Sciatic nerve blockade is typically used to treat painful conditions of the lower leg such as complex regional pain syndrome and to facilitate physical therapy by decreasing pain in the lower extremity.
A regional block of the sciatic nerve can be achieved anywhere along the course of the nerve. Most of the approaches have been developed mainly to avoid positioning problems that may be present in trauma patients or elderly people. The nerve can be blocked at the sciatic notch, at the level of the ischial tuberosity, greater trochanter, or superior aspect of the popliteal fossa.
The classic technique described by Labat (37) blocks the nerve at the level of the greater sciatic notch, using the piriformis muscle as a landmark. After informed consent is obtained, the patient is placed in the lateral Sims’ position with the side to be blocked uppermost. The upper knee is flexed, and the patient’s back is rotated slightly forward. Some patients may find this position uncomfortable, particularly those with orthopedic problems.
The landmarks are the cephalad portion of the greater trochanter and the posterosuperior iliac spine. A line is drawn connecting these two points, corresponding to the superior border of the piriformis muscle and the upper border of the sciatic notch. A perpendicular line is drawn distally from the midpoint of the first line. The point of injection is 3 to 5 cm distal on the perpendicular line. Verification of the insertion point can be made by drawing a third line connecting the cephalad portion of the greater trochanter and the sacrococcygeal joint. This third line is used to compensate for the height of the patient. The intersection of lines 2 and 3 is the point of needle insertion (Fig. 67-15).
FIGURE 67-15. Sciatic nerve block: classic approach. Classic approach for sciatic nerve injection and neural blockade.
The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A 4- to 5-in. (10- to 12-cm), 22-gauge spinal needle is introduced at right angles to the skin and advanced to a depth of 6 to 10 cm until a paresthesia is reported in the distribution of the sciatic nerve, preferably involving the foot. If periosteum is contacted, the needle is then redirected medially or superiorly. Touching the periosteum may produce a local paresthesia, which could be mistaken for a true sciatic nerve paresthesia. A nerve stimulator is extremely helpful in locating the nerve (39).
Doppler ultrasound also can be used to locate the dominant arterial structure within the sciatic notch (40). The needle is then advanced in the same orientation as the probe until a paresthesia is obtained. Successful blockade has been reported after one or two attempts in 70% of patients. After negative aspiration for blood, 20 to 30 mL of local anesthetic is injected to block the sciatic nerve.
A continuous sciatic nerve block can be performed by using a standard 16-gauge intravenous infusion cannula attached to a nerve stimulator. After obtaining muscle contraction in the lower leg, preferably dorsal or plantar flexion of the foot, an epidural catheter is advanced about 6 cm into the neurovascular space. Continuous infusion of a local anesthetic using an infusion pump can then be used to provide continuous analgesia (41). With the classic approach, both the posterior femoral cutaneous and pudendal nerves are usually blocked with the sciatic nerve.
An alternate approach may be used, with the patient positioned as above or prone. The ischial tuberosity and the greater trochanter are identified and a line drawn connecting these two points. The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A 3- to 4-in. (8- to 10-cm), 22-gauge spinal needle is inserted at the midpoint of the line until a paresthesia is elicited in the lower leg. After negative aspiration, 20 to 30 mL of local anesthetic is injected to block the sural nerve. The posterior femoral cutaneous nerve is often blocked at this level, but the pudendal nerve is frequently spared.
The anterior approach allows the sciatic nerve to be blocked without moving the patient, enabling the patient to remain in the supine position (37,42). This approach is especially helpful in trauma patients with a painful leg, but it is quite painful, and sedation is often necessary. The nerve is very deep at this point and can be difficult to locate. In adults, the sciatic neurovascular compartment is usually 4.5 to 6 cm below the surface of the femur. In children, however, the distance varies according to age and size of the child (43). The use of a nerve stimulator is advised in identifying the nerve. The posterior cutaneous nerve of the thigh may not be blocked with this approach, as tourniquet pain could result if a thigh tourniquet is applied (Fig. 67-16).
