Many studies have been conducted to evaluate the timing of brushing and methods to suppress or halt the progression of acid erosion and initial caries formation, and to restore tooth enamel. However, the analyses in these studies were done from a macroscopic point of view, such as evaluating the change in the hardness of the enamel surface and the change in X-ray permeability, the morphological change due to crystal deposits on the enamel surface has not been elucidated. The purpose of this study was to investigate how crystals on the enamel surface differ when they are in contact with oral cavity liquids such as saliva versus no contact with these liquids. In addition, we assessed the condition of initial caries in which no substantial defect is observable, from the morphological perspective by using elemental analysis and observations with a transmission electron microscope (TEM). We also considered the causes of crystal formation on the enamel surface. As a result, the outermost layer in the pre-eruptive enamel contained a large amount of Mg, which is a stabilizer of amorphous calcium phosphate, suggesting that the layer contains acidic calcium phosphates. It was suggested that tricalcium phosphate, dicalcium phosphate dihydrate, octacalcium phosphate, etc. form a layer on the surface of post-eruptive enamel with no caries and in post-eruptive enamel with caries exposed to saliva in the oral cavity and the layer containing calcium phosphate have higher acid resistance than HAp. The information from this study can be applied to basic clinical research on remineralization therapy and non-invasive treatments of initial caries.
In this study, we immersed amorphous calcium phosphate (ACP) powder in biochemical buffer solutions and performed analysis of its solubility and phase transformation of the precipitate. After preparing ACP powder that contains no impurities, we used 4-(2-hydroxyethyl)-1-piperazinetethanesulfonic acid (HEPES) buffer, one of the good buffers, as a buffer solution and measured the amount of calcium ions eluted from ACP and other calcium phosphate crystals. ACP was immersed in the buffer solution at 5°C, 20°C, and 37°C, and the amount of eluted calcium ions was measured from 15 min to 24 h thereafter. The precipitated solid phase was analyzed using X-ray diffraction and its morphology was observed using transmission electron microscopy. The precipitation of hydroxyapatite (HAp) was observed after 15 min in HEPES buffer solution. Furthermore, in this experimental group, the precipitates of the sample incubated in HEPES buffer solution at 37°C for 24 h produced the largest HAp crystals. From these results we concluded that ACP immersed in HEPES buffer solution easily releases calcium ions and phosphate ions, and a rapid phase transformation to HAp occurs. Moreover, we assume that, in addition to the thermodynamic effect, the crystal growth of HAp is enhanced by the buffer solution.
Fundamental studies on biocompatible materials were conducted using the mechanism of dentin formation as a model. Hydroxyapatite and fluorinated apatite were synthesized in a gelatin solution. Synthesized apatite was pressed under vacuum, and the mechanical strength was investigated. A hardened mold was prepared by adding amorphous calcium phosphate (ACP). Then, changes in the crystals of the hardened mold dipped in distilled water or 1 ppm fluoride solution were observed with a transmission electron microscope. The results showed that the crystal growth of apatite synthesized in 1% gelatin was suppressed. Moreover, small and homogenous granular apatite was formed in the precipitate synthesized with fluorine. Gelatin contributed to the reduction of brittleness of the hardened mold, and thus a high cutting resistance value was obtained. Furthermore, acceleration of crystal growth and aggregation of fine crystals were observed in the hardened mold when ACP was added to the solution.
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