Indications

Contraindications

Clinical anatomy

The sciatic nerve originates from the lumbar and sacral plexus and is the largest nerve in the body. The ventral rami of L4 and L5 join with those of S1, 2, and 3 to form the sciatic nerve. It is made up of two major nerves: the common peroneal and the tibial. The sciatic nerve leaves the pelvis via the sciatic foramen below the piriformis, and then passes between the greater trochanter of the femur and the ischial tuberosity. It passes posterior to the lesser trochanter of the femur and here it is blocked by this approach. Once it emerges from under cover of the gluteus maximus muscle, it becomes superficial as it passes down the posterior thigh. The site of division into the common peroneal and tibial nerves is highly variable, often two-thirds the way from the gluteal region to the popliteal fossa.

The sciatic nerve provides sensory innervation to the posterior thigh, the lateral portion of the leg below the knee (the medial aspect being supplied by the saphenous nerve), and most of the foot. It also supplies sensory innervation to the head of the femur and partially to the capsule of the hip joint. It supplies motor innervation to the hamstring muscles, and through its branches it supplies all muscle groups distal to the knee.

Surface anatomy

Important bony structures for the anterior landmark-based sciatic block include the anterior superior iliac spine, the pubic tubercle, and the greater trochanter of the femur (Fig. 25.1). A line is drawn between the anterior superior iliac spine and the pubic tubercle (along the inguinal ligament). This line is divided into equal thirds. At the junction between the medial one-third and the lateral two-thirds, a perpendicular line is drawn extending into the thigh. A further line is drawn from the greater trochanter, parallel to the inguinal ligament. Where the perpendicular line crosses this line is the needle insertion point. Rotation of the hip due to leg position or pathology can change anatomic relations; this must be remembered when the needle insertion point is chosen. Important anatomic details include the close proximity of the femoral nerve at the site of needle insertion.

Figure 25.1 Landmarks for the anterior sciatic block. The anterior superior iliac spine and pubic tubercle are marked. The measured distance between these points is divided into equal thirds. At the junction between the medial one-third and lateral two-thirds, a perpendicular line is drawn extending into the thigh. A second line is drawn, parallel to the inguinal ligament from the greater trochanter. Where this line intersects with the vertical line is the needle insertion point. 1: anterior superior iliac spine; 2: pubic tubercle; 3: inguinal ligament divided into equal thirds; 4: vertical line; 5: greater trochanter; 6: needle insertion point.

Sonoanatomy

The sciatic nerve may be difficult to visualize in this region because of the required depth of beam penetration and the use of a lower frequency transducer. A transverse transducer orientation is used (Fig. 25.2). Visualization of the sciatic nerve may be obstructed by the lesser trochanter of the femur. Perform a systematic anatomical survey from proximal to distal and from lateral to medial. First identify the femur, a curved hyperechoic line with an underlying bone shadow. Move the transducer proximally and distally to identify the lesser trochanter. Identify the anterior muscular layers: quadriceps muscles laterally and the adductor muscles medially (approximately 8 cm from inguinal crease in adults; Fig. 25.2). Identify the gluteus maximus muscle posteriorly. The gluteus maximus muscle bulk gets smaller as the transducer is moved more distally away from the inguinal crease. Locate the hyperechoic sciatic nerve deep to the adductor muscles and posterior to the femur (Fig. 25.3). The ultrasound image of the sciatic nerve in cross section is typically seen as an oval-to-circular hyperechoic structure. It is often vaguely delineated or appears isoechoic to the surrounding muscles, the latter particularly if using a tangential ultrasound beam plane.

Figure 25.2 Transverse transducer orientation for the ultrasound-guided anterior sciatic nerve block.

Figure 25.3 Transverse ultrasound image of the anterior thigh region (approximately 8 cm from inguinal crease in adult). F: femur; QM: quadriceps muscle; AM: adductor muscles; GMM: gluteus maximus muscle; SN: sciatic nerve.

If the lesser trochanter obstructs visualization of the sciatic nerve, move the transducer further on the medial aspect of the anterior thigh and orient the transducer in a slightly anterior–posterior direction. This orientation will allow the transducer to capture the best possible transverse view of the sciatic nerve behind (posterior to) the lesser trochanter. Angle the transducer slightly cephalad or caudad to optimize the angle of incidence (90°) to capture the best possible nerve image. Scan the nerve proximally (cephalad) and distally (caudad) to follow the course of the nerve and to confirm nerve identity. A longitudinal scanning approach can also be used to identify the sciatic nerve at the lesser trochanter (Fig. 25.4).

Figure 25.4 Longitudinal ultrasound image of the anterior thigh region. F: femur; QM: quadriceps muscle; AM: adductor muscles; GMM: gluteus maximus muscle; SN: sciatic nerve.

Technique

Landmark-based approach

As for all regional anesthetic procedures, after checking that emergency equipment is complete and functional, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

The patient is placed in the supine position with the leg in the neutral position. The operator stands on the side to be blocked, at the level of the patient’s thigh. The area of the needle track is anesthetized with local anesthetic. Analgesia or sedation is very desirable for this block. A 150-mm insulated needle is inserted perpendicularly with some lateral angulation in line with the lesser trochanter of the femur (Fig. 25.5). The stimulating current is set at 1.2 mA, 2 Hz, and 0.1 ms.

