Ovid: Peripheral Nerve Blocks: A Color Atlas

Ultrasound Guided Sciatic Nerve Block

Richard Brull

Vincent Chan

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

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flexion of the foot is observed. Injection of local proceeds until the nerve is surrounded by a hypoechoic ring.

Figure 40-1. Gross anatomy of the sciatic nerve along with a section or drawing illustrating the position of the nerve in transverse section.

Tips

The sciatic nerve is one of the most difficult ultrasound guided blocks to master because of its depth from the skin and the lack of easily recognized adjacent vascular structures.

Figure 40-2. Infragluteal sciatic nerve block. A: Patient and probe positioning. B: Transverse sonogram using a 2 to 5 MHz curved array transducer probe (Philips HDI 5000 system, Bothell, WA). GMM, gluteus maximus muscle; GT, greater trochanter of the femur; IT, ischial tuberosity; arrowhead, sciatic nerve.

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The infragluteal gluteal approach is the easiest technique because it is the most superficial.
Some practitioners may wish to master the gluteal and anterior approaches as well.

Gluteal Approach

Anatomically, the ultrasound guided gluteal approach to sciatic nerve block most closely resembles Labat's classic posterior technique. The patient is positioned semiprone with the operative side uppermost. A 2 to 5 MHz curved array probe is placed obliquely at the level of the ischial spine, midway between the greater trochanter of the femur and the sacral hiatus (Fig. 40-3A). In this view, the sciatic nerve is pictured as a thin hyperechoic elliptical structure deep to the gluteus maximus muscle, superficial to the linear hyperechoic shadow of the ischial bone, and lateral to the subtle curvilinear hyperechoic shadow of the ischial spine (Fig. 40-3B). The round hypoechoic pulsatile pudendal artery and compressible vein, as well as the inferior gluteal vessels, can often be identified medial to the sciatic nerve in this view (Fig. 40-3B).

Anterior Approach

Anatomically, the ultrasound guided anterior approach to sciatic nerve block most closely resembles the anterior technique described by Beck and Chelly and Delaunay. This approach is especially useful in patients who cannot be positioned semiprone (e.g., trauma). The patient rests supine with the operative leg externally rotated to facilitate needle passage posterior to the lesser trochanter of the femur. A 2 to 5 MHz curved array probe is placed obliquely approximately 8 cm distal to the inguinal crease at the medial border of the rectus femoris muscle (Fig. 40-4A). In this view, the sciatic nerve is pictured as a hyperechoic semilunar structure deep to the adductor magnus muscle and immediately posteromedial to the curvilinear bony shadow of the lesser trochanter of the femur (Fig. 40-3B). The round hypoechoic pulsatile femoral artery and compressible vein are identified much more superficial and lateral to the sciatic nerve (Fig. 40-4B).

Figure 40-3. Gluteal sciatic nerve block. A: Patient and probe positioning. B: Transverse sonogram using a 2 to 5 MHz curved array transducer probe (Philips HDI 5000 system, Bothell, WA). GMM, gluteus maximus muscle; IB, ischial bone; IS, ischial spine; arrowhead, sciatic nerve.

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Figure 40-4. Anterior sciatic nerve block. A: Patient and probe positioning. B: Transverse sonogram using a 2 to 5 MHz curved array transducer probe (Philips HDI 5000 system, Bothell, WA). AMM, adductor magnus muscle; LT, lesser trochanter of the femur; arrowhead, sciatic nerve.