FIGURE 67-16. Sciatic nerve block: anterior approach. Anterior approach for sciatic nerve injection and neural blockade. Cross section of the leg at the level of the lesser trochanter to show the relationship between the sciatic nerve and femur and the fascia separating it from the adductor magnus.
The patient is placed in the supine position with the leg in a neutral position. The anterosuperior iliac spine and the pubic tubercle are identified and marked. A line is then drawn connecting these two points, overlying the inguinal ligament, and trisected into equal parts. A perpendicular line is drawn distally from the junction of the medial and middle thirds. A third line is drawn parallel to the first, starting from the cephalad aspect of the greater trochanter. The point of intersection of this third line and the perpendicular line is the insertion point of the needle.
The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A 6-in. (15- cm), 22-gauge spinal needle is inserted and directed slightly laterally from a plane perpendicular to the skin. The needle is advanced until periosteum is contacted (usually the lesser trochanter). The needle is partially withdrawn and redirected medially and posteriorly to pass about 5 cm beyond the femur until a paresthesia is elicited. After negative aspiration, 20 to 25 mL of local anesthetic agent is injected to block the sural nerve.
The lateral approach, initially described by Ichiyanaghi (44), was found to be very difficult and never became popular. A new lateral approach described by Guardini et al. (45) is easier. It blocks the sciatic nerve just posterior to the quadratus femoris muscle in the subgluteal space.
The greater trochanter is identified, and the patient is prepared in a standard sterile fashion. A 5- to 6-in. (12- to 15-cm), 22-gauge spinal needle is advanced 3 cm distal to the maximum lateral prominence of the trochanter, close to its posterior margin. The needle is inserted until the periosteum is contacted. The needle is then partially withdrawn and redirected posteriorly and medially to slide beneath the femoral shaft until a paresthesia, or contraction of the calf or the anterior compartment muscles, occurs with the use of a nerve stimulator. After negative aspiration, 20 to 30 mL of local anesthetic is injected to block the sural nerve.
The main advantage of this technique is that the patient can remain in the supine position and the leg need not be manipulated. When using a nerve stimulator, it is important to make sure that the muscle contractions occur in the calf muscles or in the muscles of the anterior compartment. It is possible with this technique to stimulate inadvertently the nerve branch supplying the two heads of the biceps femoris muscle, producing thigh muscle contraction and misplacement of the local anesthetic.
The sciatic nerve is the largest in the body. It arises from both the lumbar and sacral plexuses. Anatomically, the sciatic nerve consists of two major nerve trunks: the tibial and common peroneal components. The tibial nerve is derived from the anterior rami of L4 to S3 nerve roots. The common peroneal nerve is derived from the dorsal branches of the anterior rami of the same roots. It leaves the pelvis along with the posterior cutaneous nerve of the thigh through the sciatic foramen beneath the inferior margin of the piriformis muscle. It passes halfway between the greater trochanter and the ischial tuberosity. It becomes superficial at the inferior border of the gluteus maximus muscle and travels down the posterior aspect of the thigh. At the superior aspect of the popliteal fossa, the sciatic nerve physically separates into the tibial and common peroneal nerves.
In the past, the sciatic nerve block was considered unreliable, technically difficult, and uncomfortable for the patient. Sedation was often required, and this interfered with the patient’s ability to provide accurate verbal feedback. This was especially the case if a paresthesia was used to identify the nerve. Reported rates of success ranged between 33% and 95% using various techniques. Today’s insulated needles and nerve stimulators have made it easier to perform this block safely in sedated or even anesthetized patients with a higher rate of success.
Although the sciatic nerve is composed of mostly somatic nerves, it has a sympathetic component. The resulting sympathetic block may allow some mild venous pooling, but this is usually insufficient to cause clinically significant hypotension. Residual dysesthesias have been reported but usually improve in 1 to 3 days. This may be the result of nerve injury from the use of long beveled needles. Using short beveled needles for regional blocks may decrease the incidence of nerve injury.