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After de- and subsequent remineralization at 0.03 and 1.0 ppm fluoride and either 0, 1, 10, 20 or 25 mM carbonate, artificial lesions in bovine enamel were demineralized again. The amount of secondary demineralization was determined after 15, 30 and 70 h of demineralization by means of quantitative microradiography. The results showed that fluoride incorporated during the remineralization period retards lesion formation during secondary demineralization. Carbonate incorporated during the remineralization period enhances secondary demineralization. An interaction between both ions was observed.

Some newly designed surface-active phosphates were investigated with respect to their capacity to inhibit caries lesion formation in vitro. On the labial or buccal surfaces of sound human third molars, windows were prepared for investigation. One half of each window was treated with the test compound at pH values of 7.5, 5.5, or 3.5. The other half served as a control. Both halves were demineralized and the generated caries lesions in both parts were compared by means of quantitative microradiography. Mono-, bis- and trisphosphates differed only marginally in their effect on the demineralization. The compounds diminished the demineralization down to 30% of that of untreated enamel. Per molecule at least one long alkyl chain (more than 12 C atoms) was required for achieving decreased demineralization, indicating the importance of a low surface tension. The results suggest that some of the tested compounds have promising properties for their use as anticaries agents, especially when applied in combination with fluoride.

To estimate permeability properties of bovine dental enamel, complex impedance measurements and radioisotope diffusion experiments were carried out before and after X-ray irradiation (single dose, 72 Gy) of enamel specimens. Neither impedance measurements nor diffusion experiments showed significant changes in permeability.

Subsurface lesions in bovine enamel slices were remineralized. The remineralization solutions contained either 0.03, 0.3, or 1.0 ppm fluoride at either pH 5.5 or 6.8. The amount of remineralization was determined after periods of up to 610 h, using quantitative microradiography. The results showed that after 126 h of remineralization in the presence of 0.03 ppm fluoride significantly (p less than 0.05) more remineralization occurred at pH 6.8 than at pH 5.5. At 0.3 and 1.0 ppm fluoride no significant differences between pH 5.5 and pH 6.8 were observed. An interaction between fluoride and pH was observed. The observed differences in the rates of remineralization are explained by the formation and subsequent transformation of the precursors octacalcium phosphate (pH 6.8) and brushite (pH 5.5) into (fluor)apatites.

Intact human enamel was demineralized in vitro to obtain artificial caries lesions. Part of the lesions was then treated with different surface-active compounds and remineralized with a remineralizing solution. The other part of the lesions was de- and remineralized in the same way, but was not treated with a surface-active compound. The results indicate that none of the surface-active compounds do affect the remineralization of the caries lesions. Possible applications of the tested compounds in combination with fluoride, for the prevention of dental caries, are discussed.

Remineralization experiments of bovine enamel with subsurface lesions were carried out with the use of a sandwich technique. The remineralizing solutions contained no or 2 ppm fluoride added to the solution. The amount of remineralization was determined after remineralization periods up to 324 by means of quantitative microradiography. After 84 h the group with fluoride showed significantly (p less than 0.05) less remineralization than the nonfluoride group. This inhibition of fluoride on the remineralization may be explained by the inhibiting effect that fluoride at certain concentrations has on the crystal growth of apatites.

This paper deals with the influence of (1) the pH, (2) the degree of saturation with respect to hydroxylapatite (OHA) and (3) the fluoride concentration of demineralizing buffer solutions on the development of surface layers of artificial carious lesions. The pH did not affect the surface layer during demineralization. With increasing saturation with respect to OHA, the mineral content of the surface layer increased and the gradient of mineral content at the inside of the surface layer became less negative. The effect of the fluoride concentration depended on the degree of saturation of the solution with respect to OHA. At the highest undersaturation used in this study (pIoha126) an increase of the fluoride concentration caused an increase of the mineral content of the surface layer, an unchanged gradient in mineral content at the inside of the surface layer, and an increased intercept of this gradient. At low undersaturation with respect to OHA (pIoha122) an increasing fluoride concentration caused a less negative gradient. An explanation for the results is given on the basis of the driving forces for dissolution of enamel mineral and reprecipitation of less soluble mineral. It is concluded that the degree of saturation with respect to fluorapatite and the fluoride concentration are factors determining the development of the surface layer.

