Crystal growth on enamel in relation to acid etching.

Crystal formation may occur during etching of enamel surfaces with phosphoric acid. In this in vitro study we observed, from a morphological standpoint, some crystals left after etching, on the surfaces, using the scanning electron microscope. More often, after water-spraying, a thin generalized precipitate remains on the surface. This precipitate may be harmful for the retention of composite resins. On few specimens we obtained needle-shaped or petal-like crystals. Crystal deposits were spread evenly, on the surface, or nucleated from different points. We tempted to identify these crystal formations by X-ray diffraction and microprobe analysis. Chemical identification seems very important because calcium phosphates solubility varies. Crystal dissolution in saliva can lead to marginal leakage and impairs the quality of esthetic restorations. On the contrary insoluble crystals may ensure microscopic retentions and crystal growth is now considered as an alternative for enamel pretreatment in bracket bonding. Crystal formation, in these first experiments, is too scarce to be used for crystal bonding. But it appears that two factors may enhance the crystal number: a preliminary topical application of fluoride and adsorption of an acidic protein, on the surface, before etching. However, further investigations are still necessary.