Comparison of Fluoride Profiles by SIMS with Mineral Density of Subsurface Enamel Lesions Treated Intra-orally with a Fluoride-releasing Device

Abstract

A variety of intra-oral model systems has evolved which allows for the study of remineralization of coronal and/or root-surface lesions following application of topical fluoride (F) agents. The problem of interpretation of the results has led to a variety of analytical methods (i.e., microhardness, F biopsy, microradiography, and polarizing light microscopy), each of which provides important but limited information related to the overall understanding of remineralization. Microhardness measures change in mineral content which is more precisely localized by microradiography and polarized light microscopy. F biopsy allows for assessment of the F uptake of lesions, but does not suggest the chemical state of the F. Previous work has demonstrated that patterns of mineral deposition during remineralization do not necessarily parallel the F uptake profiles, and fluoridated apatites cannot be distinguished from non-specifically-adsorbed F (Clark et al., 1988). Because artificial lesions demonstrate variations in depth and mineral content, complementary analytical methods that demonstrate profiles of both F content and mineral density curves on the same section are needed so that the process of remineralization can be more clearly understood. This study used secondary ion mass spectrometry (SIMS) for F profiles and quantitative microradiography for assessment of mineral deposition on the same section. These state-of-the-art methods demonstrate the precision with which information about remineralization can be obtained. Subsurface lesions in human enamel specimens were developed by immersion in 0.1M lactate buffer with 1% CMC at a pH of 4.5 for 48 h. The specimens were then mounted in a removable mandibular acrylic appliance and worn by adult subjects for 30-day control and fluoride-releasing-device (FRD, 0.5 mg F/day) treatment periods. The subjects brushed with a F-free dentifrice. After 30 days, the enamel specimens were removed, embedded, sectioned, and polished to 100 ± 5 μm. Sections were scanned for mineral content with a Zeiss VIDAS image analysis system, and density profiles were constructed. Identical areas on each section were scanned by a Cameca IMS-3fSIMS for elemental profiles of F, Ca, and P along the same paths as were scanned by image analysis. Mean F profiles suggested that low levels of salivary F from the FRD produced high F uptake in the surface layer without significant increase in mineral deposition. Conversely, significant mineral uptake occurred in the base of the lesion with decidedly lower F uptake, suggesting that fluoridated apatite formation predominates in the base of the lesion and mostly non-specific adsorbed F in the surface layer and body of the lesion. The approach used in this study enhances the understanding of the remineralization of enamel lesions by use of a low-potency F agent.