since the introduction of tooth whitening by haywood and heymann in 1989 , this trend is getting more popular .
there are a few types of bleaching methods , but all of them share a common principle of the decomposition of peroxides from hydrogen peroxide or its compounds such as carbamide peroxide ( cp ) into unstable free radicals . these radicals further breakdown into large pigmented molecules either through oxidation or reduction reaction .
the oxidation or reduction process changes the chemical structure of the interacting organic substances of the tooth , which results in the change in color .
furthermore , at 45% concentration of cp in different intracoronal tooth - bleaching procedures , maleknejad ( 2012 ) reported an increase in the diameter of dentinal tubules , and it also promoted alterations in the mineral content of the dentine .
types of bleaching methods include nonvital bleaching , in - office professional bleaching , and home bleaching .
nightguard home bleaching uses a relatively low level of whitening agent and is applied to the teeth via a custom - fabricated mouthguard and is worn at night for a duration of at least two weeks .
a lot of research has been carried out to identify the effects of bleaching on the tooth surface and dental restorative materials .
jacob ( 2007 ) reported that bleaching with cp may alter the marginal leakage of resin composite restoration ; however , amalgam restorations were not adversely affected in vitro .
it was reported that the bleaching agent , regardless of the whitening products used , will reduce the microhardness of the enamel and promote an increase in surface roughness .
hence , it is widely used for anterior and also posterior restorations . due to this
some studies concluded that application of home bleaching agent gels may cause surface softening of composite resins , whereas others stated that there are no significant changes in surface microhardness found after the application of 15% cp on composite resin .
furthermore , a study was done to evaluate the effects of bleaching on toothbrush abrasion in three types of composites .
it was concluded that nano - filled resin composites are to be used for restoration when bleaching treatment is required .
there is a significant and positive correlation found between surface roughness and vital bacterial adhesion .
it appears that bleaching agents may affect the adherence of certain cariogenic micro - organisms to the outer surfaces of composite resin restorations .
roughness has a major impact on aesthetic appearance , discoloration of restorations , plaque accumulation , secondary caries , and gingival irritation .
the experimental composite tested in this study was a locally produced dental restoration nanocomposite ( kelfil ) .
this nanocomposite was produced by the school of dental sciences ( ppsg ) , universiti sains malaysia ( usm ) , kelantan , malaysia .
the nanosilica fillers , synthesized locally by a sol - gel process , were in the size range of 1020 nm .
the experimental nanocomposite ( enc2 ) contained 35 wt% of nanosilica fillers . this material ( enc2 )
the aim of this study was to study the effects of home bleaching on the hardness and surface morphology of a universal nanocomposite , an anterior nanocomposite , and a nanohybrid composite .
two commercially available resin - based composites , based on bisphenol a - glycidyl methacrylate / triethyleneglycol dimethacrylate / ethoxylated bisphenol - a - dimethacrylate ( bis - gma)/ethoxylated bisphenol - a dimethacrylate ( bis - ema)/triethylene glycol dimethacrylate ( tegdma ) ] resin chemistry ( filtek z350 ; 3 m espe , st .
paul , mn , usa and tph 3 ; dentsply caulk , milford , de , usa ) and one experimental resin - based composite based on bis - gma / urethane dimethacrylate ( udma)/tegdma resin chemistry ( kelfil ) were used in the present study [ table 1 ] .
all composites were packed into acrylic molds ( 4 mm diameter  2 mm thick ) which were placed on a mylar strip .
the excess composites were removed using another mylar strip on top of the mold and pressed flat with a glass slab .
eighteen samples of each composite material were prepared , with a total at 54 samples .
all samples were light - cured for 20 seconds from each top and bottom surface using elipar freelight 2 ( 3 m espe st .
all samples were then polished using sof - lex ( 3 m espe , st .
four different roughness of disks were used , which were coarse ( 55 m ) , medium ( 40 m ) , fine ( 24 m ) , and ultrafine ( 8 m ) , using a mandrel on a slow - speed handpiece , 10 seconds per disk without water .
conscious efforts were made to standardize strokes , downward forces , and number of strokes for each disk .
all samples were stored in a distilled water bath at 37 c for 24 hours prior to the treatment procedure .
composites resin and bleaching agents tested the samples from each composite resin material were further divided into three groups ; groups 1 , 2 and 3 with n = 6 .
samples in group 1 as the control group were not bleached but stored in shaking water bath sw23 ( julabo , seelbach , germany ) for 14 days at 37 c of distilled water .