Third molars just erupting and premolars (before eruption or half a year or 3.5 years after eruption) were extracted and sawn vertically through the buccolingual plane. Tracings of the Ca, P, Na and Mg contents of the outer 100-µm layer of the enamel were made with an electron microprobe. Care was taken to limit the diameter of the excited volume to about 1 µm and to suppress the evaporation of Na. On the unerupted and freshly erupted surfaces the Na and Mg content varied from 50 to 80% of the content at 100 µm depth. However, half a year or 3.5 years after eruption the Na and Mg contents were negligibly small at the tooth surface and they remained smaller than in unerupted or freshly erupted teeth up to a depth of 10–30 µm. These data suggest that posteruptive maturation occurs in the mineral of the outer layer of tooth enamel whereby it builds up a higher resistance to carious breakdown. This interpretation is corroborated by clinical findings reported in the literature. The consequences of wear and abrasion on these phenomena are not known yet. The above-mentioned depth of 10–30 µm agrees with the penetration depth of pH changes estimated to result from the consumption of foods and drinks with varying pH.

Point count analyses were made with the electron microprobe for Ca, Na and Mg in a vertical cross-section through the enamel of a freshly erupted human pre-molar in the approximal area. A low excitation energy was chosen to limit the size of the excited volume and to suppress the evaporation of Na. The Na content showed variations with a wavelength of 6 ± 1 µm, which equals the distance of cross-striations. The Mg content showed variations with a wavelength of 25 ± 4 µm which equals the distance of striae of Retzius, whereby these striae probably contain a higher Mg content. On the assumption that higher Na and/or Mg contents represent mineral with a higher solubility, the observed variations in mineral composition help to explain the fine structure of carious lesions as observed frequently by microscopy and microradiography.

Artificial laminated carious lesions were made on the buccal surface of human premolars. The volume percentage of mineral was determined as a function of depth by microradiography. Using the electron microprobe, the signals for Ca, Na, Mg, P and Cl were recorded as a function of depth both through the lesions and through the adjacent sound enamel. In the demineralized parts of the lesions there was a preferential loss of Na and a preferential retention of Cl. In the surface layers and the laminations the Na/Ca and Cl/Ca ratios were practically the same as in the adjacent sound enamel. This indicates that these surface layers and laminations were not formed by gross dissolution of the original mineral followed by gross precipitation of another, less soluble calcium phosphate, but that they remained there during the continuous demineralization due to the fact that the microcrystals became resistant to dissolution, perhaps due to protection by a thin layer of precipitated fluorapatite or fluoridated hydroxyapatite. In a previous study the same was found for surface layers and laminations in natural laminated carious lesions. This indicates that during continuous in vitro demineralization tooth enamel mineral behaves similarly as during intermittent in vivo demineralization.

One of the mechanisms proposed for the caries-reducing effect of fluoride is diffusion inhibition of OH–– groups through the 63 columns of the apatite structure thereby increasing the resistance of the crystal against acidic attack. The validity of this hypothesis was checked by investigation of the reactivity of solid solutions of hydroxyapatite and oxyapatite for oxydation during storage for 3 years in air. These solid solutions provide the same number of vacancies in the OH positions as occur in the apatite of tooth enamel, whereas the mobility of the OH–– ions is the critical quantity for its reactivity in analogy with fluorhydroxyapatite solid solutions. It was found that the solid solutions were not oxydized as their lattice parameters did not change during the storage period although the experimental period was far within the limits of the theoretical estimate. This invalidates the proposed mechanism for the caries-reducing effect of fluoride, as it proves the solid state diffusion is not important in apatites at normal temperatures.

