8.1.1 Lumbar Plexus

The lumbar plexus is formed by fibers from the 12th thoracic segment and the anterior rami of the 1st to 4th lumbar nerves. Segments L1–L4 are usually involved in the formation of the femoral nerve, the obturator nerve, and the lateral cutaneous nerve of the thigh. The plexus passes peripherally after its exit from the intervertebral foramina, usually covered by the psoas major muscle (Fig. 8.4).

The genitofemoral nerve and the lateral cutaneous nerve of the thigh leave the plexus soon after the iliohypogastric and ilioinguinal nerves have split off.

Nerves relevant for anesthesia.

The individual nerves of the lumbar plexus relevant for anesthesia are as follows:

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  The lateral cutaneous nerve of the thigh (L2/3) passes over the iliacus muscle medial to the anterior superior iliac spine under the inguinal ligament; it is a purely sensory nerve innervating the skin on the lateral side of the thigh.

  
  
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  The obturator nerve (L2–L4) leaves the plexus medial to the psoas major muscle and passes through the obturator canal together with the obturator vein and artery to the inside of the thigh. An accessory obturator nerve, which innervates the capsule of the hip joint, is found in 9% of people. The obturator nerve has a very variable sensory area of innervation in the medial thigh and provides motor innervation to the adductors.

  
  
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  The femoral nerve (L1–L4) is the largest nerve of the lumbar plexus and provides the sensory innervation of the front of the thigh, while its sensory terminal branch, the saphenous nerve, innervates the inside of the lower leg as far as the ankle. The femoral nerve passes anterior to the psoas major muscle under the inguinal ligament through the muscular lacuna and is the motor nerve for the quadriceps femoris, sartorius, and pectineus muscles.

8.1.2 Sacral Plexus

The sacral plexus constitutes the lower part of the lumbosacral plexus and is the biggest nerve plexus in the human body. The plexus is formed by the junction of the anterior rami of the five sacral nerves and the coccygeal nerve. It also receives a substantial trunk, the lumbosacral trunk, from the lumbar nerves, which is composed of the entire anterior ramus of the fifth lumbar nerve and fibers from the fourth lumbar nerve (Fig. 8.5). The sacral plexus provides the nerves for the parts of the lower limb that are not supplied by the lumbar plexus, that is, for some of the hip muscles, for the flexor side of the thigh, and for all the muscles of the lower leg and foot. It also innervates the skin in part of the buttock area, the posterior side of the thigh, and the posterior, fibular, and anterior side of the lower leg and foot.

Sciatic plexus.

For anesthesia of the leg, only the so-called sciatic plexus is of importance. It derives its roots from part of the anterior ramus of the 4th lumbar nerve and from the entire anterior ramus of the 5th lumbar nerve, which together form the lumbosacral trunk, and from the anterior rami of the 1st and 2nd and part of the 3rd sacral nerve. The anterior ramus of the 1st sacral nerve is not only the biggest branch of the lumbosacral plexus, but also the biggest anterior ramus overall.

All roots of the plexus converge from their exit sites toward the greater sciatic foramen, so that the plexus forms a triangular sheet, the tip of which points toward the infrapiriform foramen where the sciatic nerve emerges. The nerve plexus lies largely on the piriformis muscle and is covered in pelvic direction by the parietal peritoneum or the tissue beneath it, the parietal fascia of the pelvis, and branches of the iliac artery. Both the superior and inferior gluteal arteries are related to the plexus in that the former passes between the lumbosacral trunk and the root of the 1st sacral nerve, the latter between the 2nd and 3rd sacral nerves.

The articular rami, which supply parts of the hip capsule, and the periosteal branches, which innervate the periosteum of the ischial tuberosity, the greater trochanter, and the lesser trochanter, are derived from the sciatic plexus.

Nerves relevant for anesthesia.

The following are the nerves relevant for anesthesia of the lower limb (Meier 2003):

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  The posterior cutaneous nerve of the thigh (S1–S3), a purely sensory nerve, leaves the pelvis minor through the infrapiriform foramen and passes a long distance subfascially, but close to the fascia lata downward on the back of the thigh toward the back of the knee.

  
  
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  The sciatic nerve (L4–S3) is the biggest nerve in the body. It derives its fibers from all the roots of the sacral plexus and innervates the entire lower leg and foot, the ischiocrural muscles of the thigh, and the small external rotators of the hip. It leaves the pelvis through the infrapiriform foramen and then passes downward in the middle third between the ischial tuberosity and the greater trochanter. The proximal part of the popliteal fossa is the furthest point at which it will divide into its two terminal branches: the tibial nerve for the flexor muscles of the ankle and sole of the foot and the common fibular nerve (also known as the common peroneal nerve) for the extensor side of the ankle and the dorsum of the foot.

  
  
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  The tibial nerve supplies the motor innervation to the flexor muscles of the ankle and is responsible for the flexors of the toes and foot. It provides sensory innervation to the skin of the lateral lower leg and the sole of the foot, and after joining the communicating branch of the fibular nerve it innervates the lateral margin of the heel and foot as the sural nerve and its terminal branch the lateral dorsal cutaneous nerve. Complete anesthesia of the tibial nerve makes plantar flexion of the foot nearly impossible while spreading and closing of the toes are rendered completely impossible.

  
  
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  The common fibular nerve (L4–S2) runs in the popliteal fossa lateral to the tibial nerve and medial to the biceps femoris muscle as far as its attachment to the head of the fibula. Distally from this point it winds around the neck of the fibula to pass posteriorly through the intermuscular septum of the leg to reach the peroneal compartment. Here it enters the gap between the origins of the fibularis longus muscle (also known as the peroneus longus) and immediately divides into its two branches. One of them is predominantly sensory (superficial fibular nerve) and the other is mainly motor (deep fibular nerve). The superficial fibular nerve supplies “little muscle and a lot of skin,” namely motor supply only to the fibular muscles but sensory innervation to the skin of the lower leg and with its medial and intermediate dorsal cutaneous nerves to the skin of the dorsum of the foot and the toes. The deep fibular nerve in contrast supplies “a lot of muscles and little skin,” namely motor supply to extensor muscles of the lower leg and sensory supply only to the sides of the first and second toes facing each other (also called the first interdigital space).