BLOCK AT A GLANCE
Block of the sciatic nerve at the gluteal, subgluteal, or proximal thigh level.
Indications: Anesthesia and analgesia for foot and ankle surgery, procedures involving the posterior aspect of the thigh and knee, and for above-knee amputation
Goal: Local anesthetic spread within the sheath containing the sciatic nerve
Local anesthetic volume: 10 to 20 mL
The sciatic nerve block is a well-established regional anesthesia technique for lower extremity surgery, both with or without combination of the saphenous nerve block. It is sometimes performed as an optional component of multimodal analgesia strategies for knee surgery. The development of ultrasound (US) guidance in regional anesthesia has increased the success rate while reducing the required local anesthetic (LA) volume for a successful block. The sciatic nerve can be accessed at several levels along its proximal course (parasacral, gluteal, subgluteal) and through different needle approaches (posterior, lateral, or anterior). However, the popularity of these techniques appears to be declining in favor of more distal approaches or motor sparing blocks of the sensory branches of the sciatic nerve components. For instance, popliteal or ankle blocks are preferred for ambulatory foot surgery (see Chapters 29 and 32) as they interfere less with early ambulation. Similarly, interventional analgesia for knee surgery is evolving toward more selective blocks targeting the terminal sensory branches of the posterior knee capsule (see Chapter 31).
Obtaining an optimal image of the sciatic nerve at the gluteal level may be challenging due to its deep location, particularly in obese patients. The distribution of anesthesia below the knee resulting from a proximal block is equivalent to that of a popliteal approach, but the resulting motor block of the posterior compartment of the thigh limits the patient autonomy. The deeper needle path toward the sciatic nerve is less tolerated requiring higher degree of sedation.
The sciatic nerve is a large nerve, originating from the lumbosacral plexus (L4-L5 and S1-S3). The nerve exits the pelvis through the greater sciatic foramen below the piriformis muscle. It courses distally along the fascial planes between the gluteus maximus muscle (posterior) and the inner group of muscles (i.e., superior and inferior gemellus, obturator internus, and quadratus femoris muscles). The nerve descends into the posterior thigh between the greater trochanter of the femur and ischial tuberosity and continues down to the popliteal fossa between the biceps femoris and adductor magnus. The posterior femoral cutaneous nerve runs along the sciatic nerve at the gluteal level until they are separated by the insertion of the biceps femoris muscle at the ischial tuberosity (Figure 28-1).
Anatomy of the sciatic nerve in the posterior thigh.
CROSS-SECTIONAL ANATOMY AND ULTRASOUND ANATOMY