Lumbar Plexus Anatomy with Application to Lateral Approaches to the Lumbar Spine

The minimally invasive lateral retroperitoneal transpsoas approach for fusion of the lumbar spine has become a popular surgical technique. However, potential complications include injury to the bowel, vasculature, and, most commonly, the lumbar plexus. ¹^, ²^, ³^, ⁴ The incidence of motor deficits following lateral transpsoas spine surgery has been reported to range between 0.7 and 33.6%, while sensory deficits following such approaches can be as high as 75%. ¹

The lumbar plexus is a network of nerve fibers located on the posterior abdominal wall, much of it lying within the substance of the psoas major muscle. The plexus is formed by the ventral rami of the first through the fourth lumbar spinal nerves. It also receives minor contributions from the 12th thoracic spinal nerve (i.e., the subcostal nerve). It produces branches that innervate structures of the lower abdomen, genitalia, and parts of the lower limb. A brief review of the salient anatomy of each branch of the lumbar plexus follows.

15.2 Anatomy

The subcostal nerve is the ventral ramus of the 12th thoracic spinal nerve. Laterally, it pierces the transversus abdominis and travels between it and the internal oblique muscle. It terminates above the terminal midline fibers of the iliohypogastric nerve and sends a lateral cutaneous branch over the iliac crest. The iliohypogastric nerve usually originates from the ventral ramus of the L1 spinal nerve. It emerges from the upper lateral border of the psoas major and crosses obliquely in front of the quadratus lumborum. It is distributed to the posterolateral gluteal skin. The anterior cutaneous branch runs between the internal oblique and transversus abdominis and innervates both muscles. It passes through the internal oblique muscle approximately 3 cm above the superficial inguinal ring and is then distributed to the suprapubic skin. The ilioinguinal nerve arises from the first lumbar ventral ramus. It emerges from the lateral border of the psoas major, along with or just inferior to the iliohypogastric nerve, passes obliquely across the quadratus lumborum and the upper part of the iliacus, and enters the transversus abdominis near the anterior end of the iliac crest. It pierces and innervates the internal oblique muscle inferior to the iliohypogastric before traversing the inguinal canal to innervate the skin of the anterior scrotum or labia majora. The lateral femoral cutaneous nerve usually arises from the posterior divisions of L2 and L3. It emerges from the lateral border of the psoas major and crosses the iliacus obliquely toward the anterior superior iliac spine (ASIS) to supply the skin of the lateral thigh. The genitofemoral nerve is formed within the substance of the psoas major, originating from the L1 and L2 ventral rami. The genital branch crosses the lower part of the external iliac artery, entering the inguinal canal through its deep ring. It supplies the cremaster and the skin of the scrotum in males. In females, it accompanies the round ligament and ends in the skin of the mons pubis and labium majus. The femoral branch provides sensory innervation to the medial upper thigh and the skin over the femoral vessels. The femoral nerve is a motor and sensory nerve derived from the posterior divisions of the ventral rami of the L2–L4 spinal nerves. Distally, in the pelvis, it is located lateral to the psoas major muscle, in the cleavage between this muscle and the iliacus. It travels deep to the inguinal ligament and lies lateral to the femoral artery. It innervates the quadriceps femoris, pectineus, sartorius, and the skin over the anterior thigh and medial leg down to the medial foot. The obturator nerve arises from the anterior divisions of the second to fourth lumbar ventral rami. It descends through the psoas major and emerges from its medial border at the pelvic brim running down between this muscle and the lumbar vertebral column to exit via the obturator foramen into the thigh.