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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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PLACEMENT

Anatomy.

The sciatic nerve is formed from the L4–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 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–L5 and S1–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. 12-1 , 12–2 , 12–3 ). The nerve continues on a downward course through the thigh to lie along the posterior medial 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 its downward course into the lower leg, whereas the common peroneal nerve travels laterally along the medial aspect of the short head of the biceps femoris muscle.

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Figure 12-1  Sciatic nerve anatomy: anterior oblique view.


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Figure 12-2  Sciatic nerve anatomy: posterior view.


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Figure 12-3  Sciatic nerve anatomy: lateral view.


Classic Approach: Sciatic Block

Position.

The patient is positioned laterally, with the side to be blocked nondependent. The flexed, nondependent leg supports the patient by placing the heel of the nondependent leg opposed to the knee of the dependent leg ( Fig. 12-4 ). The anesthesiologist is positioned to allow insertion of the needle, as shown in Figure 12-4 .

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Figure 12-4  Sciatic nerve block: classic technique and positioning.


Needle Puncture.

A line is drawn from the posterosuperior 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. 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 ( Fig. 12-5 ).

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Figure 12-5  Sciatic nerve block: technique of surface markings.


A 22-gauge, 10- to 12-cm needle is inserted at this site, as illustrated in Figure 12-4 . 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 a paresthesia is elicited or until bone is contacted. If bone is encountered before a paresthesia is elicited, the needle is redirected along the line joining the sacral hiatus and the greater trochanter until a paresthesia or motor response is elicited. During this needle redirection, the needle should not be inserted more than 2 cm past the depth at which bone was originally contacted lest the needle tip be placed anterior to the site of the sciatic nerve. Once a paresthesia or motor response is elicited, 20 to 25 mL of local anesthetic is injected.

Potential Problems.

In patients in whom the sciatic block is being used for an injury to the lower extremity, the classic position is sometimes difficult to use. The block can also be of long duration, and patients should be warned of this possibility preoperatively to prevent undue concern postoperatively. Although unsubstantiated, some think that dysesthesias are more common after this block than after other peripheral blocks.

Pearls

The keys to making this block work are adequate positioning of the patient and systematic redirection of the needle until a paresthesia is obtained.

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