enamel microabrasion is a conservative approach in order to improve surface properties , remove superficial discolorations and eliminate enamel decalcification [ 
 1 
 ] .
this procedure is used only for stains or defects that are no deeper than few tenths of a millimeter in enamel [ 
 2 
 ] .
mccloskey [ 
 3 
 ] first introduced this technique using 18% hydrochloric acid to remove superficial tooth structure in teeth affected by fluorosis .
later , croll and cavanaugh [ 
 4 
 ] recommended a technique in which a paste composed of hydrochloric acid ( hcl ) in combination with pumice was applied to the affected area with a tongue blade .
this technique was further modified by croll [ 
 5 
 ] , replacing the pumice paste with silicon carbide particles and reducing the concentration of hcl to 11% .
the combined effects of acidic solution and abrasive particles erode the enamel surface [ 
 6 
 ] . today ,
commercially available products provide improved and safer means for microabrasion [ 
 2 
 ] .
microabrasion eliminates the outer layers of enamel leaving a smooth , glossy enamel surface ; although microscopic roughness may be seen on the enamel due to acidic nature of the blend [ 
 1 
 , 
 7 
 ] .
ulukapi reported a significant reduction in enamel microhardness following microabrasion , which continued for two days and could increase the risk of enamel erosion during this period [ 
 7 
 ] . in order to enhance remineralization after microabrasion
, it is recommended to polish treated areas with fluoride prophylactic pastes and to apply topical fluoride on the tooth surface [ 
 7 
 , 
 8 
 ] .
even though fluoride can promote remineralization , the rate depends on ph of the solutions , fluoride concentration , type of fluoride salt and presence of mineral ions in the saliva [ 
 9 
 ] . applying a neutral sodium fluoride gel on enamel for four minutes after microabrasion has been suggested in a previous study [ 
 7 
 ] .
the cpp  acp nanocomplex derived from milk protein was introduced as a supplemental source of calcium phosphate ions [ 
 10 
 , 
 11 
 ] .
these nanocomplexes act as calcium and phosphate reservoirs and keep supersaturated levels of these ions in the oral cavity when applied on the tooth surface [ 
 11 
 ] . also , cpp  acp has been shown to enhance remineralization and increase the microhardness of eroded enamel [ 
 10 
 , 
 11 
 ] .
mathias et al . reported a reduction in surface roughness of micro - abraded enamel after a combination of microabrasion and cpp - acp application [ 
 1 
 ] .
a new product ( mi paste plus ) containing both cpp - acp and fluoride ( cpp - acpf ) was recently introduced .
the level of fluoride in this paste is 900 ppm , which is fairly close to that of adult toothpastes . the combination of cppacp and fluoride probably results in co - localization of calcium and phosphate ions with fluoride ions on the enamel surface and reforms the apatite crystals [ 
 12 
  
 14 
 ] .
surface microhardness measurement is a suitable technique that allows uncomplicated and nondestructive evaluation of demineralization and remineralization [ 
 15 
 ] .
therefore , the purpose of this study was to assess the effect of applying cpp - acp paste , cpp - acpf paste and sodium fluoride gel on surface microhardness of enamel after microabrasion .
in this study , 30 freshly extracted human premolars , without cracks or erosion , were selected and kept in 0.1% thymol solution for one week .
the teeth were then cleaned with water and embedded horizontally in self - cure acrylic resin so that the labial surfaces were exposed and parallel to the applied force during the microhardness test .
surface of the samples was ground successively with polishing disks ( soflex polishing disks , 3 m espe , st .
paul , mn , usa ) and diamond polishing paste ( diamond excel , diamond excel , fgm , joinville , sc , brazil ) with a particle size of 24  in order to obtain a smooth flat area of 3 mm  3 mm .
all samples were subjected to hardness indentations made with the vickers hardness machine ( mhz , koopacompany , mashhad , iran ) using a 500 g load and a dwell time of 10 seconds ( 

