Patient Position, Infragluteal Technique: The patient is positioned semiprone with the operative side up.

Indication: Surgery of the knee or leg.

Needle Size: 10-cm, 22-gauge insulated needle.

Transducer: 2 to 5 MHz, curved array.

Transducer Position: The transducer is positioned axially, midway between the greater trochanter and ischial spine, immediately inferior to the gluteal crease (Figs. 40-1, 40-2A).

Surface Landmarks: The greater trochanter and the ischial tuberosity.

Ultrasound Landmarks: The ischial tuberosity medially and the greater tuberosity laterally are seen as curvilinear hyperechoic shadows. The sciatic nerve is seen as a hyperechoic elliptical structure deep to the gluteus maximus muscle (Fig. 40-2B).

Volume: 10 to 30 mL.

Technique: Sterile prep of the skin. The needle is placed lateral to the probe nearly perpendicular to the skin along the long axis of the beam (in plane). Some practitioners prefer to use an out of plane approach. With this approach, imaging the needle can be difficult, and its position is often inferred by the movement of the tissue at its tip, or by injecting small aliquots of local as the needle is advanced toward the sciatic nerve. Because the needle tip is often difficult to see, nerve stimulation is especially useful for sciatic block. Some practitioners prefer to begin with a high current, 1 to 2 mA. Others begin closer to the threshold of stimulation, 0.5 mA. The needle is advanced until dorsi or plantar
flexion of the foot is observed. Injection of local proceeds until the nerve is surrounded by a hypoechoic ring.