Suggested Technique

Elevation of the leg and some internal rotation allow imaging of the popliteal fossa from the posterior surface.⁴ A broad linear transducer (35- to 50-mm footprint, 10 MHz center frequency) is used for most adult patients. The choice of block needle (7- to 9-cm length, 20-22 gauge) is not critical.

Popliteal block is usually performed just distal to the sciatic nerve bifurcation in the popliteal fossa for several reasons. First, the nerves are close to the posterior skin surface. This makes nerve imaging and positioning the needle tip easier. Second, the needle can be aimed at the connective tissue space between the tibial and common peroneal nerves (rather than directly aimed at the sciatic nerve).⁵ The block is performed where the tibial and common peroneal nerves are about one needle-width apart (about 1 mm). Third, there is a large amount of nerve surface area available for diffusion of local anesthetic to promote clinical block characteristics. The point of sonographic unity is closer to the knee crease than anatomic dissections would suggest because the tibial and common peroneal nerves run next to each other for some distance before visibly separating. The only potential disadvantage to this more distal popliteal block is that the popliteal vessels are closer to the nerves.

The needle bevel should face the transducer for optimal needle tip visibility (bevel down). Because the common peroneal nerve is slightly closer to the posterior surface than the tibial nerve, it is best to approach the gap between the two nerves from the femur side (i.e., a slight posterior inclination of the block needle with the lateral approach).

Studies have suggested a limited ability of ultrasound to correctly assess circumferential distribution of local anesthetic around peripheral nerves. The reported predictive value of the “doughnut” sign is only about 90% for sciatic nerve blocks.⁶ One major advantage to sciatic nerve block in the popliteal fossa is that it allows sliding assessment of the longitudinal distribution along the nerve branches (i.e., local anesthetic should not only surround the nerve but also track along the nerves).

The onset of blockade of the common peroneal nerve is usually faster than for the tibial nerve, which may reflect the smaller size of the common peroneal nerve.⁷

FIGURE 45-1 External photograph showing the approach to sciatic nerve block in the popliteal fossa (A and B). An in-plane approach from the lateral aspect of the thigh is shown. The patient is supine with the leg elevated to allow access for the transducer from the posterior aspect of the leg. Internal rotation of the leg also helps ultrasound-guided blocks in supine position.

FIGURE 45-2 Image sequence showing popliteal block. An in-plane approach is demonstrated whereby the needle tip is placed between the common peroneal and tibial nerves (A). The block needle is then advanced until tent and recoil of the adjoining connective tissue are observed. After injection, local anesthetic is distributed around both nerves (B).

FIGURE 45-3 Image sequence showing popliteal block. An in-plane approach is demonstrated in which the needle tip is placed between the common peroneal and tibial nerves (A). Local anesthetic is distributed around both nerves by injecting as the needle is withdrawn (B).

FIGURE 45-4 Sonographic assessment of popliteal block. Sliding the transducer distally along the course of the nerves demonstrates the local anesthetic distribution to both the tibial and common peroneal nerves (A). By sliding the transducer even more distally, the common peroneal nerve can be seen to give off a contributing branch to the sural nerve and lateral cutaneous nerve of the leg (B). The borders of this small nerve are clarified by the surrounding local anesthetic. When multiple rings of local anesthetic are identified that track along nerve branches, the resultant block is unequivocally established.

FIGURE 45-5 Sonographic anatomy of the popliteal fossa in short-axis (A) and long-axis (B) views. The nerves of the popliteal fossa lie closer to the posterior surface than the popliteal vessels (A). In the long-axis view (B) the usual courses of the common peroneal nerve, tibial nerve, popliteal vein, and popliteal artery are illustrated in one parasagittal plane of imaging (listed from posterior to anterior). In most cases the common peroneal nerve lies closer to the posterior surface of the thigh than does the tibial nerve. However, the location of these nerves depends on foot position, and in some cases, the tibial nerve lies closer to the posterior surface.

FIGURE 45-6 Echotexture of the common peroneal and tibial nerves. The tibial nerve is larger and has a straighter course than the common peroneal nerve. In addition, the tibial nerve has more fascicles and therefore usually appears brighter on ultrasound scans. In contrast, the common peroneal nerve can appear oligofascicular or even monofascicular with fewer, larger fascicles than the tibial nerve. Two separate examples are shown (A and B).

FIGURE 45-7 Bayonet artifact during popliteal block. Because the angle of approach is near parallel to the active face of the transducer, bayonet artifacts can be observed during popliteal blocks. This speed-of-sound artifact causes apparent bending of the needle (although no actual mechanical bending of the needle exists). Bayonet artifacts observed during popliteal blocks are caused by the presence of adipose tissue near the midline of the popliteal fossa. The artifacts occur at this transition because the speed of sound in adipose tissue is slower than the adjacent muscle. The end of the needle therefore appears to bend away from the transducer (A). As the needle is advanced the bend of the bayonet remains in the same location (B).