The superficial peroneal nerve is a branch of the common peroneal nerve that emerges from the neck of the fibula between the extensor digitorum longus and peroneal muscles to enter the subcutaneous tissue of the lateral leg. When emerging from the fibular neck, the superficial peroneal nerve most commonly lies in the lateral compartment of the leg. The superficial peroneal nerve ascends along the anterior intermuscular septum to pierce the fascia lata at the juncture of the middle and lower thirds of the leg. The nerve usually divides into its medial and lateral branches once in the subcutaneous tissue.^1,2

Some elect to infiltrate local anesthetic over the dorsum of the foot for superficial peroneal nerve block and reserve ultrasound for the deeper nerves of the ankle block.³ However, superficial peroneal nerve block in the leg can be useful when edema or infection contraindicates more distal ankle block. Proximal ultrasound-guided superficial peroneal nerve block (along with sural block) can provide surgical anesthesia for hardware removal from the lateral ankle in weight-bearing patients. In addition, ultrasound-guided superficial block in the leg is less painful than subcutaneous infiltration across the dorsum of the foot for more distal block and does not pierce the extensor tendons of the foot.

Suggested Technique

The superficial peroneal nerve can be difficult to image within the subcutaneous tissue of the distal lateral leg. By sliding the transducer along the known course of the nerve, the nerve can be identified as it emerges from the muscular compartment. An in-plane approach from either the anteromedial or posterolateral side of the leg can then be used for needle tip placement adjacent to the nerve.

More proximally, the peroneal nerves are best imaged in the prone position with the knee flexed using a pillow under the ankle. Alternatively, the leg can be elevated and internally rotated with the patient in supine position. When scanning from distal to proximal, the superficial peroneal nerve tracks along the fascia between the anterior and lateral compartments of the leg (like going down a ski jump), until it meets the acute edge of the bone of the fibula. The anterior border of the fibula is formed by the insertion of the anterior intermuscular septum. Because the nerve tracks along the anterior intermuscular septum, the anterior border of the fibula points toward the superficial peroneal nerve.

Because the superficial peroneal nerve most often lies in the lateral compartment side of the intermuscular septum, it is important for the block needle tip to puncture the septum when approaching from the anterior side. The block needle tip crosses the anterior intermuscular septum under the nerve, with injection of local anesthetic as the needle is pulled back. If the nerve is blocked close to the surface of the fascia lata, its deeper motor branches to the peroneus longus and peroneus brevis can be spared.

The key to completely surrounding the superficial peroneal nerve with local anesthetic is passing the needle over the nerve (between the superficial peroneal nerve and fascia lata). This can usually be achieved by rotating the needle bevel to face away from the transducer or performing the block proximally where there is space between the fascia lata and the nerve.

Key Points

Superficial Peroneal Nerve Block	The Essentials
Anatomy	The SPN emerges between the EDL and PL along the intermuscular septum.
	The fibular ridge points to the SPN.
	The fibular artery often lies adjacent to the SPN (zoom).
Positioning	Supine
Operator	Standing on the side of the patient
Display	Across the table
Transducer	High-frequency linear, 23- to 38-mm footprint
Initial depth setting	20 mm
Needle	25 gauge, 38 mm in length
Anatomic location	Begin imaging the lateral leg 10 cm proximal to the malleolus.
	This is where the SPN pierces the FL.
Approach	SAX view of SPN, in-plane either direction
	Place the needle tip between the FL and SPN.
Sonographic assessment	The injection should track proximally and descend along with the SPN.
Anatomic variation	The SPN can divide proximally in the leg.

EDL, Extensor digitorum longus; FL, fascia lata; PL, peroneus longus; SAX, short axis; SPN, superficial peroneal nerve.

Clinical Pearls

• Begin by scanning anterior and proximal to the lateral malleolus with the probe perpendicular to the known course of the superficial peroneal nerve.

• The superficial peroneal nerve emerges in the groove between the extensor digitorum longus and peroneal muscles. The nerve is most visible where it emerges to lie against the fascia lata (10 to 16 cm proximal to the lateral malleolus, 7 cm posterolateral to the tibial crest).

• When the superficial peroneal nerve is divided by the intermuscular septum, it continues distally as the medial and intermediate dorsal cutaneous nerves of the foot.

• When two adjacent nerves are identified within the field of view, confirm that they join proximally.

• The diameters of the superficial peroneal nerve and its branches are approximately 3 and 2 mm, respectively. The superficial peroneal nerve is often accompanied by a small fibular artery and vein.

• Conditions that contraindicate ankle block (e.g., edema and infection) are not common in outpatients. Popliteal blocks are much more commonly used than more distal blocks in these settings because patient disposition is not an issue.

FIGURE 48-1 Anatomic course of the superficial peroneal nerve in the leg. EDL, Extensor digitorum longus; PL, peroneus longus.

(Adapted from Canella C, Demondion X, Guillin R, et al. Anatomic study of the superficial peroneal nerve using sonography. AJR Am J Roentgenol 2009;193(1):174–9.)

FIGURE 48-2 External photograph showing the approach to superficial peroneal nerve block in the distal leg. An in-plane approach from the posterolateral aspect of the leg is shown (A). The corresponding sonogram shows the nerve within the subcutaneous tissue of the lateral leg (B). More proximal sonograms show the nerve emerging from the muscular compartment (C and D).

FIGURE 48-3 The superficial peroneal nerve arises from the common peroneal nerve near the neck of the fibula in the proximal leg. The edge of the bone is sharp and points to the superficial peroneal nerve because the nerve emerges within or adjacent to the intermuscular septum.

FIGURE 48-4 Image sequence showing superficial peroneal nerve block in the distal leg. An in-plane approach is demonstrated where the needle tip is placed adjacent to the nerve (A). After injection, local anesthetic is distributed around the superficial peroneal nerve (B).

FIGURE 48-5 Image sequence showing superficial peroneal nerve dividing in the distal leg. The superficial peroneal nerve is shown proximally (A) and distally (B). After injection, local anesthetic is distributed along the branches of the superficial peroneal nerve.