Sacral Plexus and Femoral Nerve: Anatomy & Surgical Management

INTRODUCTION TO THE LUMBOSACRAL NEURAL AXIS

The management of peripheral nerve injuries in the pelvis and lower extremity requires a profound understanding of the complex neuroanatomy, biomechanics, and surgical topography of the lumbosacral neural axis. The sacral plexus and the femoral nerve represent the primary motor and sensory conduits to the lower limb. Injuries to these structures—whether traumatic, iatrogenic, or compressive—can result in devastating functional deficits. For the practicing orthopedic surgeon, mastering the surgical anatomy, clinical evaluation, and operative approaches to the sacral plexus, sciatic nerve, and femoral nerve is paramount for optimizing patient outcomes.

SURGICAL ANATOMY OF THE SACRAL PLEXUS

The sacral plexus is a complex neural network formed by the anterior primary rami of L5, S1, S2, and S3. It receives a critical contribution from the lumbar plexus via the anterior primary ramus of L4, which joins with L5 to form the robust lumbosacral trunk.

Additionally, a segment of S4 joins a segment of S3 to form the pudendal nerve. Anatomically, the pudendal nerve is considered by some authorities to be an integral part of the sacral plexus, by others to be a separate pudendal plexus, and by still others to be the superior continuation of the tiny coccygeal plexus.

As the anterior primary rami converge within the true pelvis, they systematically split into anterior and posterior divisions, dictating their terminal distributions.

The Posterior Divisions

The trunk formed by the posterior divisions gives rise to several critical structures:
* Superior Gluteal Nerve: Leaves the sciatic notch proximal to the piriformis muscle. It supplies the gluteus medius, gluteus minimus, and tensor fasciae latae, which function as the primary abductors and internal rotators of the hip.
* Inferior Gluteal Nerve: Exits the sciatic notch alongside the sciatic nerve, emerging inferior to the piriformis. It exclusively supplies the gluteus maximus, the primary extensor of the hip.
* Common Peroneal Nerve: The posterior divisions ultimately coalesce to form the common peroneal part of the sciatic nerve, proceeding toward the greater sciatic notch.

The Anterior Divisions

The trunk formed by the anterior divisions becomes the tibial part of the sciatic nerve, which also proceeds toward the greater sciatic notch.

Within the pelvis, smaller branches—which are rarely of primary concern during routine orthopedic exposures but are critical for pelvic stability—are given off to the short external rotators: the quadratus femoris, obturator internus, superior gemellus, and piriformis.

The Posterior Femoral Cutaneous Nerve

Smaller branches of S1, S2, and S3 unite to form the posterior femoral cutaneous nerve (historically referred to as the posterior cutaneous nerve of the thigh or the "small sciatic nerve"). This is a relatively large sensory nerve that leaves the sciatic notch medial to the sciatic trunk. It lies just deep to the deep fascia as it courses distally in the posterior midline of the thigh, roughly overlying the sciatic trunk. It innervates the skin of the entire posterior aspect of the thigh and the popliteal fossa.

Clinical Pearl: During posterior approaches to the hip or proximal hamstring repairs, the posterior femoral cutaneous nerve is highly vulnerable. It must be identified deep to the fascia lata and protected to prevent painful postoperative neuromas and sensory deficits.

THE SCIATIC NERVE: TOPOGRAPHY AND BIOMECHANICS

The sciatic nerve is the largest peripheral nerve in the human body, boasting a transverse diameter of 2.0 to 2.5 cm at its origin. It is composed of fibers from L4, L5, S1, S2, and S3.

The nerve exits the pelvis through the greater sciatic notch, typically emerging inferior to the piriformis muscle. At this proximal level, the large trunk is easily separated into its two distinct components:
1. Common Peroneal Part: Located laterally.
2. Tibial Part: Located medially.

Frequently, along the medial aspect of the main trunk, a smaller, distinct segment—the nerve to the hamstrings—is visible and can be easily dissected from the main body of the sciatic nerve.

Surgical Warning: Anatomical variations at the greater sciatic notch are common. In approximately 15-20% of patients, the sciatic nerve or its common peroneal division may pierce the piriformis muscle or exit superior to it. Failure to recognize these variants during deep gluteal dissection can lead to catastrophic iatrogenic transection.

