Fischgold’s digastric line
FDGL=Fischgold’s digastric line, FBML= Fischgold’s bimastoid line, x+y=total overhang of C1 on C2.
see Rule of Spence.
joins the digastric notches.The normal distance from this line to the middle of the atlanto-occipital joint is 10 mm (decreased in BI) 1).
No part of odontoid should be above this line.
More accurate than the bimastoid line (FBML).
1)
Hinck VC, Hopkins CE, Savara BS. Diagnostic Criteria of Basilar Impression. Radiology. 1961; 76