Lower Extremity Block Anatomy

Anesthesiologists are more comfortable carrying out lower extremity regional block than upper extremity regional block because of the ease and simplicity of blocking the lower extremities with neuraxial techniques. Also, in no anatomic site outside the neuraxis are the lower extremity plexuses as compactly packaged as are the nerves to the upper extremity in the brachial plexus. If one compares the path of lower extremity nerves over the pelvic brim to the path of the brachial plexus over the first rib, it is clear that the four major nerves to the lower extremity exit from four widely differing sites (Figs 10-1 and 10-2). Thus, regional block of the lower extremity focuses on block of individual peripheral nerves, and my approach to anatomy will follow that concept.

Figure 10-1. Lower extremity anatomy: major nerves, anterior oblique view.

Figure 10-2. Lower extremity anatomy: major nerves, lateral view.

Two major nerve plexuses innervate the lower extremity: the lumbar plexus and the lumbosacral plexus. The lumbar plexus primarily innervates the ventral aspect, whereas the lumbosacral plexus primarily innervates the dorsal aspect of the lower extremity (see Fig. 10-2).

The lumbar plexus is formed from the ventral rami of the first three lumbar nerves and part of the fourth lumbar nerve. In approximately half of patients, a small branch from the twelfth thoracic nerve joins the first lumbar nerve. The lumbar plexus forms from the ventral rami of these nerves anterior to the transverse processes of the lumbar vertebrae deeply within the psoas muscle (Fig. 10-3). The cephalad portion of the lumbar plexus (i.e., the first lumbar nerve, and often a portion of the twelfth thoracic nerve) splits into superior and inferior branches. The superior branch redivides into the iliohypogastric and ilioinguinal nerves, and the smaller inferior branch unites with a small superior branch of the second lumbar nerve to form the genitofemoral nerve (see Fig. 10-1).

Figure 10-3. Lumbar plexus anatomy: cross-sectional view. Ao, aorta; VC, vena cava.

The iliohypogastric nerve penetrates the transversus abdominis muscle near the crest of the ilium and supplies motor fibers to the abdominal musculature. It ends in an anterior cutaneous branch to the skin of the suprapubic region and a lateral cutaneous branch in the hip region (Fig. 10-4).

Figure 10-4. Lower extremity anatomy: proximal innervation (peripheral nerves labeled on right side of the body, dermatomes on the left).

The ilioinguinal nerve courses slightly inferior to the iliohypogastric nerve. It then traverses the inguinal canal and ends cutaneously in branches to the upper and medial parts of the thigh and near the anterior scrotal nerves, which supply the skin at the root of the penis and the anterior part of the scrotum in males (see Fig. 10-4). In females, the comparable anterior labial nerves supply the skin of the mons pubis and labia majora.

The genitofemoral nerve divides at a variable level into genital and femoral branches. The genital branch is small; it enters the inguinal canal at the deep inguinal ring and supplies the cremaster muscle, small branches to the skin and fascia of the scrotum, and adjacent parts of the thigh. The femoral branch is the more medial of the two branches and continues under the inguinal ligament on the anterior surface of the external iliac artery. Below the inguinal ligament, it pierces the femoral sheath and passes through the saphenous opening to supply the skin over the femoral triangle lateral to that supplied by the ilioinguinal nerve (see Fig. 10-4). These three nerves are clinically important during regional block for inguinal herniorrhaphy or other groin procedures carried out under regional block.

Caudal to these three nerves are three major nerves of the lumbar plexus that exit from the pelvis anteriorly and innervate the lower extremity. These are the lateral femoral cutaneous, femoral, and obturator nerves (see Figs. 10-1 and 10-2).

The lateral femoral cutaneous nerve passes under the lateral end of the inguinal ligament. It may be superficial or deep to the sartorius muscle and it descends at first deep to the fascia lata. It provides cutaneous innervation to the lateral portion of the buttock distal to the greater trochanter and to the proximal two thirds of the lateral aspect of the thigh.

The obturator nerve descends along the medial posterior aspect of the psoas muscle and through the pelvis to the obturator canal into the thigh. This nerve supplies the adductor group of muscles, the hip and knee joints, and often the skin on the medial aspect of the thigh proximal to the knee.

The femoral nerve is the largest branch of the lumbar plexus. It emerges through the fibers of the psoas muscle at the muscle’s lower lateral border and descends in the groove between the psoas and the iliacus muscles. It passes under the inguinal ligament within this groove. Slightly before, or on, entering the femoral triangle of the upper thigh, the femoral nerve breaks into numerous branches supplying the muscles and skin of the anterior thigh, knee, and hip joints.

The lumbosacral plexus is formed by the ventral rami of the lumbar fourth and fifth and the sacral first, second, and third nerves. Occasionally, a portion of the fourth sacral nerve contributes to the sacral plexus. The nerve from the plexus that is of primary interest to anesthesiologists during lower extremity block is the sciatic nerve. The posterior femoral cutaneous nerve is sometimes listed as an additional branch important to anesthesiologists. In reality, the sciatic nerve is the combination of two major nerve trunks: the first is the tibial nerve, derived from the anterior branches of the ventral rami of the fourth and fifth lumbar and the first, second, and third sacral nerves, whereas the second is the common peroneal nerve, derived from the dorsal branches of the ventral rami of the same five nerves. These two major nerve trunks pass as the sciatic nerve through the upper leg to the popliteal fossa, where they divide into their terminal branches, the tibial and common peroneal nerves.

Figures 10-5 and 10-6 illustrate the cutaneous innervation of the peripheral nerves of the lower extremity. This subject is illustrated with the patient’s lower extremity in both the anatomic and the lithotomy positions for greatest clinical utility. Figure 10-7 illustrates the dermatomal innervation of the lower extremities in a similar manner. Figure 10-8 illustrates the osteotome pattern of lower extremity innervation and will be most useful to anesthesiologists who are providing anesthesia for orthopedic procedures. Figure 10-9 helps clarify the cross-sectional anatomy pertinent to regional block of the lower extremity.

Figure 10-5. Lower extremity anatomy: proximal and distal innervation.

Figure 10-6. Lower extremity anatomy in lithotomy position: proximal and distal peripheral nerves.

Figure 10-7. Lower extremity anatomy in lithotomy position: dermatomes.

Figure 10-8. Lower extremity anatomy: osteotomes.

Figure 10-9. Lower extremity anatomy: cross-sectional magnetic resonance images. A, Location of sections. B, Upper leg (below the hip). C, Middle leg (above the knee). D, Lower leg (above the ankle).