Proximal Sciatic Nerve Blocks

Four main proximal techniques^[250] are suitable for approaching the sciatic nerve: the posterior approach, the anterior approach, the frontal ischial approach, and the lateral approach.

Posterior Approach

The posterior approach is the classic proximal technique for a sciatic nerve block (after Labat). The child is placed in the semiprone position, resting on the nonoperated side. The landmarks are the greater trochanter of the femur and the caudal extremity of the coccyx ( Fig. 45-15 ). The

Figure 45-15 Proximal sciatic nerve block: posterior approach, showing the greater trochanter of the femur (1) and the coccyx (2).

Anterior Approach

The anterior approach to the sciatic nerve was developed to overcome the problem of the patient's positioning for those who should remain in the dorsal position.^{[251] [252]} The landmarks are the inguinal ligament and the greater trochanter of the femur. The location of the site of puncture is rather complicated, requiring that two lines be drawn: a line parallel to the inguinal ligament passing over the greater trochanter and a parallel line to the major axis of the body that passes over the union of the medial one third with the lateral two thirds of the inguinal ligament ( Fig. 45-16 ). The site of puncture is located at the crossing of the two lines. The needle is inserted almost perpendicular to the skin and toward the medial edge of the femur (in the direction of the lesser trochanter of the femur) until twitches are elicited in the foot. The distance at which the sciatic nerve is found can be correlated with the patient's age (see Fig. 45-8 ). This technique is consistently more difficult than the posterior approach and can result in inadvertent puncture of the femoral vessels. Several attempts have been made to simplify the landmarks, but they still remain complicated and not applicable to young children.^{[253] [254]}

Frontal Ischial Approach

Raj and colleagues^[255] described a frontal approach to penetrate the subgluteal space where the sciatic nerve runs. This technique, which was used for a long time and still is in children, especially in infants, has gained a renewed interest for use in adults.^{[256] [257]} The patient is

Figure 45-16 Proximal sciatic nerve block: anterior approach, showing the anterior superior iliac spine (1), greater trochanter of the femur (2), sciatic nerve (3), and public tubercle (4).

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Figure 45-17 Proximal sciatic nerve block: frontal ischial approach, involving the posterior aspect of the thigh.

Lateral Approach

A lateral approach to the sciatic nerve has been described for use in a patient lying supine^[258] with the leg rotated slightly medially. The only landmark is the greater trochanter of the femur. The site of puncture lies 1 to 2 cm below the lateral skin projection of the greater trochanter. The needle is inserted horizontally and toward the lower border of the femur ( Fig. 45-18 ). If bone contact is made, the needle is withdrawn and reinserted slightly more posteriorly until twitches are elicited in the leg and foot. The distance at which the sciatic nerve is found can be correlated with the patient's age (see Fig. 45-8 ). This technique has elicited several publications about its use in adult patients. It is suitable for catheter placement^[259] but requires immobilization of the lower extremity throughout the postoperative period to avoid inadvertent catheter removal (which is common in children).

All the proximal approaches to the sciatic nerve require rather large doses of local anesthetic (see Table 45-11 ). Infusion regimens for continuous blockade are displayed in Table 45-12 . With any approach, the distribution of anesthesia is the same, involving the territory supplied by the sciatic nerve and, in virtually all patients, that supplied by the posterior femoral nerve block (i.e., the dorsal part of the thigh).

Figure 45-18 Proximal sciatic nerve block: lateral approach, showing the femur (1).