The use of ultrasonographic guidance greatly expanded the options that practitioners have for accomplishing the block of the sciatic nerve because the nerve can be imaged at several convenient levels. With the transgluteal approach, the needle is inserted just distal deep to the gluteus maximus muscle to reach the sciatic nerve. The sciatic nerve at the gluteal crease is readily identified in a predictable anatomic arrangement, between two osseous landmarks (ischial tuberosity and the greater trochanter) and beneath a well-defined muscle plane. The use of ultrasound visualization decreases the need for the geometry and measurements that are required for the classic landmark-based approaches. With the subgluteal approach, the nerve simply reached a few centimeters distally, just below the level of the subgluteal crease where imaging is not interfered by the bones. The preference of one approach over the other is made based on the patient’s anatomic characteristics and personal preference.
At this transgluteal level, the sciatic nerve is visualized in the short axis between the two hyperechoic bony prominences of the ischial tuberosity and the greater trochanter of the femur (Figure 39.2-1 and 39.2-2). The gluteus maximus muscle is seen as the most superficial muscular layer bridging the two osseous structures, typically several centimeters thick. The sciatic nerve is located immediately deep to the gluteus muscles, superficial to the quadratus femoris muscle. Often, it is slightly closer to the ischial tuberosity (medial) aspect than the greater trochanter (lateral). At this location in the thigh, it is seen as an oval or roughly triangular hyperechoic structure. At the subgluteal level, however, the sciatic nerve is positioned deep to the long head of the biceps muscle and the posterior surface of the adductor magnus.
FIGURE 39.2-1. Transsectional anatomy of the sciatic nerve at the transgluteal level. Sciatic nerve (ScN) is seen between the greater trochanter of the femur and the ischium tuberosity, just below the gluteus maximus (GMM) muscle.
FIGURE 39.2-2. An ultrasound image demonstrating the sonoanatomy of the sciatic nerve (ScN). The ScN often assumes an ovoid or triangular shape and it is positioned underneath the gluteus muscle (GMM) between the ischium tuberosity (IT) and femur.
Distribution of Blockade
Sciatic nerve block results in anesthesia of the entire lower limb below the knee, both motor and sensory blockade, with the exception of a variable strip of skin on the medial leg and foot, which is the territory of the saphenous nerve, a branch of the femoral nerve. In addition, both the transgluteal and subgluteal approaches provide motor blockade of the hamstring muscles. The skin of the posterior aspect of the thigh however, is supplied by the posterior cutaneous nerve of the thigh, which has its origin from the sciatic nerve more proximal than the subgluteal approach. It is, therefore, unreliably anesthetized with subgluteal block; however, it is of relatively little clinical importance. For a more comprehensive review of the sciatic nerve distribution, see Chapter 1, Essential Regional Anesthesia Anatomy.
Equipment needed is as follows:
• Ultrasound machine with curved (phase 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
• Although a linear transducer occasionally can be used for smaller size patients for this block, the curved transducer permits the operator to visualize a wider field, including the osseous landmarks. The ischial tuberosity and greater trochanter are rarely seen on the same image when using a linear transducer.
Landmarks and Patient Positioning
Any patient position that allows for comfortable placement of the ultrasound transducer and needle advancement is appropriate. Typically for either the transgluteal or subgluteal block, this involves placing the patient in a position between the lateral decubitus and prone position (Figures 39.2-3 and 39.2-4). The legs are flexed in the hip and knee. When nerve stimulation is used simultaneously (suggested), exposure of the hamstrings, calf, and foot is required to detect and interpret motor responses. The round osseous prominences of the greater trochanter and ischial tuberosity are palpated and, if desired, marked with a skin marker. Scanning is begun in the depression between the two bones.
FIGURE 39.2-3. Patient position and transducer application for subgluteal approach to sciatic block..
FIGURE 39.2-4. Transgluteal approach to sciatic block; patient position, transducer (curved) placement and needle insertion.
The goal is to place the needle tip adjacent to the sciatic nerve, below the fascial plane of the gluteus muscles (thus, transgluteal technique) and to deposit 15 to 20 mL of local anesthetic until its spread around the nerve is documented.
