Suggested Technique

The subgluteal region has a “bright triangle” consisting of the hyperechoic sciatic nerve and adjacent tendons of the semitendinosus-biceps and semimembranosus. These proximal tendons can appear similar to the sciatic nerve on ultrasound scans. The sciatic nerve is always lateral to these ischiocrural tendons. The semimembranosus tendon is characterized by a large, flat proximal aponeurosis. The semitendinosus muscle forms a conjoint tendon with the biceps femoris that points to the medial aspect of the sciatic nerve. Short-axis view with sliding of the transducer is usually better than long-axis view to confirm nerve identity and distinguish it from the adjacent tendons.

Prone position allows the most stable access for proximal sciatic nerve block. In-plane technique from the lateral side of the leg is a relatively easy approach to proximal sciatic block. If prone positioning is difficult, lateral approach can be used for this block. A broad linear probe (5-cm footprint or larger) is best to provide a large field of view for this block because working room is not limited in this region.

Anatomic variation of the sciatic nerve in the subgluteal region primarily consists of identification of separate contributions from the common peroneal and tibial nerves.^2,3 This indicates proximal division of the sciatic nerve by the piriformis muscle. If the anomaly is correctly identified, multiple-injection technique guided by ultrasound provides complete sciatic block.

The sciatic nerve lies between the greater trochanter (lateral) and the ischial tuberosity (medial). These bony reference points are useful proximal landmarks for sciatic block in the gluteal region.⁴ In some patients the inferior gluteal artery can be identified on the medial side of the proximal sciatic nerve.

FIGURE 43-1 Course of the sciatic nerve.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Churchill Livingstone; 2004.)

FIGURE 43-2 External photograph showing the approach to sciatic nerve block in the subgluteal region. An in-plane approach from the lateral aspect of the thigh is shown (A and B). The corresponding sonogram with the sciatic nerve in transverse view is shown (C). The subgluteal sciatic nerve often has a triangular shape defined by the following three borders: the long head of the biceps femoris (posterolateral), the semitendinosus (posteromedial), and the adductor magnus (anterior). Sciatic nerve block usually targets the lateral corner of the nerve.

FIGURE 43-3 Short axis view of the proximal sciatic nerve demonstrating its internal echotexture. The “comma” or S-shaped conjoint tendon of the biceps femoris and semitendinosus points to the medial side of the sciatic nerve. The semimembranosus tendon has a “teardrop” shape and is flat medially. These ischiocrural tendons are valuable sonographic landmarks for proximal sciatic nerve block.

FIGURE 43-4 The perforating arteries have a characteristic oblique course over the anterior surface of the sciatic nerve in the proximal thigh. Because the perforator arteries have a 45-degree oblique course with respect to the sciatic nerve, the arteries appear in short-axis view regardless of whether the nerve is viewed in short axis or long axis. The perforating arteries travel through the intermuscular septum over the adductor magnus muscle.

FIGURE 43-5 The sciatic nerve can have variable morphologies. The injection distribution around the sciatic nerve in the subgluteal region can have a triangular appearance, pointing toward the conjoint tendon of the semitendinosus and the biceps femoris. The term “subgluteal” is used to refer to the region distal to the gluteal crease. Sonograms are shown before (A) and after (B) injection.

FIGURE 43-6 Short axis in-plane approach to sciatic nerve block in the proximal thigh. The lateral corner of the nerve is targeted for local anesthetic injection. Sonograms are shown before (A) and after (B) injection.

FIGURE 43-7 Because the sciatic nerve is large and straight, long-axis views can be useful for assessing the distribution along the course of the sciatic nerve. Local anesthetic should layer on both anterior and posterior sides of the nerve with the peeling edges of the distribution identified. Corresponding short-axis (A) and long-axis (B) views are shown.

FIGURE 43-8 Sciatic nerve block can be performed in the gluteal region. In this location the ischial tuberosity (medial) and greater trochanter (lateral) serve as sonographic landmarks. The sciatic nerve lies between these two bony landmarks, usually closer to the ischial tuberosity. Sonograms are shown before (A) and after (B) injection in short-axis view, with the corresponding postinjection long-axis view shown (C).