 fig . 1 

 ) .
three indentations were made for each sample and the average value was recorded as the initial surface microhardness .
the specimens were then randomly divided into three groups , with 10 samples per group . after drying the samples by mild flow of air , microabrasion paste ( opalustre , ultradent products inc . ,
south jordan , usa ) containing 6.6% hydrochloric acid and silicon carbide nanoparticles ( table 1 ) was applied on all samples at one millimeter thickness .
jordan , ut , usa ) latch - type bristle polishing cups and handpiece at a low speed for 60 seconds .
after microabrasion , the paste was removed from the surface and the samples were cleaned under running water .
microhardness test was performed with the same technique mentioned before to assess the post - abrasion hardness .
group 1 : the specimens were treated with 2% neutral ph sodium fluoride gel ( master - dent , dentonics , inc . , monroe , nc , usa ) for four minutes and were then rinsed .
indentation made with the vickers hardness machine composition and manufacturers of the materials used in this study group 2 : all the samples in this group were treated with mi paste ( gc corporation , tokyo , japan ) containing cpp - acp .
the paste was applied directly on the labial surface of the tooth for four minutes , and then washed .
group 3 : in this group , after microabrasion , mi pated plus ( gc corporation , tokyo , japan ) containing cpp - acpf was used similar to the second group . to standardize the amount of remineralizing agents applied to all samples , 0.3 ml of each was measured by a measuring spoon .
after the remineralization process was completed , the specimens were washed with water and blot dried .
all 30 specimens were subjected to microhardness test as mentioned earlier . in order to compare demineralization and remineralization between groups , the formulas mentioned below
were used to measure the softening and rehardening values [ 
 7 
 ] . softening value = b / a 100 rehardening value = c / b 100 a : initial microhardness b : microhardness after microabrasion c : microhardness after remineralization statistical analysis was performed with spss software ( version 22 ) .
one - way repeated measures anova was used to compare the surface microhardness and softening and rehardening values at baseline , after microabrasion and after remineralization with significance set at p<0.05 .
the post - hoc tukey s test was used to find significant differences between the means when anova result was significant .
the mmv at baseline , after microabrasion and after remineralization are shown in table 2 and 

 fig . 2 

 .
the baseline mmv (  standard deviation ; sd ) was 353.535 vickers hardness number ( vhn ) .
the mean standard deviation ( sd ) of surface microhardness value at baseline , after microabrasion and after remineralization ( n=30 ) different upper - case letters represent statistical significance within the row , different lower - case letters show statistical difference in the column .
after microabrasion with opalustre , the mmv ( sd ) was 302.5 48.2 vhn that shows a remarkable decrease from baseline .
this decrease was significant in all study groups . softening and rehardening values for each study group
the softening of fluoride group was significantly more than cpp - acp and cpp - acpf groups ( p<0.001 ) .
softening and rehardenig values ( % ) in the study groups after remineralization , the mmv increased significantly compared to the values after microabrasion in all groups .
the mmv of the group treated with cpp - acpf was significantly more than that of fluoride group ( p=0.027 ) . however , the difference between cpp - acpf and cpp - acp groups and the difference between fluoride group and cppacp group was not significant .
the rehardening value of fluoride group was significantly more than that of other groups ( p<0.001 ) .
in this study , the effect of microabrasion on enamel microhardness and the effect of different remineralizing agents on micro - abraded enamel were assessed . in the current study ,
the mean vhn of samples at baseline was 353.5 35 , which meets the vhn range of normal enamel [ 
 15 
 ] . according to the results obtained , enamel microabrasion with 6.6% hcl and nanoparticles of silicon carbide caused a reduction in microhardness immediately after treatment in all study groups .
this finding is in accordance with that of other studies [ 
 6 
 , 
 7 
 , 
 16 
 ] .
reported that microabrasion with hcl significantly removed the superficial enamel layer and caused mineral loss from the enamel [ 
 6 
 ] .
different amounts of enamel loss have been reported in previous studies subsequent to microabrasion [ 
 16 
  
 19 
 ] .
several parameters such as the amount of acid used , its concentration and ph , abrasive medium , time of instrumentation and the application force can affect the amount of enamel erosion during microabrasion treatment [ 
 16 
 ] .
in the current study , softening value in the fluoride group was significantly higher than in the other two groups .
this may be due to the difference in the amount of force applied . in our study , the groups were matched in terms of the acid amount and the time of instrumentation ; however the force applied was uncontrollable .
it has been shown that increased pressure results in higher substance loss . utilizing a load cell could have distributed the load equally between the samples [ 
 17 
 ] . according to the outcomes , all the remineralizing agents used in this study were able to re - harden the enamel surface after microabrasion .
our findings are consistent with the findings of many studies demonstrating remineralization effect of fluoride , cpp - acp and cpp - acpf after erosive procedures [ 
 9 
 , 
 10 
 , 
 12 
  
