Saphenous Nerve Block

Indications: 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. Complexity level: Basic

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 and perform, they are essentially devoid of compli-cations, and they can nicely complement major conduction blocks of the lower extremity. The combination of their applicability and simplicity should mandate that these blocks be in the armamentarium of every anesthesiologist.

The saphenous nerve is the largest cutaneous branch of the femoral nerve. It descends lateral 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 gracillis, 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. It innervates the skin over the medial, anteromedial, and posteromedial of the lower leg, from above the knee (part of the patellar plexus) to as low as the first metatarso-phalangeal joint, in some instances. It should be noted 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, identically to the technique for performing an ankle block.

The main landmark for this block is the tibial tuberosity, an easily recognizable and felt bony prominence on the anterior aspect of the tibia a few cm distal to the patella.

There are several techniques of saphenous nerve blockade described, however, in this chapter, we focus primarily on the one that we routinely use in our practice. With the patient in supine position, 5 to 10 mL of local anesthetic is 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.

TIPS:

• The paravenous technique has also been described, which is based on the close relationship 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 to the patella.
• In the transsartorial approach, with the patient in the supine position, a skin wheel 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 Tuohy needle is inserted at 1-finger width above the patella slightly posterior to the coronal plane and slightly caudal, 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 is injected

For surgery on the foot, the saphenous nerve is best blocked just above the medial meleolus, like in the ankle block technique. Using a 1½" needle, 6-8 mL of local anesthetic is 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.

TIP: The most effective method of blocking the saphenous nerve is a low-volume femoral nerve block. Injection of mere 10 ml of local anesthetic upon obtaining twitches of the patella or vastus medialis muscle results in nearly 100% success rate.

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. Since these blocks do not result in motor blockade, longer acting local anesthetics are most commonly chosen (e.g., 0.2-0.5% ropivacaine or 0.25% bupivacaine).

Complications of cutaneous nerve blocks are few.

Systemic toxicity of local anesthetic	- The risk is small and may be of concern only when higher volumes are used in conjuction with other high-volume major conduction blocks
Hematoma	- Avod multiple needle insertions and insertions of the needle through superficial veins
Nerve injury	- This is usually manifested as transient parestheias or dysesthesias and result due to an inadvertent intraneuronal injection - The injections should not be made when high pressures on injection are felt or when patient reports pain in the distribution of the nerve

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