Sciatic block

Placement

Anatomy. The sciatic nerve is formed from the L4 through S3 roots. These roots of the sacral plexus form on the anterior surface of the lateral sacrum and are assembled into the sciatic nerve on the anterior surface of the piriformis muscle. The sciatic nerve results from the fusion of two major nerve trunks. The “medial” sciatic nerve is functionally the tibial nerve, which forms from the ventral branches of the ventral rami of L4 to L5 and S1 to S3; the posterior branches of the ventral rami of these same nerves form the “lateral” sciatic nerve, which is functionally the peroneal nerve. As the sciatic nerve exits the pelvis, it is anterior to the piriformis muscle and is joined by another nerve—the posterior cutaneous nerve of the thigh. At the inferior border of the piriformis, the sciatic and posterior cutaneous nerves of the thigh lie posterior to the obturator internus, the gemelli, and the quadratus femoris. At this point these nerves are anterior to the gluteus maximus. Here, the nerve is approximately equidistant from the ischial tuberosity and the greater trochanter ( Figs. 14.1–14.3 ). The nerve continues downward through the thigh to lie along the posteromedial aspect of the femur. At the cephalad portion of the popliteal fossa, the sciatic nerve usually divides to form the tibial and common peroneal nerves. Occasionally this division occurs much higher, and sometimes the tibial and peroneal nerves are separate through their entire course. In the popliteal fossa, the tibial nerve continues downward into the lower leg, whereas the common peroneal nerve travels laterally along the medial aspect of the short head of the biceps femoris muscle.

Classic approach

Position. The patient is positioned laterally, with the side to be blocked nondependent. The nondependent leg is flexed and its heel placed against the knee of the dependent leg ( Fig. 14.4 ). The anesthesiologist is positioned to allow insertion of the needle, as shown in Fig. 14.4.

Needle Puncture. A line is drawn from the posterior superior iliac spine to the midpoint of the greater trochanter. Perpendicular to the midpoint of this line, another line is extended caudomedially for 5 cm. The needle is inserted through this point ( Fig. 14.5 ). As a cross-check for proper placement, an additional line may be drawn from the sacral hiatus to the previously marked point on the greater trochanter. The intersection of this line with the 5-cm perpendicular line should coincide with the needle insertion site.

At this site, a 22-gauge, 10- to 13-cm needle is inserted, as illustrated in Fig. 14.4 . The needle should be directed through the entry site toward an imaginary point where the femoral vessels course under the inguinal ligament. The needle is inserted until paresthesia is elicited or until bone is contacted. If bone is encountered before paresthesia is elicited, the needle is redirected along the line joining the sacral hiatus and the greater trochanter until paresthesia or a motor response is elicited. During this redirection the needle should not be inserted more than 2 cm past the depth at which bone was originally contacted, or the needle tip will be placed anterior to the site of the sciatic nerve. Once paresthesia or a motor response is elicited, 20–25 mL of local anesthetic is injected.