Nerve Repair and Reconstruction—Sciatic Nerve and Femoral Nerve

Gabrielle B. Davis

Catherine Curtin

SCIATIC NERVE

ANATOMY

The origin of the sciatic nerve is from the ventral divisions of L4, L5, S1, S2, and S3.
The confluence of nerve fibers joins in the pelvis and exits via the greater sciatic foramen and below the piriformis muscle (FIG 1).
In the buttocks, the sciatic nerve is located posterior to the gluteus maximus muscle and anterior to the superior gemellus, obturator internus, inferior gemellus, and quadratus femoris.

FIG 1 • Course of the sciatic nerve as it exits the pelvis below the piriformis muscle.
It courses inferiorly, midway between the greater trochanter and the ischial tuberosity.
In the posterior thigh, it travels deep to the long head of the biceps femoris.
There are two distinct divisions of the sciatic nerve traveling within the same sheath, the peroneal and tibial divisions.
In the upper thigh, the tibial division gives off braches to all of the hamstring muscles (semitendinosus, semimembranosus, biceps femoris) except for the short head of the biceps femoris (FIG 2).
The peroneal nerve distribution innervates the short head of the biceps femoris and the ischial portion of the adductor magnus muscle.
The two divisions will separate completely at the level of the popliteal fossa.
The posterior tibial nerve continues its course posteriorly to innervate the muscles in the posterior leg compartment.
The common peroneal nerve will course laterally around the head of the fibula to innervate the muscles of the anterior and lateral compartments.
The inferior gluteal artery provides majority of the blood supply to the sciatic nerve.

Innervation/Function

In the upper thigh, the sciatic nerve is responsible for motor function to the hamstrings muscles, which allow for flexion at the knee.
The common peroneal and tibial branches are responsible for all of the motor function below the knee.
It provides cutaneous sensory innervation to the posterior thigh.
Its terminal branches provide sensation to the lateral aspect of the knee, lateral lower leg, and the dorsal and plantar surfaces of the foot.

Mechanisms of Injury

The sciatic nerve is at risk for entrapment as it passes through the sciatic notch, especially in the setting of pelvic fractures.
Traumatic injuries, particularly in penetrating trauma, can occur at any level with variable grades of severity.
Iatrogenic injuries are not uncommon and result from pelvic and hip surgery, as well as injection site injuries.
Revisional hip surgeries and patients with developmental dysplasia are at higher risk.¹
Direct compression neuropathy is rare but has been described in comatose patients during prolonged anesthesia or long periods of sitting on a hard surface.² Piriformis syndrome is a compressive neuropathy of the sciatic nerve as it travels underneath the piriformis muscle.

FIG 2 • A,B. The sciatic nerve path in the posterior thigh as it divides in the popliteal fossa into the peroneal and tibial divisions.

PATIENT HISTORY AND PHYSICAL EXAM

Detailed history and physical examination must be obtained prior to the decision for operative management.
Documentation of muscle strength using the Medical Research Council (MRC) scale and sensory deficits by two-point discrimination along the cutaneous distribution of the nerve.
The extent of sensory deficit can also be measured by the “ten-test for sensation.” The patient grades the level of sensation to touch on the injured side using a scale from 1 to

10. This number is compared to the level of sensation to similar touch on the uninjured side.
Physical manifestations will be dependent on the level of injury, extent of each nerve division involved, and if it is a partial or complete injury.
High injury can lead to significant morbidity, as buttocklevel sciatic nerve lesions will result dysfunction of knee flexion, foot extension, flexion, inversion, eversion, and all movement of the toes.
Sensory losses of variable degrees result in the posterior thigh and the entire foot and lower leg except for the medial aspect of the lower leg, which is supplied by the saphenous nerve.
Injuries below the level of the hamstring can allow ambulation with an intact femoral nerve as the patient can have active flexion, extension, and locking of the knee.
Patients with piriformis syndrome complain of buttock pain and numbness. This can also result in pain radiating down the leg and thigh “sciatica.”
Patients with piriformis syndrome may have a history of buttock trauma, and symptoms are exacerbated by prolonged sitting or activities resulting in adduction and internal rotation of the leg.
On physical exam, the proximal sciatic is tested by asking the patient to flex the knee against resistance with the lower leg extended.
The peroneal branch is tested by assessing foot extension, eversion, and abduction of the toes against resistance.
The tibial branch is tested by assessing foot plantar flexion and flexion of the toes against resistance.
To test for piriformis syndrome, the “AIF” maneuver should be performed. The AIF maneuver consists of adduction, internal rotation, and flexion of the hip.
Pain on AIF maneuver is diagnostic for piriformis syndrome.

