Sciatic Nerve Block (Anterior Parafemoral Technique)

Anterior sciatic nerve block is a useful technique in patients who are unable to assume lateral decubitus or lithotomy position. In addition, anterior approach also allows performance of the femoral block in the same position and using the same skin preparation, which shortens the time required to complete both blocks. The advantages of our parafemoral approach to sciatic block over the classical anterior approach (Beck's approach) are:

• The landmarks are very simple and distinct in most patients
• The proximal extent of anesthesia of the thigh is greater than in the classical approach because the nerve is approached more proximally
• The lesser trochanter does not obstruct the path of the needle toward the sciatic nerve

The patient is positioned supine with the leg extended on the table (Figure 2).

Figure 1: Ankle fracture is an excellent indication for anterior sciatic nerve block. Since the block can be performed in the supine position, this does not require painful patient repositioning.

Figure 2: For the parafemoral approach to anterior sciatic nerve block, the patient is positioned in the supine position. As opposed to the classical approach, rotation of the leg does not have a significant effect on the ability to reach the nerve.

The inguinal area is disinfected and the femoral crease, femoral artery and insertion site 1-2 cm lateral to the lateral border to the femoral artery are marked with a pen. Local anesthetic is injected subcutaneously at the anticipated point of the block needle insertion (left figure).

With the palpating hand firmly pressed on the thigh and the middle finger on the femoral artery, a 15-cm 20G needle connected to the nerve stimulator is inserted 1-2 cm lateral to the femoral artery (middle figure).

The initial stimulating current should be set at 1.5 mA. The needle orientation should be almost in a perpendicular plane with a slight inferior and lateral angulation (5-15°) (right figure). As the needle is advanced, stimulation of one of the femoral nerve branches is often elicited at 1-3 cm depth. As the needle is further advanced, the femoral nerve twitches cease. The sciatic nerve is typically stimulated at a depth of 10-12 cm. The needle is advanced until the foot twitches are obtained at 0.5 mA or less at which point 15-20 ml of local anesthetic is injected.

This is an advanced regional anesthesia technique and it is best practiced by those with a significant experience with basic and intermediate techniques.

This is a deep block and the needle transverses multiple tissue plains. Adequate sedation and analgesia is necessary to decrease the discomfort during the block performance as well as to allow meaningful interpretation of the obtained responses to nerve stimulation.

Avoid multiple reinsertions of the needle as this may result in thigh hematoma. Anterior approach to sciatic nerve block is an advanced regional anesthesia technique and should be performed only by those with significant experience in peripheral nerve blockade.

Anterior approaches to sciatic nerve block should not be used for continuous sciatic nerve block techniques. The catheter advancement is difficult at an angle at which the sciatic nerve is stimulated.

Never use epinephrine for this block when the thigh tourniquet is planned!