Facilitating the Location of Porion to Compare True Frankfort Horizontal with Sella-Nasion

Abstract

The constructed cephalometric line sella-nasion minus seven degrees (SN-7°) has long been used as a substitute for the Frankfort horizontal in the descriptive analysis of orthodontic cases, due to the difficulty in locating anatomic porion. To eliminate this problem, the cephalostat was modified by replacing the original ear rods with hollow, cylindrically shaped shafts which permit the consistent visualization of anatomic porion. Location accuracy was tested by the double determination method. Then, using anatomic porion, the hypothesis that the constructed line SN-7° is an accurate and reliable substitute for Frankfort horizontal in the descriptive analysis of cases was tested. In an adult sample, the mean Frankfort horizontal - sella-nasion angle was 9.03° (S.D.=2.58) with a range of 2.0° to 17.5°. It was concluded that neither SN-7° nor any other constructed line is a reliable substitute for the Frankfort horizontal, due to the wide range of values determined. The mean Frankfort horizontal - mandibular plane angle was determined to test the hypothesis that when anatomic porion is utilized to construct the Frankfort horizontal, this angle is significantly different from that measured in other similar samples. The mean angle was 19.50° (S.D.=5.74) with a range of 8.0° to 35.5°. This value was significantly less than those determined for the Frankfort horizontal – mandibular plane angle in previous research.