In the upper part of its course, the sciatic nerve lies deep in the gluteus maximus muscle and rests on the posterior surface of the ischium (Figures 37–3 and 37–4). The sciatic nerve crosses the external rotators, obturator internus, and gemelli muscles, then passes on to the quadratus femoris. The quadratus femoris separates the sciatic nerve from the obturator externus and the hip joint. Medially, the posterior cutaneous nerve of the thigh and the inferior gluteal plexus accompany the sciatic nerve, whereas more distally the sciatic nerve lies on the adductor magnus. The long head of the biceps femoris crosses the sciatic nerve obliquely. The articular branches of the sciatic nerve arise from the upper part of the nerve and supply the hip joint by perforating the posterior part of its capsule. However, the articular branches are sometimes derived directly from the sacral plexus. The muscular branches of the sciatic nerve innervate the gluteus, the biceps femoris, the ischial head of the adductor magnus, the semitendinosus, and the semimembranosus muscles (Figure 37-5; Table 37-1 ). The branches of the ischial head of the adductor magnus and semimembranosus muscles arise from a common trunk. The nerve to the short head of the biceps femoris comes from the common peroneal division, whereas the other muscular branches arise from the tibial division of the sciatic nerve.
The parasacral area is delineated by the ventral aponeurosis of the piriformis muscle dorsally, by the pelvic aponeurosis medially, and by the aponeurosis of the obturator internis muscle laterally.9 The common peroneal component passes through the piriformis muscle or above it, and only the tibial component passes below the muscle.
Choice of Local Anesthetic
Despite its large size, sciatic block requires a relatively low volume of local anesthetic to achieve anesthesia of the entire trunk of the nerve.12 Generally, 20–25 mL of local anesthetic are sufficient. The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or analgesia13 (Table 37-2). When prolonged pain relief is desired long-acting local anesthetic may be more appropriate.14,15 Epinephrine is not routinely used for sciatic nerve block because of the possibility of injury due to stretching or sitting on the anesthetized nerve with the long duration of block with epinephrine-containing local anesthetics. However, its use may be justified in patients undergoing above-knee amputation, in whom these issues are not pertinent and prolonged analgesia is always beneficial.
Branches, Source, and Motor Innervation of the Sacral Plexus
N. to obturator internus m.
Lumbosacral trunk and S1
Superior gluteal n.
Lumbosacral trunk and S1
Tensor fasciae lata
N. to piriformis muscle
N. to biceps femoris superior
Anterior portion of plexus
Biceps femoris superior
N. to biceps femoris inferior and quadratus femoris
Anterior portion of plexus
Biceps femoris inferior
Branch to coxofemoral articulation
Posterior femoral cutaneous nerve (lesser sciatic nerve)
Lumbosacral trunk, SI, S2
Inferior gluteal n. to gluteus maximus muscle
Sensory branch to buttock, thigh, popliteal fossa, and lateral aspect of knee
Local Anesthetic Choices for Sciatic Nerve Block: Duration of Anesthesia and Analgesia
All approaches to the sciatic nerve block necessitate assembly of a nerve block tray before placing the block. As with all regional anesthesia techniques, the heart rate, blood pressure, and pulse oximeter are routinely monitored before performing the block. Resuscitation equipment and emergency medications must be immediately available and ready to use. Supplemental oxygen via face mask is routinely used before giving sedation. A standard regional anesthesia tray is prepared with the following equipment:
• Sterile towels and 4-in. x 4-in. gauze packs
• 20-mL syringe with local anesthetic
• Sterile gloves, marking pen, and surface electrode
• One Ր/շ-տ., 25-gauge needle for skin infiltration
• A 10-cm long, short-bevel, insulated stimulating needle (15 cm for anterior approach)
• Peripheral nerve stimulator
Common Responses to Nerve Stimulation and Action to Take
Interpreting Responses to Nerve Stimulation
Twitches of the hamstrings, calf, foot, or toes at 0.2–0.5 mA current all can be used as signs of successful localization of the sciatic plexus (nerve). Table 37-3 presents common responses to nerve stimulation and the course of action to take to obtain the proper response.16
BLOCK DYNAMICS & PERIOPERATIVE MANAGEMENT
Sciatic nerve blockade technique may result in significant patient discomfort because the needle passes through the gluteus muscles. Adequate sedation and analgesia are important to ensure patient comfort. Midazolam 2–6 mg can be given for patient positioning, and alfentanil 500–750 meg is given just before needle insertion. A typical onset time for this block is 10–25 minutes, depending on the type, concentration, and volume of local anesthetic used. The first signs of blockade onset are usually reported by the patient in the form of a feeling that the foot is “different” and/or that he or she cannot wiggle the toes.
Inadequate skin anesthesia despite an apparent timely onset of the blockade can occur.
It can take up to 30 minutes for full sensory-motor anesthesia to develop.
Local infiltration at the site of the incision by the surgeon is often all that is needed to allow the surgery to proceed.
POSTERIOR APPROACHES TO SCIATIC NERVE BLOCK
The posterior approach to sciatic blockade has wide clinical applicability for surgery and pain management of the lower extremity. In contrast to common belief, this block is relatively easy to perform and is associated with a high success rate when properly performed.17,18 It is particularly well suited for surgery on the knee, calf, Achilles tendon, 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 (Figure 37-6). When combined with a femoral nerve or lumbar plexus block, anesthesia of almost the entire leg can be achieved.
