Introduction		Techniques - The femoral nerve and lumbar pexus
Conduct of nerve blocks		Blockade of the lumbar plexus usiing the inguinal paravascular approach
Local anaesthetic drugs and dosages		The sciatic nerve
Anatomy		Acknowledgements
Surface anatomy markings		References
Indicator for specific nerve blocks		Further Reading
Performing the nerve blocks - patient preparation and techniques

Introduction

The purpose of this guide is to provide a detailed, step by step description of how to safely and reliably perform nerve blocks for surgery and pain relief on the lower limb, for the non-specialist anaesthetic practitioner. It covers femoral nerve blockade, lumbar plexus blockade using the inguinal paravascular approach and sciatic nerve blockade. No description is given of more distal blocks, since the majority of the limb is readily anaesthetised with the techniques described and as a group they are relatively simple, reliable and commonly used. 

Conduct of nerve blocks

It is recommended that all blocks on major nerves be carried out on patients that are awake or only lightly sedated, as it is believed that this decreased the risk of serious nerve damage, and will not hide the signs of unexpected local anaesthetic toxicity. Sedated patients must still be able to communicate with the operator during the nerve block procedure. It is also recommended that a nerve stimulator be used if one is available as this increases the success rate of the blocks especially in inexperienced hands. 

Local anaesthetic drugs and dosages

For fast onset and short duration blocks, lignocaine (maximum 4mg/kg) or lignocaine with adrenaline 1:200,000 (6mg/kg) can be used. When injected into a plexus or near a large nerve such as the sciatic nerve the block will come on at about 10-20 mins and last for 4-8 hours. The adrenaline will prolong the block but may possibly increase the risk of nerve damage through ischaemia. Bupivacaine in a maximum dose of 3mg/kg will give a block of a major nerve that will start at 20-30 minutes and last as long as 18 hours. There is no value in adding adrenaline to bupivacaine except for local skin infiltration. 

Anatomy

The nerve supply of the lower limb is derived from the lumbar and sacral plexuses, a network of nerves composed of the anterior primary rami of all the lumbar and the first three sacral nerve roots (and sometimes with a contribution from the twelfth thoracic nerve root). Arising from these plexuses are the five main nerves that innervate the lower limb.

The lumbar plexus: This gives rise to the femoral nerve, obturator nerve and lateral cutaneous nerve of the thigh.

The femoral nerve runs in the groove between the psoas major and iliacus muscles, with a covering of these muscles' fascia. It enters the thigh passing under the inguinal ligament, where it is lateral to the femoral artery, whose pulsations are used to help locate the nerve. The femoral nerve block is performed at this point and there are two important features of the anatomy. Firstly, below the inguinal ligament, the femoral nerve divides into anterior and posterior branches, the anterior (superficial) branch supplying sensation to the skin of the anterior and medial thigh and a posterior (deep) branch that supplies the quadriceps muscles, the medial knee joint, and the skin on the medial side of the calf and foot (via the saphenous nerve). Therefore, the block should not be performed lower than just distal to the inguinal ligament, in order not to miss one of the branches. Secondly, as it enters the thigh, the femoral nerve has two fascial layers covering it, the fascia lata and the fascia iliaca. This is in contrast to the femoral artery, which is only covered by the fascia lata alone. This means that the nerve will lie in a different tissue plane than the artery and usually a little deeper. These coverings can be used in blocking the nerve. (See techniques).

The lateral cutaneous nerve of the thigh and the obturator nerve both have important sensory distributions (to the thigh and knee). This article does not cover blocks of these nerves as single entities. However, they can readily be blocked in conjunction with the femoral nerve, using the same technique, to produce a "3-in-1" block and this is described later. (see Blockade of the lumbar plexus using the inguinal paravascular approach).

The sacral plexus: This gives rise to the sciatic nerve and the posterior cutaneous nerve of the thigh. Although these nerves are formed separately within the plexus, they pass through the pelvis and buttock together and with the techniques described here are blocked with the same injection. Hence they are considered here as a single nerve trunk, and unless specifically stated, "sciatic nerve" will refer to both the sciatic nerve and the posterior cutaneous nerve of the thigh.

The sciatic nerve leaves the pelvis and enters the buttock through the greater sciatic foramen, and then passes slightly medial to the midpoint between the greater trochanter and the ischial tuberosity, lying just posterior to the hip joint. It can be blocked at several points along this course (see techniques). The sciatic nerve leaves the buttock, passing out from under the lower border of gluteus maximus muscle and runs distally down the thigh to the popliteal fossa.

Areas supplied by the individual nerves:

A pictorial illustration of the skin areas supplied is given in figure 1. Femoral nerve: supplies skin over the anterior (front) of the thigh, the anterior knee, some of the medial (inner) thigh, via the saphenous nerve it also supplies the antero-medial aspect of the calf down to and including the medial malleolus. It has branches to the hip joint and knee joint and supplies much of the shaft of the femur. Sensation to the medial aspect of the big toe may come from the saphenous (femoral nerve). Lateral cutaneous nerve of the thigh: supplies sensation to the skin over the lateral (outside) thigh, from the greater trochanter to the knee, and on to the anterior thigh. Obturator nerve: supplies a small, variable amount of skin on the medial aspect of the knee and lower thigh. More importantly, it has a branch to the knee joint. There is also a small branch to the hip joint. Posterior cutaneous nerve of the thigh: supplies skin over the posterior (back) thigh, the popliteal fossa, the lower buttock and some of the genital area. Note that this nerve is blocked with the posterior approaches and is often missed with the anterior approach to the sciatic nerve. Sciatic nerve: via its branches supplies all the skin of the leg below the knee and all the foot, except for the medial calf and ankle, which is supplied by the saphenous (femoral) nerve. The sciatic nerve also has a small branch to the hip joint, a branch to the knee joint and fully innervates the ankle joint.

(Continued ...)

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