The femoral nerve lies outside the femoral sheath, lateral to the femoral artery. The surface marking is by palpation of the femoral artery at the midinguinal point (half way between the anterior superior iliac spine and the midline), just below the inguinal ligament. The needle is passed posteriorly, deep to the fascia lata, reaching the nerve as it enters the thigh on iliopsoas, from under the inguinal ligament (figure 44).
The sciatic nerve usually divides in the upper thigh, but may be closer to the poplital fossa. The tibial and common peroneal are closely related at the apex of the popliteal fossa, and lie in a plane between the deeply placed artery and the superficial veins. The two nerves are anesthetized with a single injection placed 5 cm above the popliteal crease and 1 cm laterally, the needle passes backwards towards the back of the femur for 3 cm (figure 45a,b). The common peroneal nerve can also be palpated and anesthetized as it crosses the neck of the fibula laterally (figure 46). The saphenous nerve may be anesthetized at a knee or ankle level (figure 47a,b), the former is over the anteromedial aspect of the tibial plateau. At the ankle, the nerve lies alongside, usually medial to, the palpable great saphenous vein on the anterior aspect of the medial malleolus.
Figure 45 a,b. Popliteal fossa: single injection anaesthetises tibial and common peroneal nerves.
3. Low division of sciatic nerve
4. Popliteal artery
5. Popliteal vein
6. Common peroneal nerve
7. Tibial nerve
8. Lateral head of gastrocnemius
9. Medial head of gastrocnemius
At the ankle, the tibial nerve (figure 48) passes with the posterior tibial vessels in the groove behind the medial malleolus. Palpation of the artery at this site allows placement of the injection just lateral to the vessel, again check for extravascular delivery of the anesthetic. The superficial peroneal and the sural nerves (figure 49) pass just behind the lateral malleolus; the deep peroneal nerve cross the ankle joint just lateral to the tibialis anterior tendon, it may be in close relationship to the anterior tibial artery (figure 50).
Digital anesthetic can be delivered from the dorsal aspects of the proximal phalanx. The needle is passed from the posterior aspect of the phalanx and then ‘walked over’ its edge, and advanced to its anterior border. The great toe usually requires separate medial and lateral injections (figure 51 a,b). Anesthetic may initially be placed centrally, over the dorsum of the toe, to reduce the discomfort of subsequent injections. It is again emphasized the need to check that the anesthetic preparation does not contain any adrenalin.
A Bier’s block (figure 52) is also applicable to the foot; first place a venous needle or cannula, exanguinate the foot over it and apply a proximal calf tourniquet. A dorsal foot vein or the great saphenous vein at the ankle, are used for the intravenous injection. The needle is placed before exanguination and the tourniquet pressure is 30 mm Hg above systolic, to prevent blood flow during the procedure. The short acting anesthetic wears off in about an hour; release within half an hour should be controlled by briefly letting down and then re-inflating the tourniquet a number of times over 4 – 5 minutes, to prevent sudden release and potential systemic toxicity.