The femoral nerve is essential for activities of daily life including walking and climbing stairs. Without it, the lower limb becomes severely impaired. In this article we will discuss its course, its motor and sensory innervation as well as clinical relevance.
The femoral nerve is the largest branch of the lumbar plexus and provides motor innervation to the anterior thigh (quadriceps). It arises from the posterior cords of the lumbar plexus (L2-L4), contrasting with the obturator nerve, which arises from the anterior cords (L2-L4) and supplies the medial compartment of the thigh (Adductor Magnus, longus and brevis muscles).
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The femoral nerve emerges lateral to psoas major, and descends to pass beneath the inguinal ligament at approximately its mid-point. It passes through the femoral triangle lateral to the femoral artery and vein.
Once it passes beneath the inguinal ligament, it divides into a deep and superficial branch. The deep branch supplies the quadriceps. The superficial branch divides into the medial cutaneous and anterior cutaneous nerve of the thigh. The nerve terminates as the sensory saphenous nerve, in the gaiter region of the leg.
It may seem counter intuitive that the femoral nerve (the nerve of the anterior thigh) arises from the posterior cords of the lumbar plexus. The answer to this contradiction can be found in embryology. Unlike the upper limb, the lower limb rotates 180 degrees. As a result, the anterior nerve derives its roots from the posterior cords, and vice versa for the obturator. In fact, the lumbar nerve roots for these nerves are also odd, as the thigh muscles are not lumbar structures. During embryological development the thigh muscles migrate from the lumbar area and carry their innervation with them, hence L1-L4.
The first motor branch innervates iliacus. This muscle, in conjunction with the psoas major, causes medial rotation of the hip. The deep branch of the femoral nerve then descends to supply the sartorius (the tailor’s muscle). Once it passes through the femoral canal (as the most lateral structure of the neurovascular bundle), it supplies the pectineus, a small muscle in the medial compartment of the thigh. Finally, the nerve supplies the four heads of the quadriceps femoris (vastus medialis, vastus lateralis, vastus intermedius and rectus femoris).
The overall functions of the femoral nerve are hip flexion (mainly via illiacus but also pectineus, sartorius and rectus femoris) and knee extension (via quadriceps). Both these movements are essential for walking and straightening one’s leg in any context (kicking a football, jumping etc). In the terminal stages of knee extension, it also causes a degree of medial rotation that results in locking of the knee (this is due to the larger, more rounded, and more forward projecting medial condyle).
The superficial branch of the femoral nerve first gives rise to the anterior and medial cutaneous nerves of the thigh. They supply sensation over the anterior and median region of the thigh. The lateral femoral cutaneous nerve is a separate sensory nerve arising from L2 and L3, and supplies sensation over the lateral thigh.
The terminal branch is the saphenous nerve, which passes through the adductor canal with the femoral vessels and supplies sensation over the region of the greater saphenous nerve in the gaiter region. As well as its sensory innervation, the femoral nerve also innervates the capsule of the hip joint, and allows for proprioceptive feedback about the joint.
Femoral nerve damage
The femoral triangle is formed by the lateral border of adductor longus, the medial border of sartorius and the inguinal ligament (with pectineus and illiopsoas forming the floor). It contains, from lateral to medial, the femoral nerve, artery and vein.
The femoral artery can be palpated 1 cm below the mid inguinal point (mid-way from the pubic symphysis to the anterior superior iliac spine or ASIS).
This is different to the mid point of the inguinal ligament (mid-way between the pubic tubercle and the ASIS). The femoral nerve can be damaged during penetrating trauma to the thigh. It can also be damaged during hip operations, particularly the anterior approach (not commonly used) where the nerve can be stretched and damaged. The fibres of the femoral nerve also mediate the knee reflex.