(Fig. 1) Lumbosacral plexus. A, The lumbar and sacral plexuses are shown here as a continuum of nerve fibers emerging from vertebral levels LI to S5. The lumbar plexus is directed at innervation of the lower anterolateral abdominal wall and the inguinal canal. The sacral plexus innervates pelvic organs and continues outside the pelvis to supply the posterior thigh and nearly all of the lower limb below the knee, (continued)
(Fig. 2) cont’d B, This schematic drawing of the sacral plexus illustrates formation of its major branches and the distribution of anterior and posterior division branches in them.
The lumbosacral trunk (L4, L5) is a thick nerve formed from the entire ventral ramus of the fifth lumbar nerve and the descending portion of the fourth. It descends distally anterior to the sacral ala posterior to the pelvic fascia and joins the ventral rami of the sacral nerves anterior to the piriformis muscle.
The sacral plexus (L4, L5, and SI to S4) is formed by the ventral rami of the fifth lumbar vertebra, the first three sacral rami, and a portion of the fourth lumbar and fourth sacral rami. The lumbosacral trunk unites with the first sacroventral ramus and, together with the second, third, and part of the fourth sacral ramus, forms the solid triangular sacral plexus. The plexus is located between the piriformis and the pelvic fascia.
The sciatic nerve (L4, L5, SI, S2, and S3), as a terminal branch of the sacral plexus, is composed of fibers from the fourth and fifth lumbar nerves and the first through third sacral nerves. The sciatic nerve later splits into the common peroneal and tibial nerves.
The pudendal nerve (S2, S3, S4) is composed of sacral branches from the second, third, and fourth sacral nerves and leaves the pelvis through the greater sciatic foramen and, as mentioned previously, reenters through the lesser sciatic foramen to enter the pudendal canal.
The coccygeal plexus (S4, S5) is formed from ventral rami of the fourth and fifth sacral nerves, as well as the coccygeal nerve, innervating the pelvic diaphragm and the skin posterior to the coccyx.
The lumbar plexus is formed by the ventral rami of the first three and the greater part of the fourth lumbar nerves. It is located dorsal to or within the fasciculi of the psoas major muscle. Although these fibers emerge from the posterior abdominal wall, many traverse the pelvis en route to their site of innervation. The branches of the lumbar plexus are listed in Table 1
|Lateral femoral cutaneous||L2, L3|
|Obturator||L2, LB, L4|
|Femoral||L2, L3, L4|
The iliohypogastric nerve emerges from the upper part of the lateral border of the psoas major muscle, penetrates the posterior part of the transversus abdominus muscle near the crest of the ilium, and divides into a lateral and an anterior cutaneous branch.
The lateral cutaneous branch is distributed to the skin of the gluteal region.
The anterior cutaneous branch continues on a course between the internal oblique and transversus muscles and penetrates the aponeurosis of the external oblique 2 cm above the external inguinal ring, where it is distributed to the skin of the hypogastric region.
The ilioinguinal nerve arises just caudal to the iliohypogastric nerve, traversing posteriorly to the crest of the ilium. It penetrates the internal oblique muscle, travels through the external inguinal ring, and is distributed to the skin of the mons pubis and labia majora.
The genitofemoral nerve passes distally through the substance of the psoas major muscle, emerging at the level of the fourth lumbar vertebra, where it is covered by peritoneum.
- The genital branch progresses to the inguinal ligament, where it accompanies the round ligament of the uterus. It provides sensory fibers to the round ligament and the labia majora.
- The femoral branch, which is lateral to the genital branch, passes under the inguinal ligament with the external iliac artery and enters the femoral sheath, penetrating the fascia lata to supply the skin of the proximal anterior thigh.
Lateral Femoral Cutaneous Nerve
The lateral femoral cutaneous nerve emerges from the lateral portion of the psoas major, where it passes under the inguinal ligament and over the sartorius muscle, dividing into an anterior and posterior branch.
- The anterior branch innervates the skin of the lateral and anterior thigh as far distally as the knee.
- The posterior branch innervates the lateral and posterior surfaces of the thigh. This innervation extends from the level of the greater trochanter to the midportion of the thigh.
The obturator nerve provides motor innervation to the adductor muscles of the thigh. It emerges from the medial border of the psoas near the pelvic brim posterior to the common iliac vessels. It continues distally lateral to the internal iliac! vessels and the ureter, then runs along the lateral wall of the pelvis to enter the obturator foramen with the obturator artery and vein.
The femoral nerve represents the largest branch of the lumbar plexus and is the principal motor innervation of the anterior thigh. It originates in the pelvis within the fibers of the psoas major, emerging from the psoas below the iliac crest. It descends between the psoas and iliacus muscles covered by the fascia. It passes underneath the inguinal ligament and remains lateral to the femoral sheath. Besides its motor function, it has sensory cutaneous function. The cutaneous branches penetrate the fascia lata of the thigh approximately 7 to 8 cm distal to the inguinal ligament, dividing into an anterior and a medial branch.