Background and aims: Establishment of natural head position (NHP) in radiography or photography by estimation, which is called natural head orientation (NHO), has been recently advocated by various studies. This study aimed to determine the effect of chin position on the accuracy and reproducibility of NHO in a more realistic way. Materials and Methods: NHP-based digital photographs of 33 adults with normal profile were modified using the computer software Supper-Goo and Photoshop technique to produce 99 profile images in three groups with normal, forward and backward chin positions. The reconstructed images were presented to three observers who rotated the images through a circular hole on a computer monitor. The horizontal angle was read on screen and recorded whenever they found the best horizontally-oriented face. This was performed at four intervals: T0: baseline; T1: after 5 minutes; T2: after one week; and T3: one month after baseline. Results: After orientation, the mean orientation angles were 0.19° ± 1.39, 0.20° ± 1.53 and -0.31° ± 1.38 for the normal, forward and backward chin groups, respectively. T-test did not show any significant differences in NHO and NHP among the three groups of chin position: P values of 0.17, 0.19, and 0.26 for normal, forward, and backward chin positions, respectively. However, one-way ANOVA demonstrated significant differences in the NHO between the normal and backward, and backward and forward chin groups. The obtained reproducibility rate of NHO calculated via Med-Calc 8.1 online software for T 0 , T 1 , T 2 and T 3 intervals was more than 60%, which is regarded significant in medical studies. Conclusion: Natural head position could be created with great reproducibility through orientation of profile images in different chin positions. NHO is under the influence of chin position; however, this influence is not clinically significant.
Objectives: Debonding of chemically bonded ceramic brackets has been mostly led to enamel damages. This study aimed to evaluate enamel cracks followed by debonding ceramic brackets with mechanical retention. Study Design: Sixty extracted human premolar teeth were randomly divided into two groups. Metallic brackets and recently available ceramic brackets with mechanical retention were bonded to the teeth in first (MB) and second (CB) group respectively. After a week, the brackets were debonded using a pair of bracket-removing pliers assembled to a testing machine. Optical stereomicroscope and trans-illumination techniques were used to calculate the cracks and measure their length before bonding and after debonding. Paired t-test, independent t-test and McNemar's test were used for statistical analysis. Results: Metallic brackets showed higher mechanical bond strength compared to ceramic brackets with mechanical locks. In both groups, higher numbers of teeth with enamel cracks and a significant increase in the length of enamel cracks were evident subsequent to debonding. Inter-group deference in the number of teeth with cracks and the length of enamel cracks before or after debonding was not significant. Conclusions:The risk of producing enamel cracks after debonding of ceramic brackets with mechanical retention is the same as metallic brackets.Key word: enamel cracks, debonding, ceramic brackets.
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