Popliteal and saphenous block

Placement

Anatomy. As illustrated in Fig. 19.1 , the cephalad popliteal fossa is defined by the semimembranosus and semitendinosus muscles medially and the biceps femoris muscle laterally. Its caudad extent is defined by the gastrocnemius muscles both medially and laterally. If this quadrilateral area is bisected, as shown in Fig. 19.1 , the area of interest to the anesthesiologist is the cephalolateral quadrant (hatched area), where both a tibial and common peroneal nerve block is possible. The tibial nerve is the larger of these two nerves; it separates from the common peroneal nerve at the upper limit of the popliteal fossa and sometimes higher. The tibial nerve continues the straight course of the sciatic nerve and runs lengthwise through the popliteal fossa immediately under the popliteal fascia. Inferiorly, it passes between the heads of the gastrocnemius muscles. The common peroneal nerve follows the tendon of the biceps femoris muscle along the cephalolateral margin of the popliteal fossa, as illustrated in Fig. 19.2 . After the common peroneal nerve leaves the popliteal fossa, it travels around the head of the fibula and divides into the superficial peroneal and deep peroneal nerves.

Position. The patient is placed in a prone position, and the anesthesiologist stands at the patient’s side to allow palpation of the borders of the popliteal fossa.

Needle Puncture. With the patient in the prone position, they are asked to flex the leg at the knee, which allows more accurate identification of the popliteal fossa. Once the popliteal fossa has been defined, it is divided into equal medial and lateral triangles, as shown in Fig. 19.1 . An “X” is placed 5–7 cm superior to the skin crease of the popliteal fossa and 1 cm lateral to the midline of the triangles, as shown in Fig. 19.1 . Through this site, a 22-gauge, 4- to 6-cm needle is advanced at an angle of 45–60 degrees to the skin while being directed anterosuperiorly ( Fig. 19.3 ). Paresthesia or a motor response is sought; when obtained, 30–40 mL of local anesthetic is injected.

When a saphenous block is added for foot and ankle surgery, the patient’s knee is bent at approximately a 45-degree angle and the medial aspect of the leg is exposed. Two primary techniques are used for a saphenous block. A superficial ring of local anesthetic may be injected just distal to the medial surface of the tibial condyle. Often 5–10 mL of local anesthetic is needed. Conversely, a more proximal technique at the cross-sectional level of the superior border of the patella is possible ( Fig. 19.4 ). In this case, a 22- to 25-gauge, 3- to 4-cm needle is inserted immediately deep to the sartorius muscle in the plane between the vastus medialis and the sartorius muscles, and 10 mL of local anesthetic is injected.