Table 2.

Orthopedic parameters, including landmarks and definitions

Atlas inclination (modified)	Angle from atlas plane to Frankfort horizontal plane.
Atlas plane (AT)	Line drawn through the most anterior and most posterior sites of the atlas.
Chamberlain’s distance	Distance from dens tip to Chamberlain’s line.
Chamberlain’s line (palato-occipital line)	Line drawn from posterior edge of hard palate to posterior edge of foramen magnum.
Clivus-dens angle	Angle between dorsal ends of clivus and dens axis.
Dorsal clivus boundary	Dorsal end of clivus
Dorsal dens boundary	Dorsal end of dens axis
McGregor’s line (palato-suboccipital line)	Line from posterior edge of hard palate to most inferior point of squama occipitalis.
McRae’s line (foramen magnum line)	Line between anterior and posterior edges of foramen magnum.
OPT line	Line drawn against the anterior cranial base to define the craniocervical angle.
Powers ratio	Ratio between distances (i) opisthion to dens tip and (ii) Ba to projection center of arcus posterior atlantis.
Ranawat’s line	Distance between a line connecting the projection centers of the anterior/posterior arches of the atlas and the center of the shadow of the axis vertebra.
Redlund-Johnell/Petersson line	Distance between McGregor’s line and center of inferior endplate of second cervical vertebra (mC2)
Solow/Tallgren sum (modified)	Sum of angles formed by linear parameters OPT, CVT (line through spC2 und pC2), NL, NSL and ML to the Frankfort horizontal plane.