 14 
 ] .
the mean rehardening value in the group treated with neutral sodium fluoride was noticeably higher than that in the other groups , which means that the neutral sodium fluoride was the most effective remineralizing agent .
many studies showed convincing evidence for the preventive effect of fluoride by promoting remineralization [ 
 13 
 , 
 14 
 , 
 20 
 ] .
one of the protective mechanisms of fluoride is formation of a calcium fluoride ( caf2)-like layer on the tooth surface enhancing reconstruction of minerals such as fluorapatite or fluorohydroxyapatite on the surface of enamel crystals [ 
 21 
 ] .
lata et al . studied the remineralization potential of fluoride and cpp - acp for enamel lesions .
similar to the current study , they found that fluoride was more effective in remineralization although cpp - acp had the ability to re - harden the enamel [ 
 15 
 ] .
found no difference in the remineralizing potential of fluoride toothpaste , cpp - acp and cpp - acpf dental products .
this may be due to higher concentration of fluoride in our study ( 9000 ppm ) compared to fluoride toothpaste ( 1000 ppm ) in their study [ 
 14 
 ] .
the cpp - acp paste caused an increase in enamel microhardness after microabrasion with opalustre .
the remineralization effect of cppacp paste involves the incorporation of cpp part of this complex to the enamel and biofilm , in order to deliver the calcium and phosphate ions to the enamel rods and reconstruct the apatite crystals [ 
 12 
 ] .
our findings are similar to those of tantbirojn et al , who showed that softened enamel was significantly hardened after being treated with cpp - acp paste [ 
 10 
 ] . in cpp - acpf treatment group ,
it is speculated that the difference in the percentage of microhardness recovery is caused by the fluoride ions used in cpp - acpf paste .
our findings are consistent with those of rirattanpong et al who reported no additional effect for use of cpp - acp plus fluoride ions in comparison with using cpp - acp alone [ 
 13 
 , 
 14 
 ] .
contrary to our results , the synergistic effect of cpp - acp and fluoride and greater remineralizing potential of this compound for eroded enamel were confirmed in the study by srinivasan et al [ 
 22 
 ] .
future clinical studies are recommended since remineralization in vitro may be quite different from what usually occurs in vivo .
within the limits of the present study , it can be concluded that all the remineralizing agents used were effective in rehardening enamel after microabrasion .
the cpp - acp and cpp - acpf pastes are effective , but to a lesser extent than neutral sodium fluoride gel for remineralizing the enamel surface .
incorporation of fluoride in the cpp - acpf formulation does not provide any additional remineralizing potential when compared to cpp - acp paste .
 objectives : this study aimed to assess the effect of applying casein phosphopeptide amorphous calcium phosphate ( cpp - acp ) paste , casein phosphopeptide  amorphous calcium phosphate fluoride ( cpp - acpf ) paste and sodium fluoride gel on surface microhardness of enamel after microabrasion.materials and methods : thirty freshly extracted human premolars were selected . 
 all samples were subjected to hardness indentations made with the vickers hardness machine and the average value was recorded as the initial surface microhardness . 
 the specimens were then randomly divided into three groups ( n=10 ) of cpp - acpf , fluoride and cppacp . 
 the teeth were micro - abraded with opalustre . 
 microhardness test was performed to assess the post - abrasion hardness . 
 three remineralization modalities were performed on samples of each group . 
 the enamel surface microhardness measurements were performed . to compare the difference between groups , the rehardening and softening values were defined . 
 one - way anova and tukey s post hoc test at a significance level of 5% were used for statistical analysis.results:the mean microhardness value ( mmv ) had a significant decrease after microabrasion from baseline . 
 the mmv had a significant increase after remineralization in all groups . 
 the mmv of cpp - acpf group was significantly more than that of fluoride group ( p=0.027 ) . 
 the rehardening value of fluoride group was significantly more than that of other groups ( p<0.001).conclusion : all the remineralizing agents were effective for rehardening the enamel after microabrasion . 
 the cpp - acp and cpp - acpf pastes are effective , but to a lesser extent than neutral sodium fluoride gel in remineralizing enamel surface . 
 incorporation of fluoride to cpp - acp formulation does not provide any additional remineralizing potential . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION