The Sciatic Nerve

Anatomy The sciatic nerve is the largest nerve in the body, measuring about 2 centimetres in thickness in its proximal portion. In this portion it is actually made up of the sciatic nerve and the posterior cutaneous nerve of the thigh. This "double nerve" contains contributions from lumbar nerve roots 4 and 5 and sacral nerve roots 1,2 and 3. In the techniques that are described here, this large "double nerve" is considered effectively as a single nerve and blocked with the one injection. For simplicity, it is referred to just as the sciatic nerve.

The important bony landmarks that one needs to be able to identify for blocking the sciatic nerve via the two posterior approaches described are the greater trochanter, posterior superior iliac spine, the ischial tuberosity and the sacral hiatus. For the anterior approach, the landmarks are the anterior superior iliac spine and the pubic symphysis on the pelvis and the greater trochanter on the femur.

Technique It will be appreciated from the description of these landmarks that there are several possible routes to block the sciatic nerve. Three of the most common approaches are described here. The first is the classical posterior approach of Labat⁵ performed with the patient in the lateral position. The second is another posterior approach, but the patient is supine and the leg is flexed at the hip and at the knee. Finally, the anterior approach is described where the patient is supine with the legs lying naturally extended. The choice of technique will to some extent be influenced by the position that is easiest for the patient to assume. However, the success rate is higher with the posterior approaches unless a nerve stimulator is used⁴. Furthermore, the anterior technique tends to be technically more difficult⁴ and therefore it is suggested that every effort should be made to position the patient for one of the posterior approaches.

Posterior approach of Labat5 (figure 3) The patient is first placed in the lateral position with the side to be blocked uppermost. While the lower leg is kept straight, the upper leg is flexed at the knee so that the ankle is brought over the knee of the lower leg. Another way of achieving the correct degree of hip and knee flexion is to have the posterior superior iliac spine, the greater trochanter and the knee in a straight line.

The point of injection is identified as follows:

A line is drawn between the greater trochanter and the posterior superior iliac spine (the line lies approximately over the upper border of the piriformis muscle). From the midpoint of this line, at right angles to it, draw a second line passing down over the buttock. The point of injection is 3 - 5 centimetres along this perpendicular line. It can be more precisely identified by drawing a third line between the greater trochanter and sacral hiatus, the point of injection being where this third line intersects with the second, perpendicular line. Having identified this point, place a small wheal of local anaesthetic at the site.

The needle that is required for this block needs to be quite long. A standard adult lumbar puncture needle is usually sufficient (9cm, 22G). In a very large person, an extra long needle, (10 - 12cm) may make the location of the nerve an easier task.

The needle is inserted perpendicular to the skin and slowly advanced until either bone is encountered, or paraesthesiae are elicited. (For the block to be successful, paraesthesiae below the knee should be felt.) If bone is encountered, the needle is withdrawn approximately 1-3cm and redirected slightly, either medially or laterally. Gentle probing within this single (transverse) plane should enable the nerve to be located by producing paraesthesiae as described. If the needle has been inserted as far as possible and neither paraesthesiae elicited nor bone encountered then the tip may have entered the greater sciatic notch. Should this occur, then the needle should be withdrawn almost fully, until the tip is just beneath the skin and then redirected in a slightly medial or lateral plane as described above.

Having located the nerve by paraesthesiae, the needle should be fixed in position and a syringe containing approximately 20ml of local anaesthetic connected. Aspiration is performed to exclude intravascular placement of the needle and the local anaesthetic is then injected. Repeat aspiration half way through the injection, in case the tip of the needle has moved). It is important that if severe pain occurs or if there is significant resistance to injection then the operator should stop immediately and reposition the needle, as these may be signs of intraneural injection.

Alternative posterior approach (of Raj) If it is not possible to have the patient lying on their side, then a variation of the posterior approach may be performed with the patient supine, although the hip is still manipulated.

To perform this block, the operator stands by the patient's bed, on the side to be blocked. The hip is then flexed as much as possible with knee bent. This position can be held stable by bracing the foot against the front of the operator's shoulder as they face towards the head of the bed. Alternatively, an assistant can hold the leg steady. The greater trochanter is palpated on the outside of the leg and the ischial tuberosity is also located, being the main prominence at the base of the buttock. It is often possible to palpate these two bony protuberances at the same time using the thumb and middle finger of the same hand. The midpoint between these two landmarks is the injection point. It is sometimes possible to identify this line joining the greater trochanter and ischial tuberosity as a depression between two muscle bellies - semitendinosus and biceps femoris.

