral position (Figure 5a). A linear transducer is placed transversely just above the intermalleolar line to identify the anterior tibial artery, which is seen as a pulsatile anechoic structure on the surface of the tibia (hyperechoic white curve) [Figure 5b]. The deep peroneal nerve is identified as a hyperechoic structure medial to the artery (Figure 5c). The needle is introduced in-plane from medial to lateral (or out-of-plane) to approach adjacent to the nerve (Figure 5d). Local anesthetic (2-4 ml 0.5% ropivacaine) is injected around the nerve.

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Figure 5. (a) Patient position for deep peroneal nerve block; (b) Ultrasound scan of the structures slightly proximal to the intermalleolar line; (c) Transverse placement of transducer and in-plane needling proximal to the intermalleolar line; (d) In-plane injection to block the nerve. (Yellow arrowhead denotes deep peroneal nerve; red arrowhead denotes anterior tibial artery; white arrowhead denotes the needle.)

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Optional advanced (small nerves) ultrasound-guided blocks

The next section describes the ultrasound-guided techniques of three superficial nerves (superficial peroneal, saphenous, and sural nerves).

Superficial peroneal nerve (superficial fibular nerve) (L5, S1, S2)

The patient is positioned supine or semi-lateral with the hip rotated internally to expose the lateral aspect of the leg (Figure 6a). A linear transducer is placed transversely just above the lateral malleolus to identify the fibula. The superficial peroneal nerve is situated superficially between the peroneus brevis (PB) and the extensor digitorum longus (EDL) as a triangular hyperechoic shadow (Figure 6b) under the crural fascia (an oval hyperechoic structure).⁹ The EDL is anterior to the nerve, while the PB is posterior to the nerve. The needle is introduced in-plane (or out-of-plane), from anterior to posterior direction (Figure 6c), to approach adjacent to the superficial peroneal nerve. Local anesthetic (5-6 ml 0.5% ropivacaine) is sufficient for an effective block (Figure 6d).

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Figure 6. (a) Patient position for superficial peroneal nerve block;(b) Ultrasound scan of the structures slightly proximal to the intermalleolar line; (c) Transverse placement of transducer and in-plane needling proximal to the intermalleolar line (d) In-plane injection to block the nerve. (PB - peroneus brevis, EDL - extensor digitorum longus, SPN - superficial peroneal nerve. Yellow arrowhead denotes superficial peroneal nerve.)

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Sural nerve (S1, S2)

The patient is positioned with the knee flexed and internal rotation at the hip, making the posterolateral aspect of the leg accessible. A linear transducer is placed transversely above the lateral malleolus, identifying the peroneus brevis anteriorly and the Achille's tendon posteriorly. The sural nerve (hyperechoic oval) is located anterior to the Achilles tendon (Figure 7a). The needle is introduced in-plane from anterior to posterior (or out-of-plane), proximal to the lateral malleolus (Figure 7b) to advance adjacent to the nerve and local anesthetic (2-4 ml 0.5% ropivacaine) is injected (Figure 7c).

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Figure 7. (a) Ultrasound scan of the sural nerve (yellow arrowhead) and short saphenous vein (blue arrowhead) just proximal to the lateral malleolus; (b) Patient positioning, transducer orientation, and needling; (c) In-plane needling to block sural nerve (yellow arrowhead) block.

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Saphenous nerve (L2, L3, L4)

The hip is externally rotated. A linear transducer is placed transversely above the medial malleolus to identify the great saphenous vein with the saphenous nerve anterior to it (hyperechoic oval) (Figure 8a). Application of a tourniquet would enhance the appearance of the vein. The needle is introduced in-plane from posterior to anterior (or out-of-plane), proximal to the medial malleolus to approach the saphenous nerve (Figure 8b).¹⁰ Local anesthetic (2-4 ml 0.5% ropivacaine) is injected (Figure 8c).

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Figure 8. (a) Ultrasound scan of the saphenous vein and nerve proximal to the medial malleolus; (b) In-plane needling for saphenous nerve block; (c) Ultrasound-guided in-plane approach to the saphenous nerve block. (Yellow arrowhead denotes saphenous nerve; blue arrowhead indicates excellent saphenous vein.)

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Alternate Approach to Block Superficial Nerves

Ultrasound-guidance is most beneficial for the tibial and deep peroneal nerve blocks. Should one wish to use the landmark technique for the other nerves, the superficial peroneal and saphenous nerves can be anesthetized by subcutaneous infiltration of 10-15 ml local anesthetic anteriorly between medial and lateral malleoli. This can be achieved by using a single anterior injection entry point. Similarly, 4-5 ml of subcutaneous infiltration between lateral malleolus and Achilles tendon provides adequate sural nerve coverage.

Please note that ankle block does not cover the tourniquet pain around the thigh or the calf; therefore, it is only suitable as an analgesic block and perhaps surgical anesthesia for surgeries of short duration.

 

Conclusion

Ultrasound-guided ankle block enhances the block's reliability, safety, and overall success rate. It also helps to minimize the local anesthetic volume required, thus permitting bilateral ankle block performance. It can easily be performed in most patients for selected surgeries and allows for early ambulation while providing pain relief.

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Clinical Pearls

• Compared to a landmark-based ankle block, an ultrasound-guided ankle block is usually done more proximally to the ankle joint as the malleolar prominences hinder appropriate transducer-skin contact.
• We recommend starting with the tibial nerve, as this is a larger nerve and benefits from a longer "soak" time. We advocate a proximal block of the tibial nerve because a distal block may lead to a medial calcaneal branch's sparing. Flexing and extending the great toe generates a “sea-saw” movement of the nerve on the flexor hallucis longus, helping to identify it.
• Care should be exercised not to inject deep into the periosteum as subperiosteal injections are painful. An out-of-plane approach may be easier to perform.
• The superficial peroneal nerve can be blocked either shallow (distal-leg) or deep (mid-leg) to the crural fascia. However, a distal injection can result in missing one of the branches (intermediate or medial branch).¹¹ Therefore, we favor a more proximal injection before the division of the nerve.
• The sural nerve may be omitted if the surgical site is confined to the medial of the foot. If the nerve cannot be easily identified, a perivenous injection may be performed within the fascial compartment. An out-of-plane approach may be easier to perform.
• The saphenous nerve divides into several articular branches proximal to the ankle. Therefore, while a block at or above ankle sufficiently blocks the distal foot, a proximal block (at distal tibia) is needed to provide analgesia for ankle joint surgery itself.¹² Application of a tourniquet at mid-calf can help to identify the great saphenous vein in cases where it is not easily visible.
• Keep it simple. Ultrasound-guidance is most beneficial for the tibial and deep peroneal nerve blockage. Skin infiltration on the anterior aspect of the intermalleolar line will block saphenous and superficial peroneal nerves. Infiltration between the lateral malleolus and Achilles tendon will block the sural nerve.