• Jerry D. VIoka, MD

       INTRODUCTION

Blocks of the lateral femoral cutaneous, posterior femoral cutaneous, saphenous, sural, and superficial peroneal nerves are useful anesthetic techniques for a variety of superficial surgical procedures. These blocks are simple to learn Blocks of the lateral femoral cutaneous, posterior femoral and perform. They are essentially devoid of complications cutaneous, saphenous, sural, and superficial peroneal nerves and can nicely complement major conduction blocks of the lower extremity.^{1, 2} The combination of their applicability and simplicity mandate that these blocks should be in the armamentarium of every anesthesiologist.

Figure 40–1. Cutaneous innervation of the lower extremity, anterior view.

Indications & Contraindications

Cutaneous nerve blocks of the lower extremity can be used to anesthetize patients for a variety of surgical procedures. The lateral femoral cutaneous nerve block has been used to provide anesthesia for pediatric patients undergoing muscle biopsy³ and to provide analgesia after femoral neck surgery in older patients.^{4, 5} The posterior femoral cutaneous nerve block is used for any surgical procedure performed on the posterior aspect of the thigh.⁶ The saphenous, sural, and superficial peroneal nerve blocks can be used as part of an ankle block to provide complete anesthesia to the foot and ankle, or they can be used separately to provide anesthesia to specific portions of the foot and ankle. These blocks can be used for a variety of foot and ankle procedures.

Figure 40–2. Cutaneous innervation of the lower extremity, posterior view.

        The contraindications to performing cutaneous nerve blocks of the lower extremity are few, but include local infection at the sites of needle insertion, preexisting central or peripheral nervous systems disorders, and allergy to local anesthetic.

Functional Anatomy

The cutaneous nerves of the extremities are blocked by injection of local anesthetic in the subcutaneous layers above the muscle fascia. The subcutaneous tissue contains a variable amount of fat, superficial nerves, and vessels. Deep into this area lies a tough membranous layer, deep fascia of the lower extremity enclosing muscles of the leg. Deep fascia is penetrated by numerous superficial nerves and vessels.

        The cutaneous innervation of the lower extremity is accomplished by nerves that are part of the lumbar and sciatic plexuses (Figures 40–1 and 40–2). The largest cutaneous nerves are the lateral femoral cutaneous nerve, the posterior femoral cutaneous nerve, the saphenous nerve, the sural nerve, and the superficial peroneal nerve. A more detailed review of the relevant anatomy is provided with a description of the individual block procedures and in Chapter 3.

Table 40–1.

Choice of Anesthetic for Cutaneous Nerve Block of the Lower Extremity

Choice of Local Anesthetic

Any local anesthetic can be used for cutaneous blocks of the lower extremity. The choice of local anesthetic is based primarily on the desired duration of the blockade. Because these blocks do not result in motor blockade, longer-acting local anesthetics are most commonly chosen (eg, 0.2–0.5% ropivacaine and 0.5% bupivacaine). When performing blocks in the ankle area, it is always prudent to avoid using epinephrine owing to the risk of decreasing blood flow to the toes. Onset time for the block depends on the local anesthetic used⁷ (Table 40–1).

        LATERAL FEMORAL CUTANEOUS NERVE BLOCK

General Considerations

Lateral femoral cutaneous nerve block can be used to provide complete anesthesia in patients undergoing small skin grafts on the lateral aspect of the thigh, or it can be combined with femoral block^8–10 or sciatic block to complement them and extend sensory coverage for tourniquet pain. Its use has also been reported as a diagnostic tool for meralgia paresthetica, neuralgia of the lateral femoral cutaneous nerve of the thigh.

Distribution of Anesthesia

The lateral femoral cutaneous nerve provides sensation to the anterolateral aspect of the thigh (see Figure 40–1).

Patient Positioning

The patient is in a supine position with the anesthesiologist at the patient’s side. The anterior superior iliac spine is palpated and marked.

Anatomic Landmarks

The main landmark for lateral femoral cutaneous nerve blockade is easily identified in most patients; it is the anterior superior iliac spine. The lateral femoral cutaneous nerve emerges from the lateral border of the psoas major muscle and crosses the iliacus muscle obliquely toward the anterior superior iliac spine, where it supplies the parietal peritoneum of the iliac fossa. The nerve then passes into the thigh behind or through the inguinal ligament, variably medial to the anterior iliac spine (typically about 1 cm) or through the tendinous origin of the sartorius muscle, dividing into anterior and posterior branches.

