Anatomy

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

1. The lumbar plexus (Fig. 18.14) is formed within the psoas muscle from the anterior rami of the L1–L4 spinal nerves, with a contribution from the 12th thoracic nerve. The most cephalad nerves of the plexus are the iliohypogastric, ilioinguinal, and genitofemoral nerves. These nerves pierce the abdominal musculature anteriorly before supplying the skin of the hip and groin. The remainder of the lower abdomen is supplied by intercostal nerves. The three caudal nerves of the lumber plexus are the lateral femoral cutaneous (LFC), femoral, and obturator nerves.
   1. The LFC nerve passes under the lateral end of the inguinal ligament, supplying sensory innervation to the lateral thigh and buttock.
   2. The femoral nerve passes under the inguinal ligament just lateral to the femoral artery and 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. 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 femoral nerve, supplying the skin of the medial leg and foot. It is the only nerve of the lumbar plexus that innervates the leg below the knee.
   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 is formed from the anterior rami of the L4–L5 lumbar nerves and the S1–S3 sacral nerves. The two major nerves of the sacral plexus are the sciatic nerve and the posterior cutaneous nerve of the thigh.
   1. The posterior cutaneous nerve of the thigh travels with the sciatic nerve in its proximal extent and supplies the skin of the posterior thigh. Techniques for blocking the sciatic nerve will also block the posterior cutaneous nerve of the thigh.
   2. The sciatic nerve 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. 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.
      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 anterior foot.
         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.
         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.