Enamel Erosion: A Time-Resolved in situ AFM study

Objective: The primary objective of this work was to explore the time dependant nature of enamel erosion using ‘in situ time-resolved' AFM. A secondary objective was to explore the influence of pH and fluoride on the erosion process. Methods: Enamel specimens were cut from the buccal sides of bovine incisors and polished using standard procedures. A Nanoscope-IV AFM operating in contact mode was employed to detect changes in surface topography as a function of demineralisation solution and exposure time. AFM images were acquired under continuous stable fluid flow conditions (1ml/minute flow rate). Perfusion solutions consisted of artificial saliva (AS), 1000 ppm fluoride (F), or demineralisation solutions (DS) of 1x10^-3M citric acid, adjusted to a pH of (i) 3.3, (ii) 4.5, or (iii) 5.5. The treatment regime is summarised in Table 1. Results: Upon exposure to the demineralisation solutions the outer ‘debris' layer of the enamel surface was removed. Characteristic rod structure and needle like crystallites within the rod structure were exposed. In situ time-resolved AFM imaging revealed dissolution to be initiated at the rod core/rod sheath interface and preferentially at defects inherent within the enamel surface. Dissolution was observed to preferentially propagate at the interface between the dissolved and undissolved sections of the enamel rods in the x-y direction. In the absence of surface defects dissolution was observed to propagate anisotropically along the c-axis of the enamel rods. Dissolution from the rod sheath walls was suppressed relative to the cores. Table 1 details RMS roughness values from the enamel surface obtained as a function of treatment group at a fixed time point. Fluoride was observed to suppress enamel rod dissolution. Conclusion: In situ time-resolved AFM has successfully been employed to study the initial stages of enamel dissolution. Fluoride is observed to inhibit the dissolution of the enamel rod core.