MEASUREMENTS
MEASUREMENTS AND WHAT THEY MEAN
Atlanto-dental interval - ADI
The ADI measures the distance between:
-
Anterior Arch of the Atlas (C1): The front part of the first cervical vertebra (C1).
-
Dens of the Axis (C2): The peg-like structure on the second cervical vertebra (C2), which fits into the atlas and allows rotation of the head.
An abnormal ADI suggests Atlantoaxial Instability (AAI) due to transverse ligament laxity
.png)
Basion-axial interval - BAI
The Basion-Axial Interval (BAI) measures the distance between the basion (the midline bony prominence at the anterior margin of the foramen magnum) and the posterior aspect of the odontoid process (dens) of the axis (C2). It is assessed in lateral radiographs of the cervical spine.
-
Basion: This is the point at the midline of the anterior part of the foramen magnum, located where the base of the skull meets the cervical spine.
-
Odontoid process (dens): This is the peg-like projection of the second cervical vertebra (C2), which articulates with the atlas (C1) to enable head rotation.
​​
An abnormal BAI suggests instability at the craniocervical junction
​
.png)
Basion-dens interval - BDI
The Basion-Dens Interval (BDI) measures the distance between two structures:
-
Basion: The anterior part of the foramen magnum, which is a bony prominence at the base of the skull.
-
Dens (Odontoid Process): The peg-like structure that projects upward from the second cervical vertebra (C2) and fits into the atlas (C1) to allow rotation of the head.
​
An abnormal BDI suggests instability at the craniocervical junction
.png)
Clivo-axial angle - CXA
The Clivo-Axial Angle (CAA) measures the angle formed by two lines:
-
Clivus Line: This line is drawn along the clivus, a sloped bony plate at the base of the skull. The clivus forms part of the posterior surface of the sphenoid bone and extends to the basilar part of the occipital bone, anterior to the foramen magnum.
-
Axis Line: This line is drawn along the body of the axis (C2 vertebra), which is the second cervical vertebra that supports the dens (odontoid process). The dens fits into the atlas (C1) and allows for head rotation.
The angle formed by the intersection of these two lines (the Clivo-Axial Angle) reflects the relationship between the base of the skull and the cervical spine, which is crucial for craniovertebral junction alignment.
​
An abnormal CXA indicates ventral brainstem compression or potential stretching/kinking of the brainstem
.png)
Power's ratio
The Powers Ratio measures the relative alignment of the dens (odontoid process) of the axis (C2) vertebra with respect to the posterior arch of the atlas (C1).
The measurement is typically used in lateral radiographs or CT scans of the cervical spine to assess for any displacement or instability in the craniovertebral junction.
​
An abnormal Power's ratio suggests atlanto-occipital dislocation (AOD)
.png)
The Grabb-Oakes Measurement specifically evaluates the relationship between the odontoid process (dens) of the second cervical vertebra (C2) and the posterior arch of the atlas (C1). The measurement looks at how far the dens protrudes into the atlas, which is an important indicator of whether the atlantoaxial joint is unstable or not.
​
An abnormal GOL suggests a high risk of ventral brainstem compression due to instability
Grabb-oakes line(GOL)
.png)
Translational measurements are done on dynamic imaging to calculate the difference between flexion and extension. Translational BAI is used to assess how much the skull slides between different positions which indicates instability.
Translational measurements
Upright dynamic imaging vs supine
​When you’re upright, gravity pulls the head downward, placing the most stress on the ligaments and joints of the upper cervical spine (occiput–C1–C2).
If those ligaments are lax or damaged (as in CCI or AAI), the skull can subtly sink or shift, causing:
-
brainstem or spinal cord compression,
-
obstruction of cerebrospinal fluid (CSF) flow,
-
stretching of the vagus or accessory nerves, and
-
mechanical strain on vascular or neural structures.
In a supine MRI, gravity is off-loaded — the head is supported by the table, and much of that pathological movement disappears, leading to false negatives.
​
-
Static MRIs or CTs (taken while lying still) only show the bones, ligaments, and alignment in one neutral position.
-
But instability means abnormal movement between vertebrae — something that often only becomes visible when the head and neck move (flexion, extension, rotation).
-
Therefore, someone may have “normal” alignment on a neutral MRI, but their joints could sublux or shift abnormally when the neck moves, causing brainstem or cord compression that isn’t seen in neutral imaging.