Having located the injection point a small skin wheal is raised and the same needle as above is inserted at right angles to the skin. Once again the aim is to elicit paraesthesiae below the knee and this is achieved exactly as described above - by gentle probing in the transverse ("side to side") plane. The sciatic nerve is most likely to lie slightly medial to the path of the needle if these landmarks are used. The injection of local anaesthetic is then also performed in exactly the same manner as for the classical technique of Labat.

It should be noted that this alternative approach would block the sciatic nerve several centimetres more distally than the first approach described. However, it is rare to miss the posterior cutaneous nerve of the thigh, even with the alternative approach.

Anterior approach to the sciatic nerve Occasionally, it will be not be possible to move the patient's leg from the neutral position with them lying supine. In this case a sciatic nerve block can be performed using an anterior approach although as already stated, this tends to be more of a technical challenge! This technique will also result in the sciatic nerve being blocked at a relatively distal point (just beyond the hip joint) and hence it is possible to miss the posterior cutaneous nerve of the thigh, which becomes separated from the sciatic nerve by the hamstring muscles just below the buttock.

The landmarks for the point of injection are as follows: (see figure 4) firstly, trace a line over the inguinal ligament (from the anterior superior iliac spine to the pubic tubercle) and divide it into thirds. From the junction of the inner and middle thirds draw another line at right angles to the first, going down the leg. The next step is to find the greater trochanter and from this draw another line parallel to the line over the inguinal ligament (the first line drawn). Where this last line crosses the perpendicular line (the second line) is the point of injection. A small skin wheal of local anaesthetic is then injected at this site.
The needle used for this block is a standard adult lumbar puncture needle (9 cm long). However, the depth of insertion required for this block is commonly greater than for the posterior approaches and a longer needle is often required. The needle is then inserted perpendicular to the skin, which means aiming in a slightly lateral direction. The operator should aim to strike bone, close to the medial edge of the femur. This will be at about the level of the lesser trochanter. The needle is then withdrawn slightly, redirected more medially (it should be more towards the vertical) and advanced, this being repeated until the needle is "walked off" the bone, the aim being to just slip past the medial edge of the femur. The operator should note the depth at which the needle initially strikes the femur. Normally this is done by carefully observing the portion of needle shaft remaining at the skin. Once the needle has been directed off the bone, as described above, it should be inserted a further 5 cm (2 inches). The tip of the needle should now be in the region of the neuromuscular bundle. (See figure 5)

Aspiration to check for intravascular needle placement is particularly important if this approach is used, as there is a higher likelihood of vascular puncture. Having aspirated, the needle is immobilised in the usual fashion and approximately 20 ml of local anaesthetic injected.

If there is resistance to injection, then the tip of the needle may still be within muscle substance. If this occurs then the needle should be slowly advanced until injection is easily accomplished.

This technique does not require that paraesthesiae are found, but if they are elicited this is a positive sign of correct needle placement.

If one wishes to be positive about the needle placement and it is not possible to elicit paraesthesiae using the anterior approach as just described, then it is sometimes helpful to use a more medial injection point (about 1 - 2cm inside of the one described). This means that the needle will pass the medial edge of the femur at more of an angle than before and the tip will end posterior to the femur. This may help find the nerve, which tends to lie slightly behind the femur at this level. (When using this more medial injection point, it may help to place the free hand under the buttock and palpate the ischial tuberosity. The needle is then aimed at a point estimated to be 1 - 2cm lateral to the ischial tuberosity.)

Performing a sciatic nerve block using a nerve stimulator A nerve stimulator may be used in conjunction with any of the approaches to the sciatic nerve that have been described above. The techniques for determining the point of injection and locating the nerve are no different, except that one will look for muscle contraction. The best indicator of proximity to the nerve is dorsiflexion of the foot at the ankle and one should aim to achieve this at a stimulating current of 0.3 - 0.5 mA. However, when using the posterior approaches, one may also see contraction of the "hamstring" muscles down the back of the thigh, which may be taken as a sign of proximity to the sciatic nerve. Having achieved muscle contraction at the required stimulating current, injection of local anaesthetic is performed in the usual manner. 

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