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.

Figure 31.4 Cadaver structures of the lateral aspect of the ankle illustrating anatomy pertinent to the ankle block. 1: Lateral malleolus; 2: Achilles’ tendon; 3: sural nerve; 4: short saphenous vein.

Surface anatomy

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.

Figure 31.5 Landmarks for the ankle block include medial and lateral malleoli: intermalleolar line: dorsalis pedis pulse: and Achilles’ tendon.

Sonoanatomy

Tibial nerve

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°.

Figure 31.6 A high frequency ultrasound transducer placed immediately above and behind the medial malleolus to locate the tibial nerve in the transverse (short axis) view.

Figure 31.7 Transverse ultrasound image of the sonoanatomy relevant to the tibial nerve block using a 10–15 MHz linear transducer. TP: tibialis posterior; FDL: flexor digitorum longus; PTA: posterior tibial artery; TN: tibial nerve; FHL: flexor hallucis longus; MM: medial malleolus.

Deep peroneal nerve

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 31.8 A high frequency ultrasound transducer is placed on the dorsum of the foot along the intermalleolar line to locate the dorsalis pedis artery in the transverse view.

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.

Superficial peroneal nerve

Position the patient supine and bolster the foot with a pillow to expose the anterior and lateral portion of the lower leg and foot. The lower limb is internally rotated for examination (Fig. 31.10). The superficial peroneal nerve can be identified sonographically between the peroneus longus and extensor digitorum longus muscles separated by the anterior crural intermuscular septum, using a 10–15 MHz transducer (Fig. 31.11). Distal to this, the nerve is superficial. The nerve pierces the fascia at a variable distance above the intermalleolar line (6–16 cm). The nerve typically divides below the level it pierces the fascia. Its detection is facilitated by the presence of fat around it.

Figure 31.10 Lower limb and transducer positioning for superficial peroneal nerve identification.

Figure 31.11 Transverse ultrasound image of the sonoanatomy relevant to the superficial peroneal nerve block using a 10 MHz linear transducer. SPN: superficial peroneal nerve; PL: peroneus longus; EDL: extensor digitorum longus.

Sural nerve

Position the patient supine and bolster the foot with a pillow to expose the anterior and lateral portion of the lower leg and foot. The lower limb is internally rotated for examination. A tourniquet is placed around the proximal tibia to distend the lesser saphenous vein. A transverse image of the lesser saphenous vein with a 10–15 MHz transducer above the lateral malleolus is obtained (Fig. 31.12). The sural nerve is usually only identified after injection of local anesthetic around the lesser saphenous vein (Fig. 31.13).

Figure 31.12 A high frequency ultrasound transducer is placed in the transverse orientation above and behind the lateral malleolus for sural nerve identification.

Figure 31.13 The sural nerve is identified in proximity to the lesser saphenous vein. SN: sural nerve; AT: Achilles’ tendon; LSV: lesser saphenous vein.

Saphenous nerve

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 for examination. A tourniquet is placed around the proximal tibia to distend the greater saphenous vein. A transverse image of the greater saphenous vein with a 10–15 MHz transducer above the medial malleolus is obtained (Fig. 31.14). The saphenous nerve is usually only identified after injection of local anesthetic around the greater saphenous vein (Fig. 31.15).

Figure 31.14 A high frequency ultrasound transducer is placed in the transverse orientation above the medial malleolus for saphenous nerve identification.

Figure 31.15 The saphenous nerve is identified in proximity to the greater saphenous vein. SN: saphenous nerve; GSV: greater saphenous vein: T: tibia.

Technique

Landmark-based approach

As for all regional anesthetic procedures, after checking that emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

Tibial nerve block

The block is performed by needle insertion on a line between the medial malleolus and Achilles’ tendon (Fig. 31.16), just posterior to the tibial artery. If paresthesia is elicited, the needle should be withdrawn slightly and 5 mL of local anesthetic be injected. If paresthesia is not reported, the needle is advanced to the bone and withdrawn slightly, and 10 mL of local anesthetic injected.

Figure 31.16 Tibial nerve block technique. The needle is inserted adjacent to the Achilles’ tendon and toward the superior aspect of the medial malleolus. On bony contact, the needle is withdrawn and injection is made.

Sural nerve block

The sural nerve block is performed by needle insertion on a line between the lateral malleolus and the Achilles’ tendon (Fig. 31.20). If paresthesia is reported, the needle is withdrawn slightly and injection made. Otherwise, the needle is advanced until bony contact is made, withdrawn slightly, and the injection made.

Figure 31.20 Sural nerve block technique. The needle is inserted adjacent to the Achilles’ tendon and toward the superior aspect of the lateral malleolus. On bony contact, the needle is withdrawn and injection is made.

The ankle block using the above technique uses three injection points. Alternately, all five nerves can be blocked individually.

Ultrasound-guided approach

The ultrasound machine and block tray should be placed in positions which allow the operator to simultaneously scan the patient and take items from the block tray with minimal movement. This setup may take some forethought but is a worthwhile exercise, and will facilitate successful regional anesthesia.

The operator stands at the patient’s feet, with the ultrasound machine on the side to be blocked, (Fig 31.21). The skin is disinfected with antiseptic solution and draped. A sterile sheath (CIVCO Medical Instruments, Kalona, IA, USA) is applied over the ultrasound transducer with sterile ultrasound gel (Aquasonic, Parker Laboratories, Fairfield, NJ, USA). Another layer of sterile gel is placed between the sterile sheath and the skin. The ultrasound screen should be made to look like the scanning field. That is, the right side of the screen represents the right side of the field. Adjustable ultrasound variables such as scanning mode, depth of field, and gain are optimized.

Figure 31.21 Global view of the block field for the ultrasound-guided ankle block.

Tibial nerve block

Both In Plane (IP) and Out of Plane (OOP) approaches can be used. The IP approach is commonly used for single-shot injection. Insert a 4–5 cm 22–25 G needle in-line with the ultrasound transducer (Fig. 31.22). Aim to place the needle tip on each side of the tibial nerve without puncturing the posterior tibial artery. 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’ (Fig. 31.23).

Figure 31.22 Transducer and needle positioning during ultrasound-guided tibial nerve block. Note the needle orientation is perpendicular to the ultrasound beam.

Figure 31.23 Local anesthetic surrounding the tibial nerve at the ankle. TN: tibial nerve; FHL: flexor hallucis longus; MM: medial malleolus; LA: local anesthetic.

Deep peroneal nerve block

A 25-G 2.5-cm needle can be inserted using the OOP approach (Fig. 31.24). 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 (Fig. 31.25).