Surgical Anatomy and Approaches to the Nerves of the Lower Limb

Keywords: lumbar plexus, sacral plexus, lateral femoral cutaneous nerve, femoral nerve, sciatic nerve

Fernando Martínez and Federico Salle

2.1 Introduction

Innervation of the lower limbs follows a basic pattern: two nerve plexuses (lumbar and sacral) give rise to a number of nerves that enter the extremity through three anatomical regions—the inguinal, gluteal, and obturator—to distribute themselves throughout the muscular, cutaneous, bony, and vascular structures of the limb. ^{1,​ 2,​ 3,​ 4,​ 5,​ 6,​ 7,​ 8}

In this chapter, we review the anatomy of the nerves of the lower limb, especially focusing on: (1) how this anatomy can cause clinical disorders, and (2) how it influences surgical approaches to treatment.

2.2 Lumbar Plexus

The lumbar plexus is formed by the union of the anterior branches of spinal roots L1–L4, with additional nerve fiber contributions from T12 ( ▶ Fig. 2.1). The anterior branches of the aforementioned roots emerge from their corresponding neural foramina and, thereafter, remain inside the psoas major muscle which has two fascicles of insertion. The anterior insertions correspond to the lumbar vertebral bodies, while the posterior ones can be found at the level of the transverse processes of the same vertebrae. This is how a V-shaped interstice is created between the two fascicles. ³

Within the substance of the muscle, L1–L4 exchange fibers and form the lumbar plexus as follows: L1 mostly supplies the ilioinguinal, iliohypogastric, and genitofemoral nerves; L2 and L3 contribute to the lateral femoral cutaneous nerve of the thigh; and L2–L4 give rise to the obturator and femoral nerves. Conceptually, Russell has divided the terminal branches of the lumbar plexus into two groups of three nerves each: (1) the inguinal group, composed of the ilioinguinal, iliohypogastric, and genitofemoral nerves; and (2) the femoral group, composed of the lateral cutaneous, femoral, and obturator nerves. From an anatomical point of view, the first four nerves are considered collateral branches of the lumbar plexus, while the last two are considered terminal branches.

2.2.1 Inguinal Group

The three nerves within the inguinal group originate within the psoas major muscle and run across the anterior abdominal wall to reach the inguinal region. These nerves can suffer direct trauma, can be damaged by traction or kinking, and can even be injured by sutures placed during operative procedures involving the lower anterior abdominal wall (such as, appendectomies, C-sections, etc.) or lateral wall (lumbotomy), giving rise to sensory disturbances or pain syndromes across their territory of distribution (i.e., the inguinal area and genitalia). ^{9,​ 10,​ 11,​ 12}

Iliohypogastric Nerve

The iliohypogastric nerve has its origins in L1, although it also receives a T12 anastomosis. After emerging from under the psoas muscle, it runs outward across the quadratus lumborum muscle to finally rest between the transversus abdominis and internal oblique muscles. In the region of the anterior superior iliac spine, it divides into two branches. The outer branch becomes superficial and innervates the lateral gluteal region. The internal branch continues its descending path, passing through the inguinal canal to innervate the inguinal region. The sensory distributions of all the nerves of the inferior limb are shown in ▶ Fig. 2.2.

Ilioinguinal Nerve

The ilioinguinal nerve stems from a branch of the L1 root and has a trajectory which is similar to that of the iliohypogastric nerve, albeit somewhat more caudal. It runs at the level of the oblique muscles of the abdomen and innervates structures, such as, the spermatic cord and cremaster muscle. Along with the iliohypogastric nerve, it provides sensory innervation to the inguinal and genital regions.

Genitofemoral Nerve

The genitofemoral nerve receives nerve fibers from both L1 and L2. It has a deeper trajectory than the aforementioned two nerves within the lumbar region. Before reaching the inguinal ligament, it divides into two branches: genital and femoral. ⁹ The femoral branch runs beneath the inguinal ligament, lateral to the common femoral artery, and innervates the region of the femoral triangle. The genital branch enters the inguinal canal and terminates in the skin of the external genitalia and cremaster muscle (in males).

Surgical Approach

These three nerves are not generally approached per se, except when they are affected by postoperative fibrosis or iatrogenic lesions secondary to different kinds of surgery in the region. Ideally, a thorough clinical examination will reveal the source of pain, guiding the location and extent of the surgical incision.

2.2.2 Femoral Group

The femoral group of nerves is composed of two mixed (both motor and sensory) nerves and one that is purely sensory.

Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve (LFCN) is an exclusively sensory nerve that has its origins in branches from L2 and L3. From its origin, it runs lateral and downward, relative to the iliac muscle. When it reaches the inguinal ligament, anatomical variations may be evident. ^{5,​ 13,​ 14} It usually passes below the outermost sector of the ligament, and traverses the superficial fascia 2.5 cm below and medial to the anterior superior iliac spine. ¹⁴ After entering the anterior region of the thigh, it then divides into two branches that innervate the anterolateral thigh, from the gluteal region to the knee. ²

Among the anatomical variations that have particular clinical relevance, we note the following: passage of the nerve between the fibers of the inguinal ligament; entrance into the thigh lateral to the anterior superior iliac spine; absence of the main nerve trunk in the thigh with two or more branches already divided; and a nerve that pierces the sartorius muscle to become superficial. ^{14,​ 15}

In its passage from the inguinal region to the thigh, the LFCN can be trapped by aponeurotic fibers or can suffer direct trauma. Iatrogenic trauma can arise from specific surgical positions (ventral decubitus) and interventions, such as, obtaining bony grafts from the anterior superior iliac spine. ¹⁶ Compressive neuropathy affecting the LFCN typically is associated with pain in the anterolateral part of the thigh.

Clinical Exploration

Since these nerves are largely sensory, their examination requires an assessment of sensory function throughout their territory of cutaneous distribution.

Surgical Approach

Consistent with the anatomical features just reviewed, the LFCN is approached by creating an incision parallel to the inguinal ligament, 2 cm below and 2 cm medial to the anterior superior iliac spine ( ▶ Fig. 2.3, ▶ Fig. 2.4, ▶ Fig. 2.5).

Obturator Nerve

The obturator nerve has its origin in L2–L4. From its origin, it runs medially along the internal border of the psoas muscle into the retroperitoneum, to finally enter the lower pelvis. ^{17,​ 18} In the pelvis, it follows the lateral wall until the subpubic canal, through which the nerve enters the obturator region ( ▶ Fig. 2.6). Just before exiting the pelvis, it divides into an anterior and posterior branch.

Related posts:

Clinical Aspects of Traumatic Peripheral Nerve Lesions in the Lower Limb Surgical Repair of Nerve Lesions: Neurolysis and Neurorrhaphy with Grafts or Tubes Nerve Injuries: Anatomy, Pathophysiology, and Classification Compressive Lesions of the Lower Limb and Trunk Ultrasound in Peripheral Nerve Surgery Nerve Anatomy of the Upper Limbs

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