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Topic

design image
Posterior arch of atlas

The posterior arch of the atlas is the bony arch forming the posterior portion of the C1 vertebra, connecting the lateral masses and enclosing the spinal canal at the level of the atlas. It provides a protective roof for the upper cervical spinal cord and serves as an attachment site for ligaments and muscles, including the posterior atlanto-occipital membrane and rectus capitis posterior minor muscles. The posterior arch is horseshoe-shaped, with a thin midline portion known as the posterior tubercle, and lateral extensions that articulate with the lateral masses. This arch contributes to the stability and mobility of the atlantoaxial and atlanto-occipital joints, allowing rotation and flexion-extension movements of the head while maintaining protection for the spinal cord.

Synonyms

  • C1 posterior arch

  • Posterior arch of the first cervical vertebra

  • Atlas posterior arch

Function

  • Forms the posterior boundary of the spinal canal at C1, protecting the spinal cord

  • Provides attachment for cervical muscles and ligaments

  • Supports the posterior atlanto-occipital and atlanto-axial joints

  • Contributes to cervical spine stability and head mobility

MRI Appearance
T1-weighted images:

  • Posterior arch appears as low signal intensity due to compact cortical bone

  • Surrounded by intermediate to high signal soft tissues and paraspinal muscles, which provide contrast

  • Marrow within the arch may show intermediate signal, distinguishable from cortical bone

  • Fractures or lytic lesions appear as disruption of the hypointense cortical outline

T2-weighted images:

  • Cortical bone remains hypointense (black)

  • Bone marrow and soft tissues appear intermediate to hyperintense, allowing detection of edema or inflammation

  • Useful for identifying traumatic injuries or marrow changes

STIR (Short Tau Inversion Recovery):

  • Fat suppression enhances marrow edema, ligamentous injury, or soft tissue pathology

  • Normal posterior arch cortical bone is hypointense, while marrow edema or fractures appear hyperintense

CT Appearance

  • Posterior arch appears as a dense, well-corticated bony structure

  • Axial, coronal, and sagittal planes clearly show cortical continuity and curvature

  • Surrounding air in the pharynx may appear hypodense, providing natural contrast

MRI images

Posterior arch of atlas axial mri

MRI images

Posterior arch of atlas sag mri