- Indications: saphenous vein stripping or harvesting, supplementation for medial foot/ankle surgery in combination with a sciatic nerve block
- Transducer position: transverse on anteromedial mid thigh or below the knee at the level of the tibial tuberosity, depending on the approach chosen (proximal/distal).
- Goal: local anesthetic spread lateral to the femoral artery and deep to the sartorius muscle or more distal, below the knee, adjacent to the saphenous vein.
- Local anesthetic: 5–10 mL
Needle insertion to block the saphenous nerve at the level of the mid thigh (A) or below the knee (B).
The saphenous nerve is the terminal sensory branch of the femoral nerve. It supplies innervation to the medial aspect of the leg down to the ankle and foot. Blockade of the nerve can be sufficient for superficial procedures in this area; however, it is most useful as a supplement to a sciatic block for foot and ankle procedures that involve the superficial structures in medial territory. The use of ultrasound guidance has improved the success rates of the saphenous blocks, compared with field blocks below the knee and blind transsartorial approaches. The ultrasound-guided techniques described here are relatively simple, quick to perform, and fairly reproducible.
The sartorius muscle forms a "roof" over the adductor canal in the lower half of the thigh in its descent laterally-to-medially across the anterior thigh. The muscle appears as an oval shape beneath the subcutaneous layer of adipose tissue. Often, the femoral artery, which passes beneath the muscle, can also be palpated. The sides of the triangular canal are formed by the vastus medialis laterally and adductor longus or magnus medially (depending on how proximal or distal the scan is). The saphenous nerve is infrequently seen on the ultrasound image; however, sometimes it is visualized as a small round hyperechoic structure medial to the artery. A femoral vein accompanies the artery and saphenous nerve, which are all typically visualized at 2 to 3 cm depth. When attempting to identify the saphenous nerve on ultrasound image, the following anatomic considerations should be kept in mind:
- Above the knee: The saphenous nerve pierces the fascia lata between the tendons of the sartorius and gracilis muscles before becoming a subcutaneous nerve.
- The saphenous nerve also may surface between the sartorius and vastus medialis muscles. (Figure 38-2 A, B, and C).
- Below the knee, the nerve passes along the tibial side of the leg, adjacent to the great saphenous vein subcutaneously (Figure 38-3A, B, and C).
- At the ankle, a branch of the nerve is located medially next to the subcutaneously positioned saphenous vein.