The femoral nerve is the largest terminal branch of the lumbar plexus. It originates from the spinal nerves L2-L4. Exiting the pelvis at the level of the inguinal crease the femoral nerve lies lateral to the femoral vessels but not in the same compartment. The nerve lies deep to the fascia iliaca on the iliopsoas muscle. Starting scanning as cranially as possible often makes it easier to identify the nerve because of the clearer anatomy and the superficial position of the nerve. Moving the ultrasound-transducer caudally the iliopsoas muscle with the femoral nerve will be covered by the sartorius muscle. In this position it might be difficult to start looking for the nerve.
On its course down the thigh the femoral nerve gives off several branches to the different muscles. Already at the level of the inguinal crease this branches might be individual nerves with separate epieneural layer around each of them and the femoral nerve more an assembly of its branches witch travel in close approximation for some distance before physical separating and heading to their respective destinations. Both the pressure of the ultrasound probe and the rotation of the leg in the hip joint can influence the arrangement of these Neve braches and thus the appearance of the femoral nerve. This anatomical fact is the reason why it may be difficult to define a normal shape or clear normal values of the cross-sectional area of the femoral nerve at the level of the inguinal crease.
At the level of the apex of the iliopectineal fossa (the intersection of the medial border of the sartorius muscle and the lateral border of the adductor longs muscle – mostly where the femoral artery has just passed beneath the medial border of the sartorius) the muscular branches to the rectus femoris, vastus intermedius und vastus lateralis have already pierced through fascia iliaca to reach their respective muscle and only the muscular branch to the vastus medialis runs together with the terminal sensory branch of the femoral nerv, the saphenous nerve. Most of the time at this position they run on the latero-ventral border of the femoral artery.
At the level of the apex of the femoral triangle (the intersection of the medial border of the sartorial muscle ant the medial border of the adductor longs muscle) the saphenous nerve enters the adductor canal deep to the vasoadductor membrane together with the femoral artery. The muscular branch to the vastus medialis moves away ventral to the adductor canal, running between the sartorius muscle and the vastus medialis. In addition to the muscular branches sensory end branches innervates anterior and medial parts of the knee joint capsule.
At the distal end of the adductor canal the saphenous nerve pierces the vasoadductor membrane and runs between the sartorius and gracilis muscle to become subcutaneous. In the distal course along the medial lower leg the saphenous nerve or its branches is found subcutaneous in close proximity to the great saphenous vein. In addition tho the innervation of the skin of the medial lower leg an the medial aspect of the foot with varying distal extension, deep branches contribute to the bony innervation of the medial distal tibia (medial malleolus) and the medial capsule of the ankle joint. To be aware of the special course of the saphenous nerve helps to find and scan the nerve in its entire course; in contrast to its position on the medial leg, both on the distal thigh and on the proximal lower leg, in between at the level of the knee it runs further dorsally on the medial hollow of the knee.
The origin of the infrapatellar branch (or even multiples branches) out of the saphenous nerve might be very variable; distal at the level of the knee or much proximal at the thigh, even piercing through the sartorius muscle. So it can be very challenging to identify this branch in some individuals. The infrapatellar branch provides cutaneous sensory innervation of the anterior aspect of the knee and the medial aspect of the knee joint (with their corresponding articular branches).