The anterior approach to sciatic block is an advanced nerve block technique. The block is well-suited for surgery on the leg below the knee, particularly on the ankle and foot. It provides complete anesthesia of the leg below the knee with the exception of the medial strip of skin, which is innervated by the saphenous nerve. When combined with a femoral nerve block, this procedure results in anesthesia of the entire knee and leg. It should be noted that the anterior approach may have less utility compared with the posterior approach. The sciatic nerve is blocked more distally, and a higher level of skill is required to achieve reliable anesthesia. Consequently, we reserve the use of this block for patients who cannot be repositioned into the lateral position needed for the posterior approach. This technique is not ideal for catheter insertion because of the deep location and perpendicular angle of insertion required to reach the sciatic nerve.
The sciatic nerve is formed from the L4 through S3 roots. The roots of the sacral plexus combine on the anterior surface of the sacrum and are assembled into the sciatic nerve on the anterior surface of the piriformis muscle. The course of the nerve can be estimated by drawing a line on the back of the thigh from the apex of the popliteal fossa to the midpoint of a line joining the ischial tuberosity to the apex of the greater trochanter. The nerve exits the pelvis through the greater sciatic notch and gives off numerous articular (hip, knee) and muscular branches. Once in the upper thigh, it continues its descent behind the lesser trochanter and becomes completely covered by the femur. The only part of the nerve accessible to blockade through an anterior approach is a short segment slightly above and below the lesser trochanter. The muscular branches of the sciatic nerve are distributed to the biceps femoris, semitendinosus, and semimembranosus, and to the ischial head of the adductor magnus.
• Because the level of the blockade with the anterior approach to sciatic block is often below the departure of the muscular branches, twitches of the hamstring muscles cannot be accepted as a reliable sign of localization of the main trunk of the sciatic nerve.
Distribution of Blockade
A sciatic nerve block through the anterior approach results in anesthesia of the hamstring muscles below the blockade and the entire leg below the knee (including the ankle and foot) except for a strip of skin over the medial aspect. The distal two thirds of the hamstring muscles are also anesthetized. Neither the posterior cutaneous nerve of the thigh and articular branches of the hip are anesthetized, nor the skin over the medial aspect of the leg below, because it is innervated by the saphenous nerve, a branch of the femoral nerve. Consequently, the anterior approach to sciatic block should be chosen for selected patients undergoing knee or below-knee surgery who also are unable to be positioned for the posterior approach. A proximal thigh tourniquet should be reconsidered with this technique because of the risk of prolonged ischemia of the sciatic nerve, particularly when epinephrine-containing solutions of local anesthetics are used.
A standard regional anesthesia tray is prepared with the following equipment:
• Sterile towels and gauze packs
• One 20-mL syringe containing local anesthetic
• A 3- to 5-mL syringe plus a 25-gauge needle with local anesthetic for skin infiltration
• A 1.5-mm, 22-gauge short-bevel insulated stimulating needle
• Peripheral nerve stimulator
• Sterile gloves; marking pen
Landmarks and Patient Positioning
The patient is in the supine position with both legs fully extended.
• Placing a pillow underneath the patient’s hips can be useful to optimize access to the groin and landmarks for the block.
The following landmarks should be outlined routinely using a marking pen:
1. Femoral crease (Figure 19.2-1).
FIGURE 19.2-1. Landmarks for anterior sciatic block. Femoral crease is outlined as a line connecting anterior superior iliac spine (semicircle) and the finger palpating the pubic bone.
2. Femoral artery pulse (Figure 19.2-2).
FIGURE 19.2-2. Landmarks for the anterior sciatic nerve block. The index finger is on the pulse of the femoral artery.
3. Needle insertion point is marked 4 to 5 cm distally to the femoral crease on a line passing through the pulse of the femoral artery and perpendicularly to the femoral crease (Figures 19.2-3 and 19.2-4).
FIGURE 19.2-3. A line passing through the pulse of the femoral artery and perpendicular to the femoral crease is drawn.
FIGURE 19.2-4. Point of needle insertion is labeled 4-5 cm from the femoral crease on the perpendicular line that passes through the femoral pulse
• Avoid displacing the soft tissues laterally or medially during palpation of the femoral artery. The skin and subcutaneous tissue in this area are highly movable, and lateral or medial displacement of the tissues can skew the femoral artery landmark.
After cleaning the area with an antiseptic solution, local anesthetic is infiltrated subcutaneously at the determined needle insertion site. The operator should stand on the side of the patient to be blocked and have the ipsilateral foot in the line of vision to be able to monitor the patient and the responses to nerve stimulation.
