Ankle

Ankle blocks are commonly used for surgery involving the foot below the ankle.  Five nerves provide sensation to the foot.  The posterior tibial, deep peroneal, superficial peroneal, and sural nerves all arise from the sciatic nerve, while the saphenous nerve is a terminal branch of the femoral nerve.  The posterior posterior tibial, deep peroneal, sural, and saphenous nerves can all be blocked in the ankle with ultrasound guidance.  Performing this block can be challenging as it requires identification and injection of local anesthetic at multiple sites.  Additionally, each of these nerves is located close to either arteries or veins, increasing the potential for vascular injection. All injections should be accomplished using a short-axis (cross-section) view and an in-plane needle technique.  As with all ultrasound-guided blocks, observing circumferential spread around the nerve with ultrasound while injecting will give the best results for a complete block.

The posterior tibial nerve is the most prominent and easily identifiable nerve with in the ankle.  It provides sensation to the heel, medial sole, and part of the lateral sole.  The patient is placed in the supine position, with the lower extremity elevated.  The transducer is placed just above the medial malleolus on the medial aspect of the leg.  The nerve will appear as a hyperechoic circle just posterior to the posterior tibial artery (see figure below).  As you scan cephalad, the nerve will become larger, and is often best imaged with the transducer perpendicular (90 degrees) to the skin.  A high frequency, small-footprint transducer, such as a “hockey stick” is ideal for performing this block.  Once all anatomical structures are identified and the US image is optimized, a 22-25 gauge needle is inserted either in-plane or out-of-plane, and 5-8 cc’s of local anesthetic are injected so that circumferential spread around the nerve is achieved.

The image below shows circumferential spread of local anesthetic following a posterior tibial nerve block of the right ankle.  Color Doppler shows the posterior tibial artery (white arrow) superior to the nerve.  Circumferential spread of local anesthetic is noted around the posterior tibial nerve (yellow arrow).

The deep peroneal nerve is a continuation of the common peroneal nerve and runs along the anterior leg, entering the ankle between the flexor halucis longus and extensor digitorum longus tendons.  It supplies sensation to the medial half of the dorsal foot; in particular the first and second digits.  With the patient in the supine position, the deep peroneal can be identified by placing a high frequency transducer 1-2 cm above the superior border of the medial malleolus and identifying the doralis pedis artery (below left).  The nerve will appear as a small hyperechoic circle just lateral to the artery.  After optimizing the ultrasound image, a 25 gauge needle is inserted from a lateral to medial aspect, using an in-plane technique.  Under ultrasound-guidance 5-8 mls of local anesthetic are injected to achieve circumferential spread around the nerve.

The saphenous nerve is the terminal branch of the femoral nerve supplying sensation to the medial aspect of the lower leg and ankle.  It is often blocked in conjunction with the sciatic nerve at the level of the popliteal for surgeries of the ankle or foot.  Distal to the knee, the saphenous nerve is within close proximity to the saphenous vein, which generally courses along the medial aspect of the tibia.  With the patient in the supine position, a high frequency transducer is placed on the medial aspect of the lower tibia (below right).  When scanning, care must be taken not to apply to much pressure with the transducer as it can occlude the saphenous vein, eliminating the major reference point for identifying the nerve.  Using color Doppler when performing this block may help to visualize the saphenous vein.  Once the vein is located, the nerve will appear as a hyperechoic circle medial to the vein.

The image below shows an in-plane approach to a saphenous nerve block.  The needle, indicated by the white arrows, is placed just below the saphenous nerve (yellow arrow).  A tourniquet was applied to the proximal calf prior to the procedure to dilate the saphenous nerve (SV), making it more visible.

The sural nerve is the most difficult of the four nerves to identify with ultrasound, because of its small size and relation to the lesser saphenous  nerve.  It is formed by the union of branches of the tibial and common peroneal nerves, coursing posteriorly in the calf and traveling to the lateral malleolus.  The sural nerve provides sensation to the lateral and posterior part of the lower leg as well as cutaneous sensation to the lateral portion of the foot and fifth digit.  With the patient in the supine position and the foot bolstered by a pillow, a high frequency transducer is placed proximal to the lateral malleolus and the lesser saphenous vein is identified (below left).  A tourniquet may be placed on the proximal calf to distend the vein.  The imaged is optimized, and 5 mls of local anesthetic is injected to achieve circumferential spread.

The superficial peroneal nerve provides cutaneous sensation to the dorsum of the foot and five toes.  It is a branch of the common peroneal nerve and courses along the lateral tibia towards the ankle just lateral to the extensor digitorum tendon.  Because it is not located next to any vascular structures, there are no easily identifable landmarks making it almost impossible to see with ultrasound.  It is most commonly blocked by infiltrating local anesthetic at the malleolar level along the lateral aspect of the foot.  In the image below of a lateral ankle open-reduction internal-fixation procedure, the superficial peroneal nerve has been isolated (yellow arrows).  Note its course anterior to the lateral malleolus.

Because of their small nature, nerves in the ankle are best located by starting with a more identifiable structure such as an artery or vein.  Using color-flow Doppler can often help identify these vessels.  While arteries tend to be readily seen because of their muscular make up and pulsatile flow, veins are easily compressable and often not seen at this level.  Using a tourniquet proximal to the scanning area may help dilate the veins making them more visible on ultrasound.  Additionally, using distal augmentation, the process of squeezing the extremity distal to the scanning area and forcing flow through the veins, will increase the visibility of the vessels while using color-flow Doppler.  The two images below are of the saphenous vein and nerve.  The image on the left shows the saphenous vein dilated from the use of a tourniquet, while the image on the right shows flow through saphenous vein while performing distal augmentation.

 It is important to remember that aggressive injections at multiple locations in the ankle increase the potential of hydrostatic damage to the small nerves, particularly those located within closed ligamentous spaces.


References

Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 4th ed. New York, NY: Lange Medical Books/McGraw-Hill; 2006:352-353.

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:177-181.

Gray AT. Atlas of ultrasound-guided regional anesthesia. Philadelphia, PA. Saunders, Elsevier; 2010:180-197.

Bigeleisen PE, ed, Orebaugh SL, Moayeri N, et al. Ultrasound-guided regional anesthesia ad pain medicine. Baltimore, MD. Lippincott Williams & Wilkins; 2010:140-143.

Redborg K E, Sites B D, Chinn C D, Gallagher J D, Ball P A, Antonakakis J G, Beach M L. Ultrasound improves the success rate of a sural nerve block at the ankle. Regional Anesthesia and Pain Medicine 2009;34(1):24-8.

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