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I 25 research _ bleaching I cosmeticdentistry 2_2011 Figs. 6a & b_ACP control area 46 µm subsurface (a). ACP-treated 46 µm subsurface (b). the intercrystalline spaces would be easier for per- oxide, since the Dextran dye we used in our study has a high molecular weight of 3,000 to 70,000, while that of a hydroxyl radical is 17. Our study did not find any evidence to support the claim that the addition of ACP reduces tooth sensitivity by decreasing the size of the pores. Another study looking at the simultaneous appli- cation of admixture solutions containing citric acid and sodium fluoride did find formation of CaF2 globulesdepositedontheenamelsurface.However, the CaF2 globules deposited on the enamel surface appeared to be unable to prevent the alteration of the apatite structure during further exposure to acidic agents. No evidence of fluoride-induced re- covery of the modified apatite structure was found. As for whether ACP can repair the pores simultane- ously, previous studies of the repair of etched enamel reported that partial recovery from such damage takes several months invivo.24 It is unlikely that our study duration of 14 days could have offered noticeable repair. Hypothetically, ACP could reduce the effect of sensitivity if the pore sizes were reduced. However, such reduction would also prevent efficient dif- fusion of hydroxyl radicals to effect bleaching. Instead,ACPcouldactviadifferentmechanisms,for examplebyinfluencingthetypeofanionorradicals formed. Hydrogen peroxide can form a number of different active oxygen species, depending on re- action conditions, including temperature, pH, light andpresenceoftransitionmetals.Whetheronetype of anion or radical is less prone to causing sensi- tivity deserves further study. It has also been suggested that ACP may have the ability to directly depolarise nerve endings.25 Additionally, in the generation of ACP by combin- ing calcium nitrate and potassium phosphate, 0.25 % of potassium nitrate is generated.26 A re- cent study on traditional low sensitivity whiteners raised the question of whether the low level of potassium nitrate generated as a by-product was, in fact, clinically relevant.27 Clinical studies have shown that ACP was effective in reducing sensitivity.16,25 However, a recent in vivo study of 9 % H2O2 whitening strips with fifty subjects indicated that daily use of a Casein phosphopeptide-amorphous calcium phosphate paste in conjunction with tooth whitening has a minimal effect on tooth sensitiv- itycomparedwithaplacebopaste.Ourstudyresult raised the question of whether ACP might exert its desensitising effect through a mechanism other than decreasing the size of subsurface pores. It is recognised that our study was limited by its small sample size and we only evaluated the enamelsubstrate.Severesensitivitycanbeinitiated through exposed dentine substrate. Another criti- cism may stem from the argument that cracks, which were shown in this study, may have been induced by extraction forces. Further studies need to be performed to explain the mechanism of ACP desensitisation better. _Conclusion Uptake of dye through enamel intercrystalline spaces exists in both control and bleached enamel surfaces. Whitening products with and without ACP did not appear to decrease the size of these spaces. Subclinical enamel defects/cracks may be the cause of severe bleaching sensitivity. _Acknowledgement We would like to thank Dr Katsuya Miyake and Mr Darren Baker, Cell Imaging Core Laboratory at Georgia Health Sciences University, for their help with the confocal microscopy. This project was funded by a grant from Ultradent._ Editorial note: A complete list of references is available fromthepublisher. Dr Daniel C.N.Chan University ofWashington School of Dentistry Office of Clinical Services Health Sciences Center,D322 P.O.Box 356365 Seattle,WA 98195,USA dcnchan@u.washington.edu cosmeticdentistry _contact Fig. 6a Fig. 6b