In vitro comparative study of two different bleaching agents on micro-hardness dental enamel

Objective: To evaluate the effect of home-use bleaching agent containing 16% Carbamide Peroxide [CP] and in-office bleaching agent containing 38% Hydrogen Peroxide [HP] on enamel micro-hardness

Study Design: An in vitro experimental study

Place and Duration of Study: Department of Operative Dentistry and Science of Dental Materials at Dr. Ishrat-ul-Ebad Khan Institute of Oral Health Sciences, Dow University of Health Sciences and Material Engineering Department of NED University of Engineering and Technology, Karachi, from July to December 2014

Methodology: A total of 90 enamel slabs from 45 sound human 3rd molar were randomly divided into 3 groups. Each group contained 30 specimens [n=30]. Group 1 was kept in artificial saliva at 37[degree]C in incubator during the whole experiment. However, Groups 2 and 3 were treated with power whitening gel and tooth whitening pen respectively. After bleaching session, specimens were thoroughly rinsed with deionized water again for 10 seconds and then stored in artificial saliva at 37[degree]C in incubator. Artificial saliva was changed after every 2 days. The Vickers hardness tester [Wolpert 402 MVD, Germany] was adjusted to a load of 0.1 kg [100 gm] and dwell time of 5 seconds. Three Vickers were performed on each specimen using a hardness tester according to the ISO 6507-3:1998 specification. Micro-hardness measurements were performed before and after bleaching at day 1, 7 and 14

Results: In the control group, the baseline micro-hardness was 181.1 +/- 9.3 which was reduced after the storage on day 1, 7 and 14 [p = 0.104]. In Group 2, baseline micro-hardness was 180.4 +/- 10.1 which was reduced to 179.79 +/- 10.0 units after day 1. Whereas, on day 7 and 14, the values of micro-hardness were 179.8 +/- 10 and 179.7 +/- 10.29, respectively [p=0.091]. Furthermore, the baseline micro-hardness in Group 3 was 174.0 +/- 22.9 units which was reduced to 173 +/- 23 on day 1, 170 +/- 30 on day 7 and 173 +/- 23 on day 14 [p = 0.256]. The statistically insignificant difference was found among micro-hardness values of different bleaching agents [p = 0.118]

Conclusion: Bleaching with 38% Hydrogen Peroxide [HP] and 16% Carbamide Peroxide [CP] resulted in insignificant effect on surface micro-hardness of enamel

Effect of different irradiation times on microhardness and depth of cure of a nanocomposite resin

To determine the microhardness and depth of cure of nanocomposite using different irradiation times on both upper and lower surfaces of composite material. In-vitro experimental study design. Dr. Ishrat-ul-Ebad Khan Institute of Oral Health Sciences, Dow University of Health Sciences and NED University of Engineering and Technology, Karachi, from March to May 2010. Total 120 cylinder shaped specimens; 60 specimens for depth of cure test and 60 specimens for micro hardness test were fabricated using A3 shade of nanocomposite [Filtek Z350 XT, 3M ESPE]. For each irradiation time four groups were made [Group 1 = 20s] [Group 2 = 30s], [Group 3 = 40s] and [Group 4 = 60s]. For each group fifteen specimens were used. The resin was placed and polymerized into a cylindrical plastic mold. Depth of cure was measured by using micrometer. Micro Vickers hardness was measured on both top and bottom surfaces. SPSS-16.0 was used for statistical analysis. There was statistically significant difference in the depth of cure between all groups showing the highest value in group 4 [p < 0.001]. For hardness on top surface, there was a statistically significant difference in between groups 1 and 2 [p=0.001], groups 1 and 3 [p < 0.001], groups 1 and 4 [p < 0.001] There was no statistically significant difference between groups 2 and 3, groups 2 and 4 and in between groups 3 and 4. For hardness on bottom surface, there was statistically significant difference in between all groups showing the highest value in group 4 [p < 0.001]. Depth of cure and hardness was increased by increasing irradiation time. Hardness on the top surface was higher than bottom surface values