The description of the technique in this chapter will focus primarily on the transgluteal approach. However, since the subgluteal approach is performed just a few centimeters more distal and it is technically easier, the reader can easily perform either approach by using general guidelines provided and referring to Figure 39.2-3, Figure 39.2-4, and algorithms at the end of the chapter. With the patient in the described position, the skin is disinfected and the transducer is positioned so as to identify the sciatic nerve (Figure 39.2-4). If the nerve is not immediately apparent, tilting the transducer proximally or distally can help improve the contrast and bring the nerve “out” of the background of the musculature. Often, the nerve is much better imaged after the injection of local anesthetic (Figure 39.2-5). Alternatively, sliding the transducer slightly proximally or distally can improve the quality of the image and allow for better visualization. Once identified, the needle is inserted in-plane, typically from the lateral aspect of the transducer and advanced toward the sciatic nerve. If nerve stimulation is used (1.0 mA, 0.1 msec), the passage of the needle through the anterior fascial plane of the gluteus muscles often is associated with a motor response of the calf or foot. Once the needle tip is positioned adjacent to the nerve (Figure 39.2-6A) and after careful aspiration to rule out an intravascular needle placement, 1 to 2 mL of local anesthetic is injected to document the proper injection site. Such injection often displaces the sciatic nerve away from the needle; therefore, an additional advancement of the needle 1 to 2 mm toward the nerve may be necessary to ensure the proper spread of the local anesthetic. When injection of the local anesthetic does not appear to result in a spread around the sciatic nerve, additional needle repositions and injections may be necessary. Assuring the absence of high resistance to injection is of utmost importance because the needle tip is difficult to visualize on ultrasound due to the steep angle and depth of the needle placement.
FIGURE 39.2-5. Sciatic nerve (yellow arrows) as seen in the subgluteal position (linear transducer), needle path (white arrows) and local anesthetic (turquoise arrows) in the intramuscular tunnel surrounding the sciatic nerve.
FIGURE 39.2-6. (A) Ultrasound image demonstrating the simulated needle path to reach the sciatic nerve (ScN) using an in-plane technique in transgluteal approach. The simulated needle (1) is shown transversing the gluteus muscle with its tip positioned at the lateral aspect of the sciatic nerve. (B) Needle path and distribution of local anesthetic (blue shaded area) to block the ScN through the transgluteal approach.
• Never inject against high resistance to injection (>15 psi) because this may signal an intraneural injection.
• The ability to distinguish the sciatic nerve from its soft tissue surroundings often is improved after the injection of local anesthetic; this can be used as a marker to confirm the proper identification of the nerve when injection begins.
In an adult patient, 15 to 20 mL of local anesthetic is usually adequate for successful blockade of sciatic nerve (Figure 39.2-6). Although a single injection of such volumes of local anesthetic suffices, it may be beneficial to inject two to three smaller aliquots at different locations to ensure the spread of the local anesthetic solution around the sciatic nerve. The block dynamics and perioperative management are similar to those described in Chapter 19.
Continuous Ultrasound-Guided Subgluteal Sciatic Block
The goal of the continuous sciatic block is similar to the non-ultrasound-based techniques: to place the catheter in the vicinity of the sciatic nerve between the gluteus maximus and quadratus femoris muscles. The procedure consists of three phases: needle placement, catheter advancement, and securing the catheter. For the first two phases of the procedure, ultrasound visualization can be used to ensure accuracy in most patients. The needle typically is inserted in-plane from the lateral to medial direction and underneath the fascia to enter the subgluteal space.
Advancement of the needle until the tip is adjacent to the nerve and deep to the gluteus maximus fascia should ensure appropriate catheter location. Proper placement of the needle also can be confirmed by obtaining a motor response of the calf or foot at which point, 4 to 5 mL of local anesthetic is injected. This small dose of local anesthetic serves to ensure adequate distribution of the local anesthetic as well as to make the advancement of the catheter easier. This first phase of the procedure does not significantly differ from the single-injection technique. The second phase of the procedure involves maintaining the needle in the proper position and inserting the catheter 3 to 5 cm beyond the needle tip into the subgluteal space in the vicinity of the sciatic nerve. Insertion of the catheter requires an assistant when it is done under ultrasound guidance. Alternatively, the catheter can be inserted using a longitudinal view. With this approach, after successful imaging of the sciatic nerve in the cross-sectional view, the transducer is rotated 90° so that the sciatic nerve is visualized in the longitudinal view. However, this approach requires significantly greater ultrasound imaging skills.
The catheter is secured by either taping it to the skin or tunneling. A common infusion strategy includes ropivacaine 0.2% at 5 mL/minute with a patient-controlled bolus of 5 mL/hour.
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