In describing the lower limb, the lumbosacral plexus may be said to be formed from the ventral rami of the first lumbar to third sacral nerves (L1 to S3), with a common small contribution from the 12th thoracic nerve (T12) (see Plates 2-2 to 2-4). The lumbar portion of the plexus arises from the four upper lumbar nerves and gives rise to the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, obturator, accessory obturator, and femoral nerves.
As with the brachial plexus, the spinal nerves contributing to the lumbar plexus divide into anterior and posterior branches, but the plexus lacks some of the complexity of the brachial plexus, because the definitive nerves usually arise from combinations of looping contributions from adjacent spinal nerves. The lumbar plexus is formed deep to the psoas major muscle and lies anterior to the transverse processes of the lumbar vertebrae (see Plate 2-2). Only the first two lumbar nerves contribute preganglionic sympathetic fibers to the sympathetic chain through white rami communicantes; all lumbar nerves receive postganglionic fibers through gray rami communicantes.
The iliohypogastric nerve arises from L1 together with a frequent contribution from T12. It emerges from the psoas major muscle at its lateral border and crosses the quadratus lumborum muscle to penetrate the transverse abdominal muscle near the iliac crest. This nerve, primarily motor to abdominal musculature, ends in an anterior cutaneous branch to the skin of the suprapubic region and a lateral cutaneous branch that crosses the iliac crest to distribute in the hip region. A lateral cutaneous branch of the subcostal nerve (T12) also supplies the upper thigh.
The ilioinguinal nerve from L1 has a similar course to the iliohypogastric nerve in the abdominal wall but enters the lateral end of the inguinal canal and accompanies the spermatic cord through that canal. Emerging at the superficial inguinal ring, it ends as the anterior scrotal (or anterior labial) nerve as a cutaneous nerve to the scrotum and adjacent area of the thigh. In about 35% of cases, the ilioinguinal nerve combines with the genitofemoral nerve in the abdomen, running with the latter on the surface of the psoas major muscle, but distributes finally in its typical cutaneous distribution.
The genitofemoral nerve arises by a union of branches from the anterior portions of L1 and L2. In the abdomen, it descends on the ventral surface of the psoas major muscle and then divides into genital and femoral branches. The genital branch innervates the cremaster muscle and gives twigs to the scrotum and adjacent thigh; the more medial femoral branch descends under the inguinal ligament on the surface of the external iliac artery to supply the skin of the femoral triangle.
The lateral femoral cutaneous nerve arises from the posterior branches of L2 and L3 (see Plates 2-2 and 2-5). The obturator nerve is the largest nerve formed from the anterior divisions of the lumbar plexus, specifically from those of L2 to L4. The accessory obturator nerve is small and is present in only 9% of cases. (The obturator nerve is fully described in Plate 2-6.)
The femoral nerve, the largest branch of the lumbar plexus, is formed from the posterior branches of L2 to L4. Passing under the inguinal ligament, it shortly breaks up in the femoral triangle into its numerous branches. (This nerve is fully described in Plate 2-5.)
Muscular branches of the lumbar plexus distribute to the quadratus lumborum muscle (T12; L1, 2, 3 ), the psoas major muscle ([L1], 2, 3, 4), the psoas minor muscle (L1, 2), and the iliacus muscle (L2, 3, 4).
The sacral portion of the lumbosacral plexus, commonly designated the sacral plexus, is formed from the ventral rami of a part of the fourth lumbar nerve (L4) and the fifth lumbar and first, second, and third sacral nerves (L5; S1, 2, 3) (see Plate 2-3). The descending portion of L4 joins L5 over the ala of the sacrum to form the lumbosacral trunk, which then descends across the sacroiliac articulation to join the ventral ramus of S1. The lumbosacral trunk contains anterior and posterior branches of the ventral rami of L4 and L5. The ventral rami of S1, 2, 3 pass lateralward from the pelvic sacral foramina and divide into anterior and posterior branches in front of the piriformis muscle. The largest nerves of the sacral plexus are L5 and S1, and the superior gluteal artery usually leaves the pelvis by passing between them.
