this technique offers an effective treatment which is rather easy , safe , and cost - effective [ 2 , 3 ] . however , it has some possible side effects   such as hypersensitivity , soft tissue burning sensation , and recurrent staining of tooth surface in short term . the employed concentrations of carbamide peroxide in home bleaching do not appear to cause any macroscopic changes in surface enamel . however , microscopic alterations such as increased surface roughness and formation of shallow erosions have been reported [ 2 , 6 ] . these changes in the surface topography are associated with sacrificing the color and glossy appearance of the enamel . the defects mentioned above are the result of a shift in the composition of bleached enamel   leading to reduction of calcium , phosphate , and fluoride contents . since this damage leads to more staining susceptibility after vital bleaching [ 10 , 11 ] , it might be possible to compensate for this problem by employing mineralizing agents . accordingly , it has been documented that if the enamel surface is recovered with fluoride , casein phosphopeptide - amorphous calcium phosphate ( cpp - acp ) , or hydroxyapatite ( hap ) after bleaching , the microstructural defects might be repaired . therefore , the bleaching effect would last longer and staining would be prevented [ 1315 ] . furthermore , it has been discovered that , by reducing the particle size down to nanorange , the remineralization process would be amplified . indeed , due to the increased surface to volume ratio and proportion of atomicity , the interaction as well as the adhesion of nanoparticles with tooth structure would be improved [ 10 , 16 ] . on this ground , in recent applications of hap for biomimetic repair of damaged enamel , it has been confirmed that the nano - hap ( n - hap ) would lead to a considerably superior remineralization [ 10 , 17 ] . on the other hand , tricalcium phosphate ( tcp ) this complex consists of some structural sites that can be activated by various organic molecules , leading to very good remineralization [ 18 , 19 ] . tcp has a specific form that overcomes the limited bioavailability of other insoluble calcium phosphates for the remineralization process . application of bioactive glass or n - hap in conjunction with carbamide peroxide bleaching has been investigated in very few recent studies [ 7 , 21 ] . some of them reported that these complexes do not affect tooth whitening efficacy   while others concluded that it would prevent restaining after dental bleaching . it should be noted , however , that the available literature on this object especially comparing the effect of nanoparticles on tooth color stability is extremely scarce . therefore the aim of this study was to evaluate the effect of nano - tcp ( n - tcp ) and n - hap to prevent restaining of enamel surface after dental bleaching . rod - like hydroxyapatite particles ( diameter < 100  nm , aspect ratio 2 - 3 ) were purchased from nanoshel co. ( panchkula , india ) . meanwhile , plate - like -tcp nanoparticles ( diameter ~ 100  nm ) were synthesized in a previous study . forty caries - free bovine incisors were selected and after cleaning with aqueous slurry of pumice , they were stored in 1% thymol solution . subsequently , a plastic cover was fabricated for the crowns of the teeth using a vacuum forming machine . finally , a digital photograph was taken in a standard method for determining the color of each tooth . all the digital photographs were taken under a standard condition in a complete dark chamber while the distance of the camera ( canon eos d40 ) was fixed . in order to work in a constant environment , the background was black while the samples were put in a silicon box . for exposure metering , a circular punch of the gray card with a reflectance value of 18% was put near each sample and the same manual exposure mode was selected for the whole samples . carbamide peroxide 20% gel ( opalescence , ultradent , usa ) was inserted in each plastic cover and the crowns of the teeth were exposed to one daily application of the bleaching agents for two hours for fourteen consecutive days . finally , another digital photograph was taken for recording the color . in order to mimic the oral condition during bleaching , after each daily treatment , the specimens were thoroughly rinsed with air / water spray and stored in distilled water until the next day . following the bleaching process five 30  100 mm glass boxes were prepared and the specimens were immersed in the boxes as follows.the control group specimens ( the first group ) were daily immersed for 10 minutes in a standard tea solution ( boiling 1  gr of tea in 100  ml of water for 2 minutes and then passing the solution through gauze in order to remove the tea leaves ) ,   fixed in air for another 10 minutes , and rinsed , respectively . then they were stored in distilled water until the next day.the second group specimens were daily immersed in 10% tcp for 3 minutes prior to receiving the treatments similar to the control group.the third group specimens were daily immersed in 5% tcp for 3 minutes prior to receiving the treatments similar to the control group.the fourth group