Sciatic nerve : Overview

Anatomy and Physiology Of Sciatic Nerve :

Origin and Branches of the Sciatic Nerve :

Sciatic nerve starts in the lower spine and follows a long path through the buttock, down the back of the thigh and leg, and finally ends in the foot.

Origin of Sciatic Nerve :

The sciatic nerve is formed by the combination of 5 nerves in the lumbar (lower) and sacral spine—L4, L5, S1, S2, and S3. These nerve fibers are typically responsible for motor and sensory functions of the lower body. The 5 nerves group together near the front surface of the piriformis muscle deep in the buttock and form the large, thick sciatic nerve. At its thickest portion, the nerve measures around 2 cm in diameter.2,3

The nerve leaves the pelvis along with its surrounding nerves and blood vessels through the greater sciatic foramen below the piriformis muscle. It progresses downward between the muscles of the thigh and is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides. In around 10% of the population, the nerve may divide above the knee.

Branches of Sciatic Nerve :

At the popliteal fossa near the back of the knee, the sciatic nerve divides into 2 main branches: the tibial nerve and the common peroneal nerve. The popliteal fossa is a rhomboid-shaped space that serves as a conduit for blood vessels and nerves in the leg.

The tibial nerve continues down the back of the leg to the heel and sole of the foot.
The common peroneal travels sideways along the outer part of the knee to the outer border of the leg and foot.
Both these nerves finally terminate into sural nerves.

Illustration of the Sciatica Nerve variants :

This nerve is usually undivided and leaves the pelvis through the greater sciatic foramen below the piriformis muscle (left). Sometimes, the nerve may be divided, with one part passing through the piriformis and the other below the muscle (right).

Anatomic Variants of the Sciatic Nerve
It is estimated that around 16% of the population may have variations in the anatomical structure of the sciatic nerve.1,4

These variations include :

The sciatic nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvis below the muscle. This variant is the most common among others.
This nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvic area above the muscle.
The sciatic nerve divides above piriformis, one portion travels in front of it, the other travels behind it.
An undivided sciatic nerve exits through the piriformis muscle.
An undivided sciatic nerve exits from behind the top part of the piriformis.
In cases where the sciatic nerve divides, both portions of the nerve immediately merge again and course downward as a single nerve.

While the variants are considered normal, these individuals may be at a higher risk of developing sciatica pain due to impingement, entrapment, or irritation of the nerve.

Motor functions of the sciatic nerve :

The motor functions include:

• Knee flexion: Bending the knee
• Hip adduction: Bringing the thighs together/movement of the leg toward the midline of the body
• Plantar flexion: Pointing the foot downward
• Flexion of toes: Pointing the toes downward
• Dorsiflexion of the foot: Pointing the foot upward
• Extension of toes: Pointing the toes upward
• When the sciatic nerve is compressed, it is common to experience reduced motor functions and weakness in the affected leg.

Sensory functions :

This nerve provides sensations to the skin (dermatome) over the following areas:

• Front, back, and outer part of the thigh
• Front, back, and outer part of the lower leg
• The top and outer side of the foot
• Sole of foot
• The web between the first and second toes
• Sensory symptoms such as burning, tingling, and/or numbness may be experienced when the sciatic nerve is inflamed or irritated.

Innervation of Sciatic Nerve :

Tibial division

motor

• semitendinous
• semimembranous
• long head biceps femoris
• gastrocnemius
• popliteus
• plantaris
• soleus
• tibialis posterior
• flexor digitorum longus
• flexor hallucis longus
• sensory innervation
• none
• Peroneal division
• motor
• abductor hallucis
• flexor digitorum brevis
• flexor hallucis brevis
• foot lumbricals
• quadratus plantae
• flexor digiti minimi
• adductor hallucis
• foot interossei
• abductor digiti minimi
• sensory
• articular branch to knee joint
• sural nerve branch
• runs distal with the small saphenous vein
• anastomoses with a branch from the peroneal nerve
• continues distal on lateral aspect of the Achilles tendon
• terminates as the lateral dorsal cutaneous nerve of the foot
• common peroneal nerve

motor

• short head of biceps femoris
• deep peroneal nerve

leg

• tibialis anterior
• extensor digitorum longus
• peroneus tertius
• extensor hallucis longus

foot

• extensor digitorum brevis (lateral terminal branch)
• extensor hallucis brevis (lateral terminal branch)

sensory

• articular branch to the ankle joint
• medial terminal branch: 1st dorsal webspace
• superficial peroneal nerve

motor

• lateral compartment of leg
• peroneus longus
• peroneus brevis

sensory

• majority of skin on the dorsum of foot, excluding webspace between hallux and second digit (deep peroneal nerve)
• anterolateral distal 1/3 of leg

Origin

• The nerve originates from lumbosacral plexus L4-S3
• tibial division
• orginates from anterior preaxial branches of L4,L5,S1,S2,S3
• peroneal division
• originates from from postaxial branches of L4,L5,S1,S2

Course

• Exits sciatic notch
• runs anterior or deep to piriformis
• runs posterior or superficial to short external rotators (superior gemellus, inferior gemellus, obturator internus)

Posterior leg

• It then runs down the posterior leg where it breaks into its three main divisions at the level of the mid thigh

Terminal branches

• common peroneal nerve
• tibial nerve

Clinical Significance of Sciatic Nerve :

• 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 lumbar 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.
• 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.

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