It is the distance (in mm) between the basion and the superior extension of the posterior cortical margin of the body of the axis in the median (midsagittal) plane.
Normal values are <12 mm on plain radiographs and were not found to be reliable of on CT 1 where they range from -8.7 mm to +26 mm in normal subjects.
- 1. Rojas CA, Bertozzi JC, Martinez CR et-al. Reassessment of the craniocervical junction: normal values on CT. AJNR Am J Neuroradiol. 2007;28 (9): 1819-23. AJNR Am J Neuroradiol (full text) - doi:10.3174/ajnr.A0660 - Pubmed citation