specimens were daily immersed in 10% hap for 3 minutes prior to receiving the treatments similar to the control group.the fifth group specimens were daily immersed in 5% hap prior to receiving the treatments similar to the control group.all these treatments were repeated for five days and finally a new photograph was taken for recording the color . the control group specimens ( the first group ) were daily immersed for 10 minutes in a standard tea solution ( boiling 1  gr of tea in 100  ml of water for 2 minutes and then passing the solution through gauze in order to remove the tea leaves ) ,   fixed in air for another 10 minutes , and rinsed , respectively . the second group specimens were daily immersed in 10% tcp for 3 minutes prior to receiving the treatments similar to the control group . the third group specimens were daily immersed in 5% tcp for 3 minutes prior to receiving the treatments similar to the control group . the fourth group specimens were daily immersed in 10% hap for 3 minutes prior to receiving the treatments similar to the control group . the fifth group specimens were daily immersed in 5% hap prior to receiving the treatments similar to the control group . first , the global color cast of the images was eliminated according to the piece of gray card in the pictures . in order to compare the color , we incorporated the commission internationale de l'eclairage ( cie ) system in the form of l , a , and b obtained by the software . in the cie system , l  characterizes the lightness and can range from 0 ( dark ) to 100 ( light ) . the value of  a  represents the red ( + ) green (  ) spectrum and  b  represents the yellow ( + ) blue (  ) spectrum . subsequently , l , a , and b values of the selected area were metered and the histogram information was obtained . the photoshop l , a , and b values were transformed into the cie l , a , and b values using the following formulas : ( 1)l=l100250,a=a128240250,b=b128240250.ultimately , e that represents the total color difference in cie system was calculated as(2)e=l2a2b21/2 . one sample of each group was observed to evaluate the changes in the surface morphology after five days of treatment . the samples were mounted on the aluminum stub using carbon - coated double sided adhesive tape and then coated with gold using a sputter coater . subsequently , the superficial microstructure of the specimens was analyzed using scanning electron microscopy ( sem ) ( tescan , vegaii , xmu , czech republic ) . after exploring the normal distribution , wilcoxon test was used to compare the baseline color parameters with the results of after bleaching . meanwhile , the data regarding the specimens receiving mineralizing agents were analyzed using one - way anova and post hoc bonferroni test . color parameters of the teeth before and after bleaching ( l , a , and b values ) are demonstrated in figure 1 . the baseline l , a , and b values underwent significant changes after bleaching ( p < 0.001 ) . therefore , it was revealed that the bleaching process had significantly improved the enamel color of the examined teeth . as it is summarized in table 1 , after five days of restaining regimens , there were no significant differences between l , a , and b values of different groups ( p > 0.05 ) . however , e of 10% tcp was significantly lower than the control group ( p = 0.02 ) while there were no significant differences between the other groups , indicating that immersing in 10% tcp solution significantly compensated for the demineralization effect of bleaching process and leading to longer stability of tooth color . moreover the representative images of experimental and control groups are demonstrated in figure 2 . as figure 3 illustrates , numerous porosities were observed on the surface of the control and the 5% hap groups . in contrast , all the samples treated by other remineralizing agents ( 10% tcp , 5% tcp , and 10% hap ) had considerably smoother surfaces compared to the control . it could be concluded that application of all the studied remineralizing agents , except for the 5% hap solution , led to an observable remineralization and smoothening of the enamel surface . the results of the current study showed that application of n - hap or n - tcp on tooth surface after bleaching protocol would decrease the restaining of enamel . the results of the present study are consistent with previous reports incorporating other agents such as fluoride , nanocarbonate apatite , or ccp - acp after bleaching [ 10 , 15 ] . accordingly , it has been documented that n - hap   and nanocarbonate apatite   penetrate into the intercrystalline spaces and rod sheaths [ 25 , 26 ] . therefore , these nanoparticles enhance the superficial enamel smoothness and block up surface defects . resembling our study , pedreira de freitas et al . compared the effect of 2% neutral sodium fluoride and nano - hap after bleaching treatment and reported that the surface gloss increased only in the nano - hap group . since the scattering or reflection of the light strongly depends on the surface texture , their investigation could suggest that n - hap noticeably recovered the surface irregularities caused by bleaching which consequently