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Y. SINGH, S. BAUMRIND, S. CARLSON, and R. BOYD, University of the Pacific School of Dentistry, San Francisco, CA, USA
Objectives: In the analysis of lateral skull x-ray images, the Sella-Nasion frame of reference is frequently used in preference to the Frankfort Plane frame of reference in the belief that Sella and Nasion, the landmarks that determine SN are more reliably located than are Porion and Orbitale, the landmarks that determine Frankfort Plane. This study tested the appropriateness of this belief by comparing the reliability of location of the four landmarks. Methods: The sample consisted of data from pre-treatment lateral cephalograms for 103 randomly chosen orthodontic patients (31 males and 72 females, aged 13.1±7.3 years). All four landmarks were identified independently on each image by 3 trained judges. The standard deviations of the replicate assessments were compared to estimate relative reliability of landmark location.
The widely held belief that a constant can be used as a conversion factor between the SN and Frankfort frames of reference was also tested by measuring the angle between the two lines for the same 103 patients. Results: Standard deviations for each landmark in the horizontal (X) direction were: Sella 0.37, Nasion 0.39, Porion 0.51, Orbitale 0.68 (all values in mm). The corresponding values in the vertical (Y) direction were: Sella 0.28, Nasion 0.84, Porion 0.23, Orbitale 0.75. The standard deviation for the angle between SN and Frankfort Plane was 3.5 degrees (mean=7.3o). Conclusions: In this sample, the reliability of location of Porion and Orbitale did not appear markedly different from the reliability of location of Sella and Nasion, particularly in the more important vertical (Y) direction. The angular relationships between SN and Frankfort Plane appeared quite variable, implying that the use of any constant to adjust between them is likely to incur substantial error.
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