Unlike the upper extremity, there are two nerve plexuses innervating the lower extremity; the lumbar plexus innervates the ventral aspect, whereas the lumbosacral plexus primarily innervates the dorsal aspect of the lower extremity.
The three major branches of the lumbar plexus are the lateral femoral cutaneous, femoral, and obturator nerves. They exit from the pelvis anteriorly and innervate the ventral aspect of the lower extremity.
The sciatic nerve is the combination of two major trunks: the tibial and common peroneal nerves.
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 with 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. 12.1 and 12.2 ). Thus regional block of the lower extremity focuses on block of individual peripheral nerves, and my approach to anatomy will follow that concept.
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. 12.2 ).
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. 12.3 ). 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. 12.1 ).