- Line is drawn from posterosuperior margin of hard palate to most inferior surface of occipital bone

- Lateral skull/ Lateral cervical spine 

- Odontoid apex should not lie above this line >8mm males and >10mm in females

- Most accurate and reproducible for basilar impression 

- Indicator of basilar impression

- Line drawn from posterior aspect of hard palate to posterior aspect of foramen magnum

- Lateral skull/ lateral cervical 

- Tip of odontoid process should not be more >7mm above the line; normal variation up to 3mm above

 - Best done on lateral skull w/ CT

- Indicator of basilar impression

- Lateral neutral/ flexion extension cervical spine

- Distance b/w posterior margin of anterior tubercle and anterior margin of dens

- Normal measurements 1-3mm adults; 1-5mm children

- Flexion is best view to assess the ADI b/c it puts the most stress on the transverse ligament

- Decreases ADI attributed to: age, DJD,

- Increases ADI attributed to: trauma, occipitalization, Down's syndrome, pharyngeal infections, inflammatory arthropathies

Line drawn connecting the digastric grooves which are medial to the base of the mastoid process.

Digastric line odontoid apex measurement ranges 1-21mm with average 11mm.  Digastric line atlanto-occipital joint ranges 4-20mm and averages 12mm.

Measurement decrease with basilar impression.