THE FEMORAL NERVE: ANATOMY AND VULNERABILITY

While the sacral plexus dominates the posterior neural axis, the anterior thigh is governed by the femoral nerve (L2, L3, L4), the largest branch of the lumbar plexus. After passing deep to the inguinal ligament, it arborizes rapidly to supply the anterior thigh musculature, specifically the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.

Mechanisms of Injury

The femoral nerve is highly susceptible to various mechanisms of injury:
* Penetrating Trauma: Often injured by penetrating wounds to the lower abdomen or groin. In such cases, concomitant injury to the small intestine or the adjacent iliac/femoral artery is common.
* Iatrogenic Injury: Because of their intimate proximity, the iliac artery and femoral nerve may be injured together during anterior pelvic or retroperitoneal surgery. Furthermore, during operations in which the patient is positioned prone (e.g., complex spine surgery), excessive compression of the anterior thigh against unpadded bolsters can cause ischemic neuropraxia of the femoral nerve.
* Hematomas: Femoral neuropathies frequently result from retroperitoneal or iliacus hematomas caused by hemophilia, aggressive anticoagulant therapy, or blunt trauma. The nerve becomes compressed within the rigid fascial compartment of the iliopsoas.
* Pelvic Fractures: Branches of the femoral nerve may be contused, stretched, or lacerated by displaced superior pubic ramus fractures.

Pitfall: Complete division of the femoral nerve is often overlooked in the acute trauma setting. Because the patient may still exhibit weak active extension of the knee (via compensatory mechanisms) and because the surgical team is often focused on life-threatening hemorrhage from adjacent vessels, the nerve injury is missed. This mirrors the frequently overlooked injuries to the musculocutaneous nerve in the upper extremity.

CLINICAL EVALUATION AND DIAGNOSTIC PATHWAYS

A meticulous clinical examination is the cornerstone of diagnosing lumbosacral plexus and peripheral nerve injuries.

Evaluating the Sacral Plexus and Sciatic Nerve

• Inferior Gluteal Nerve (Gluteus Maximus): Paralysis results in profound difficulty rising from a squatting or sitting position, and in ascending steps or a steep slope. The patient will often demonstrate a "gluteus maximus lurch," thrusting the trunk posteriorly at heel strike to maintain hip extension.
• Superior Gluteal Nerve (Gluteus Medius/Minimus): Injury results in a positive Trendelenburg sign and a compensatory abductor lurch during the stance phase of gait.
• Sciatic Nerve: Complete lesions result in a flail foot (loss of all dorsiflexion, plantarflexion, inversion, and eversion) and loss of knee flexion (hamstring paralysis).

Evaluating the Femoral Nerve

• Motor Examination: Atrophy of the anterior thigh muscles (quadriceps) becomes obvious within weeks of the injury. Surprisingly, the patient is usually able to extend the knee slightly against gravity and can often stand and walk, especially on level surfaces. This is because the gastrocnemius, tensor fasciae latae, gracilis, and gluteus maximus act synergistically to stabilize the limb and lock the knee in extension. However, the patient will find it nearly impossible to ascend a hill or climb stairs without the knee buckling.
• Sensory Examination: The autonomous zone of sensory supply for the femoral nerve usually consists of a small, distinct area just superior and medial to the patella. The broader anterior aspect of the thigh and the medial leg (supplied by the saphenous nerve, the terminal sensory branch of the femoral nerve) may show only varying degrees of hypesthesia due to overlapping dermatomal innervation.
• Electrodiagnostics: Electrical stimulation with needle electrodes (Electromyography - EMG) and Nerve Conduction Studies (NCS) inserted near the femoral nerve or its target muscles are invaluable. They help differentiate between a preganglionic root avulsion, a plexus injury, and a peripheral nerve lesion, while also establishing a baseline for reinnervation.

SURGICAL TREATMENT AND OPERATIVE APPROACHES

The management of sacral plexus, sciatic, and femoral nerve injuries depends heavily on the mechanism of injury, the continuity of the nerve, and the time elapsed since the insult.

Indications for Surgery

1. Open Wounds: Sharp, penetrating injuries with suspected nerve transection demand immediate or early (within 2-3 weeks) surgical exploration and primary repair.
2. Iatrogenic Injuries: If a nerve is known to be transected or ligated during a procedure, immediate repair is indicated. If a deficit is noted postoperatively without known transection, a period of observation (3-6 months) with serial EMG is warranted.
3. Compressive Lesions: Expanding hematomas (e.g., iliacus hematoma compressing the femoral nerve) causing progressive neurologic deficit require urgent surgical decompression and hematoma evacuation.
4. Failure to Improve: Closed traction injuries that show no clinical or electromyographic signs of recovery by 3 to 6 months should be explored.

