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Changes in heated and in laser-irradiated human tooth enamel and their probable effects on solubility

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Summary

Enamel of intact human teeth laser irradiatedin vitro under certain conditions is known to have less subsurface demineralization than unirradiated enamel on exposure to acid; consequently, the potential use of laser irradiance to reduce caries is apparent. The laser-induced physical and/or chemical changes that cause this reduced subsurface demineralization are not known. A laser-irradiated tooth enamel surface will have a temperature gradient that decreases towards the dentin junction. Dependent on irradiant conditions, the temperature may range from >1400°C at the surface to near normal at the dentin-pulp junction. Along this steep temperature gradient, different compositional, structural, and phase changes in the tooth enamel are to be expected. Identification of changes occurring along this gradient has bearing on understanding the dissolution reduction mechanism and, in turn, optimizing its effect. Changes in laser-irradiated material from the highest temperature region have been characterized, but those occurring in sequential layers of decreasing temperatures have not. Since the laser-induced changes are expected to primarily arise from localized heating, previously reported theramlly induced changes in tooth enamel on heating in conventional furnaces were utilized to infer corollary changes along the gradient in laser-irradiated tooth enamel. These thermally inferred changes which resulted in modifications in the tooth enamel apatite and/or newly formed phases were correlated with their probable effects on altering solubility. A temperature gradient range from 100–1600°C was considered with subdivisions as follows: I, 100–650°C; II, 650–1100°C; and III,>1100°C. Two of the products formed in range III, α-Ca3(PO4)2 and Ca4(PO4)2O, and also identified in the fused-melted material from laser-irradiated tooth enamel, are expected to markedly increase solubility in those regions that contain considerable amounts of these compounds. Products and changes occurring in range II, separate phases of α- and/or β-Ca3(PO4)2, and a modified phase of apatite, may increase or decrease the solubility depending on the Ca/P ratio and the resultant amounts of α-, β-Ca3(PO4)2 formed. Modifications in tooth enamel apatite effected in range I are expected to decrease its solubility; the formation of pyrophosphate in this range may have a substantial effect on reducing the solubility rate. It appears that laser-irradiant conditions that produce localized temperatures above about 650°C may have a deleterious effect on tooth enamel solubility unless calcium is introduced to increase the Ca/P ratio to near that of hydroxyapatite. Other important considerations of laser-irradiant treatment and interactions of tooth enamel, including enhanced uptake of fluoride, more effective irradiant wavelengths, selective reactions, directional absorption in crystals, and threshold irradiant conditions are briefly discussed and/or reviewed.

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  1. Mineralized Tissue Research Branch, National Institute of Dental Research, National Institutes of Health, 20892, Bethesda, Maryland, USA

    B. O. Fowler

  2. Division of Inorganic Materials, Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo, Japan

    S. Kuroda

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Fowler, B.O., Kuroda, S. Changes in heated and in laser-irradiated human tooth enamel and their probable effects on solubility. Calcif Tissue Int 38, 197–208 (1986). https://doi.org/10.1007/BF02556711

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