Projection of the tip of the dens more than 5 mm above a line joining the hard palate to the posterior lip of the foramen magnum (Chamberlain's line) or the tip of the dens is greater than 7 mm above McGregor's line (the back of the hard palate to the lowest point of the occipital squama). [ http://www.ncbi.nlm.nih.gov/pubmed/10084535 ]