62<strong>Update</strong> <strong>in</strong> <strong>Anaesthesia</strong>the psoas major muscle and is <strong>in</strong>vested <strong>in</strong> a fascialsheath derived from these two muscles. This sheathforms a cont<strong>in</strong>uous cover<strong>in</strong>g around the plexus,extend<strong>in</strong>g down to the femoral nerve just below the<strong>in</strong>gu<strong>in</strong>al ligament. Therefore if local anaesthetic<strong>in</strong>jected around the femoral nerve at the <strong>in</strong>gu<strong>in</strong>alligament can be made to spread proximally, then theother two nerves can be simultaneously blocked at theirorig<strong>in</strong>s from the lumbar plexus.Technique The <strong>in</strong>jection po<strong>in</strong>t and method for correctneedle placement are exactly as described for the femoralnerve. It is most practical to use the s<strong>in</strong>gle <strong>in</strong>jectiontechnique, as this will enable placement of the large volumesrequired.There are two differences; the ma<strong>in</strong> difference comes withthe actual <strong>in</strong>jection of the local anaesthetic. Hav<strong>in</strong>g aspiratedon the needle to check that the tip is not <strong>in</strong>travascular, thehand is then moved to apply firm pressure on the thigh(with the thumb) about 2 - 4 cm. below the <strong>in</strong>sertion po<strong>in</strong>tof the needle. The <strong>in</strong>jection is then performed, all the whilema<strong>in</strong>ta<strong>in</strong><strong>in</strong>g the pressure. The pressure can be releasedabout thirty seconds after the <strong>in</strong>jection has been completed.(The <strong>in</strong>jection will require an assistant, as the operator willhave one hand immobilis<strong>in</strong>g the needle and the otherapply<strong>in</strong>g pressure.) This procedure encourages spread ofthe local anaesthetic upwards, towards the lumbar nerveroots.The second difference is that larger volumes of localanaesthetic are used to achieve the necessary spread. Them<strong>in</strong>imum volume to block all three nerves is 20 mls.However, many texts suggest larger volumes such as 25 -30 mls. When us<strong>in</strong>g these volumes, particularly <strong>in</strong>comb<strong>in</strong>ation with a sciatic nerve block, it may be necessaryto dilute the concentration of local anaesthetic solution used<strong>in</strong> order to limit the total dose given.Use of a nerve stimulator: as for the femoral nerve.The Sciatic NerveAnatomy The sciatic nerve is the largest nerve <strong>in</strong> the body,measur<strong>in</strong>g about 2 centimetres <strong>in</strong> thickness <strong>in</strong> its proximalportion. In this portion it is actually made up of the sciaticnerve and the posterior cutaneous nerve of the thigh. This“double nerve” conta<strong>in</strong>s contributions from lumbar nerveroots 4 and 5 and sacral nerve roots 1,2 and 3. In thetechniques that are described here, this large “doublenerve” is considered effectively as a s<strong>in</strong>gle nerve andblocked with the one <strong>in</strong>jection. For simplicity, it is referredto just as the sciatic nerve.The important bony landmarks that one needs to be ableto identify for block<strong>in</strong>g the sciatic nerve via the twoposterior approaches described are the greater trochanter,posterior superior iliac sp<strong>in</strong>e, the ischial tuberosity andthe sacral hiatus. For the anterior approach, thelandmarks are the anterior superior iliac sp<strong>in</strong>e and thepubic symphysis on the pelvis and the greatertrochanter on the femur.Technique It will be appreciated from the descriptionof these landmarks that there are several possible routesto block the sciatic nerve. Three of the most commonapproaches are described here. The first is the classicalposterior approach of Labat 5 performed with the patient<strong>in</strong> the lateral position. The second is another posteriorapproach, but the patient is sup<strong>in</strong>e and the leg is flexed atthe hip and at the knee. F<strong>in</strong>ally, the anterior approach isdescribed where the patient is sup<strong>in</strong>e with the legs ly<strong>in</strong>gnaturally extended. The choice of technique will to someextent be <strong>in</strong>fluenced by the position that is easiest for thepatient to assume. However, the success rate is higherwith the posterior approaches unless a nerve stimulator isused 4 . Furthermore, the anterior technique tends to betechnically more difficult 4 and therefore it is suggested thatevery effort should be made to position the patient for oneof