CHAPTER 25 Anterior sciatic block
Surgical procedures on the lower limb from the knee distally, including Achilles’ tendon repair and most foot surgery (excluding the area supplied by the saphenous nerve); in combination with a lumbar plexus block for knee surgery, including total knee replacement and cruciate ligament repair.
Hemorrhagic diathesis; anticoagulation therapy; local neural injury; risk of lower extremity compartment syndrome (e.g. fresh fractures of the tibia and fibula, or especially traumatic and extensive elective orthopedic procedures of the tibia and fibula); and distorted anatomy due to fractured femur.
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.
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.
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.