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Anulus fibrosus of intervertebral disc

The anulus fibrosus (AF) is the dense, outer fibrous ring of an intervertebral disc that surrounds the gelatinous nucleus pulposus. It is composed of concentric layers of fibrocartilage and type I collagen fibers, arranged in alternating oblique orientations. This structure provides tensile strength, flexibility, and resistance to rotational, compressive, and shear forces. The AF attaches firmly to the vertebral endplates and adjacent vertebral bodies, maintaining disc integrity and spinal stability.

The AF is thicker anteriorly and thinner posteriorly, particularly in the lumbar spine, which explains the predilection for posterolateral disc herniation. The lamellae of the AF gradually become less organized and more vascularized at the outermost layers, allowing some healing potential in case of injury.

Location

  • Encircles the nucleus pulposus centrally

  • Extends from the superior to inferior vertebral endplates

  • Thicker anterolaterally, thinner posterolaterally

  • Present throughout cervical, thoracic, and lumbar spine

  • Outer layers attach to adjacent vertebral bodies, inner layers blend with nucleus pulposus

Function

  • Encases and protects the nucleus pulposus

  • Maintains disc height and spinal stability

  • Distributes mechanical load across vertebrae

  • Resists torsion, compression, and tensile forces

  • Limits excessive vertebral motion

Synonyms

  • Annulus fibrosus

  • Fibrous ring of intervertebral disc

  • AF

MRI Appearance

T1-weighted images:

  • The AF appears as a thin, low signal intensity (hypointense) ring surrounding the slightly higher signal nucleus pulposus.

  • Outer AF may show mild enhancement in vascularized layers if contrast is used.

  • Degenerative changes or annular fissures may appear as focal hyperintense areas along the ring.

T2-weighted images:

  • AF is uniformly low signal (hypointense) due to dense fibrocartilage and low water content.

  • Nucleus pulposus is high signal intensity (hyperintense) in healthy discs.

  • Posterolateral thinning or radial tears appear as linear or crescent-shaped hyperintensities extending from the nucleus pulposus.

STIR (Short Tau Inversion Recovery):

  • AF remains low signal, while edema, inflammation, or annular tears appear hyperintense.

  • Excellent for detecting acute injury, disc inflammation, or post-traumatic changes.

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal AF may show mild, homogeneous enhancement of outer layers due to sparse vascularity.

  • Pathological AF (degenerative, inflamed, or torn) demonstrates enhancement along fissures or tears, highlighting vascularized fibrous tissue or inflammation.

CT Appearance:

  • AF appears as a soft tissue density ring surrounding the slightly lower-density nucleus pulposus.

  • Calcified or degenerated AF is hyperdense, easily seen on CT.

  • Disc contour, height, and annular calcifications are clearly evaluated.

  • CT is valuable for assessing osteophytes, endplate irregularities, and disc degeneration.

MRI images

Anulus fibrosus, Intervertebral disc mri axial image

MRI images

Anulus fibrosus, Intervertebral disc mri sagittal image

CT images

Annulus fibrosus of Intervertebral disc ct axial