The sciatic nerve is the largest nerve in the human body. It is composed of the tibial and peroneal segments and is derived from L4-S3 anterior rami. The sciatic nerve runs through the greater sciatic foramen to the gluteal region deep to gluteus maximus muscle. Most of the conventional approaches for sciatic nerve blockade at this level are relatively deep and therefore not attractive for ultrasonography in much of the adult surgical population. Once the sciatic nerve enters the dorsal side of the thigh between the caudal border of the gluteus maximus and the biceps muscles, its position is more superficial, appears as an oval or flat hyperechoic structure, is more easily seen with current ultrasound technology, and is an easier block to perform under ultrasound guidance. Posterior cutaneous nerve of the thigh is in a medial position relative to the sciatic nerve at the subgluteal level. Distally, the sciatic nerve components (tibial and common peroneal) run between the semitendinosus (medial) and the biceps femoris muscle (lateral) where division into the two end branches can be observed at 5-10 cm superior to the popliteal crease and lateral and more superficial to the popliteal vessels (artery and vein).
With the patient positioned in the semi-lateral to lateral decubitus position (typically), the ultrasound probe is placed to scan in the horizontal plane between the ischial tuberosity and the greater trochanter of the femur (Fig. 21.10a). Subcutaneous fat and the gluteus maximus muscle are superficial to the sciatic nerve, and the nerve is usually a flattened or ovoid hyperechoic structure at this level and somewhat closer to the ischial tuberosity than to the femur. The conjoint tendon of the long head of the biceps muscle, tendon of the semitendi-nosus muscle and tendon of the semimembranosis muscle insert on the ischial tuberosity which may make it difficult to distinguish from the sciatic nerve since there appearance in the transverse plane may all seem similar (Fig. 21.10b).
Ultrasound visualization and blockade of the sciatic nerve is a theoretically attractive technique that should be possible in clinical practice due to the large size of this nerve. However, this block may be challenging for two reasons: first, the depth and distinctive
anisotropy, where appropriate ultrasonographic visualization is only possible when the ultrasound beam is perpendicular to the nerve and the lack of adjacent vascular structures to guide approximation of anatomical nerve location; second, in obese individuals the depth of the nerve is increased and the superficial adipose layer creates a hyperechoic layer that impedes visualization of deeper nerve structures. Therefore, when performing a sciatic nerve block, two major considerations must be considered: the choice of an appropriate needle insertion site (including the necessity to block the posterior cutaneous nerve of thigh) and the best level to visualize the nerve pending the body habitus of the patient.
Usually the best ultrasound image of the proximal sciatic nerve can be achieved in the subgluteal region (described above). At the subgluteal level the sciatic nerve lies in a more superficial position and can be identified between the gluteus maximus and biceps muscles and superficial to the quadratus femoris and adductor magnus muscles between the two boney landmarks of the greater trochanter and ischial tuberosity (Fig. 21.10c). At this level, the sciatic nerve appears as round to oval to flat and only moderately hyperechoic structure. The sciatic nerve is often easier to find more distal as a hyperechoic structure medial to biceps femoris in the popliteal region and then followed cephalad to the subgluteal region.
When performing a subgluteal approach to the sciatic nerve, a curvilinear ultrasound probe is most often used in adults. Depending upon the depth of the sciatic nerve, ultrasound probe frequencies between 2 and 5 MHz are appropriate for optimal visualization. With the patient placed either in the supine position with the hip flexed to 90° or in the lateral or prone position, the block can be performed with a 50- to 70-mm short bevel needle and a needle insertion point with an out-of-plane technique in the transverse plane (Fig. 21.10a). Currently, larger volumes of local anesthetic are being used for ultrasound-guided blockade of the sciatic nerve (20-30 ml); however, these volumes may decrease as appropriate evidence for efficacy and safety continues to be determined. In thinner adults and in pediatric patients, it is often possible to use an in-plane technique and this facilitates the approach of the needle
both deep and superficial to the nerve to obtain optimal local anesthetic spread around the sciatic nerve.
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