samples in group 2 were subjected to opalescence pf ( ultradent product , south jordan , ut , usa ) 10% cp for eight hours per day , for 14 days according to the suggested wear time of the manufacturer .
samples in group 3 were bleached with opalescence pf ( ultradent product , south jordan , ut , usa ) 20% cp for two hours per day for a total of 14 days . in groups 2 and 3 ,
prior to each bleaching procedure , the samples were taken out from the distilled water bath , air - dried with an air jet spray for 60 seconds .
the bleaching agent was applied on one surface of the sample using a microbrush ( kerr corporation , california , usa ) and left for the duration suggested by the manufacturer . after each bleaching procedure
, the samples were washed with a water jet spray for 60 seconds before they were stored back in distilled water bath and ready for the next bleaching procedure the next day .
all samples were subjected to surface roughness testing using atomic force microscopy ( afm ) ( ambios technology , california , usa ) .
five different areas were selected randomly with a scan area of 40  40 m and resolution 512  512 pixels to obtain surface roughness values ( ra ) .
then , three - dimensional ( 3d ) images with 10  10 m sizes were acquired for each group of materials .
after surface roughness testing , all samples were subjected to hardness testing using a vickers hardness tester , model vm50 ( fie , maharashtra state , india ) .
the samples were placed underneath the indenter and 1 kg load was applied for a dwell time of 15 seconds .
every sample was indented three times at three different points , and the mean readings were recorded .
all statistical analysis was conducted at a significance level of p < 0.05 using mann - whitney test and kruskal - wallis test .
all statistical analysis was conducted at a significance level of p < 0.05 using mann - whitney test and kruskal - wallis test .
the results of the statistical analysis using kruskal - wallis test are presented in tables 2 and 3 .
table 4 reveals the result of mann - whitney test analysis of kelfil and table 5 is the result of mann - whitney test of tph 3 .
median roughness number ( ra , nm ) and interquartile range of the three tested composite resins after 14 days of bleaching with 10 and 20% cp median vickers hardness number and interquartile range of the three tested composite resins after 14 days of bleaching with 10 and 20% cp median vickers hardness number and interquartile range for kelfil after 14 days of bleaching with 10 and 20% cp median vickers hardness number and interquartile range for tph 3 after 14 days of bleaching with 10 and 20% cp table 2 shows the roughness number ( ra ) of the three types of composites that were bleached with 10 and 20% cp home bleaching agents .
there was no significant change in roughness in all three composites after 14 days of bleaching with 10 and 20% cp compared to the control group .
table 3 illustrates the results of all the samples that were subjected to the vickers hardness test .
kelfil showed significant change in hardness with a p value of 0.001 and there was also a significant change in tph 3 with a p value of 0.021 .
table 4 shows that there was a significant change in kelfil after 14 days of bleaching .
there was a significant increase in the surface hardness after bleaching with 10% cp home bleaching agent .
however , there was a significant decrease after bleaching with 20% cp home bleaching agent .
there was also a significant decrease in the surface hardness of kelfil after bleaching with 20% cp when compared to the group of samples bleached with 10% cp .
table 5 reveals a significant change in the surface hardness of tph 3 after being subjected to 14 days of bleaching .
there was a significant decrease in the surface hardness in the groups bleached with 10 and 20% cp compared to the control group .
however , there was no significant change when the group was bleached with 10% cp compared to the group bleached with 20% cp .
in this study , filtek z350 , a commercialized and widely used nanocomposite did not show any significant changes in surface hardness when compared to its control group , and also to groups of samples bleached with either 10 or 20% cp .
however , there were significant changes in the vickers hardness number for both kelfil and tph 3 that had been subjected to 14 days of bleaching with either 10 or 20% cp .
for kelfil , there was a significant increase in hardness after bleaching for 14 days with 10% cp but a significant decrease in hardness with 20% cp .
after the curing process of a composite resin , a postpolymerization process continues to occur up to a certain period of time , which increases the hardness of the composite . in this study , the bleaching process was possibly being carried out at the same time as the postpolymerization process .
the postpolymerization process in kelfil was observed with the 10% cp , and it was more prominent than the bleaching process ; hence the increase in its surface hardness .