the posterior approaches.Posterior approach of Labat 5 (figure 3) The patient isfirst placed <strong>in</strong> the lateral position with the side to be blockeduppermost. While the lower leg is kept straight, the upperleg is flexed at the knee so that the ankle is brought overthe knee of the lower leg. Another way of achiev<strong>in</strong>g thecorrect degree of hip and knee flexion is to have theposterior superior iliac sp<strong>in</strong>e, the greater trochanter andthe knee <strong>in</strong> a straight l<strong>in</strong>e.The po<strong>in</strong>t of <strong>in</strong>jection is identified as follows:A l<strong>in</strong>e is drawn between the greater trochanter and theposterior superior iliac sp<strong>in</strong>e (the l<strong>in</strong>e lies approximatelyover the upper border of the piriformis muscle). From themidpo<strong>in</strong>t of this l<strong>in</strong>e, at right angles to it, draw a secondl<strong>in</strong>e pass<strong>in</strong>g down over the buttock. The po<strong>in</strong>t of <strong>in</strong>jectionis 3 - 5 centimetres along this perpendicular l<strong>in</strong>e. It can bemore precisely identified by draw<strong>in</strong>g a third l<strong>in</strong>e betweenthe greater trochanter and sacral hiatus, the po<strong>in</strong>t of<strong>in</strong>jection be<strong>in</strong>g where this third l<strong>in</strong>e <strong>in</strong>tersects with thesecond, perpendicular l<strong>in</strong>e. Hav<strong>in</strong>g identified this po<strong>in</strong>t,place a small wheal of local anaesthetic at the site.
<strong>Update</strong> <strong>in</strong> <strong>Anaesthesia</strong> 63Figure 3: Anatomical landmarks for Labat’s technique for sciatic nerve blockThe needle that is required for this block needs to bequite long. A standard adult lumbar puncture needle isusually sufficient (9cm, 22G). In a very large person,an extra long needle, (10 - 12cm) may make thelocation of the nerve an easier task.The needle is <strong>in</strong>serted perpendicular to the sk<strong>in</strong> andslowly advanced until either bone is encountered, orparaesthesiae are elicited. (For the block to besuccessful, paraesthesiae below the knee should befelt.) If bone is encountered, the needle is withdrawnapproximately 1-3cm and redirected slightly, eithermedially or laterally. Gentle prob<strong>in</strong>g with<strong>in</strong> this s<strong>in</strong>gle(transverse) plane should enable the nerve to be locatedby produc<strong>in</strong>g paraesthesiae as described. If the needlehas been <strong>in</strong>serted as far as possible and neitherparaesthesiae elicited nor bone encountered then thetip may have entered the greater sciatic notch. Shouldthis occur, then the needle should be withdrawn almostfully, until the tip is just beneath the sk<strong>in</strong> and thenredirected <strong>in</strong> a slightly medial or lateral plane as describedabove.Hav<strong>in</strong>g located the nerve by paraesthesiae, the needleshould be fixed <strong>in</strong> position and a syr<strong>in</strong>ge conta<strong>in</strong><strong>in</strong>gapproximately 20ml of local anaesthetic connected.Aspiration is performed to exclude <strong>in</strong>travascular placementof the needle and the local anaesthetic is then <strong>in</strong>jected.Repeat aspiration half way through the <strong>in</strong>jection, <strong>in</strong> casethe tip of the needle has moved). It is important that ifsevere pa<strong>in</strong> occurs or if there is significant resistance to<strong>in</strong>jection then the operator should stop immediatelyand reposition the needle, as these may be signs of<strong>in</strong>traneural <strong>in</strong>jection.Alternative posterior approach (of Raj) If it is notpossible to have the patient ly<strong>in</strong>g on their side, then avariation of the posterior approach may be performed withthe patient sup<strong>in</strong>e, although the hip is still manipulated.To perform this block, the operator stands by the patient’sbed, on the side to be blocked. The hip is then flexed asmuch as possible with knee bent. This position can beheld stable by brac<strong>in</strong>g the foot aga<strong>in</strong>st the front of theoperator’s shoulder as they face towards the head of thebed. Alternatively, an assistant can hold the leg steady.The greater trochanter is palpated on the outside of theleg and the ischial tuberosity is also located, be<strong>in</strong>g the ma<strong>in</strong>prom<strong>in</strong>ence at the base of the buttock. It is often possible
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