Surgical Approach to the Proximal Sciatic Nerve and Sacral Plexus

Positioning: The patient is placed in the prone position. The entire lower extremity must be prepped and draped free to allow for intraoperative manipulation and observation of muscle twitch during nerve stimulation.

Incision and Dissection:
1. A curvilinear "question mark" incision is utilized, starting along the posterior third of the iliac crest, curving distally to the greater trochanter, and extending distally along the posterior thigh.
2. The deep fascia is incised. The gluteus maximus is split in line with its fibers or elevated as a flap by releasing its insertion on the femur and the iliotibial tract.
3. The piriformis muscle is identified. The sciatic nerve is located as it exits the greater sciatic notch, typically inferior to the piriformis.
4. If access to the sacral plexus is required, the piriformis may be tenotomized and reflected medially.
5. Caution: The inferior gluteal artery and nerve, which exit adjacent to the sciatic nerve, must be meticulously preserved to prevent devascularization and denervation of the gluteus maximus.

Surgical Approach to the Femoral Nerve

Positioning: The patient is placed supine with a bump under the ipsilateral hip.

Incision and Dissection:
1. An ilioinguinal incision is made, extending from the anterior superior iliac spine (ASIS) medially toward the pubic tubercle, and can be extended distally over the anterior thigh.
2. The external oblique aponeurosis is divided, and the inguinal ligament is identified.
3. The femoral nerve lies deep to the inguinal ligament, lateral to the femoral artery, and outside the femoral sheath, resting on the iliopsoas muscle.
4. For proximal injuries, the inguinal ligament may need to be divided (and later repaired) to trace the nerve into the retroperitoneal space.

Techniques of Nerve Reconstruction

• Neurolysis: For nerves encased in dense scar tissue (e.g., post-pelvic fracture), external neurolysis is performed under microscopic magnification to free the nerve from the surrounding fibrotic bed.
• Direct Neurorrhaphy: If the nerve is transected and the ends can be approximated without tension, an epineurial repair is performed using 8-0 or 9-0 non-absorbable monofilament suture. The joints may be positioned to reduce tension (e.g., hip extension and knee flexion for the sciatic nerve), but extreme positioning should be avoided to prevent postoperative traction injury upon mobilization.
• Nerve Grafting: If a tension-free primary repair is impossible, interfascicular nerve grafting is mandatory. The sural nerve is the standard autograft donor. The grafts must be reversed to prevent axonal loss down sensory branches. Given the large diameter of the sciatic and femoral nerves, multiple cables (often 4 to 6) are required to bridge the defect.

POSTOPERATIVE PROTOCOLS AND REHABILITATION

Postoperative management is critical to the success of peripheral nerve surgery.

Immobilization

Following direct repair or grafting, the limb is immobilized in a custom orthosis or cast for 3 to 4 weeks to prevent tension on the neurorrhaphy site. For sciatic nerve repairs, this typically involves a hip spica or a hinged knee brace locked in flexion. For femoral nerve repairs, the hip is slightly flexed.

Rehabilitation and Orthotics

Once immobilization is discontinued, a rigorous, phased physical therapy program is initiated.
* Passive Range of Motion (PROM): Commences to prevent joint contractures, progressing gradually to avoid sudden stretch on the regenerating nerve.
* Orthotic Support:
* Sciatic Nerve Lesions: An Ankle-Foot Orthosis (AFO) is essential to prevent equinus contracture and assist with toe clearance during the swing phase of gait.
* Femoral Nerve Lesions: A Knee-Ankle-Foot Orthosis (KAFO) with a drop-lock knee joint may be required initially to prevent knee buckling during weight-bearing, though many patients compensate well enough to use only a cane or crutches.
* Monitoring Recovery: Axonal regeneration occurs at a rate of approximately 1 mm per day (1 inch per month). Serial clinical examinations assessing advancing Tinel's sign and follow-up EMGs at 3, 6, and 12 months are utilized to track reinnervation. If proximal muscle groups fail to reinnervate within 12 to 18 months, secondary reconstructive procedures, such as tendon transfers or joint arthrodesis, must be considered.