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Annulus fibrosus of intervertebral disc

The annulus fibrosus is the outer fibrous ring of the intervertebral disc, surrounding the nucleus pulposus. It is composed of multiple concentric lamellae of collagen fibers arranged in alternating oblique directions, giving it both strength and flexibility. This unique arrangement allows the annulus to resist torsional, compressive, and shear forces in the spine while stabilizing adjacent vertebrae.

It plays a crucial role in maintaining spinal integrity and biomechanics. With age or injury, the annulus fibrosus undergoes degenerative changes, predisposing it to fissures, tears, or herniation of the nucleus pulposus, which can compress neural structures.

Synonyms

  • Fibrous ring of the disc

  • Annulus fibrosus intervertebralis

  • Peripheral fibrous zone of intervertebral disc

Structure and Location

  • Composition: Made of 15–25 concentric lamellae of type I collagen fibers

  • Fiber arrangement: Fibers run obliquely and alternately in opposite directions between layers, strengthening resistance to rotation

  • Outer layers: Firmly attach to vertebral endplates and periosteum of vertebral bodies

  • Inner layers: Blend with the nucleus pulposus, forming a transitional zone

  • Location: Forms the peripheral margin of every intervertebral disc, surrounding the nucleus pulposus centrally

Relations

  • Anteriorly: Anterior longitudinal ligament (ALL) reinforces the annulus

  • Posteriorly: Posterior longitudinal ligament (PLL) lies along its posterior aspect

  • Superior/Inferior: Vertebral endplates anchor the annulus to vertebral bodies

  • Medially: Encircles the nucleus pulposus

Function

  • Provides structural support and stabilizes the intervertebral disc

  • Resists compression, torsion, shear, and bending forces in the spine

  • Protects the nucleus pulposus and prevents its extrusion

  • Maintains proper spacing between vertebral bodies, preserving spinal flexibility

Clinical Significance

  • Annular tears/fissures: Can cause low back pain and allow nucleus pulposus herniation

  • Disc herniation: Posterolateral annulus is weaker, common site for lumbar disc herniation

  • Degeneration: Age-related dehydration and fissuring reduce load-bearing capacity

  • Trauma: Acute disruption may lead to herniated disc with neural compression

  • Surgical relevance: Seen in discectomy, artificial disc replacement, and spinal fusion

MRI Appearance

T1-weighted images:

  • Annulus fibrosus shows low signal intensity

  • Degeneration or fatty infiltration may produce focal bright areas

  • Annular tears may show linear hyperintensity after contrast

T2-weighted images:

  • Annulus appears as a low signal intensity rim around the disc

  • Nucleus pulposus appears brighter, allowing differentiation

  • Annular tears may appear as focal high-signal zones within the dark annulus

STIR (Short Tau Inversion Recovery):

  • Normal annulus: low signal intensity

  • Pathology (inflammation, edema, tear): bright hyperintensity in affected annular fibers

T1 Fat-Sat Post-Contrast:

  • Normal annulus: minimal to no enhancement

  • Annular fissures/tears: linear or focal enhancement

  • Discitis or infection: diffuse enhancement of annulus and adjacent endplates

CT Appearance

Non-Contrast CT:

  • Annulus fibrosus appears as a dense fibrous rim around the disc

  • May show calcification in chronic degeneration

  • Tears or herniation not well visualized without secondary effects

Post-Contrast CT:

  • Annulus shows little to no enhancement normally

  • Pathologic tears or infection may demonstrate enhancement of annular margins

  • CT discography (with contrast injection into disc): annular tears become outlined by contrast extension

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