Nanoindentation study of human premolars subjected to bleaching agent.

Bleaching of teeth is gaining popularity due to cosmetic reasons. However, the effect it has on teeth is still largely unknown. This paper seeks to evaluate the effect of a bleaching agent, 30% hydrogen peroxide, on the nanomechanical properties of dentin and enamel using the nanoindentation technique. The Young's modulus and hardness obtained from nanoindentation before and after bleaching were compared. Five newly extracted human premolars were used. Nanoindentation was first done on the sliced enamel and dentin regions to determine their mechanical properties. One batch of samples was kept in Hank's balanced salt solution as control while the other was bleached in 30% hydrogen peroxide for 24h. The same number of nanoindentations was then done near the previously indented regions for both the control and bleached samples and the results compared. Using paired sample t-tests with alpha=0.05, it was found that there were no significant differences in both the Young's modulus and hardness of dentin and enamel kept in control. However, the mechanical properties of the bleached dentin were significantly decreased. For intertubular dentin, the mean hardness decreased by 29-55% and the mean Young's modulus decreased by 19-43%. For enamel, the mean hardness decreased by 13-32% while the mean Young's modulus decreased by 18-32%. The exact mechanism by which hydrogen peroxide affects the dentin and enamel has yet to be fully elucidated. However, it is observed to have an undermining effect on the nanomechanical properties of teeth.

Effect of hydrogen peroxide on intertubular dentine.

OBJECTIVES: To evaluate the effect of 30% hydrogen peroxide on the surface changes and nanomechanical properties of intertubular dentine. METHODS: Five freshly extracted human premolars were sectioned and two specimens were obtained from each tooth. Atomic force microscopy (AFM) images of dentine were taken. Baseline hardness and Young's modulus of intertubular dentine were determined using nanoindentation testing and recorded. The specimens were then treated with either 0.5 ml of Hank's balanced salt solution (control group) or 30% hydrogen peroxide at 24 degrees C for 24 h. AFM imaging and determination of hardness and Young's modulus of intertubular dentine were then repeated. The images were compared and statistical analyses of hardness and Young's modulus were carried out using paired sample t-tests. RESULTS: The AFM images showed recession of the intertubular dentine surface post-bleaching. For the control group, there was no significant change in hardness (p=0.124) and Young's modulus (p=0.438) of intertubular dentine. However, a significant decrease in hardness (p=0.002) and Young's modulus (p=0.001) of intertubular dentine was observed for the experimental group. CONCLUSIONS: Exposure to 30% hydrogen peroxide for 24 h caused surface changes to intertubular dentine and significantly decreased the hardness and Young's modulus of intertubular dentine.

X-ray diffraction analysis of mineral trioxide aggregate and Portland cement.

AIM: To compare the major constituents present in ProRoot mineral trioxide aggregate (MTA), ProRoot MTA (tooth coloured formula), ordinary Portland cement and white Portland cement using powder X-ray diffractometery. METHODOLOGY: X-ray diffractometery of the four materials was carried out with the divergence and scatter slits set at 1 degree and the receiving slit at 0.10 mm. The scan range was set at 5-70 degrees and continuous scans for the theta-2theta range were run with a scan speed of 2 degrees min(-1). The patterns obtained were then compared with the Powder Diffraction Files (PDF) found in the International Centre for Diffraction Data database. The three strongest peaks were used for the identification of the constituents. The relative intensities were plotted against the angle 2theta and compared with the plots in the PDF. RESULTS: The main constituents were found to be tricalcium silicate, tricalcium aluminate, calcium silicate, and tetracalcium aluminoferrite in all the four cements with the additional presence of Bi2O3 in ProRoot MTA and ProRoot MTA (tooth coloured formula). CONCLUSIONS: The four cements had similar major constituents. Data on Portland cement may be used for the further development or modification of ProRoot MTA in order to improve its physical characteristics and expand its scope of clinical applications.

Effect of traditional and alternative intracoronal bleaching agents on microhardness of human dentine.

The purpose of this study was to compare the effect of traditional and alternative bleaching agents on microhardness of human dentine when used intracoronally. Thirty-six premolars were divided into six groups and bleaching agents were sealed into the pulp chambers as follows: group 1--distilled water (control), group 2--30% hydrogen peroxide solution, group 3--sodium perborate mixed with distilled water, group 4--sodium perborate mixed with 30% hydrogen peroxide solution, group 5--35% carbamide peroxide gel, group 6--35% hydrogen peroxide gel. Access cavities were sealed and the teeth were stored in distilled water at 37 degrees C. After 7 days, each tooth was sectioned at the cemento-enamel junction level and microhardness testing was carried out on dentine. The results showed that treatment with 35% hydrogen peroxide gel, 30% hydrogen peroxide solution and 35% carbamide peroxide gel reduced the microhardness of outer dentine to a small extent while treatment with sodium perborate mixed with water and sodium perborate mixed with 30% hydrogen peroxide solution did not significantly alter the microhardness of dentine.