الفهرس 
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Abstract
A total of 130 freshly extracted, sound, single rooted human teeth were selected. Teeth were cleaned from calculus deposits and soft tissue. They were grouped and divided according to the different procedures adopted.
In the first part of the study, i.e., surface topography examination 30 teeth were used they were grouped and subgrouped to examine normal enamel and dentine as well as the effect of carbamide peroxide 16% and 22% on enamel, and the effect of sodium perborate mixed either with water or hydrogen peroxide on dentine surface topography. Samples were prepared from the facial surface of maxillary anterior teeth by disking with carborandom disk in conventional speed handpiece. Samples were covered with gold in a sputter coater and examined under electron microscope.
In the second part of the study, i.e. enamel and dentine microhardness, 30 teeth were selected and they were grouped and subgrouped to examine the effect of sodium perborate mixed with water or hydrogen peroxide on dentine microhardness and the effect of carbamide peroxide 16 or 22% on the microhardness of enamel. Blocks of enamel and dentine with a size of 4X4X2 mm were prepared from the facial surface of maxillary anterior teeth by disking. Blocks were highly polished, covered by gold in a sputter and placed in acrylic mould. For enamel microhardness the load was 200 grams and time of indentation was 10 seconds, while for dentine the load was 50 grams and the time was 15 seconds, five indentations were made for each block. For enamel blocks subsurface microhardness were calculated by grinding the samples on wet silicone carbide paper and ± 100 um were removed from enamel surface. After the desired amount of enamel had been removed the blocks were coated again with gold in a sputter coater, another five vicker hardness measurements were done with the same load and indentation time to denote subsurface microhardness.
In the third part of the study i.e., dentine permeability 30 roots were splitted into two halves longitudinally pulp tissue was removed and samples were grouped according to the material being applied 10 samples were bleached with sodium perborate mixed with water while 10 samples was bleached with sodium perborate mixed with hydrogen peroxide and 10 samples control. All surface of the specimen except the treated surface were covered with varnish and wax. Leishman dye was applied in each root and left for one hour. Roots were disked and the disked parts were seen and photographed under stereomicroscope.
In the fourth part of the study, i.e. color change, 40 teeth were used, lingual access cavity was done in each crown and lingual access was enlarged by Gates Glidden bur size 2 through 6. Teeth were photographed under standardized condition and their shades were recorded. Teeth were stained under human blood and centrifuged at 2500 rpm for 20 min, the staining procedure was repeated 6 times. Teeth shades were then recorded and teeth were photographed in standardized conditions. Teeth were grouped according to the bleaching procedure i.e., external or internal bleaching for external bleaching carbamide peroxide 16% and 22% were applied on labial surface of teeth for 1 week and was refreshed daily, while for internal bleaching sodium perborate was mixed with water or hydrogen peroxide and placed in pulp chamber and closed with cavit and left for two weeks, both mixtures were refreshed weekly, teeth were photographed as described before and color shades were recorded.
SEM examination of enamel bleached with 16% carbamide peroxide showed mild alternation of the surface in terms of shallow depressions with a porous areas and slight loss of prism cores. While enamel bleached with 22% carbamide peroxide showed a more aggressive effect as surface porosity increased with marked loss of prism cores.
SEM examination of dentine bleached with sodium perborate mixed with water showed minimal changes, as some deposits of the material were found, while dentine bleached with sodium perborate mixed with hydrogen peroxide showed funneling of dentinal tubules orifices.
Vicker hardness values for enamel surface and subsurface decreased slightly in samples bleached with 16% carbamide peroxide while samples bleached with 22% vicker hardness values decreased dramatically. For dentine permeability the leishman dye was more permeable in dentine bleached with sodium perborate mixed with hydrogen peroxide than dentine bleached with sodium perborate mixed with water.
For color change teeth bleached with 22% carbamide peroxide showed lighter color than samples bleached with 16% carbamide peroxide for external bleaching, while for internal bleaching teeth bleached with sodium perborate mixed with hydrogen peroxide showed lighter color than teeth bleached with sodium perborate mixed with water.
Conclusion:
1) External and internal bleaching causes surface topographic changes in both enamel and dentine.
2) Internal bleaching decreases dentine microhardness and increased dentine permeability depending on the mixing of sodium perborate either with water or hydrogen peroxide.
3) There was a direct relation between increasing the strength of bleaching material and reduction of surface hardness.
4) As the percentage of carbamide peroxide increases teeth gets lighter in color.
5) Sodium perborate mixed with hydrogen peroxide gets teeth lighter in color than sodium perborate mixed with water.
6) Subsurface hardness always reduced more than surface using same bleaching material.