Download Update 11 - Update in Anaesthesia - WFSA

64Update in Anaesthesiato palpate these two bony protuberances at the sametime using the thumb and middle finger of the samehand. The midpoint between these two landmarks isthe injection point. It is sometimes possible to identifythis line joining the greater trochanter and ischialtuberosity as a depression between two muscle bellies- semitendinosus and biceps femoris.Having located the injection point a small skin wheal israised and the same needle as above is inserted at rightangles to the skin. Once again the aim is to elicitparaesthesiae below the knee and this is achieved exactlyas described above - by gentle probing in the transverse(“side to side”) plane. The sciatic nerve is most likely to lieslightly medial to the path of the needle if these landmarksare used. The injection of local anaesthetic is then alsoperformed in exactly the same manner as for the classicaltechnique of Labat.It should be noted that this alternative approach wouldblock the sciatic nerve several centimetres more distallythan the first approach described. However, it is rare tomiss the posterior cutaneous nerve of the thigh, even withthe alternative approach.Anterior approach to the sciatic nerve Occasionally,it will be not be possible to move the patient’s leg fromthe neutral position with them lying supine. In thiscase a sciatic nerve block can be performed using ananterior approach although as already stated, this tendsto be more of a technical challenge! This techniquewill also result in the sciatic nerve being blocked at arelatively distal point (just beyond the hip joint) andhence it is possible to miss the posterior cutaneousnerve of the thigh, which becomes separated from thesciatic nerve by the hamstring muscles just below thebuttock.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 andmiddle thirds draw another line at right angles to the first,going down the leg. The next step is to find the greatertrochanter and from this draw another line parallel to theline over the inguinal ligament (the first line drawn). Wherethis last line crosses the perpendicular line (the secondline) is the point of injection. A small skin wheal of localanaesthetic is then injected at this site.The needle used for this block is a standard adult lumbarFigure 4: Landmarks for the anterior approach to the sciatic nerve.