The fingers of the palpating hand should be firmly pressed against the quadriceps muscle to decrease the skin-nerve distance and stabilize the needle path. The needle is introduced at an angle perpendicular to the skin plane (Figure 19.2-5). Initially, the nerve stimulator should be set to deliver a 1.5-mA current, as with all “deep” blocks. The current of higher intensity results in an exaggerated motor response, decreasing the chance of missing this twitch of the foot or toes during nerve localization.. The twitch of the foot or toes typically occurs at a depth of 10 to 12 cm. After obtaining negative results from an aspiration test for blood, 15 to 20 mL of local anesthetic is slowly injected. Any resistance to the injection of local anesthetic should prompt cessation of the injection attempt, followed by slight withdrawal. Persistent resistance to injection should prompt complete needle withdrawal and flushing of the needle before reattempting the block; see “tips” for more explanation of the importance of this strategy.
FIGURE 19.2-5. Needle insertion for anterior sciatic block.
• Because the needle transverses muscle planes, it is occasionally obstructed by muscle fibers. However, when resistance to injection is met, it is never correct to assume the needle is obstructed. The proper action is to withdraw the needle and check its patency by flushing before reinserting it.
Visible or palpable twitches of the calf muscles, foot, or toes at 0.2 to 0.5 mA.
• Local twitches of the quadriceps muscle are elicited often during needle advancement. The needle should be advanced past these twitches.
• Although there is a concern about femoral nerve injury with further needle advancement, this concern is theoretical. At this level, the femoral nerve is divided into smaller terminal branches that are mobile and are unlikely to be penetrated by a slowly advancing, short-beveled needle.
• Resting the heel on the bed surface may prevent the foot from twitching even when the sciatic nerve is stimulated. This can be prevented by placing the ankle on a footrest or by having an assistant continuously palpate the calf or Achilles’ tendon.
• Because branches to the hamstring muscle may leave the main trunk of the sciatic nerve before the level of needle insertion, twitches of the hamstring should not be accepted as a reliable sign of sciatic nerve localization Figure 19.1-2.
• Bone contact is frequently encountered during needle advancement, indicating the needle has contacted the femur (usually the lesser trochanter) (Figure 19.2-6). When the needle is stopped by the bone, the following algorithm is used:
FIGURE 19.2-6. Needle pass required to reach the sciatic nerve through the anterior approach. Note that the lesser trochanter of the femur partially obscures the sciatic nerve. Internal rotation of the leg (arrow) is beneficial in allowing access of the needle to the sciatic nerve .
Withdraw the needle back to the subcutaneous tissue.
Rotate the foot inward (internal rotation).
Advance the needle to bypass the lesser trochanter. The internal rotation of the leg also rotates the lesser trochanter posteriorly and away from the path of the needle and often allows passage of the needle toward the sciatic nerve.
When steps 1-3 fail to facilitate passage of the needle, the needle is withdrawn back to the skin and reinserted 1-2 cm medial to the initial insertion site and at a slightly medial angulation (Figure 19.2-7).
FIGURE 19.2-7. When the needle fails to pass by the trochanter minor despite internal leg rotation, the needle is inserted 1–2 cm medial to the initial insertion and advanced in a slight medial to lateral direction to reach the sciatic nerve.
Some common responses to nerve stimulation and the course of action to take to obtain the proper response are given in Table 19.2-1.
TABLE 19.2-1 Common Responses to Nerve Stimulation and Course of Action for Proper Response
• We avoid the use of epinephrine for the anterior approach to a sciatic nerve block because of the perceived risk of nerve ischemia due to the combined effects of the vasoconstrictive action of epinephrine and application of a tourniquet.
Block Dynamics and Perioperative Management
Performance of the anterior approach to a sciatic block is associated with patient discomfort because the needle must transverse multiple muscle planes on its way to the sciatic nerve. The administration of midazolam 2 to 4 mg after the patient is positioned and alfentanil 500–1000 mg just before infiltration of local anesthetic is beneficial to allay anxiety and decrease discomfort during the procedure in most patients. A typical onset time for this block is 20 to 30 minutes, depending on the type, concentration, and volume of local anesthetic used. The first sign of blockade onset is usually a report by the patient that the foot “feels different” or an inability to wiggle the toes.
• When indicated, the femoral block is performed first, resulting in anesthesia of the skin and muscle overlying the needle path for the anterior sciatic block and less patient discomfort.
Complications and How to Avoid Them
Table 19.2-2 lists some general and specific instructions on possible complications and methods that can be used to avoid them.
TABLE 19.2-2 Complications of Anterior Approach to Sciatic Nerve Block and Preventive Techniques
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