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Anterior Approach to the Sciatic Nerve Block

(Originally posted 7 May 2001 on About Anesthesiology)

Few surgical procedures can be performed with the sciatic block by itself. However, when combined with the femoral nerve block (a relatively simple block to perform), the entire lower extremity can be blocked - providing surgical anesthesia and good postoperative pain relief. Both blocks are usually required even for surgery on the lateral foot or ankle (an area which the sciatic block alone would cover) because most surgeons use a thigh tourniquet - which can cause significant discomfort and pain to the patient if the thigh is not numb.

Sciatic block can be used with or without the addition of the femoral nerve block for postoperative pain control following surgery of the foot or ankle. It should be noted that some surgeons prefer that this not be done - this is due to the concern that a high level of pain relief may mask symptoms (pain) associated with developing compartment syndrome after surgery.

There are four classically described approaches to the sciatic nerve block - the posterior approach, the lateral approach, the supine lithotomy approach, and the anterior approach. While each has its place and advantages, the anterior approach is one that few practitioners perform due to lack of familiarity or training.

To be fair, the anterior approach is associated with the highest failure rate of the four approaches. This might be attributed to lack of familiarity with the technique and/or the anatomy involved with the block. Also, some practitioners attempt the block without the use of the nerve stimulator - something that can obviously decrease success rates.

At the same time, this approach offers some distinct advantages: it is convenient because it can be performed with the patient in the supine position, it does not require the patient to be placed in what may be an impossible or painful lateral position, it allows the femoral nerve block to be performed in the same position and with the same skin preparation...

, having stated that the anterior approach may offer some benefits and may also be a bit of a technical challenge, let's proceed on to how it is done:

1. Identify the landmarks - A line connecting the anterior-superior iliac spine (ASIS) and the medial pubic tubercle represents the inguinal ligament. This line is divided into thirds. Another line parallel to the inguinal ligament and passing through the greater trochanter is identified. A third line is drawn perpendicular to these two lines and passing through where the medial third of the inguinal ligament meets the middle third. The intersection of this third line with the transtrochanteric line (second line drawn) is the needle insertion point.

2. The area is prepared with sterile technique. Local anesthetic is used to raise a skin wheal in preparation for injection.

3. A 9 cm or 15 cm (depending on the size of the patient) needle is used for the block. This needle should be an insulated needle that is used with a nerve stimulator. The needle is inserted perpendicularly to the skin (which means that a slight lateral direction is taken) and stimulation of the sciatic nerve is sought. Proper stimulation should give plantar or dorsal flexion.

4. More than likely, the femur will be contacted with the needle. The needle is then "walked off" the femur's medial edge. Once off the bone, another 3-5 centimeters is usually required for maximal stimulation.

5. Twitches of the hamstring should not be accepted. The endpoint is twitches in the foot.

6. Once the needle is properly positioned, aspirate to verify that the needle is not intravascular. Then 20-30 cc's of local anesthetic is injected.

It should be noted that, due to the size of the sciatic nerve, the block may require a good thirty minutes or more to set up fully. In addition, this technique results in a fairly distal block of the nerve - and it is therefore possible to miss the posterior cutaneous nerve of the thigh. This is usually not a big issue, but awareness of it may be important.

Obviously, a good knowledge of anatomy and the willingness to insert a fairly long needle are essential for this block. However, familiarity with this technique can come in handy in selected patients that require a sciatic nerve block without change in position.

 

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