IMAGING

Detailed physical exam detailing sensory and motor deficits will help delineate the level of injury.
Nerve conduction studies can also assist with localization of the injury.
Of note, electromyography (EMG) has limited utility in the acute setting, as muscles may not demonstrate signs of denervation for several weeks.
For piriformis syndrome, imaging may demonstrate hypertrophy of the piriformis muscle, anomalous vessels, or bands around the muscle.

FIG 3 • MRI neurogram showing bright fascicles of the sciatic nerve.
Magnetic resonance (MR) neurogram is an emerging technique in which edema or inflammatory changes around a nerve could help identify location of injury (FIG 3).

NONOPERATIVE MANAGEMENT

If the injury is closed or due to compression, a trial of conservative management should first be attempted.
Physical therapy should be initiated early, focusing on ambulation with bracing and orthotics.
Pain management for relief of neuropathic pain includes pharmacologic modalities and therapy for desensitization.
If no significant return of function is noted by 3 months, then surgical exploration is warranted.
Unrelenting pain could indicate neuroma formation and may warrant surgical exploration even if signs of spontaneous recovery are evident.

Treatment for piriformis syndrome is typically conservative including stretching of the muscle and image-guided steroid injections.

FIG 4 • Patient prepared for surgery of distal sciatic injury after knee dislocation. Arrow marks the area of nonadvancing Tinel. Note the needles in place for baseline motor responses during the intraoperative nerve monitoring.

If conservative treatment fails, then surgical release of the piriformis can be considered.

SURGICAL MANAGEMENT

Concern for laceration to one or both branches as demonstrated by significant motor or sensory deficits warrant early operative exploration.³
Intraoperative electrophysiologic studies can serve as a guide to determine the area of injury for compression, stretch, or blunt injuries that have not spontaneously recovered (FIG 4).
If nerve action potentials are detected across the injured site, external neurolysis should be performed.
Nerve repair or reconstruction should be performed in the setting of a nerve not in continuity or intraoperative nerve testing that finds no recovery across the zone of injury.
Primary repair should always be attempted if there is not significant tension on the repair.
If primary repair without tension cannot be performed, reconstruction should be performed with nerve grafting.
Neuropathic pain is a common concern. The use of antineuropathic pain medications should be part of the patient’s multimodal treatment.

TECHNIQUES

▪ Upper Sciatic Nerve: Hip and Buttock Pathology

The patient should be placed in the prone position.
A bump can be placed beneath the pelvis to provide flexion at the hips.
The contralateral lower extremity should also be prepped and draped in the event a sural nerve graft is required for repair.
A curvilinear incision is made starting at the posterior superior iliac spine, traveling laterally to the greater trochanter, and inferiorly to the gluteal crease.
The dissection is carried down to the level of the gluteus maximus muscle.
The superior and lateral aspect of the gluteus maximus muscle is released from the iliotibial tract.
A 2- to 3-cm cuff of the muscle is left to the superolateral attachment to facilitate closure.
An avascular plane below the gluteal maximus muscle should be developed by retracting the muscle medially to expose the sciatic nerve.
Careful hemostasis using bipolar electrocautery is critical for good visualization.
Care must be taken not to injure the hamstring branches as they typically run superior and medial to the main sciatic trunk.
The gluteal neurovascular structures travel along with the sciatic nerve, and care must be taken to preserve these vessels.
In some cases, the piriformis muscle will need to be detached from the femur laterally and retracted medially for improved exposure of the nerve at the sciatic notch.