Anesthetic Blockade of the Femoral Nerve

The femoral nerve is the largest branch of the lumbar plexus. It arises from the dorsal branches of the second to fourth ventral rami (▶ Fig. 22.1), descends on the iliopsoas muscle, and enters the thigh deep to the inguinal ligament at the lateral edge of the femoral sheath, which separates it from the femoral artery. ¹ The inguinal ligament is a convergent point of the transversalis fascia (fascial sac lining the deep surface of the anterior abdominal wall) and iliac fascia (fascia covering the posterior abdominal wall). As the femoral nerve passes beneath the inguinal ligament, the nerve is positioned lateral to and slightly deeper than the femoral artery between the psoas and iliacus muscles (▶ Fig. 22.2). At the inguinal crease, it is on the surface of the iliacus muscle and covered by the fascia iliaca or sandwiched between two layers of fascia iliaca.

Fig. 22.1 Proximal portion of the right femoral nerve and its relationships.

(Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)

Fig. 22.2 Distal portion of the right femoral nerve and its relationships.

Conventionally, the femoral nerve passes behind the inguinal ligament and divides into anterior and posterior divisions in the thigh. ² The femoral nerve block is performed on the main trunk of the femoral nerve just below the inguinal ligament. ³ The higher division of the nerve in the iliac fossa results in incomplete femoral nerve block.

The pectineus nerve is the most medial branch of the femoral nerve in the thigh. A branch to the pectineus muscle is given off as the femoral nerve enters the femoral triangle, beyond the inguinal ligament, innervating the anterior thigh muscles, hip and knee joints, and skin on the anteromedial thigh. ³^, ⁴ The origin of this nerve from the femoral nerve in relation to the pectineus is more important morphologically than clinically.

There are instances in which the femoral nerve ⁵^, ⁶^, ⁷ is split by an aberrant slip of the iliacus or psoas major muscle. The branches from the anterior divisions are mainly to the sartorius muscle and two cutaneous branches: the medial cutaneous nerve of the thigh and the intermediate cutaneous branch of the thigh. The branches from the posterior division ⁸ are the saphenous nerve, the nerve to quadriceps femoris, and the branch to the knee joint.

22.2 Variations

The femoral nerve can arise from the ventral rami of T12 to L4 (prefixed) or from L1 to L5 (postfixed). It is rarely found within the iliopsoas muscle or between the femoral vessels.

Aizawa reported that the medial cutaneous nerve branch and the adductor longus branches were the first two branches to leave the femoral nerve in the thigh. ¹

The medial cutaneous nerve branches and vastus medialis, vastus intermedius, vastus lateralis, and rectus femoris branches are arranged medially to laterally, respectively. ¹ Gustafson et al ⁹ reported this pattern except in two specimens where the sartorius branch from the femoral nerve was located between two cutaneous branches.

22.2.1 Applied Anatomy: Dermatomal Innervations

The femoral nerve innervates the skin over the anteromedial aspect of the thigh and knee, and the medial border of the leg and medial malleolus (via the saphenous nerve).

22.2.2 Myotomal Innervations

The femoral nerve innervates the following muscles: sartorius, quadriceps femoris (rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis), iliopsoas, and pectineus.

22.2.3 Articular Innervations

The femoral nerve innervates the anterior wall of the hip joint, the anterior aspect of the femur, and the anteromedial walls of the knee joint.

22.2.4 Femoral Nerve Block

Indications

Indications for a femoral nerve block include operations on the anterior thigh (i.e., lacerations, skin graft, muscle biopsy), pin or plate insertion/removal (femur), and femur fractures.