The Femoral Nerve is most commonly approached and understood as a group of nerves lying lateral to the femoral artery and vein in the upper thigh. The nerve lies lateral to the vascular compartments (medial compartment contains the femoral vein and lateral compartment holds the artery) and below 2 layers of fascia beneath the skin (superficially the fascia lata and deeper the iliacus fascia).
The femoral nerve begins sending branches laterally soon after passing under the inguinal ligament. The first branch usually innervates the sartorius muscle.
No muscles overlie the femoral nerve at its position at the top of the thigh.
Identification of the nerve is greatly helped by first locating the femoral artery and vein and then examining the space lateral to the artery. The nerve appears as a roundish or oblong, bright (echo-dense) structure when viewed transversely with ultrasound. Sometimes it can be seen to contain bright dots which represent the nerve fascicles it holds inside.
When using a nerve stimulator needle under ultrasound guidance, you will notice that a great deal of the time placing the needle directly on the main nerve mass will not produce a motor response leading you to wonder whether this is indeed the nerve. By persistently re-directing the stimulating needle about the space you can usually elicit the standard quadriceps muscle twitch, but ultrasound observation of the needle position which produces the twitch shows that usually at this level exposed motor fibers lie lateral to the main nerve bundle mass.
There is nothing to suggest that injection of the local anesthetic in the area which contains these motor fibers produces a block which is in any way superior to injection of the local in the area of the main nerve mass. On the contrary, practice has shown that a very solid block can be attained by injecting local anesthetic slowly with the needle tip adjacent to the main femoral nerve mass. Injecting under ultrasound guidance allows the operator to observe the spread of the anesthetic solution and re-direct the needle as appropriate to insure that the target nerve mass is surrounded with the anesthetic.
As with many other nerve blocks performed under ultrasound guidance, the ultrasound femoral technique minimizes the number of physical passes with the needle therefore minimizes the amount of tissue trauma and this, coupled with low pressure injecting, periodic aspiration and direct observation of the accumulation of anesthetic in the tissues, reduces the chance of inadvertent intra-vascular injection or absorption to a minimum.
In conclusion, the femoral nerve block is a procedure ideally suited to the use of ultrasound guidance.