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Posterior atlantooccipital membrane

The posterior atlantooccipital membrane (PAOM) is a broad, thin ligamentous sheet that connects the posterior arch of the atlas (C1) to the posterior margin of the foramen magnum. It forms the posterior boundary of the atlantooccipital joint, bridging the space between the occipital bone and C1, and contributing to craniovertebral stability. The PAOM is continuous with the ligamentum flavum inferiorly, extending down the cervical spine, and serves as a protective layer for the upper cervical spinal cord and vertebral arteries.

The membrane is elastic and flexible, allowing flexion and extension at the atlantooccipital joint while limiting excessive motion. It plays a crucial role in posterior craniovertebral stability and is an important landmark during posterior craniovertebral surgical approaches or trauma assessment.

Function

  • Stabilizes the atlantooccipital joint

  • Limits excessive flexion and extension of the occiput on C1

  • Protects the upper cervical spinal cord and vertebral arteries

  • Serves as a surgical landmark for posterior craniovertebral procedures

  • Provides attachment continuity to the ligamentum flavum and posterior cervical ligaments

Synonyms

  • Posterior atlantooccipital ligament

  • PAOM

  • Posterior atlanto-occipital membrane

MRI Appearance

T1-weighted images:

  • The PAOM appears as a thin, low signal intensity (hypointense) band posterior to the spinal cord at the craniovertebral junction

  • Surrounding cerebrospinal fluid (CSF) and fat provide contrast, making the membrane distinguishable

  • Thickening, tears, or edema may appear as focal hyperintensity or irregularity

T2-weighted images:

  • The PAOM is low signal intensity (hypointense) due to dense fibrous tissue

  • CSF anterior to the membrane is high signal, allowing clear visualization of the membrane as a hypointense posterior boundary

  • Pathological changes such as ligamentous injury, edema, or degeneration may appear as hyperintense areas within or along the membrane

STIR (Short Tau Inversion Recovery):

  • Normal PAOM remains low signal

  • Edema, inflammation, or traumatic injury appears hyperintense, highlighting ligamentous disruption or hemorrhage

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal PAOM shows no enhancement

  • Inflammation, infection, or vascularized scar tissue may demonstrate mild focal enhancement along the ligament

CT Appearance:

  • The PAOM is not directly visualized on conventional CT due to its soft tissue nature

  • Indirect assessment is possible by evaluating the posterior atlantooccipital space, alignment of C1, and occipital bone margins

  • Bony landmarks (posterior C1 arch and foramen magnum) are clearly seen

  • CT is useful for assessing associated fractures or bony abnormalities at the craniovertebral junction

MRI images

Posterior atlantooccipital membrane mri sagittal image