Sciatic Nerve Block

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Sciatic nerve originates from the lumbosacral plexus (L4, L5, S1, S2, and S3) and is the largest nerve of the lower extremity. The sciatic nerve innervates the lower extremity as indicated in Table 20.9.

Indications

As the primary anesthetic and/or postoperative pain management with or without a continuous catheter.

Table 20.9 Sciatic nerve innervation of the lower extremity.

Motor

Sensory

Biceps femoris and ischial head of adductor magnus muscle

Posterior aspect of the thigh

Semitendinous and semimembranous muscles

The entire leg except the medial aspect of lower leg

Articular branches to hip and knee

Clinical Uses Surgical

• Foot and ankle surgery (not effective for sensory distribution on the medial portion of ankle or foot)

• Short saphenous vein stripping

• Surgery below the knee and posterior thigh (spares sensory on medial calf)

Supplemental (in Combination with a Femoral Nerve Block, May Also Require An Obturator or Lumbar Plexus Blockade)

• Total knee replacement

• Tibia plateau fracture repair

• Total ankle replacement

• Above and below knee amputation

• Ankle surgery

Postoperative Analgesia

For the surgeries indicated above.

Technique Preparation

Arrange sterile towels, sterile gloves, gauze pads, marking pen, antiseptic solution, peripheral nerve stimulator, syringes, and needles for local infiltration and nerve block placement.

Dose

20-30 ml syringes of local anesthetic. Needles

25 g 1.5 in. needle for skin infiltration and 10 or 15 cm short bevel insulated stimulation needle.

Agents

3% chloroprocaine, 2% lidocaine, 0.5% ropivacaine, 0.5% bupivacaine.

Surface Anatomy and Landmarks (Classical Approach of Labat) Landmarks

Landmarks include greater trochanter (GT), posterior superior iliac spine (PSIS), and the sacral hiatus (SH) (Fig. 20.8).

The patient lies in a lateral decubitus position with the side to be blocked non-dependent. The knee of the non-dependent leg is flexed 45-60° so as to rest the plantar portion of the foot or ankle on the knee of the dependent limb. Mark the palpable location of the GT, PSIS, and the SH. Then draw two lines, one connecting the PSIS to the GT and another connecting the SH to the GT. At the midpoint of the line between the PSIS and the GT, another 4-5 cm long perpendicular line is drawn which should approximate the line between the SH and the GT; this point will serve as the block needle insertion site. Subsequent to aseptic precautions, a local anesthetic skin wheal is raised with the infiltrating needle at the point marked. A 10 or 15 cm short bevel insulated needle is connected to the nerve stimulator set at 1.0 mA and introduced through the skin wheal perpendicular to the skin. Appropriate muscle twitch responses include the lower leg and/or foot that remains consistent as the current on the nerve stimulator is decreased to a range of 0.3-0.5 mA. If appropriate and sustained muscle twitches are not obtained, then the needle is withdrawn to the skin and reintroduced in a logical and sequential fanwise fashion in a path perpendicular to the course of the sciatic nerve in the hip. Following confirmation of negative aspiration for blood, inject the chosen local anesthetic through the needle once an appropriate and sustained muscle twitch is obtained.

Figure 20.8 Posterior anatomical landmarks for the classical approach (Labat's) to the sciatic nerve block. The greater trochanter (GT) is identified as #1 and a straight line is drawn from the midpoint of the GT (1) to the posterior superior iliac spine (3). A 4-5 cm long second line is drawn caudomedially perpendicular to the midpoint between these two landmarks (1 and 3) and will serve as the needle insertion site (X). Another line is drawn connecting the midpoint of the GT (1) to the sacral hiatus which should bisect the X (needle insertion site) and serve as a cross-check for proper needle insertion. The dashed line (2) represents the furrow formed between the long head of the biceps femorus muscle and the medial edge of the gluteus maximus muscle. This dashed line (2) represents the course of the sciatic nerve toward the leg and should intersect at the needle insertion site (X) serving as another cross-check for proper needle placement.

Figure 20.8 Posterior anatomical landmarks for the classical approach (Labat's) to the sciatic nerve block. The greater trochanter (GT) is identified as #1 and a straight line is drawn from the midpoint of the GT (1) to the posterior superior iliac spine (3). A 4-5 cm long second line is drawn caudomedially perpendicular to the midpoint between these two landmarks (1 and 3) and will serve as the needle insertion site (X). Another line is drawn connecting the midpoint of the GT (1) to the sacral hiatus which should bisect the X (needle insertion site) and serve as a cross-check for proper needle insertion. The dashed line (2) represents the furrow formed between the long head of the biceps femorus muscle and the medial edge of the gluteus maximus muscle. This dashed line (2) represents the course of the sciatic nerve toward the leg and should intersect at the needle insertion site (X) serving as another cross-check for proper needle placement.

Pitfalls and Pearls Pitfalls

• Patient discomfort secondary to large muscle group twitching

• Local anesthetic toxicity must be considered when combining additional PNBs

• Paresthesia

Pearls

• Adequate patient positioning is key to the successful block

• Systematic redirection of the needle is necessary until the appropriate twitch response is obtained

• Twitches of the quadriceps muscle often occur, but the needle should be advanced deep to this response

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