        The anterior branch becomes superficial about 10 cm distal to the anterior superior iliac spine supplying innervation to the skin of the anterior and lateral thigh as far as the knee. It connects terminally with the cutaneous branches of the anterior division of the femoral nerve and the infrapatellar branch of the saphenous nerve, forming the patellar plexus. The posterior branch pierces the fascia lata higher than the anterior, dividing to supply the skin on the lateral surface from the greater trochanter to about the middle of the thigh and occasionally also supplying the gluteal skin.

Technique

A 4-cm, 22-gauge needle is inserted 2 cm medial and 2 cm caudal to the anterior superior iliac spine (Figure 40–3). The needle is advanced until a loss of resistance is felt as the needle passes through the fascia lata. A short-bevel needle is suggested to exaggerate the loss of resistance as the needle passes through the fascia. Because this fascia “give” is not consistent and its perception may vary among performers, local anesthetic is injected in a fanwise fashion both above and below the fascia lata from medial to lateral direction. A volume of 10 mL of local anesthetic is injected for this block. Although the lateral femoral cutaneous nerve is a sensory nerve, relatively higher concentrations of long-acting local anesthetic are useful to increase the success rate (0.5% ropivacaine or bupivacaine) because this is essentially a “blind” technique.

Figure 40–3. Lateral femoral cutaneous nerve block. The landmark for this block is the anterior superior iliac spine.

        When used to provide anesthesia for a skin graft harvest site on the lateral thigh, the peripheral innervation of the lateral femoral cutaneous nerve in specific patients is outlined before beginning skin harvesting.

        Because no larger vascular structures or other organs are nearby, blockade of the lateral femoral cutaneous nerve carries a minimal risk of systemic toxicity due to an inadvertent intravascular injection.

       POSTERIOR CUTANEOUS NERVE BLOCK OF THE THIGH

General Considerations

The posterior cutaneous thigh nerve block has been used in burn patients for donor skin grafting taken from the posterior thigh or as part of a popliteal/posterior femoral cutaneous nerve block in short saphenous vein stripping.¹¹

Distribution of Anesthesia

The posterior cutaneous nerve of the thigh innervates the skin over the posterior thigh between the lateral femoral cutaneous and anterior femoral cutaneous nerves (see Figure 40–2).

Patient Positioning

The patient can be positioned prone, in the lateral decubitus position (shown in Figures 40–4 and 40–5), or supine with the leg elevated 90 degrees.

Anatomic Landmarks

The posterior femoral cutaneous nerve originates from the dorsal branches of the first and second and from the ventral branches of the second and third sacral rami. It runs through the greater sciatic foramen below the piriformis and descends under the gluteus maximus muscle with the inferior gluteal vessels, posterior or medial to the sciatic nerve. The nerve then descends in the back of the thigh deep to the fascia lata. Its branches are all cutaneous and are distributed to the gluteal region, the perineum, and the flexor aspect of the thigh and leg.

Figure 40–4. Posterior cutaneous nerve of the thigh block, subgluteal approach.

Technique

The gluteal fold is identified and 10 mL of local anesthetic are injected subcutaneously to raise a skin wheal (Figure 40–4). In addition, at the midpoint of the gluteal crease, 5 mL of local anesthetic are injected at a deeper level, using a fan technique to reach the nerve that has not emerged through the deep fascia.

Figure 40–5. Posterior cutaneous nerve of the thigh block, midthigh approach.

        To block the posterior cutaneous nerve of the thigh above the knee level, as for short saphenous vein stripping (as a complement to popliteal block),¹¹ 10 mL of local anesthetic are injected subcutaneously along a line 5 cm above and parallel with the popliteal crease (Figure 40–5). The patient is in the prone position for both blocks.

       SAPHENOUS NERVE BLOCK

General Considerations

The saphenous nerve block is most commonly used in combination with a sciatic nerve block or popliteal block to complement anesthesia of the lower leg for various vascular, orthopedic, and podiatry procedures. The saphenous nerve is a terminal cutaneous branch of the femoral nerve. Its course is in the subcutaneous tissue of the skin on the medial aspect of the ankle and foot. All cutaneous nerves of the foot should be thought of as a neuronal network rather than single strings of nerves with a well-defined and consistent anatomic position.

