[Comparative evaluation of elastic modulus of orthodontic elastomeric ligatures]

The application of elastomeric ligatures is one of the methods of archwires ligation to orthodontic brackets. Knowledge about modulus of elasticity of these ligatures is useful for selecting a specific brand of elastomeric ligature for a special condition. The purpose of this study was to compare elastic modulus of seven commercially available elastomeric ligatures in the initial condition and after 28 days immersion in a simulated oral environment. Changes in the modulus of elasticity were also surveyed for each brand of ligatures after 28 days. In this experimental laboratory-based study, elastomeric ligatures were obtained from different companies and their modulus of elasticity were measured in the initial condition and after 28 days immersion in a simulated oral environment using Zwick testing machine at 5mm/mm cross head speed. The data were analyzed using SPSS software and ANOVA, Duncan and student's t-test. Different brands of elastomeric ligatures showed significant differences in elastic modulus both in the as-received condition and after 28 days immersion in the simulated oral environment [p<0.05]. Ligatures from American Orthodontics and Ortho Technology showed the lowest and ligatures from Dentaurum revealed the highest elastic modulus in the as-received condition. Modulus of elasticity in the ligatures from American Orthodontics, Ortho Technology and ITO increased significantly while in ligatures from Dentaurum it was decreased significantly after 28 days immersion in the simulated oral environment [p<0.05]. Ligatures from American Orthodontics and Ortho Technology are suitable for immediate use after bonding and in sliding mechanics while ligatures from Dentaurum are not suitable to use immediately after bonding

[ relationship between lower facial height and maxillary and mandibular lengths based on Harvold analysis in Mashhad high school students]

Introduction: at the end of an orthodontic treatment, the coordination between different facial dimensions is very important. The changes in facial height cause changes in other dimensions. In the present study, the relationship between lower third of facial height and maxillary and mandibular lengths was evaluated and compared with the previous studies

Materials and Methods: in this study, 38 lateral cephalograms of Mashhad high school students aged 16- 19 with mean age of 17 years were selected and traced manually. The profile, occlusal relationship, overjet and overbite in these cases were normal and molar and canine relationships were class I. The data were analysed using SPSS software [version 10] and t-student test was used for analysis

Results: the findings were as follow: *the relationship between lower third of facial height and both the length of I mandibular base and the length of mandible based on Harvold analysis was significant. *The relationship between the lengths of both the maxilla and anterior cranial base, and lower third of facial height were not significant. *No relationship was found between the length of maxillary base [based on Harvold analysis] and lower third of facial height

Conclusion: according to this investigation, it is concluded that the length of mandibular base has a significant correlation with the lower third of facial height, but such a correlation does not exist between maxilla and lower third of facial height