[Reference lines on diagnostic imagings of the head Part 1: Reference lines on cephalogram]

The relationship between three major reference lines including Reid's base line (RBL), canthomeatal line (CML) and orbitomeatal line (OML), which pass the center of orbital margin and porion, was investigated in 141 cephalograms of normal healthy volunteers. The difference between CML and OML is so small (mean 1.0 degrees +/- S.D. 1.3 degrees) that these two lines are considered almost identical. It is said that RBL and CML meet about 10 degree, but present study shows the angle is about 13 approximately 14 degree. The relationship between these two major reference lines (RBL and CML) and the other four reference lines including nasion-clition line (NC), clition-basion line (CB), nasion-basion line (NB) and nasion-opisthion line (NO) is also evaluated. NO shows the most stable relationship to RBL (22.7 degrees +/- 2.1 degrees) and NB shows the most stable relationship to CML (15.0 +/- 2.1 degrees).