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Topic

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Sciatic nerve

The sciatic nerve is the largest and longest peripheral nerve in the human body. It arises from the lumbosacral plexus (L4–S3 nerve roots). The nerve exits the pelvis through the greater sciatic foramen, usually below the piriformis muscle, and courses along the posterior thigh before branching into the tibial nerve and common peroneal (fibular) nerve near the popliteal fossa.

The sciatic nerve innervates the posterior thigh muscles (hamstrings), part of the adductor magnus, and all muscles below the knee (through its terminal branches). It also carries sensory fibers from the posterior thigh, leg, and foot.

Anatomically, the sciatic nerve is about 2 cm wide at its origin, making it easily identified on imaging. Variations exist, such as a high division into tibial and common peroneal components, or passage through the piriformis (piriformis syndrome).

Clinically, it is significant in sciatica, nerve entrapment syndromes, traumatic injuries, iatrogenic damage during hip surgery, tumors (schwannomas, neurofibromas), and inflammatory neuropathies.

Synonyms

  • Nervus ischiadicus

  • Ischiatic nerve

  • Great sciatic nerve

Function

  • Provides motor innervation to posterior thigh and entire leg/foot (via tibial & peroneal branches)

  • Provides sensory innervation to posterior thigh, leg, and foot

  • Major conduit for lower limb motor and sensory function

  • Plays key role in clinical syndromes such as sciatica and neuropathy

MRI Appearance

T1-weighted images:

  • Nerve appears as a bundle of hypointense fascicles with thin hyperintense surrounding fat planes

  • Clear visibility at greater sciatic notch and posterior thigh

T2-weighted images:

  • Normal nerve: fascicular architecture with intermediate-to-low signal

  • Pathology (edema, neuritis, entrapment): hyperintense signal within fascicles

STIR:

  • Suppresses fat, highlighting nerve edema, inflammation, or tumors as bright hyperintense regions

  • Sensitive for sciatic neuropathy and radiculitis

T1 Fat-Saturated Post-Gadolinium:

  • Normal sciatic nerve: minimal or no enhancement

  • Abnormal enhancement indicates neuritis, tumors (schwannoma/neurofibroma), or infectious/inflammatory processes

  • Helps differentiate nerve from surrounding vessels and soft tissue

3D T2-weighted Imaging:

  • Provides isotropic, high-resolution visualization of the nerve’s entire course from pelvis to thigh

  • Fascicular architecture preserved, seen as alternating hypointense fascicles and hyperintense perineurium

  • Useful for surgical mapping and evaluating entrapment, traumatic injury, or variant anatomy

3D T1 Post-Gadolinium Imaging:

  • Enhances the perineural sheath and abnormal lesions

  • Demonstrates tumor margins, inflammatory changes, and perineural spread of malignancy

  • Provides precise multiplanar reconstructions for neurosurgical and orthopedic planning

CT Appearance

Non-contrast CT:

  • Sciatic nerve appears as a soft-tissue density structure posterior to hip joint and along posterior thigh

  • Limited contrast with surrounding tissues unless enlarged or surrounded by fat planes

  • Useful in trauma for detecting adjacent bony injury compressing the nerve

CT Post-Contrast:

  • Sciatic nerve itself does not enhance

  • Surrounding inflammatory masses, tumors, or collections enhance and outline the nerve

  • May help identify nerve sheath tumors or extrinsic compression

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MRI images

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