Distribution of Anesthesia
Sciatic nerve blockade results in anesthesia of the skin of the posterior aspect of the thigh, hamstrings, and biceps muscles, part of the hip and knee joints, and the entire leg below the knee, with the exception of the skin of the medial aspect of the lower leg (see Figure 37-6). Depending on the level of surgery, the addition of a saphenous or femoral nerve block may be required.
Classic Posterior Approach Anatomic Landmarks
Landmarks for the posterior approach to sciatic blockade are easily identified in most patients (Figure 37-7). Proper palpation technique is of utmost importance because the adipose tissue over the gluteal area may obscure these bony prominences. The landmarks are outlined by a marking pen:
1. Greater trochanter
2. Posterior superior iliac spine
3. Needle insertion site 4 cm distal to the midpoint between the two landmarks
The patient is in the lateral decubitus position with a slight forward tilt. The foot on the side to be blocked should be positioned over the dependent leg so that twitches of the foot or toes can be easily noted. After cleaning with an antiseptic solution, local anesthetic is infiltrated subcutaneously at the determined needle insertion site. The anesthesiologist performing the block should assume an ergonomic position to allow precise needle maneuvering and monitoring of the responses to nerve stimulation.
Raise the height of the bed enough to allow a comfortable and stable position for the patient during block placement and for observation of the muscle twitches obtained during nerve stimulation.
The fingers of the palpating hand should be firmly pressed on the gluteus muscle to decrease the skin-nerve distance (Figure 37-8). The skin below the index and middle finger is stretched for greater precision during block placement. The palpating hand should not be moved during block placement; even small movements of the palpating hand can substantially change the position of the needle insertion site because the skin and soft tissues in the gluteal region are highly movable. The needle is introduced at an angle perpendicular to the spherical skin plane (Figure 37-8). The nerve stimulator should be initially set to deliver 1.5 mA current (2 Hz, 100 psec) to allow detection of twitches of the gluteal muscles and stimulation of the sciatic nerve.
As the needle is advanced, the first twitches observed are from the gluteal muscles. These twitches merely indicate that the needle position is still too shallow. The goal is to achieve visible or palpable twitches of the hamstrings, calf muscles, foot, or toes at 0.2–0.5 mA current. Twitches of the hamstrings are equally acceptable because this approach blocks the nerve proximal to the separation of the nerve branches to the hamstrings muscle. Once the gluteal twitches disappear, brisk response of the sciatic nerve to stimulation is observed (hamstrings, calf, foot, or toe twitches). After the initial stimulation of the sciatic nerve is obtained, the stimulating current is gradually decreased until twitches are still seen or felt at 0.2–0.5 mA current. This typically occurs at a depth of 5–8 cm.
After negative aspiration for blood, 15–25 mL of local anesthetic is injected (Figure 37-9). Any resistance to the injection of local anesthetic should prompt needle withdrawal by 1 mm. The injection is then reattempted. Persistent resistance to injections should prompt complete needle withdrawal and flushing to ensure needle patency before réintroduction.
Since the level of the blockade with this approach is above the departure of the branches for hamstring muscles, twitch of any of the hamstring muscles can be accepted as a reliable sign of localization of the sciatic nerve.
When the first needle pass does not result in nerve localization, do not regard it as a failure. Instead, use a systematic approach to troubleshooting:
1. Ascertain a functional nerve stimulator that is properly connected to the patient and needle and ensure that it is set to deliver the desired intensity of current.
2. Mentally visualize the plane of the initial needle insertion, and redirect the needle in a slightly caudal direction (5–10 degrees) to the initial insertion plane.
3. If the above maneuver fails, withdraw the needle to the skin and redirect it slightly cephalad (5–10 degrees) to the initial insertion plane.
4. Failure to obtain foot response to nerve stimulation should prompt a reassessment of the landmarks and patient position.
The continuous sciatic nerve block is an advanced regional anesthesia technique, and experience with the single-shot technique is recommended to ensure its efficacy and safety. Continuous sciatic nerve block was described by Gross in 1956.19 The current technique used is similar to the singleshot injection; however, slight angulation of the needle in the caudal direction is necessary to facilitate threading of the catheter. Securing and maintenance of the catheter are easy and convenient. This technique can be used for surgery and postoperative pain management in patients undergoing a wide variety of lower leg, foot, and ankle surgeries. Perhaps the single most important indication for use of this block is for amputation of the lower extremity.
The continuous sciatic block technique is similar to the singleshot technique. A standard regional anesthesia tray is prepared and an 8–10 cm long, insulated stimulating needle (preferably Tuohy-style tip) is used. Proper positioning at the outset and maintenance of the position during the continuous sciatic nerve block are crucially important to allow for precise catheter placement. A slight forward pelvic tilt prevents the “sag” of the soft tissues in the gluteal area and significantly facilitates block placement.
With the patient in the lateral decubitus position and a slight forward pelvic tilt, the landmarks are identified and marked with the pen. After a thorough skin cleaning with antiseptic solution, the skin at the needle insertion site is infiltrated with local anesthetic. A 10-cm long continuous- block needle is connected to the nerve stimulator (1.5 mA) and inserted at an angle perpendicular to the skin sphere. The opening of the needle should face distally (pointing toward the patient’s foot) to facilitate catheter insertion. The initial intensity of the stimulating current should be 1.0–1.5 mA.
It is useful to inject some local anesthetic intramuscularly to prevent pain on advancement of larger-gauge and blunt-tipped needles typically used for this block.