Sciatic Nerve Block

Indications

Few surgical procedures can be performed with sciatic block alone. It is usually combined with femoral ("three-in-one") or psoas compartment nerve blocks to produce surgical anesthesia of the entire lower extremity. Application of a thigh tourniquet or involvement of the medial foot and ankle in the surgical field (saphenous nerve distribution) usually necessitates supplementary blockade of the femoral nerve. The advantages of sciatic-femoral blockade as a sole anesthetic technique include avoidance of general anesthesia, avoidance of neuraxial blockade, minimization of hemodynamic effects¹, and provision of long-lasting postoperative analgesia.

Sciatic block with or without femoral nerve block can be used for postoperative pain control following foot and ankle surgery. Neural blockade produces high quality analgesia following open reduction of calcaneal fractures and avoids the side effects related to PCA morphine use ². Duration of analgesia may be longest, and patient satisfaction highest, when the block is performed at the end of surgery immediately following application of the splint while the patient is still under general anesthesia. The risk of masking the symptoms of developing compartment syndrome with a dense regional block should be considered when the block is initiated following open reduction and internal fixation of tibia and fibula fractures.

Anatomy

The sciatic nerve is the largest single nerve trunk of the body and in the average adult has a diameter about as large as the thumb (16-20 mm). It arises from the L4, L5, S1, S2, S3 spinal roots and exits the pelvis posteriorly through the greater sciatic foramen and runs laterally along the posterior surface of the ischium anterior to the piriformis muscle. The posterior cutaneous nerve of the thigh accompanies the sciatic nerve as it exits the greater sciatic foramen. The sciatic nerve has medial and lateral components which separate into the tibial and the common peroneal nerves in the superior aspect of the popliteal fossa.

Approach

There are four approaches to blocking the sciatic nerve: the classic posterior approach of Labat, and the anterior, lateral, and supine lithotomy approachs. The anterior, lateral, and lithotomy approaches have the advantage of keeping the patient supine. A higher failure rate characteries the anterior approach ³. The classic approach requires placing the patient in a lateral (Sim's) position which may be painful in the multiply-injured patient and contraindicated in the presence of a potential spine injury.

Classic Posterior Approach: Technique

The patient is placed in the lateral (Sim's) position, with the operative side nondependent. The operative extremity is flexed 45 degrees at the hip and 90 degrees at the knee and rests against the dependent lower extremity.

The posterior superior iliac spine (PSIS), greater trochanter, and sacral hiatus are identified and marked with a skin marker (Figure 1). Since the greater trochanter is a large landmark, marking the most superior and posterior aspect of the greater trochanter helps maintain consistency in landmarks between patients. Consistency in positioning is also critical for success of the block and can be checked by placing the PSIS, most supero-posterior aspect of the greater trochanter, and the head of the fibula along a straight line.

A line is drawn with a skin marker between the greater trochanter and PSIS (Figure 2). This line is bisected. A perpendicular is dropped 3-5 cm from the midpoint of this line to the point of needle insertion. The point of needle insertion should lie along a third line drawn between the greater trochanter and the sacral hiatus (Figure 3).

The area of needle insertion is sterilely prepped and draped. In the awake patient a wheal of local anesthetic is placed and a 6 inch 22-gauge short-bevel teflon-coated nerve stimulator needle is advanced perpendicular to the skin. The nerve lies about 6-8 cm deep (Figure 4). Stimulation intensity is initially set at 1.5-2.0 mA and adjusted downward as the evoked motor response increases (Figure 5). Plantar flexion (downgoing toes) at less than 0.5 mA is the desired motor response and indicates placement of the needle near the medial part (tibial component) of the nerve (Figure 6, Movie: QuickTime, 554 K).

After a negative aspiration and negative epinephrine-containing test dose, the needle is held immobile and local anesthetic is injected incrementally, with attention paid to the presence of paresthesias, reflex movement, and resistance to injection (Figure 7, 8). Although sciatic blocks can be performed without the use of a nerve simulator by seeking paresthesias in the awake patient, use of a nerve stimulator results in high success rates and improves the success of the block^{4 , 5, 6}. The nerve stimulator also allows one to perform the block with patients under general or spinal anesthesia. Since the nerve trunk is large, onset time and efficacy may be improved by injecting local anesthetic in more than one location, such as both laterally (peroneal component) as well as medially (tibial component) ⁷.

Choice Of Local Anesthetic

Twenty to 30 cc of 0.375% to 0.5% bupivacaine or ropivacaine with epinephrine is injected in divided doses. Use of higher concentrations of local anesthetic may decrease the latency of onset of anesthesia and motor block⁸. A large mass and volume of long-acting agent is chosen to obtain the maximum duration of analgesia. The mean duration of analgesia following 20 cc of 0.5% bupivacaine is 14 hours but can range up to 24 hours. Alkalinization of bupivacaine reduces the time to onset and prolongs the duration of useful analgesia⁹. The transition from the sciatic block to parenteral opioid analgesics when the block dissipates should be anticipated.

When surgical anesthesia rather than postoperative analgesia is the goal shorter-acting local anesthetics can be used. Lidocaine with epinephrine has been used successfully for outpatient procedures with reasonable recovery times to ambulation and discharge¹⁰.

Complications

As in other peripheral nerve blocks, local anesthetic toxicity from intravascular injection is a risk but is minimized by the use of epinephrine test doses and incremental injections. It should be kept in mind that epinephrine test doses are less sensitive in patients under general anesthesia. Nerve injury from intraneural injection is avoided by use of a nerve stimulator and avoidance of paresthesias in the awake patient, and with careful attention paid to stimulation intensity. Strong motor evoked responses to low stimulation currents (<4 mA) may indicate intraneural placement. Local anesthetic should flow easily into the perineural space on injection: resistance suggests intraneural needle placement. Tactile feedback may be improved through the use of smaller (10 cc or less) syringes rather than larger (20 cc) syringes. Neural ischemia should be considered in patients with severe peripheral vascular disease who present with nerve injury following sciatic block ¹¹.

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