The digastric line, also known as the biventer line, has been described and used to evaluate basilar invagination on frontal skull plain film and coronal CT images.
The digastric line is drawn between right and left digastric grooves, which are located medial to the mastoid apices. These are considered constant in relative position. The distance from this digastric line to either the tip of the dens or a line bisecting the atlanto-occiptal joints has been described as between 11 mm and 10-12 mm, respectively. A measured distance below these thresholds may suggest basilar invagination.
As with many techniques for assessing for basilar invagination, there are known limitations :
- given variable dens morphology, comparison of digastric line to the atlanto-occipital joint may be more reliable
- margins of the atlanto-occipital joints are not always obvious, which may limit accuracy