Anatomy

Anatomy. There are two major plexuses that innervate the lower extremity: the lumbar plexus and the sacral plexus.

1. The lumbar plexus (Figure 21.9A) is formed within the psoas muscle from the anterior rami of the L1-4 spinal nerves, with contribution from the 12th thoracic nerve (subcostal nerve). The most cephalad nerves of the plexus are the iliohypogastric,ilioinguinal, and genitofemoral. These nerves pierce the abdominal musculature anteriorly before supplying the skin of the hip, groin, and genital regions. The three caudal nerves of the lumber plexus are the lateral femoral cutaneous (LFC),femoral, and obturator.
   1. The LFC nerve passes under the lateral end of the inguinal ligament, supplying sensory innervation to the lateral thigh and buttock.
      

      Figure 21.9 Diagram of the lumbar (A) and sacral plexus (B).

      (From Horn JL,Nasiri M.Regional blocks—A brief overview. In: Jaffe RA,Schmiesing CA,Golianu B, eds. Anesthesiologist’s Manual of Surgical Procedures.6th ed.2020:1763-1799.)

   2. The femoral nerve passes under the inguinal ligament just lateral to the femoral artery and vein, under the fascia iliaca, on the surface of the iliacus muscle. It supplies the muscles and skin of the anterior thigh as well as the knee and hip joints. From there, it courses with the femoral artery and vein through the adductor canal, an anatomic tunnel bounded by the fascial planes of the sartorius, the vastus medialis, and the adductor longus and magnus muscles. The nerve and vessels exit the canal at the adductor hiatus just above the medial femoral condyle. The saphenous nerve is the cutaneous termination of the nerve, supplying the skin of the medial leg and foot. It is the only nerve of the lumbar plexus that innervates below the knee (Figure 21.3).
   3. The obturator nerve exits from the pelvis through the obturator canal of the ischium, innervating the adductor muscles of the thigh, the hip and knee joints, and a portion of the skin of the medial thigh.
2. The sacral plexus (Figure 21.9B) is formed from the anterior rami of the L4-5 nerves and the S1-3 nerves (lumbosacral trunk). The two major nerves of the sacral plexus are the sciatic nerve (SCN) and the posterior cutaneous nerve of the thigh (PCNT). Other branches of the sacral plexus are the pudendal, superior, and inferior gluteal nerves.
   1. The PCNT travels with the SCN in its proximal extent and supplies the skin of the posterior thigh, extending sometimes to the posterior calf.
   2. The SCN passes out of the pelvis through the greater sciatic foramen, becomes superficial at the lower border of the gluteus maximus, descends along the medial aspect of the femur supplying branches to the hamstrings, and becomes superficial again at the popliteal fossa, where it lies between the semimembranosus and semitendinosus muscles medially and biceps femoris muscle laterally. There, it divides into the tibial nerve and the common peroneal nerve.
      1. The tibial nerve travels down the posterior calf and passes under the medial malleolus before dividing into its terminal branches. It supplies the skin of the medial and plantar foot and causes plantar flexion (Figure 21.3).
      2. The common peroneal nerve winds around the head of the fibula before dividing into the superficial and deep peroneal nerves.
         1. The superficial peroneal nerve is a sensory nerve that passes down the lateral calf, dividing into its terminal branches just medial to the lateral malleolus, supplying the dorsal aspect of the foot (Figure 21.3).
         2. The deep peroneal nerve enters the foot just lateral to the anterior tibial artery, lying at the superior border of the malleolus, in between the anterior tibialis tendon and the extensor hallucis longus tendon. Although primarily a motor nerve causing dorsiflexion of the foot, it also sends a sensory branch to the web space between the first and second toes (Figure 21.3).
         3. The sural nerve is a sensory nerve formed from branches of the common peroneal and tibial nerves. It passes under the lateral malleolus, supplying the lateral foot (Figure 21.3).