Ankle Block

ANATOMY

Terminal branches of the sciatic nerve in the ankle region are:

1. tibial nerve
2. superficial peroneal nerve
3. deep peroneal nerve
4. sural nerve

The tibial nerve is most prominent branch and can be blocked easily under ultrasound at the level of the medial malleolus. This nerve is often located posterior and lateral to the posterior tibial artery and medial to the flexor hallucis longus tendon. Anterior to the posterior tibial artery lie the tibialis posterior and flexor digitorum longus tendons.

The superficial and deep peroneal nerves as well as the sural nerve are superficial in the subcutaneous tissue plane. The small deep peroneal nerve may be difficult to locate. This nerve is expected to lie adjacent to the anterior tibial vessels (above the ankle) and the dorsalis pedis artery (lower down at the ankle).

Transverse View of the Ankle

SCANNING TECHNIQUE

• Position the patient supine and bolster the foot with a pillow to expose the anterior and medial portion of the lower leg and foot.
• After skin and transducer preparation, place a 10-15 MHz transducer immediately above the medial malleolus to locate the tibial nerve in the transverse (short axis) view.
• It is also easy to visualize this nerve longitudinally (long axis).
• Optimize machine imaging capability. Select the appropriate depth of field (usually within 1-2 cm), focus range (usually within 1-2 cm) and gain.

A 12 MHz hockey stick transducer over the left medial malleolus

ANATOMICAL CORRELATION

Transverse View of the Tibial Nerve at the Ankle

Arrowhead = tibial nerve   FDL = flexor digitorum longus tendon   FHL = flexor hallucis longus muscle   MM = medial malleolus   PTA = posterior tibial artery   TP = tibialis posterior tendon

NERVE LOCALIZATION

Tibial Nerve

• Perform a systematic anatomical survey in the medial aspect of the ankle.
• The bony medial malleolus is easily identified (bony shadow).
• Move the transducer slightly posteriorly to identify the tibialis posterior and flexor digitorum longus tendons. Both tendons are found within the flexor retinaculum of the ankle. They display a sliding movement with ankle flexion and are often hyperechoic.
• Then identify the pulsatile posterior tibial artery (Doppler use is optional).
• The tibial nerve at the ankle is often round to oval with a honeycomb appearance. It is expected to lie posterior to the posterior tibial artery.
• Trace the tibial nerve proximally. The nerve is larger and is easier to identify more cephalad in the leg. It is also easy to image the nerve longitudinally by rotating the transducer 90 degrees.

Transverse View   Arrowheads = tibial nerve   PTA = posterior tibial artery

Longitudinal View   Arrowheads = tibial nerve

NEEDLE INSERTION APPROACH

• Ultrasound guided ankle block is considered a BASIC skill level block because this is a superficial block.
• Both In Plane (IP) and Out of Plane (OOP) approaches can be used. The IP approach is commonly used for single shot injection.

IN PLANE NEEDLE INSERTION APPROACH

• With the patient lying supine and the leg bolstered by a pillow, insert a 4-5 cm 22-25 G needle inline with the ultrasound transducer as seen in picture below.

Aim to place the needle tip on each side of the tibial nerve without puncturing the posterior tibial artery.

• Nerve stimulation is usually not necessary.

LOCAL ANESTHETIC INJECTION

• Once satisfied with the needle position, inject 5-8 mL of local anesthetic.
• Observe local anesthetic injection in real time to judge adequacy of spread. Aim to see circumferential spread of hypoechoic local anesthetic solution around the nerve “donut sign”.
• Circumferential spread usually results in a complete block.
• If local anesthetic spread is deemed suboptimal, move the needle to either side of the nerve before completing the second half of the injection.
• Scan the nerve in the transverse and longitudinal planes proximally and distally to check the extent of local anesthetic spread.

Transverse View   Arrowheads = tibial nerve   LA = local anesthetic

Longitudinal View   Arrowheads = tibial nerve   LA = local anesthetic

• The deep peroneal nerve is a superficial branch that is located adjacent to the dorsalis pedis artery at the ankle region.
• After skin and transducer preparation, place a 10-15 MHz transducer on the dorsum of the foot along the intermalleolar line to locate the dorsalis pedis artery in the transverse (short axis) view.

Aim to find the predominantly hypoechoic deep peroneal nerve lateral to the dorsalis pedis artery and the extensor hallucis longus tendon. This nerve is small thus visualization can be difficult.

• A 25 G 2.5 mm needle can be inserted using the out of plane approach.

If the deep peroneal nerve is clearly visualized, inject 2-3 mL of local anesthetic on each side of the nerve.

• If the nerve is not clearly visualized, inject 2-3 mL of local anesthetic on each side of the artery in the subcutaneous plane.
• Observe local anesthetic spread around the nerve circumferentially in the subcutaneous plane above bone and at approximately the same level as the artery.

Pre-injection   Arrowhead = deep peroneal nerve   DPA = dorsalis pedis artery

Post-injection   Arrowhead = deep peroneal nerve   DPA = dorsalis pedis artery   LA = local anesthetic

VIDEO GALLERY

Ankle Block (In Plane Approach)

Lateral Decubitus Position Ankle Block (In Plane Approach)

SELECTED REFERENCES

• Schabort D, Boon JM, Becker PJ, Meiring JH. Easily Identifiable Bony Landmarks As an Aid in Targeted Regional Ankle Blockade. Clinical Anatomy 2005;18:518–526