The nerves blocked in the popliteal fossa—the tibial and peroneal nerves—are extensions of the sciatic nerve. The principal use of this block is for foot and ankle surgery. The addition of a saphenous nerve block improves comfort, because medial lower leg and ankle sensory blockade makes tourniquets and medial ankle surgery more comfortable.
Patient Selection. To use the classic form of this block, the patient must be able to assume the prone position. Elicitation of paresthesia or a motor response is desirable but not essential; however, block effectiveness decreases without these endpoints.
Pharmacologic Choice. The principal use of these blocks is to provide sensory analgesia; thus lower concentrations of a local anesthetic are practical in contrast to situations in which motor blockade is essential. Concentrations of 1% lidocaine, 1% mepivacaine, 0.25% to 0.5% bupivacaine, and 0.2% to 0.5% ropivacaine are effective.
Traditional block technique
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.
Although vascular structures also occupy the popliteal fossa, intravascular injection should be infrequent if the usual precautions are taken. Hematoma formation is possible.
The sciatic nerve courses through the popliteal fossa where it is blocked. It is beneficial to use ultrasound for this block because the division of the sciatic nerve into posterior tibial and common peroneal nerves occurs at variable distances from the popliteal crease. The goal is to block the sciatic nerve before it divides and inject the local anesthetic within the epineurium. This allows for a more consistent blockade of both divisions and use of lower volumes of local anesthetic. The same anatomic landmarks that are used for the nerve stimulator-guided technique are also used for the ultrasound-guided block. However, with the ultrasound, the goal is to find the popliteal vessels first ( Figs. 19.5 and 19.6 ).