prevented restaining . their finding is also in agreement with singh et al . who reported that the restaining of bleached teeth would be prevented by applying fluoride or cpp - acp after bleaching . one of the most interesting outcomes of the current research was the stronger effect of n - tcp compared to n - hap . this finding is in agreement with researchers who studied the soluble compound of calcium and phosphate ( amorphous acp ) . they reported that amorphous acp is more soluble and is more similar to bone or tooth structure compared to crystalline hap . therefore , acp dissolves readily in the oral cavity and redeposits on the damaged enamel . similar to acp , the water solubility of tcp is higher than hap which is quite effective for remineralization . for bone regeneration , the highly crystalline hap particles are classified as nonresorbable materials while tcp is resorbable . therefore , the higher solubility of n - tcp could be the reason for its stronger effect compared to n - hap . this hypothesis has been confirmed by previous publication in which the calcium component of resorbable calcium phosphate materials was introduced as a major factor for local mineralization and also the surrounding calcium pool . however , our results demonstrated a dose - dependent effect for n - tcp because it was not meaningfully effective at 5% but a significant restaining inhibition occurred at 10% . similarly , in published literatures , dose dependency has been reported frequently for remineralizing agents [ 29 , 30 ] . it has been documented that , as the calcium content in treatment solution increase , the remineralization would increase too [ 29 , 30 ] . moreover , the sem micrographs showed noticeably smoother surface in the groups receiving remineralizing treatment ( except for n - hap 5% ) compared to the control . accordingly , it has been reported that n - hap may be deposited onto the enamel surface [ 31 , 32 ] . our sem micrographs demonstrated that many surface defects were produced on the enamel surface as a result of bleaching process , while the nanoparticles reconstructed the surface topography . in a similar study , gjorgievska and nicholson applied toothpaste containing bioactive glass on the bleached enamel surface . their sem micrographs also represented changes in enamel surface morphology after the bleaching procedure , whereas , by incorporation of the toothpaste , the irregularities were repaired . although our sem micrograph showed signs of remineralization in n - hap groups , our color analysis did not demonstrate significant restaining prevention for n - hap groups . this finding is consistent with some investigators while it is against some other ones [ 7 , 33 ] . surveyed bleached enamel and claimed significant increase in surface gloss after polishing with n - hap . the results of this study revealed that restaining by tea solution was strongly prevented by 10% n - tcp . although in previous researches different solutions were used for restaining , it has been indicated that , compared to coffee or chlorhexidine , teeth have higher susceptibility to staining by tea . overwhelmingly , as it is shown in table 1 , the results of this study showed that all the treatment solutions decreased the color change compared to the control . therefore , it can be concluded that some of nanoparticle which has a tooth - like structure would be beneficial for longer efficacy of the bleaching protocol . after bleaching , all the experimental solutions prevented the restaining of enamel to some extent . however , only the 10% n - tcp could significantly maintain the resultant color compared to the control , indicating the recovery of the damaged enamel surface by the calcium phosphate compound .
objective . 
 this study was designed to evaluate the effect of nano - tricalcium phosphate ( n - tcp ) and nanohydroxyapatite ( n - hap ) on prevention of restaining of enamel after dental bleaching . 
 methods . 
 forty bovine incisors were bleached with 20% carbamide peroxide for two weeks . 
 afterward , they were divided into five groups based on remineralization solution : no treatment ( control ) , 10% n - tcp , 5% 
 n - tcp , 10% n - hap , and 5% n - hap . 
 each group was daily immersed for 10 minutes in the restaining solution ( tea ) and for 3 minutes in the remineralization agent , respectively . 
 this protocol was repeated for five days . 
 subsequently , three digital photographs ( baseline , after bleaching , and after restaining ) were analyzed by adobe photoshop software . 
 the obtained l , a , b , and e parameters were compared using anova and wilcoxon and bonferroni tests . 
 results . 
 after bleaching , there were significant changes in tooth colors ( p < 0.001 ) while , after restaining and immersion in remineralization solutions , there were no significant differences in l , a , and b values of different groups ( p > 0.05 ) . 
 however , e of 10% tcp was significantly lower than the control ( p = 0.02 ) while there were no significant differences between the other groups ( p > 0.05 ) . 
 conclusion . 
 10% n - tcp could significantly maintain the resultant color and reconstruct the enamel structure after bleaching .