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).
FIGURE 39.1-2. Cross- sectional anatomy of the sciatic nerve (ScN). Shown are femoral artery (FA), adductor longus muscle (ALM), pectineus muscle, adductor magnus muscle (AMM), adductor brevis muscle (ABM), gracilis muscle (GsM), and the femur. The sciatic nerve is seen posterior to the AMM.
FIGURE 39.1-3. Ultrasound anatomy of the sciatic nerve. From superficial to deep; femoral artery (FA) and femur laterally, adductor magnus muscle (AMM) and sciatic nerve (ScN) laterally. The sciatic nerve is typically located at a depth of 6 to 8 cm.
Distribution of Blockade
Sciatic nerve block results in anesthesia of the posterior aspect of the knee, hamstrings muscles, and entire lower limb below the knee, both motor and sensory, with the exception of skin on the medial leg and foot (saphenous nerve). The skin of the posterior aspect of the thigh is supplied by the posterior cutaneous nerve of the thigh, which has its origin from the sciatic nerve more proximal than the anterior approach. It is, therefore, not blocked by the anterior approach. Practically, however, the lack of anesthesia in its distribution is of little clinical consequence. For a more comprehensive review of the sciatic nerve distribution, see Chapter 1.
Equipment needed is as follows:
• Ultrasound machine with curved (phased array) transducer (2–8 MHz), sterile sleeve, and gel
• Standard nerve block tray (described in the equipment section)
• One 20-mL syringe containing local anesthetic
• A 100-mm, 21 to 22 gauge short-bevel insulated stimulating needle
• Peripheral nerve stimulator
• Sterile gloves
Landmarks and Patient Positioning
Anterior approach to sciatic nerve block is performed with the patient in the supine position. The hip is abducted to facilitate transducer and needle placement (Figure 39.1-4 and 39.1-5). When feasible, the hip and knee should be somewhat flexed to facilitate exposure. If nerve stimulation is to be used at the same time (recommended), exposure of the calf and foot are required to observe motor responses. In either case, it is useful to expose the entire thigh to appreciate the distance from the groin to knee.
FIGURE 39.1-4. Transducer position to visualize the sciatic nerve through the anterior approach.
FIGURE 39.1-5. A simulated needle path using an out of plane technique to reach the sciatic nerve (ScN) through the anterior approach.
The goal is to place the needle tip immediately adjacent to the sciatic nerve, between the adductor muscles and biceps femoris muscle, and deposit 15 to 20 mL of local anesthetic until spread around the nerve is documented.
With the patient in the proper position, the skin is disinfected and the transducer positioned so as to identify the sciatic nerve. If the nerve is not immediately apparent, sliding and tilting the transducer proximally or distally can be useful to improve the contrast and bring the nerve “out” of the background from the musculature. Finally, if the patient is able to dorsiflex and/or plantar flex the ankle, this maneuver often causes the nerve to rotate or otherwise move within the muscular planes, facilitating identification. Once identified, the needle is inserted in-plane or out of plane (more common in our program) from the medial aspect of the thigh and advanced toward the sciatic nerve (Figure 39.1-5). If nerve stimulation is used (1.0 mA, 0.1 msec), the contact of the needle tip with the sciatic nerve is usually associated with a motor response of the calf or foot. Once the needle tip is deemed to be in the proper position, 1 to 2 mL of local anesthetic is injected to confirm the adequate distribution of injectate. Such injection helps delineate the sciatic nerve within its intramuscular tunnel, but it may displace the sciatic nerve away from the needle. Improper spread of the local anesthetic or nerve displacement may require an additional advancement of the needle. When injection of the local anesthetic does not appear to result in a spread around the sciatic nerve, additional needle repositions and injections are necessary.
• Insertion of the needle in an out-of-plane manner with hydro-dissection/localization is often a more logical method to accomplish this block.
In an adult patient, 15 to 20 mL of local anesthetic is usually adequate for successful blockade (Figure 39.1-6). Although a single injection of such volume of local anesthetic suffices, it may be beneficial to inject two to three smaller aliquots at different locations to assure the spread of the local anesthetic solution around the sciatic nerve. The block dynamics and perioperative management are similar to those described in the nerve stimulator technique section, Chapter 19.
FIGURE 39.1-6. Simulated needle path using an out-of-plane technique with local anesthetic and proper distribution of local anesthetic to anesthetize the sciatic nerve (ScN).
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