Lumbar and sacral plexus anatomy

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CHAPTER 23 Lumbar and sacral plexus anatomy

Dominic Harmon

The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus. The plexus is usually divided into three parts: the lumbar, sacral, and pudendal plexuses, for ease of description. The lumbar plexus primarily innervates the ventral aspect, whereas the sacral plexus innervates the dorsal aspect of the lower limb.

Lumbar plexus

The lumbar plexus (Fig. 23.1) lies deep within the psoas major muscle in front of the transverse processes of the lumbar vertebrae. It is formed by the ventral rami of the first three lumbar nerves and the greater part of the ventral ramus of the fourth nerve. All the branches of the plexus emerge from the substance of the psoas major.

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Figure 23.1 Lumbar plexus anatomy.

The first lumbar nerve, frequently supplemented by the 12th thoracic, splits into an upper and a lower branch; the upper and larger branch divides into the iliohypogastric and ilioinguinal nerves, the lower and smaller branch unites with a branch of the second lumbar to form the genitofemoral nerve.

The remainder of the second nerve, and the third and fourth nerves, divide into ventral and dorsal divisions. The ventral division of the second unites with the ventral divisions of the third and fourth nerves to form the obturator nerve. The dorsal divisions of the second and third nerves divide into two branches, a smaller branch from each uniting to form the lateral cutaneous nerve of the thigh, and a larger branch from each joining with the dorsal division of the fourth nerve to form the femoral nerve.

The iliohypogastric and ilioinguinal nerves

The iliohypogastric (L1) and ilioinguinal (L1) nerves emerge from the upper part of the lateral border of the psoas major muscle, initially together or separate throughout, with the former above the latter. They both pass laterally in front of the quadratus lumborum to enter the neurovascular plane between the transverse abdominis and internal oblique muscles.

The iliohypogastric nerve pierces the internal oblique about 2 cm medial to the anterior superior iliac spine and goes on to pierce the external oblique about 3 cm above the superficial inguinal ring (anterior cutaneous branch). It supplies sensation to suprapubic skin; a lateral cutaneous branch supplies posterolateral gluteal skin.

The ilioinguinal nerve pierces the lower border of the internal oblique to enter the inguinal canal, which it leaves through the superficial ring to supply the skin of the anterior scrotum (mons pubis and labium majus), root of penis (clitoris), and upper medial thigh.

The genitofemoral nerve

The genitofemoral nerve (L1, 2) emerges from the anterior surface of the psoas major. Its genital branch enters the inguinal canal through the deep ring, and runs in the spermatic cord, supplying cremaster and a small area of scrotal skin in males. In females, it accompanies the round ligament. The femoral branch passes down behind the inguinal ligament with the femoral artery (superficial and lateral to it), and pierces the femoral sheath and fascia lata to supply the skin over the femoral triangle.

The lateral cutaneous nerve of thigh

The lateral cutaneous nerve of the thigh (lateral femoral cutaneous nerve) arises from the dorsal divisions of the second and third lumbar nerves. It emerges from the lateral border of the psoas major about its middle, and crosses the iliacus muscle obliquely toward the anterior superior iliac spine. It passes under the inguinal ligament and over the sartorius muscle into the thigh, where it divides into two branches.

The anterior branch becomes superficial about 10 cm below the inguinal ligament and divides into branches, which are distributed to the skin of the anterior and lateral parts of the thigh as far as the knee. The posterior branch pierces the fascia lata and subdivides into branches, which pass backward across the lateral and posterior surfaces of the thigh, supplying the skin from the level of the greater trochanter to the middle of the thigh.

The femoral nerve

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