Mechanical properties of in situ demineralised human enamel measured by AFM nanoindentation

Diet-induced demineralisation is one of the key factors in surface changes of tooth enamel, with soft drinks being a significant etiological agent. Th e first step in this dissolution process is characterised by a change in th e mechanical properties of the enamel and a roughening of the surface. The objective of this pilot study was to measure early stages of in situ induce d hardness changes of polished human enamel surfaces with high accuracy usi ng a nanoindenter attached to an atomic force microscope (AFM). Human unerupted third molars were cleaned. sterilised with sodium hypochlor ite, sectioned and embedded in epoxy resin. The outer enamel surface was po lished and the samples partly covered with a tape, allowing a 2-mm-wide zon e to be exposed to the oral environment. Samples were fitted in an intra-or al appliance, which was worn from 9 a.m. to 5 p.m. for one day. During this time the volunteer sipped 250 ml of a drink over 10 min periods at 9.00, 1 1.00, 13.00 and 15.00 h. Three different drinks, mineral water, orange juic e and the prototype of a blackcurrant drink with low demineralisation poten tial were used in this study. At the end of the experiment the samples were detached from the appliance, the tape removed and the surfaces chemically cleaned. The surface hardness and reduced Young's modulus of the exposed an d unexposed areas of each sample were determined. In addition, high resolut ion topographical AFM images were obtained. This study shows that by determining the hardness and reduced Young's modul us, the difference in demineralisation caused by the drinks can be detected and quantified before statistically significant changes in surface topogra phy could be observed with the AFM. The maximum decrease in surface hardnes s and Young's modulus occurred in the samples exposed to orange juice, foll owed by those exposed to the blackcurrant drink, while exposure to water le d to the same values as unexposed areas. A one-way ANOVA showed a statistic ally significant difference between the changes in hardness for the drinks at a 95% confidence level (p = 0.0000), while a Kruskal-Wallis test proved a statistically significant difference between the changes in the reduced Y oung's modulus at a 95% confidence level (p = 0.0000). Thus, it was possibl e to detect differences in demineralisation potential in an in situ study a t a very early stage. Further investigations with greater subject numbers a nd a larger quantity of samples are necessary to fully evaluate the potenti al of this method. (C) 2001 Elsevier Science B.V. All rights reserved.