Converging toward the lower portion of the greater sciatic foramen, the plexus forms a broad triangular band, the apex of which passes through the foramen into the gluteal region. The pelvic splanchnic nerves, arising from the ventral rami of S2 to S4, represent the important sacral part of the craniosacral (parasympathetic) portion of the autonomic nervous system. They join the inferior hypogastric plexus and have a largely pelvic and perineal distribution. All the nerves of the plexus receive gray rami communicantes from the sympathetic chain ganglia or trunk.
The principal nerve of the sacral plexus is the sciatic nerve (see Plate 2-7). It is composed of an anteriorly derived nerve, the tibial segment, and a nerve formed from posterior branches, the common peroneal nerve. These two nerves are usually combined in a single sheath, but in 10% of cases the two parts are separated in the greater sciatic foramen by all or part of the piriformis muscle. They are occasionally separate through-out the thigh. The nerves of the plexus and their sources are listed in tabular format on Plate 2-3.
NERVES OF GLUTEAL REGION
The sciatic nerve usually emerges from the pelvis at the lower border of the piriformis muscle and enters the thigh in the hollow between the ischial tuberosity and the greater trochanter of the femur (see Plate 2-7).
The nerve to the piriformis muscle may be represented by separate contributions from S1 and S2. Twigs arise from the dorsal aspect of these nerves and immediately enter the pelvic surface of the muscle. Muscular nerves to the levatorani and coccygeus muscles arise from the loop between the rami of S3 and S4 and descend to enter the pelvic surface of these muscles.
The superior gluteal nerve, from the posterior branches of L4, 5 and S1, passes from the pelvis above the piriformis muscle. Deep to the gluteus maximus and gluteus medius muscles, the nerve accompanies the superior gluteal vessels anteriorly over the surface of the gluteus minimus muscle. It supplies the gluteus medius and minimus muscles and, continuing beyond them, the tensor fasciae latae muscle.
The inferior gluteal nerve, formed from the posterior branches of L5 and S1, 2, passes from the pelvis below the piriformis muscle. It enters the deep surface of the gluteus maximus muscle, to which it is the sole supply. The nerve to the quadratus femoris and inferior gemellus muscles is formed from the anterior branches of L4, 5 and S1. In the gluteal region, it is deep to the sciatic nerve and descends over the back of the ischium anterior to the gemellus muscles and the tendon of the internal obturator muscle. It provides articular branches to the hip joint and a branch to the inferior gemellus muscle and ends in the anterior surface of the quadratus femoris muscle.
The nerve to the obturator internus and superior gemellus muscles arises from anterior branches of L5 and S1, 2. In the gluteal region, it is inferomedial to the sciatic nerve and on the lateral side of the internal pudendal vessels. It crosses the superior gemellus muscle and supplies a small nerve to it. The remaining nerve to the obturator internus muscle crosses the ischial spine and enters the ischiorectal fossa through the lesser sciatic foramen. It ends in the perineal surface of the muscle. The posterior femoral cutaneous nerve is a mixed nerve, formed by posterior branches from S1 and S2 and anterior branches from S2 and S3; its cutaneous distributions are described on page 57. In the gluteal region, it lies alongside the sciatic nerve and descends in the midline of the thigh. It also provides perineal branches that are cutaneous in the perineum and the back of the scrotum.
The perforating cutaneous nerve arises from posterior branches of S2 and S3 and is associated at its origin with the lower roots of the posterior femoral cutaneous nerve. Its cutaneous distribution is described on page 57.
DERMATOMES OF LOWER LIMB
As in the upper limb, the serial order of distribution of lower limb nerves as seen in the lumbosacral plexus is retained in the cutaneous zones of appropriate nerves in the limb. The lumbar nerves have cutaneous terminals that distribute from above down and lateromedially. Sacral segments are restricted to the posterior aspect of the limb and the lateral side of the foot. The spiraling of nerve distribution is a consequence of the medial rotation of the lower limbs of almost 90 degrees that takes place in development, so that the future knees point ventrolaterally (see Plates 2-5 to 2-7). There is always an overlap of adjacent segments; therefore, the lines of separation are indistinct.