Subgluteal Sciatic Nerve Block

This block is indicated for surgery at and below the knee, including Achilles tendon repair and foot surgery. Preference for this technique over the more proximal gluteal or Labat approach may be guided by the more superficial and hence approachable nature of the nerve at this location. It may also be less painful to block the nerve at this more distal location.
Anatomy

The nerve leaves the pelvis through the greater sciatic foramen below the piriformis muscle and descends just medial to the midpoint of a line between the greater trochanter of the femur and the ischial tuberosity along the back of the thigh. This is the target location for the subgluteal approach.

Proximally, the nerve lies deep to the gluteus maximus, resting first on the posterior ischial surface (gluteal / Labat block location) and then coursing inferiorly posterior to the obturator internus, gamelli, and quadratus femoris muscles to enter into the back of the thigh where it divides. Inferior to the lower border of the gluteus maximus the nerve is relatively superficial, after which it lies under cover of the long head of the biceps femoris muscle.


Patient Positioning and Surface Anatomy

The patient is positioned semiprone with the hip and knee flexed and the foot resting on the dependent knee. A horizontal line is drawn joining the medial aspect of the greater trochanter to the ischial tuberosity. The traditional puncture site is located on this line just medial to its midpoint.

Scanning Technique

The highly hyperechoic bony structures, ischial tuberosity (medial) and greater trochanter (lateral), with underlying hypoechoic shadows, are excellent landmarks for localization of the sciatic nerve. A curved, lower frequency 2- to 5-MHz probe or a linear 4- to 7-MHz probe is used for scanning the subgluteal region. The required depth of penetration is often 3 to 4 cm from the skin surface. In the transverse/short-axis plane the image may show the inner borders of both bony landmarks and the sciatic nerve slightly medial to the midline.

If the sciatic nerve is hard to localize at the subgluteal region, it can be traced proximally from the bifurcation point at or near the apex of the popliteal fossa. The sciatic nerve in the subgluteal region appears predominantly hyperechoic (bright) and is often elliptical in short axis on ultrasound. It may also be wide and flat, almost comet shaped, rendering it difficult to visualize.


Local Anesthetic Application

Both in plane and out of plane needling approaches are possible using a 5 to 10 cm insulated needle. It is more difficult to follow the needle path with an out of plane approach particularly with a curved array probe. Nerve localization is confirmed with a nerve stimulator. The goal is to deposit 20 – 30 ml of local anesthetic within the vicinity of the nerve. A hypoechoic fluid collection may be seen around the hyperechoic nerve when successful. A number of needle direction changes may be necessary to surround the nerve.



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