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Annular epiphysis

The annular epiphysis (vertebral ring apophysis) is a secondary ossification center forming a thin bony ring at the superior and inferior margins of the vertebral body endplates. It plays a key role in vertebral growth, endplate stability, and attachment of the outer fibers of the intervertebral disc.

It is an important structure in pediatric and adolescent spine anatomy and a common site involved in traction-related changes and ring apophyseal variants seen on imaging.

Synonyms

  • Vertebral ring apophysis

  • Ring epiphysis of the vertebral body

Location

  • Situated at the peripheral margins of the vertebral body endplates

  • Forms a circumferential ring along the superior and inferior endplates

  • Between the vertebral body and the outer annulus fibrosus

  • Present at each vertebral level from cervical to lumbar spine

  • Deep to the anterior and posterior longitudinal ligaments

Anatomical components

  • Thin circumferential bony rim

  • Superior annular epiphysis

  • Inferior annular epiphysis

  • Attachment site for:

    • Outer fibers of the annulus fibrosus

    • Sharpey-type collagen fibers

Relations

Superiorly:

  • Intervertebral disc endplate

Inferiorly:

  • Vertebral body cancellous bone

Anteriorly:

  • Anterior longitudinal ligament

  • Anterior annulus fibrosus

Posteriorly:

  • Posterior annulus fibrosus

  • Posterior longitudinal ligament (near midline)

Laterally:

  • Peripheral annulus fibrosus

Developmental anatomy

  • Not present at birth

  • Appears between 5–10 years of age as a secondary ossification center

  • Progressive ossification during adolescence

  • Fusion with vertebral body: Usually between 18–25 years

  • May remain partially visible in young adults as a normal variant

X-ray appearance

Spine radiographs (AP / lateral views):

  • Annular epiphysis: Thin, linear bony rim along vertebral endplate margins

  • Appearance: Smooth, well-corticated ring

  • Age dependence: More conspicuous in children and adolescents

  • Fusion: Disappears after complete fusion with vertebral body

CT appearance

Pre-contrast CT:

  • Annular epiphysis: Thin hyperdense bony ring at the vertebral endplate periphery

  • Margins: Well-defined cortical outline

  • Relationship: Clearly separated from central endplate in skeletally immature patients

  • After fusion: Appears continuous with the vertebral body cortex

MRI appearance

T1-weighted images:

  • Annular epiphysis: Low signal cortical rim at the vertebral margin

  • Adjacent marrow: Intermediate-to-high signal in the vertebral body

  • Disc interface: Clear delineation between bone and annulus

T2-weighted images:

  • Annular epiphysis: Low signal intensity line

  • Endplate cartilage (in younger patients): Intermediate signal

  • Intervertebral disc: High signal nucleus pulposus

STIR:

  • Annular epiphysis: Low signal cortical rim

  • Adjacent marrow: Suppressed fat signal with preserved bony outline

  • Utility: Highlights marrow and endplate interface in developing spine

MRI image

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CT image

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