. nevertheless , with 20% cp , the bleaching process was more dominant than the postpolymerization process in all the three composites tested .
the increase in hardness with 10% cp was observed in both filtek z350 and kelfil , and it maybe due to the similarities in their filler structure . in tph 3 , there was a significant decrease when comparison was made between the control group and 10% cp group , and also between the control group and 20% cp .
this significant decrease is consistent with the conclusion drawn from other studies . as a general rule , the higher the filler loading , the greater the physical properties of the composite resins .
it has a lower filler loading of 29% by volume compared to filtek z350 with the filler loading of 78.5% by weight or 59.5% by volume which is suitable for anterior and also posterior composite resin restorations .
the lower filler loading of kelfil may lead to loosely packed inorganic fillers , which in turn caused the significant decrease in the vickers hardness number .
for tph 3 , there was a significant decrease in vickers hardness number in samples that were subjected to 14 days of bleaching with both 10 and 20% cp .
it was concluded that nanohybrid resins generally presented inferior properties compared to the nanofilled composite .
tph 3 has 58% volume filler loading , which is similar to filtek z350 . the possible explanation for the significant reduction in the surface hardness might be due to the presence of bis - gma monomer .
resin composites are reported to be highly susceptible to chemical softening due to the presence of bis - gma monomer if the chemicals have a solubility parameter ranging from 1.82  10 to 2.97  10 ( j / m ) . in the case of tph 3
, the content of bis - gma in its matrix might be higher than in filtek z350 .
another possible explanation is the degree to which the filler is bonded to the resin matrix .
although there was a significant decrease in the vickers hardness numbers of kelfil and tph 3 , there were no significant changes in the roughness number in all specimens after 14 days of bleaching with either 10 or 20% cp opalescence pf home bleaching agents .
this is consistent with the conclusion that no statistically significant differences were detected for any material .
the cantilever sensor of afm senses any irregularities at the surface of the specimen ; however , as all the composites tested have similar nano - sized fillers , approximately 20 nm , there was no significant difference between them .
furthermore , all samples were polished with the same polishing system , sof - lex disks which may contribute to a similar surface finish in the composite resins even after bleaching treatment .
the 3d images of afm [ figures 13 ] show that there was no plucking out of filler particles after the bleaching treatment .
afm topography 40  40 m of z350 after bleaching with 20% cp afm topography 40  40 m of kelfil after bleaching with 20% cp afm topography 40  40 m of tph 3 after bleaching with 20% cp generally , the surface roughness of all composites tested has a reading below 0.2 m or 200 nm .
ra above 0.2 m results in an increase in the accumulation of plaque and a higher risk for caries and periodontal inflammation . according to chung in 1994 , when ra was lower than 1m ,
therefore , all composites surfaces evaluated after bleaching treatment have demonstrated a smooth surface , which from the clinical point of view , present no risk of accumulation of plaque . from this study
, it can be inferred that following bleaching , the experimental anterior composite and nanohybrid composite may need to be replaced , due to the reduction in their surface hardness .
however , this has to be further investigated with in vitro studies evaluating the effects of saliva , and controlled clinical trials are necessary to determine any clinical implication .
based on the result of this study , it can be concluded that 14 days of bleaching with 10 or 20% cp home bleaching agents did not cause any changes in surface roughness of the three tested composites .
the afm evaluation of surface roughness showed that both bleaching agents yielded ra values below 200 nm for all materials tested , which poses no risk of the accumulation of plaque .
the performance of kelfil after bleaching treatment is similar to that of the commercial nanocomposites .
the afm evaluation of surface roughness observed in the 3d images promises to be an effective technique .
nanocomposite and nanohybrid composite resins bleached with 10 or 20% cp bleaching agents result in the same outcome . on the other hand ,
 objective : home bleaching agents may not be safe for composite resins . the purpose of this study was to evaluate the effects of 10 and 20% opalescence pf home bleaching agents on the surface roughness and hardness of universal nanocomposite ( filtek z350 ) , anterior nanocomposite ( kelfil ) , and nanohybrid composite 
 ( tph 3).materials and methods : fifty - four composite resin samples with 18 samples for each type of composite resin were prepared using acrylic molds ( 4  2 mm ) . 
 each type of composite resin was further divided into three groups [ n = 6 controls were placed in distilled water for 14 days and the other two groups of n = 6 were bleached with 10 and 20% carbamide peroxide ( cp ) , respectively for 14 days ] . 
 surface hardness of the composite resin was tested with a vickers hardness tester , whereas surface roughness was tested with atomic force microscopy ( afm).results : there were significant changes in the surface hardness of kelfil and tph 3 
 . however , all the tested materials showed no significant changes in the surface roughness.conclusion:after 14 days of home bleaching treatment , there was no adverse effect on the surface roughness of all three composite resins , although the surface hardness for kelfil and tph 3 were significantly reduced . 
INTRODUCTION
MATERIALS AND METHODS
Statistical analysis
RESULTS
DISCUSSION
CONCLUSIONS