Chapter 10 – Lower Extremity Anatomy
When lower extremity regional block is contrasted with upper extremity regional block, it is clear that anesthesiologists are more comfortable carrying out the former. In large part this is due to the ease and simplicity of blocking the lower extremities with neuraxial techniques. Additionally, 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, in a fashion similar to the routing of brachial plexus over the first rib, it is clear from Figures 10-1 and 10–2 that the four major nerves to the lower extremity exit from four widely differing sites. Thus, regional block of the lower extremity necessarily focuses on blocking individual peripheral nerves. My approach to anatomy follows that concept.
When considering lower extremity innervation, it is essential to understand that two major nerve plexuses innervate the lower extremity; the lumbar plexus and the lumbosacral plexus. The lumbar plexus is primarily involved in innervating the ventral aspect, whereas the lumbosacral plexus is primarily involved with innervating 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 12th 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 12th 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 ).
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 ).
The ilioinguinal nerve has a course slightly inferior to that of the iliohypogastric nerve. In males, 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 (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 via 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 upon 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 nerves 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 nerves and the first, second, and third sacral nerves. The second major portion of the sciatic nerve 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 through the upper leg to the popliteal fossa, where they divide into their terminal branches, tibial and common peroneal.
Figures 10-5 and 10–6 illustrate the cutaneous innervation of the peripheral nerves of the lower extremity. I have chosen to illustrate this subject with the patient’s lower extremity in both the anatomic and lithotomy positions. These positions should provide a unique and clinically useful perspective. 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 is most useful to anesthesiologists who are providing anesthesia for orthopedic procedures. Figure 10-9 helps in understanding the cross-sectional anatomy pertinent to regional block of the lower extremity.