Anatomy, Sciatic Nerve

Introduction

The sciatic nerve is the largest nerve in humans, originating in the lower back and traveling posteriorly through the lower limb as far down as the heel of the foot. The sciatic nerve innervates a significant portion of the skin and muscles of the thigh, leg, and foot.[1][2]

The nerve originates from the ventral rami of spinal nerves L4 through S3 and contains fibers from both the posterior and anterior divisions of the lumbosacral plexus. After leaving the lower vertebrae, the nerve fibers converge to form a single nerve. It exits the pelvis through the greater sciatic foramen inferior to the piriformis muscle along with the pudendal nerve and vessels, inferior gluteal nerve and vessels, nerve to obturator internus, and posterior cutaneous nerve. The sciatic nerve then progresses down the posterior compartment of the thigh deep to the long head of the biceps femoris muscle, superficial to adductor magnus and short head of biceps femoris muscle, and laterally to semitendinosus and semimembranosus muscles. Just before reaching the popliteal fossa, it divides into 2 important branches. One branch is the tibial nerve, which continues to descend in the posterior compartment of leg and foot. The other branch is the common peroneal nerve, which travels down the lateral and anterior compartment of the leg and foot.

Structure and Function

The sciatic nerve provides motor innervation to the posterior compartment of the thigh. This includes the biceps femoris, semimembranosus, semitendinosus, and the ischial portion of the adductor magnus which allow for knee flexion and hip adduction. The tibial nerve innervates the posterior compartment of the leg and foot which includes the gastrocnemius, soleus, plantaris, popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior. These muscles are primarily responsible for plantarflexion and flexion of the toes. The common peroneal nerve innervates the anterior and lateral compartments of the leg and foot. The anterior compartment includes tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. These muscles are primarily responsible for dorsiflexion of the foot and extension of toes. The lateral compartment includes peroneus longus and brevis, which are responsible for foot eversion.[3]

The sciatic nerve also provides sensory innervation to the skin of the foot and the lower leg (except for the medial leg which is innervated by the saphenous nerve). The tibial nerve further divides into the medial and lateral plantar nerves, which are responsible for the sensation of the sole. The common peroneal nerve further divides into the superficial peroneal nerve and deep peroneal nerve. The superficial peroneal nerve provides sensory innervation to the lateral leg and dorsum of the foot. The deep peroneal nerve is responsible for sensation in between the first and second toes. The medial and lateral sural nerves are made up of collateral branches from both the tibial and common peroneal nerves and provide sensation to the calf and a small lateral portion of the foot.

Embryology

At the beginning of the third week after fertilization, a process called neurulation begins. It starts when a region of cells overlying the ectoderm begins to thicken and form the neural plate. The neural plate leads to the neural tube and neural crest cells. The neural crest cells will lead to the neurons in the peripheral nervous system, including the sciatic nerve. Additionally, Schwann cells which are responsible for myelination of the peripheral nervous system are also derived from neural crest cells.

Blood Supply and Lymphatics

Peripheral nerves, including the sciatic nerve and its divisions, receive their blood supply from 2 sources, the extrinsic and intrinsic systems. The extrinsic system consists of contributions from nearby arteries and veins which make up the vasa nervorum. The intrinsic vasculature of the sciatic nerve includes arteries and veins run longitudinally just deep to the epineurium. This vasculature connects with extrinsic vasculature at various junction points. The flow within the nerve's vasculature is highly variable and consists of many collateral networks. When comparing the branches of the sciatic nerve, tibial portion of the sciatic nerve has a richer blood supply than the peroneal branches.

Physiologic Variants

Most often the undivided sciatic nerve exits the greater sciatic foramen inferior to the piriformis, but this is not always the case.[4] In the most common anatomic variant, the sciatic nerve splits above the piriformis and one division exits through the piriformis while the other exits below. There are 6 anatomic variants described in the current literature:

Type I: sciatic nerve exits inferior to piriformis, undivided

Type II: sciatic nerve divides above piriformis, one portion exits through piriformis, the other inferior to it

Type III: sciatic nerve divides above piriformis, one portion courses anterior to piriformis, the other posterior to it

Type IV: sciatic nerve exits through piriformis, undivided

Type V: sciatic nerve divides above piriformis, one portion exits through piriformis, the other superior to it

Clinical Significance

Sciatica is defined as irritation or compression of the sciatic nerve that causes pain in the buttock area with radiation to the lower leg. The vast majority of cases have a spinal cause, such as disc herniation or rupture causing impingement of L5 or S1 nerve roots. Other common causes include spinal stenosis, degenerative disc disease, and spondylolisthesis. There are also many non-spinal causes or sciatica, which are less common. These include piriformis syndrome, trauma, post-operative complications, gynecologic conditions, and herpes zoster.[5][6][7]

Symptoms include mid-buttock pain that radiates down the leg, most often unilaterally. The patient will often experience paresthesias in a dermatomal distribution, either L5 or S1 depending on where the sciatic nerve is being irritated. Weakness and changes in reflexes are less commonly seen.

One test that should always be done in patients with low back pain is straight leg raising with the patient supine. The test is positive if there is ipsilateral pain between 10 to 60 degrees. This test is quite sensitive (90%), but not specific. If this test elicits pain in the contralateral leg, one should suspect disc herniation and order an imaging test. MRI is very useful when the exact level of compression in the spine is unclear. MRI can also assess the soft tissues or presence of an infection or a neoplastic cause. If there is urine retention or dysesthesia around the anus in a patient with sciatica, one should suspect cauda equina syndrome. Prompt surgical decompression is indicated for this disorder.

The majority of cases of sciatica are treated conservatively with rest for a few days, pain medication and some physical therapy. Surgery may be required in cases where there is significant stenosis, or there is evidence of irritation of the nerve.[8]