Effects of bleaching and remineralising agents on the surface hardness of enamel

Statement of Problem: Home bleaching is a common method for whitening the teeth. However, bleaching may lead to a decrease in the hardness of the enamel

Purpose: The purpose of this study was to investigate the effects of two different concentrations of carbomide peroxide [CP] on the hardness of the enamel and also to evaluate the effects of the remineralising agents on the hardness of bleached enamel

Materials and Method: Crowns of 100 intact extracted human anterior teeth were resected from their roots and mounted in acrylic resin in a way that the buccal surface was parallel to the floor [horizontal]. The samples were then divided into 10 groups. The baseline hardness in the middle of the buccal surface was measured through Vickers Micro-hardness test and at a load of 500 gram per second. Then five groups were bleached with 10% carbomide peroxide and other five groups with 22% carbomide peroxide. The bleaching was performed for 21 days and was applied 4 hours each day. The samples were kept in distilled water in the interval time between the applications of bleaching agents. In the next phase the enamel micro-hardness was measured and different demineralising agents were used for each group. The Tooth Mousse [TM] paste; MI paste plus [MI]; and Crest fluoridated toothpaste was applied for 4 hours to the surface of the enamels in three groups. In the forth group, samples were embedded in fresh cow milk for the same period and the fifth group was kept in distilled water as a control group. Then, the final hardness was measured and the collected data were analyzed by t-test, paired sample t-test and One-way ANOVA test

Results: Bleaching with the aforementioned concentration of CP had no effects on enamel microhardness. In the groups with a 10% CP, none of the demineralising agents had any effect on the hardness value. However, the application of milk increased the hardness. In the groups with a 22% CP, TM paste reduced the enamel microhardness value while Crest, increased it. MI paste and milk didn't have any effect on it

Conclusion: The use of TM paste results in lower hardness of the bleached enamel. It seems that the high concentration of fluoride in MI paste may be responsible for increased microhardness of enamel. Milk and fluoridated toothpaste have propensity to increase the enamel hardness

[Comparison of residue materials after retreatment of filled canals with resilon and gutta-percha]

Retreatment procedures in endodontics requires the complete removal of the original root filling materials. The most common materials that must be removed during retreatment are gutta-percha and sealers. The aim of this in vitro-experimental study was to compare the amount of residual filling material after retreatment of the root canal walls which were obturated with gutta-percha or Resilon. Thirty extracted premolar single root human adult teeth were selected and prepared by race rotary tiles up to MAF#35. Then, they were divided into two groups of 15 [A and B]. Group A was obturated with gutta-percha and AH26 sealer and group B with Resilon system by lateral condensation technique. After 4 weeks all the canals were retreated by Race rotary files and the use of chloroform and master apical file increased to #40 file. Then, the teeth were split longitudinally and the remaining materials on the surface of the canals were visualized with streomicroscope with 20 magnification and digital photographs captured. The amount of residual materials and time of retreatment were analyzed with Mann-Whitney and Freidman tests. The highest residual material value was in Resilon group and the difference between the two groups was significant [p <0.001]. The time required for retreatment was not significant between the two groups [p =0.381]. Orthograde retreatment of Resilon was less efficient than gutta- percha with more residual materials on the surfaces of the root canals that were filled with Resilon

[Comparison of canal transportation after preparation by 3 different rotary systems]

Cleaning and shaping has been recognized as one of the most important steps of root canal treatment. Various Ni-Ti rotary systems have been introduced for reduction of complications and facilitating the root canal shaping process. The purpose of this study was to determine the amount of transportation by using the three rotary systems, Profile, Flex master and Race. This ex-vivo randomized controlled trial was carried out on mesial mandibular root canals [type 3 Weine] with mild curvature [15-35°]. The roots were randomly divided into three groups of 20 canals each. The roots were sectioned horizontally at 3mm, 6mm from the apex and then, reassembled before instrumentation according to the manufacture's guidelines. Photographs were taken from the cross sections before and after instrumentation. Image analyses were performed by Photoshop software, version 8. The ability to preserve canal shape centrally as well as the direction of transportation was determined next. Data was analyzed by ANOVA and Tukey tests. The difference was significant in mid apical parts when comparing the three groups. Most transportation was in the furcation area. Race had the most and profiles showed the least transportation in both parts of the roots [P=0.001]. The profile system was found to be the best for preserving the centering position of the root canal among the three rotary systems

[Comparison of air driven handpiece and electromotor on distortion or fracture rate of nickel titanium rotary instruments]

One of the affecting factors in adverse prognosis of root canal therapy is procedural accidents as broken files, facing with difficulty in removing. Many manufacturers have designed and marketed various electromotors with the ability of controlling rotational speed and torque. On the other hand, these expensive motors have encouraged other manufacturers to marketing different air driven handpieces. Despite of presenting various vast instruments for this purpose, only limited comparative studies have been carried out between electromotor and air driven handpiece. The purpose of this study was to evaluate the effect of air driven handpiece versus electromotor on distortion or fracture rate of NiTi rotary files. This in vitro randomized controlled trial was carried out on 160 canals of human's matured molars with mild curvature [15-33°]. After initial preparation of samples and considering the inclusion criteria, in the first group, preparation was carried out with air driven handpiece and in groups 2 Endo IT [VDW, Germany] was used as electromotor. In both groups M two files with crown down technique were used for canal preparations. Data on file distortion or fracture were collected and analyzed using Mann Whitney, Mantel Cox, Kaplanmeiere and T tests. No significant differences on distortion or fracture rate of files between the two groups were found [p>0.05]. Based on survival analysis safety probability of files after preparation of 9 canals was%64.1 in group 1 and%69.9 in group 2. There was no significant differences between this safety probability in the two groups [P=0.272]. These findings showed that both electromotor and air driven handpiece has nearly similar effect on fracture rate or distortion of NiTi rotatory instruments

[Evaluation of different clearing techniques: an auxiliary method for studying root canal anatomy]

Various techniques for clearing the root canal system have been used in different aspects of dentistry. Poor quality of the final cleared tooth may affect accurate evaluation of the anatomy or other characteristics of the canals. The purpose of this study was to compare a number of currently used clearing methods in order to identify the most effective procedure. Fifty human extracted premolars were included in this experimental study. A routine clearing protocol was employed for all teeth. The specimens were divided into 5 groups of 10, applying different combinations of decalcifying [HCL, HNO3] and clearing agents [eugenol, methyl salicylate]. The samples were examined by two endodontists. Data were subjected to statistical analysis using Chi-square and Kruskal Wallis tests. A significant difference was observed between the study groups. The most effective decalcifying material and clearing agent was%10 HC1 and methyl salicylate, respectively. According to the results of the present investigation, application of%10 HC1 along with methyl salicylate is suggested for the evaluation of the root canal system