- Indications: foot and ankle surgery; analgesia following knee surgery
- Transducer position:
- ANTERIOR APPROACH: transverse on the proximal medial thigh
- TRANSGLUTEAL APPROACH: transverse on the posterior buttock, between the ischial tuberosity and greater trochanter
- Goal: local anesthetic spread adjacent to the sciatic nerve
- Local anesthetic: 15–20 mL
(A) Needle insertion to block the sciatic nerve using an anterior approach. Note that a curved (phased array) lower frequency transducer is used either in-plane (shown) or out-of-plane needle insertion can be used. (B) Transgluteal approach to sciatic block; patient position, transducer (curved) placement and needle insertion.
The anterior approach to sciatic block can be useful in patients who cannot be positioned in the lateral position due to pain, trauma, presence of external fixation devices interfering with positioning, and other issues. It also may be well-suited to patients who require postoperative blocks for analgesia following a total knee arthroplasty. Ultrasonography adds the benefit of no requirement for the palpation of a femoral pulse or the use of geometry for identification of the skin puncture point. In addition, using the ultrasound-guided approach should reduce the risk of puncture of the femoral artery as 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 insertion of catheters because a large needle must traverse several muscles (causing pain and possibly hematomas), an awkward catheter location (medial thigh), and catheter insertion at 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 39.1-2 and 39.1-3). Beneath the superficial sartorius muscle is the femoral artery, and deep and medial to this vessel is the profunda femoris artery. Both of these can be identified with color Doppler ultrasound for orientation. The femur is easily seen as a hyperechoic rim with the corresponding shadow beneath the vastus intermedius. Medial to the femur is the body of the adductor magnus muscle, separated by the fascial plane(s) of the hamstrings muscles. The sciatic nerve is visualized as a hyperechoic, slightly flattened oval structure sandwiched between these two muscle planes. The nerve is typically visualized at a depth of 6 to 8 cm (Figure 39.1-3).
Cross-sectional anatomy of the sciatic nerve (ScN). Shown are femoral artery (FA), adductor ...