This is a peripheral nerve block of the sciatic nerve performed more distal in the leg proximal to the popliteal crease. With the patient in the supine or prone position, the ultrasound probe is placed to scan in the transverse plane on the posterior surface of the popliteal fossa 5 cm or more proximal to the popliteal flexion crease of the knee (Fig. 21.11a). Subtle inclination of the ultrasound probe in the caudad or cephalad direction may be needed to produce ideal visualization of the nearly round, hyperechoic sciatic nerve or sciatic nerve components (Fig. 21.11b) (within this proximity of the popliteal crease, the sciatic nerve will often take on the appearance of separate tibial and common peroneal nerve branches). The sciatic nerve is surrounded by a large amount of fat, and, laterally, the typical muscular pattern of the biceps femoris can be recognized with its medial surface concave toward the nerve. Medial to the nerve, the muscle/tendon of the semimembranosus is identifiable, with its lateral surface convex toward the nerve. The ultrasound view of the sciatic nerve at the popliteal site is often so distinct as to allow discrimination of the peroneal and tibial nerve components, and these components will separate as the ultrasound probe is slid caudad along the popliteal fossa (Fig. 21.11c). At, or just proximal to the popliteal flexion crease of the knee, scanning for the popliteal artery pulsation at this site can help locate the tibial nerve since the tibial nerve is typically immediately lateral to the popliteal vein(s) and artery. Once the tibial nerve is identified, it can be followed cephalad by sliding the probe (still in the transverse plane) until the peroneal nerve is seen to join the tibial nerve on its lateral side (Fig. 21.11c). In addition to imaging the sciatic nerve in the transverse plane just described, asking the patient to dorsiflex and/or plantarflex the foot produces a rotation of the nerve components allowing for easier nerve confirmation/identification ("seesaw" sign) (Schafhalter-Zoppoth et al. 2004).
Figure 21.11a Photograph demonstrating ultrasound probe placement and needle insertion point for sciatic nerve block in the popliteal fossa. ST and SM semimembranosus and semitendenosus muscle tendons, BF biceps femoris muscle tendons. Nerve block needle is in an out-of-plane orientation.
Figure 21.11b Ultrasound image of sciatic nerve in the popliteal fossa. Note the biceps muscle/tendon lateral to the nerve (N). The femur bone can be identified by the hyperechoic arched image within the sonogram.
The optimal selection for distal sciatic nerve blockade depends on the best ultrasound visualization of the nerve. This site typically occurs just proximal to sciatic nerve division into tibial and peroneal nerve branches and with the probe positioned in the transverse plane of the popliteal fossa. The popliteal sciatic nerve block can be performed with either an inplane (Fig. 21.11d) or an out-of-plane technique (Fig. 21.11a). In both techniques, the patient can remain in a supine position with flexed hip and knee (keeping the tib-fib portion of the
Figure 21.11d Sonogram of "in-plane" popliteal sciatic nerve block. Note the spread of local anesthetic (LA) that is spreading around the nerve (N). LA local anesthetic, N nerve, arrows identify the peripheral nerve block needle.
operative leg parallel to the floor with blanket or pillow support), in the lateral position with straightened lower limb or in the prone position (slightly flexed hip and knee). By using the in-plane technique, ultrasound image illustrated in Fig. 21.11d, the puncture site is above the biceps muscle with a 70- to 80-mm short bevel needle. The block needle tip should be placed both above and then below the sciatic nerve to achieve an optimal spread of local anesthetic and reliably sufficient sciatic nerve block.
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