Figure 25.5 Anterior sciatic block technique. The needle is inserted with a slightly lateral orientation. If the lesser trochanter is contacted, the needle is withdrawn and aimed more medially. The sciatic nerve is usually contacted at 8–12 cm from the skin, depending on thigh thickness.

On contact with the lesser trochanter, the needle is withdrawn, angulated in a less lateral orientation, and advanced; the sciatic nerve is usually stimulated 3–4 cm deeper (Fig. 25.6). Total needle depth to the sciatic nerve is approximately 70% of thigh thickness; usually 8–12 cm. The needle is advanced slowly until the appropriate muscle response is obtained: tibial nerve component produces plantar flexion and inversion of the foot, while common peroneal stimulation produces dorsiflexion and eversion of the foot. The needle position is adjusted while reducing the current to 0.35 mA with maintenance of the muscle response. Hamstring contraction response is not accepted. On its path to the sciatic nerve, femoral nerve stimulation may be initially found. To ensure safe practice, the femoral nerve should not be blocked before sciatic nerve block.

Figure 25.6 Axial T1-weighted MR image showing anatomy of proximal thigh at site of injection for anterior approach to sciatic nerve block. Note proximity of femoral nerve and vessels to track of needle. 1: Femoral nerve and vessels; 2: sciatic nerve; 3: gluteal vessels; 4: gluteus maximus.

Incremental injection of 20–30 mL of local anesthetic is made with repeated aspiration.

Ultrasound-guided approach

Intravenous access, electrocardiogram (ECG), pulse oximetry and blood pressure monitoring are established. Maximized comfort for the operator and patient is an important step in pre-procedure preparation. For the ultrasound-guided anterior sciatic block, the patient is placed in the supine position with the knee flexed and the ipsilateral hip externally rotated. The operator stands on the side to be blocked with the ultrasound screen on the same side above the hip (Fig. 25.7).

Figure 25.7 Global view of the block field for the ultrasound-guided anterior sciatic nerve block.

The skin is disinfected with antiseptic solution and draped. A sterile sheath (CIVCO Medical Instruments, Kalona, IA, USA) is applied over the ultrasound transducer with sterile ultrasound gel (Aquasonic, Parker Laboratories, Fairfield, NJ, USA). Another layer of sterile gel is placed between the sterile sheath and the skin. The anterior thigh region is scanned in the transverse plain with a low frequency transducer. The ultrasound screen should be made to look like the scanning field. That is, the right side of the screen represents the right side of the field. Adjustable ultrasound variables such as scanning mode, depth of field, and gain are optimized.

A skin wheal of local anesthetic is raised at a distance from the medial aspect of the ultrasound transducer to facilitate sterility and allow a shallow angle of approach to improve needle visualization. The needle bevel should face the active face of the transducer to improve visibility of the needle tip. A free hand technique rather than the use of a needle guide is preferred. A 22-GA × 120-mm insulated regional block needle (B. Braun, Bethlehem PA) is inserted within the plane of imaging to visualize the entire shaft and bevel along the path of the ultrasonic beam (Fig. 25.8). Advance the needle in a medial to lateral direction as well as an anterior to posterior direction when the thigh is externally rotated.

Figure 25.8 Ultrasound transducer and needle positioning during the ultrasound-guided anterior sciatic block. Note the needle orientation in the same plane as the ultrasound beam.

The needle is attached to sterile extension tubing, which is connected to a 20-mL syringe and flushed with local anesthetic solution to remove all air from the system. The operator can slide and tilt the transducer to maintain the needle tip within the plane of imaging as much as possible. The needle tip should be clearly identified within the plane of imaging before advancing the needle. Because of the steep angle of needle advancement, it may be difficult to clearly visualize the block needle. Often, needle and tissue (muscle) movements are observed without a clear view of the needle shaft and tip. Also adjustment in needle direction can be technically challenging. Change in needle angle often requires needle withdrawal and a second needle attempt through the adductor muscles.

Figure 25.9 Needle end point for ultrasound-guided anterior sciatic block before injection. F: femur; QM: quadriceps muscle; AM: adductor muscles; GMM: gluteus maximus muscle; SN: sciatic nerve.

The needle is brought in contact with the nerve, taking care not to puncture it. If an electrical nerve stimulation technique is used (recommended here) characteristic motor activity of either dorsi- or plantar-flexion is elicited in the foot. After negative aspiration, 1 mL of local anesthetic is injected. If spread of local anesthetic is satisfactory, then a further 3–4 mL of local anesthetic is injected. The needle tip is readjusted posterior to the nerve and another aliquot of local anesthetic is deposited around it. The needle tip is manipulated if required to deposit anesthetic on all sides of the sciatic nerve.

A hypoechoic (fluid) expansion can be seen during local anesthetic injection (Fig. 25.10). Fluid expansion in the adductor muscles indicates intramuscular injection. Appropriate needle advancement is required. Inject 15–20 mL of local anesthetic for postoperative analgesia.

Figure 25.10 Needle end point for ultrasound-guided anterior sciatic block after injection. F: femur; QM: quadriceps muscle; AM: adductor muscles; GMM: gluteus maximus muscle; SN: sciatic nerve, LA: local anesthetic.

Adverse effects