Sciatic Nerve Block: Anterior / Transgluteal / Subgluteal Approach

Figure 1: (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.

PART 1: ANTERIOR APPROACH

Figure 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 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 8cm.

General Considerations

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.

Ultrasound Anatomy

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 1-2 and 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 1-3).

Figure 1-4: Transducer position to visualize the sciatic nerve through the anterior approach.

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.

Equipment

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 glove

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 1-4 and 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.

Technique

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 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.

In an adult patient, 15 to 20 mL of local anesthetic is usually adequate for successful blockade (Figure 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.

PART 2: TRANSGLUTEAL AND SUBGLUTEAL APPROACH

Figure 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 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.

Figure 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.

Equipment

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 glove

Figure 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.

In an adult patient, 15 to 20 mL of local anesthetic is usually adequate for successful blockade of sciatic nerve (Figure 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.

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 degrees 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.