SCIATIC NERVE BLOCK AT A GLANCE
Indications: foot and ankle surgery, below-knee amputation, analgesia following knee surgery involving the posterior compartment (Figure 33F–1)
Anterior approach: transverse on the proximal medial thigh
Transgluteal approach: transverse on the posterior buttock, between the ischial tuberosity and greater trochanter
Subgluteal approach: transverse on the gluteal crease
Other approaches (eg, parasacral, lateral) have been described but will not be detailed here.
Goal: local anesthetic spread within the sciatic nerve sheath
Local anesthetic: 10–20 mL
Distribution of sensory and motor after sciatic nerve blockade at gluteal and subgluteal level.
PART I: ANTERIOR APPROACH
The anterior approach to the sciatic nerve block can be useful in patients who cannot be positioned in the lateral position due to pain, trauma, the presence of external fixation devices that interfere with positioning, or other issues. The ultrasound (US)-guided approach may reduce the risk of femoral artery puncture compared with the landmark-based approach. The actual scanning and needle insertion are performed on the anteromedial aspect of the proximal thigh, rather than the anterior surface, and may require a slight abduction and external rotation of the thigh. This block is not well-suited to catheter insertion because a large needle must traverse several muscles (discomfort during procedure and risk of hematoma), it is an awkward catheter location (medial thigh), and catheter insertion at an approximately perpendicular angle to the sciatic nerve is difficult.
The sciatic nerve is imaged approximately at the level of the minor trochanter. At this location, a curved transducer placed over the anteromedial aspect of the thigh will reveal the musculature of all three fascial compartments of the thigh: anterior, medial, and posterior (Figures 33F–2 and 33F–3). Beneath the sartorius muscle is the femoral artery, and deep and medial to this vessel is the deep artery of the thigh. Both can be identified with color Doppler US for orientation. The femur is seen as a hyperechoic rim with a corresponding shadow beneath the vastus intermedius. Medial to the femur is the adductor magnus muscle, anterior to the hamstring muscles. The sciatic nerve is visualized as a hyperechoic oval structure sandwiched between these two muscle. The nerve is typically visualized at a depth of 6–8 cm (see Figure 33F–3).
Cross-sectional anatomy of the sciatic nerve (ScN). Shown are the femoral artery (FA), adductor longus muscle (ALM), adductor magnus muscle (AMM), adductor brevis muscle (ABM), and femur. The sciatic nerve is seen posterior to the AMM.
Ultrasound anatomy of the sciatic nerve. From superficial to deep, visualized laterally: femoral artery (FA), femur, adductor magnus ...