CHAPTER 31 Ankle block
The ankle and foot are innervated by five nerves. One, the saphenous nerve, is the terminal branch of the femoral nerve, whereas the remaining four are branches of the sciatic nerve. These are the tibial nerve, the sural nerve, and the superficial and deep peroneal nerves.
The tibial nerve runs deep to the flexor retinaculum and posterior to the posterior tibial vein and artery, between the Achilles’ tendon and medial malleolus (Figs 31.1 and 31.2). It divides into medial and lateral plantar nerves providing sensory innervation to the medial side of the sole of the foot and heel. The tibial nerve provides motor supply to the intrinsic muscles of the foot. It is the largest nerve at the ankle, requiring the longest block onset, and thus should be blocked first.
Figure 31.1 Cadaver structures of the medial aspect of the ankle illustrating anatomy pertinent to the ankle block. 1: medial malleolus; 2: Achilles’ tendon; 3: tendon of tibialis posterior; 4: tendon of flexor digitorum longus; 5: posterior tibial artery and vein; 6: tibial nerve.
Figure 31.2 Axial T1-weighted MR image showing relevant anatomy of ankle. 1: site of deep peroneal nerve; 2: superficial peroneal nerve; 3: tibia; 4: fibula; 5: posterior tibial artery; 6: tibial nerve; 7: sural nerve; 8: Achilles’ tendon.
The deep peroneal nerve runs deep to the extensor retinaculum and superficial to the tibia, lateral to the anterior tibial artery. It is bounded medially by the anterior tibial artery and tendon of the extensor hallucis longus muscle, and laterally by the extensors of the second toe. It provides sensory innervation to the tarsal and metatarsal joints and the first interdigital space.
The superficial peroneal nerve travels distally with the peroneus brevis muscle, becoming superficial above the lateral malleolus, and runs over the dorsum of the foot, to which it provides sensory innervation (Figs 31.2 and 31.3).
Figure 31.3 Cadaver structures of the ventral aspect of the ankle illustrating anatomy pertinent to the ankle block. 1: medial malleolus; 2: tendon of tibialis anterior; 3: tendon of extensor hallucis longus; 4: tendon of extensor digitorum longus; 5: anterior tibial artery; 6: superficial peroneal nerve branches. The deep peroneal nerve is deep to the fascia and cannot be seen here.
The saphenous nerve runs superficially with the great saphenous vein. It divides into terminal branches at the ankle. It provides sensory innervation to the medial aspect of the ankle and dorsum of the foot in a wedge shape toward the great toe.
The sural nerve runs superficially with the small saphenous vein and lies subcutaneously between the lateral malleolus and Achilles, tendon (Figs 31.2 and 31.4). It provides sensory innervation to the lateral aspect of the ankle and foot.
Important bony structures for the ankle block include the medial and lateral malleoli and the calcaneum. Other landmarks include the Achilles’ tendon, and on the ventral aspect of the ankle, the anterior tibial artery pulse and extensor hallucis longus tendon (Fig. 31.5). These tendons can be accentuated if the patient dorsiflexes the foot against resistance. A single needle insertion site at the midpoint of the intermalleolar line on the ventral aspect of the ankle is used for block of the superficial and deep peroneal nerves and saphenous nerve. Needle insertion for sural and tibial block is adjacent to the Achilles tendon, at the level of the superior aspect of the medial and lateral malleoli, respectively.
Position the patient supine and bolster the foot with a pillow to expose the anterior and medial portion of the lower leg and foot. The lower limb is externally rotated. Place a 10–15 MHz transducer immediately above the medial malleolus to locate the tibial nerve in the transverse (short axis) view (Fig. 31.6). 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 (Fig. 31.7). 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°.
Position the patient supine and bolster the foot with a pillow to expose the anterior and medial portion of the lower leg and foot. The lower limb is in the neutral position. The small deep peroneal nerve may be difficult to locate. This nerve lies adjacent to the anterior tibial vessels (above the ankle) and the dorsalis pedis at the ankle region. A 10–15 MHz transducer is placed on the dorsum of the foot along the intermalleolar line to locate the dorsalis pedis artery in the transverse view (Fig. 31.8) The predominantly hypoechoic deep peroneal nerve is found lateral to the dorsalis pedis artery and the extensor hallucis longus tendon (Fig. 31.9).
Figure 31.9 Transverse ultrasound image of the sonoanatomy relevant to the deep peroneal nerve block using a 10–15 MHz linear transducer. DPN: deep peroneal nerve; T: tibia; DPA: dorsalis pedis artery.