Distribution of Anesthesia

The saphenous nerve innervates the skin over the medial, anteromedial, and posteromedial aspect of the lower leg from above the knee (part of the patellar plexus) to as low as the first metatarsophalangeal joint in some instances (Figures 40–1 and 40–6).

Patient Positioning

The patient is placed supine with the leg to be blocked supported by a footrest.

Figure 40–6. Saphenous nerve anatomy. Saphenous nerve pierces through the sartorius muscle (1), subpatellar branch (2), saphenous nerve in its descent on the medial aspect of the thigh (3).

Figure 40–7. Tibial tuberosity. Palpation of the landmark for the saphenous nerve block.

Anatomic Landmarks

The main landmark for this block is the tibial tuberosity, an easily recognizable and easily felt bony prominence on the anterior aspect of the tibia a few centimeters distal from the patella (Figure 40–7). The saphenous nerve is the largest cutaneous branch of the femoral nerve. It descends laterally to the femoral artery into the adductor canal, where it crosses anteriorly to become medial to the artery. It proceeds vertically along the medial side of the knee behind the sartorius, pierces the fascia lata between the tendons of the sartorius and gracilis, and then becomes subcutaneous. From here, it descends on the medial side of the leg with the long saphenous vein along the medial tibial border. Note that the saphenous nerve branches into numerous small branches as it enters the subcutaneous space, and, as such, it is often difficult to achieve blockade of the entire extensive saphenous nerve network. For this reason, it is always preferable to block the saphenous nerve as distally as possible. For instance, to achieve anesthesia of the foot, the saphenous nerve is best approached at the level of the ankle, which is identical with the technique for performing an ankle block.

Techniques

The below-knee field block is performed with the patient in supine position. Five to 10 mL of local anesthetic are injected as a ring deeply subcutaneously, starting at the medial surface of the tibial condyle and ending at the dorsomedial aspect of the upper calf (Figure 40–8).

        The paravenous technique has also been described, which is based on the close relation of the saphenous vein and nerve, to achieve a higher success rate. First, the saphenous vein is identified using a tourniquet around the leg in dependent position. The technique involves injection of 5 mL of local anesthetic in a fan-like fashion around the vein on the medial side of the leg just distal from the patella.¹² This technique, however, carries a small risk of creating a hematoma when the saphenous vein is punctured.

Figure 40–8. Saphenous nerve block. Shown is a subcutaneous injection of 10 mL of local anesthetic in a circumferential fashion on the medial aspect of the leg at the level of the tibial tuberosity.

        In the transsartorial approach, with the patient in the supine position, a skin wheal is raised over the sartorius muscle belly. The sartorius muscle can be palpated just above the knee with the leg extended and actively elevated. The needle is inserted at one finger-width above the patella slightly posterior to the coronal plane and slightly caudad through the muscle belly of the sartorius until a loss of resistance identifies the subsartorial adipose tissue. The depth of insertion is typically between 1.5 and 3 cm. After negative aspiration for blood, 10 mL of local anesthetic are injected.

        For surgery on the foot, the saphenous nerve is best blocked just above the medial malleolus, similar to the technique in ankle block (Figure 40–9). Using a 1½-in. needle, 6–8 mL of local anesthetic are injected subcutaneously immediately above the medial malleolus in a ring-like fashion. The most commonly reported complication of this block is a painless hematoma of the saphenous vein at the injection site.

        The saphenous nerve can also be blocked by using a nerve stimulator technique and performing a low-volume femoral nerve block. Injection of 10 mL of local anesthetic after obtaining either a medial muscle response, signified by contraction of the vastus medialis muscle, or an anterior muscle response, signified by contraction of the rectus femoris muscle and elevation of the patella, results in a high rate of block success.^13,14 Neurostimulation of the medial compartment of the femoral nerve requires even less volume of local anesthetic, compared with that of a standard femoral block.^1,15

Figure 40–9. Saphenous nerve block, distal approach above the medial malleolus.

Clinical Pearls

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Ultrasound-Assisted Nerve Blocks in Adults. Histology of Peripheral Nerves. Supraclavicular Brachial Plexus Block.