In this study evidence is provided for the existence of a gradient in solubility and rate of dissolution of human tooth enamel. These gradients were obtained quantitatively by applying calcium and pho

Sound human premolars were cut transversely and polished. The polished surface of the buccal enamel was exposed to buffer solutions of different degrees of saturation with respect to hydroxyapatite. The mineral content of the surface layers of artificial lesions created by one particular solution depended on the gradient in solubility and susceptibility for transformation of the enamel mineral. In a series of lesions created by a series of buffer solutions with increasing degree of saturation, a particular mineral content of the surface layer moved inwards, proportional to the maintenance of the strength of the driving force for dissolution and for reprecipitation.

Slices of bovine enamel and intact human enamel were treated with (a) a two-step treatment consisting of phytate followed by hexadecyl amine, or (b) 2-O-stearoylglycerol-1,3-diphosphate (Glydip), or (c) phosphatidic acid. The effect of these agents on the rate of caries lesion formation and on the permeability of dental enamel was quantitatively determined. Glydip had the most pronounced effect on lesion formation and showed a strong inhibiting effect on the rate of demineralization, combined with a reduction of the permeability. The results indicate that at least a part of the effect of Glydip on the demineralization was caused by a reduction of the rate of dissolution of the enamel mineral. The reduction in permeability may have had an additional effect.

In this study a buffer system containing calcium and phosphate was used to produce caries-like lesions. The influence of the pH on the lesion characteristics was investigated. The degree of saturation with respect to hydroxylapatite and the concentration of the undissociated acid were kept constant. The pH was either 4.0, 4.5, 5.0 or 6.0. The lesions were examined microradiographically. The lesion characteristics studied were: the mineral content of the surface layer; the mineral content of the body of the lesion; the depths at which these mineral levels were reached, and the depth of the lesion. The most important influence of the pH was that on the rate of demineralization. A striking phenomenon was the sudden change in the rate at which the different levels of demineralization were reached. In solutions with a pH lower than or equal to 4.5, the time needed to reach a certain demineralization was considerably shorter than for solutions with a pH higher than or equal to 5.0.

The rate of dissolution of whole human enamel was studied by carrying out artificial caries experiments in buffers of known pH and known degree of undersaturation with respect to hydroxyapatite and by

Extensive ‘white spot’ and ‘brown/white spot’ lesions with intact surfaces in 11 molars were investigated by microradiography and electron microprobe analysis for Fe, Ca, Na and Cl. At the surface of the brown spots some Fe was found. Within the lesions a preferential dissolution of Na and a slight enrichment of Cl was found. This suggests that Na and Cl are sited in different parts of the mineral and the Na-containing part appears to be more soluble. The Na and Cl contents of a lamination approach those of adjacent sound enamel. This indicates that laminations do not result from major reprecipitation of a less soluble calcium phosphate within the formed lesion, but that the mineral is protected from local dissolution early in the process. A possible mechanism is discussed.

This paper gives a detailed description of the development of the surface layer during demineralization of human enamel. Optical inspection of microradiograms of lesions in different stages of development can give the impression that the surface layer increases in thickness with time. Using a more objective method of inspection together with an explicit description, the development of the surface layer was followed with time more quantitatively. This method was based on the changes occurring at the inside of the surface layer. It appeared that the surface layer thickness was constant with time in all lesions investigated. These observations indicate that at constant composition of the buffer solutions, no dissolved material from the body of the lesion is deposited at the inside of the surface layer.

The classical theory on the solubility of ionic compounds has been extended to solid solutions and deviations from stoichiometry by combination of thermodynamical and solid state chemical principles.

In this study a mathematical model for the caries process is presented. It includes diffusion of ions, dissolution of mineral and complexation of ions. The conclusions from experiments with the model

Sound buccal surfaces of human premolars were exposed to buffer solutions which were slightly under- or supersaturated with respect to hydroxyapatite as well as fluorapatite (FA). It appeared to be possible to create subsurface lesions under these conditions, and fluoride appeared not to be necessary for the formation of a surface layer. The mineral content of the surface layer was dependent on the degree of saturation with respect to FA. A comparison is made with literature data, and the differences found are discussed.