The anterior approach is useful in patients who are unable to assume a lateral decubitus position (Figure 44.1). It is a deep block that often requires a 15-cm needle and is associated with slightly more discomfort than other approaches. The proximity of the femoral artery anterior to the sciatic nerve in this position increases the risk of inadvertent arterial puncture. Continuous block techniques have been described with this approach, but are somewhat unpopular in the United States.
With a supine patient, draw a line from the anterior superior iliac spine to the pubic tubercle, and divide it into thirds. Draw a second line, parallel to the first, medial from the cephalad aspect of the greater trochanter. Then, draw a third line perpendicular from the medial third of the first line until it intersects with the second line. This intersection, located over the lesser trochanter of the femur, will serve as the point of initial needle insertion (Figures 44.2 and 44.3). The lesser trochanter obstructs the route to the sciatic nerve when the leg and foot are in neutral position; internal rotation of the leg by 45° exposes the nerve and allows the needle easy passage to the nerve.
• 21-gauge, 15-cm insulated needle.
• 18-gauge, 15-cm insulated Tuohy needle for catheter placement. Catheter placed 5 cm beyond needle tip. Catheters are not recommended.
Set nerve stimulator at 1.5 mA and advance the needle perpendicularly. If bone is contacted, withdraw and rotate the leg internally. Advance in the same plane as before until a twitch is elicited. If bone is contacted again, the initial insertion site may be distal to the lesser trochanter. In this situation withdraw the needle and externally rotate the leg 45° and re-advance the needle. Elicitation of plantar flexion/inversion or dorsiflexion/eversion at 0.5 mA or less are the sought-after endpoints for correct placement. Stimulation of the hamstring suggests the needle is deep to the nerve (Figure 44.4).
3. Ultrasound-Guided Anterior Sciatic Nerve Block
The patient is placed supine, with the hip and the knee mildly flexed to facilitate exposure. The transducer is placed approximately 7 cm distal to the inguinal crease in a transverse fashion on the anteromedial aspect of the thigh (Figure 44.5). Obtain a view of the femur and visualize the lesser trochanter. Then slide the probe medially. The sciatic nerve should appear as an hyperechoic structure between the adductor magnus muscle anteriorly and the gluteus maximus muscle posteriorly (Figure 44.6). 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.
3.3. Approach and injection
With the sciatic nerve identified, insert a 10- to 15-cm needle in plane from the medial aspect of the thigh and advanced toward the sciatic nerve. Once the needle tip is in position near the nerve, inject 1 to 2 mL of local anesthetic to confirm the adequate distribution. The needle position may need to be adjusted some if there is improper spread of local anesthetic or nerve displacement. Twenty milliliters of local anesthetic is usually adequate for successful blockade.
Teaching Points. This approach typically does not reliably block the posterior cutaneous nerve of the thigh.