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Alar ligament

The alar ligaments are a pair of strong, short fibrous bands that extend from the lateral aspects of the dens (odontoid process) of the axis (C2 vertebra) to the medial sides of the occipital condyles. They are oriented obliquely and function as critical stabilizers of the craniocervical junction, limiting excessive rotation and side-bending of the head relative to the cervical spine. Each ligament is composed of dense collagen fibers, making it resistant to tensile stress while allowing normal physiological motion.

The alar ligaments are located posterolateral to the transverse ligament of the atlas, forming part of the atlanto-odontoid complex. They play a vital role in maintaining atlanto-occipital stability, protecting the upper cervical spinal cord and vertebral arteries, and preventing over-rotation injuries.

Function

  • Restrains excessive rotation of the head

  • Limits lateral bending at the atlanto-occipital joint

  • Contributes to craniocervical stability

  • Protects the upper cervical spinal cord and vertebral arteries

  • Provides ligamentous support for the atlanto-odontoid complex

Synonyms

  • Alar ligaments of odontoid

  • Cruciate-stabilizing ligaments (part of craniocervical ligaments)

  • Ligamenta alaria

MRI Appearance

T1-weighted images:

  • Alar ligaments appear as low signal intensity (hypointense) linear bands extending obliquely from the dens to the occipital condyles

  • Surrounded by intermediate-to-high signal intensity fat, which provides contrast

  • Ligament integrity is visible; tears or disruption appear as focal hyperintensity or discontinuity

T2-weighted images:

  • Ligaments remain low signal intensity (hypointense) due to dense fibrous composition

  • Edema or ligament injury may appear as linear or patchy hyperintense signal

  • Adjacent synovial joints and cerebrospinal fluid provide natural high-signal contrast

STIR (Short Tau Inversion Recovery):

  • Normal alar ligaments remain low signal

  • Acute trauma, inflammation, or ligamentous edema appear hyperintense, allowing detection of partial or complete tears

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal alar ligaments generally show no enhancement

  • Pathological ligaments (trauma, inflammation, or infection) demonstrate enhancement along disrupted fibers, highlighting abnormal vascularized tissue

CT Appearance:

  • Ligaments are not directly visualized on conventional CT due to low soft tissue contrast

  • Indirect assessment possible via alignment of dens and occipital condyles and evaluation of associated fractures or bony avulsions

  • Thin-section CT with 3D reconstruction helps evaluate bony attachment sites and assess for avulsion injuries

MRI images

Alar ligament mri axial image

MRI images

Alar ligament mri sag image