RESUMEN
Contemporary cephalometric analysis in orthodontics is based on comparison between craniofacial portions and reference planes. One of these planes is Frankfort Horizontal [FH] Plane that can be made by anatomic portion or machine portion. The aim of this study was to determine and compare the angle between Anatomic Frankfort Horizontal plane [AFH] and True Horizontal [TH] with the angle formed by Machine Frankfort Horizontal plane [MFH] and TH. Materials. In this Analytical-descriptive study, digital lateral cephalograms were taken in Natural Head Position from 50 orthodontic patients with the minimum age of 14 and class I malocclusion [Viazis analysis on Onyx Ceph software was employed to confirm the class I malocclusion]. Then on each cephalogram, two FH planes were constructed using machine and anatomic portions. Then the angle between each FH plane and TH plane was measured and recorded. Finally, the data were analyzed by paired Mest [a=0.05]. Machine Frankfort Horizontal plane [MFH] and Anatomic Frankfort Horizontal plane [AFH] showed significant differences of-5,66 and -1.58 degrees with True Horizontal plane [TH] respectively [P<0.001]. For cephalometric evaluation in orthodontic patients if the Frankfurt plane be used, it is better to use anatomical portion, because the difference between the Anatomic Frankfort Horizontal plane [AFH] and True Horizontal plane [TH] is significantly less than the difference between the Machine Frankfort Horizontal plane [MFH] and True Horizontal plane [TH]