CHAPTER 24 Posterior sciatic block

Clinical anatomy

The sciatic nerve originates from the lumbar and sacral plexes 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 arises on the pelvic surface of the piriformis muscle. It then passes out of the pelvis into the gluteal region through the greater sciatic foramen below the piriformis muscle, and descends between the greater trochanter of the femur and the ischial tuberosity (Figs 24.1 and 24.2). Once it emerges from under cover of the gluteus maximus, it becomes superficial as it passes down the posterior thigh.

Figure 24.1 Cadaver structures illustrating anatomy pertinent to the sciatic block technique. 1: retracted gluteus maximus; 2: gluteus medius; 3: piriformis; 4: superior and inferior gemelli; 5: quadratus femoris; 6: sciatic nerve; 7: posterior cutaneous nerve of thigh; 8: inferior gluteal nerve and artery; 9: superior gluteal artery.

Figure 24.2 Axial T1-weighted MR image showing anatomy of sciatic nerve at site of block using the classical posterior approach of Labat. 1: greater trochanter; 2: quadratus femoris muscle; 3: ischial tuberosity; 4: sciatic nerve; 5: gluteus maximus muscle.

It divides at a variable distance, often two-thirds of the way down the posterior thigh, into the common peroneal and tibial nerves.

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, a branch of the femoral nerve), and most of the foot. It also supplies innervation to the head of the femur and partially to the capsule of the hip joint. It supplies motor innervation to the hamstrings and all muscle groups distal to the knee.

Surface anatomy

Important bony structures for the posterior sciatic block include the greater trochanter of the femur, the posterior superior iliac spine, and the sacral hiatus. The greater trochanter can be difficult to identify exactly. It can be identified by palpating the lateral aspect of the proximal femur; ‘walking’ upward, one’s finger tends to ‘fall off’ the bone when the apex of the greater trochanter is reached. The apex of the greater trochanter lies approximately a hand’s breadth below the lateral aspect of the iliac crest. It is easier to palpate when the patient’s hip is passively abducted to relax the gluteus medius and maximus. The posterior superior iliac spine is the bony prominence at the posterior end of the iliac crest. It is directly below the ‘sacral dimple’ (dimple of Venus), a depression in the skin visible above the buttock, close to the midline. Palpation from the iliac crest can help to correctly identify the posterior superior iliac spine.

A consistent method of outlining the greater trochanter is needed because this is a large structure and variations can affect further marking. It is suggested that the outer perimeter of the greater trochanter is used for line drawings. A line is drawn between the posterior superior iliac spine and the greater trochanter (Fig. 24.3). This line is bisected and a perpendicular line is drawn passing downward from its midpoint. A further line is drawn from the sacral hiatus to the greater trochanter. The point at which this line intersects with the perpendicular line marks the point for needle insertion. The intersection is usually 5 cm along the perpendicular line.

Figure 24.3 Landmarks for the posterior sciatic block. The greater trochanter (1) and posterior superior iliac spine (2) are marked. The measured distance between these points is divided equally. A perpendicular line is drawn extending into the thigh; 5 cm down this line is the needle insertion point (3). A line drawn from the greater trochanter to the sacral hiatus (4) should intersect this point.

Sonoanatomy

The patient is positioned laterally, with the side to be anesthetized uppermost and with the hip and knee flexed (Fig. 24.4). Perform a systematic anatomical survey from cephalad to caudad and from superficial to deep using a low-frequency, 5–2 MHz, curved array ultrasound transducer. The prominence of the greater trochanter and the ischial tuberosity are identified. These are seen as hyperechoic lines with shadowing beneath. On a sonogram, the ‘subgluteal space’ is seen as a hypoechoic area between the hyperechoic perimysium of the gluteus maximus and the quadratus femoris muscles (Fig. 24.5). It extends from the greater trochanter laterally to the ischial tuberosity medially. At this level, the sciatic nerve is seen as an oval hyperechoic nodule approximately 1.5–2 cm in diameter within the subgluteal space. It is often difficult to see the sciatic nerve due to tissue depth and reflections of muscles and fascial coverings. Follow the nerve by scanning proximally (cephalad) and distally (caudad) to follow the course of the nerve. It may be necessary to first identify the nerve in the posterior thigh region and then trace the nerve proximally, should visualization be difficult.