| dc.description.abstract | The purpose of this study was to assess the SN-FH angle in three ethnicities- North American Caucasian, Black and Mexican- while also providing insight on the variability of both the sella-nasion and Frankfort horizontal reference planes between and within these populations. Secondarily, the study aimed to determine if the SN-FH angle correlates to facial divergence and growth tendencies observed radiographically.
Materials and Methods
This retrospective, cross-sectional study included 309 de-identified radiographs gathered from the Texas A&M College of Dentistry Orthodontic Department patient archives. The radiographs consisted of three ethnic groups and included both males and females. A custom cephalometric analysis was created using Dolphin Imaging Software (Version 11.9) and radiographs were subsequently digitized. Linear and angular measurements were derived from the tracings in order to compare the SN-FH angle within and between the three groups. Additional measures of facial divergence, head posture, and growth forecast were also analyzed and compared to the SN-FH angle. SPSS was used for statistical analysis. A two-way analysis of variance was used to evaluate group and sex differences, controlling for age. In order to consolidate all of the variables analyzed for this study and discover particular underlying trends related to the SN-FH angle, a principal component factor analysis with varimax was completed.
Results
Coefficients of variation indicated that SN contributed more to the variability seen in the SN-FH angle than the FH reference plane. The entire sample had an average SN-FH angle of 8.6°. There was no difference in the SN-FH angle between sexes. In regards to the ethnic differences, there was a significant difference in the SN-FH angle between Caucasians and African-Americans (p<0.001). African-Americans had larger SN-FH angles than Caucasians. There was no difference in the angle between Mexican-Americans and Caucasians or African-Americans. Seven factors contributed to the overall variation seen in the sample; of those factors, five were significantly correlated to the SN-FH angle: maxillo-mandibular relationship to SN, upper to lower face height ratio, cranial base angulation, and growth pattern. The maxillo-mandibular relationship to SN factor explained over forty percent of variation seen in the SN-FH angle.
Conclusions
The average SN-FH angle is not 7°, though this value has come to be accepted in the orthodontic community. The value is actually larger than 7° in most populations. There is little or no sex difference in the SN-FH angle. The larger SN-FH angles observed in African-Americans are due to both genetic and environmental factors that may lead to a flatter anterior cranial base and more superior position of porion. The SN-FH angle is influenced by growth pattern, the anterior-posterior position of the jaws, and upper to lower face height. The strong correlation between the SN-FH angle and the Mx-Mn relationship to SN indicates that SN is a more prominent component in determining the individual variation seen in the SN-FH angle, rather than the FH reference plane. | en |
