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Although enamel demineralization is important for our understanding of caries formation, no consensus has been reached regarding the possible differences in susceptibility of primary and permanent enamel. We used the constant composition (CC) technique to investigate the acid-induced demineralization of these tissues at a relative undersaturation with respect to hydroxyapatite (HAP) of 0.902, pH = 4.5, and ionic strength = 0.15 mol L(-1). The demineralization rates showed significant differences, primary enamel having the greater susceptibility to dissolution during an initial linear stage: 1.5 +/- 0.5 x 10(-10) mol mm(-2) min(-1) compared with 2.6 +/- 0.5 x 10(-11) mol mm(-2) min(-1) for permanent enamel. During the reactions, we observed nanosized crystallites which attached to the enamel surfaces or escaped into the bulk solution. These nanosized crystallites were kinetically protected against further dissolution, even though the solutions remained undersaturated. It is hypothesized that they may contribute to the remarkable mechanical and dynamic characteristics of enamel.

The influence of 9.6 microm CO2 laser irradiation on enamel dissolution kinetics was investigated using a constant composition method designed for rate measurements of enamel dissolution as a function of depth, on a micrometer scale. In contrast to lower irradiation intensities (< or =1.0 J cm(-2)), which consistently showed reduced dissolution rates, higher fluences (energy per surface area) resulted in initially increased dissolution rates, which rapidly decreased, after dissolution times corresponding to removal of a few micrometers, to rates similar to those acquired using lower fluences. It was also demonstrated that surface damage during laser irradiation could be limited to the first 1-2 microm by lowering the number of pulses per spot during the irradiation procedure. The constant composition method can be used to measure detailed kinetics of inhibition of acid dissolution of dental enamel that has been treated by low fluence 9.6-microm CO2 laser irradiation., (Copyright (c) 2005 S. Karger AG, Basel.)

The interfacial free-energy is an important factor in the regulation of mineralization and dissolution at the surfaces of dental hard tissues. However, few thermodynamic studies have been aimed at the elucidation of the interfacial terms. Contact angle measurements (sessile drop and thin layer wicking) and kinetic dissolution and growth techniques have been used to study the interfacial properties of root dentin (D), human enamel (E), and hydroxyapatite (HAP). The interfacial tensions between water (w) and each of these phases were calculated from contact angle data according to surface tension components theory. The values gamma wD = 4.5 x 10(-3) J m-2, gamma wE = 8.8 x 10(-3) J m-2, and gamma w,HAP = 10.4 x 10(-3) J m-2 were of the same order of magnitude as those obtained from dissolution kinetic data (pH = 4.5): gamma wD = 1.4 x 10(-3) J m-2, gamma wE = 3.2 x 10(-3) J m-2, and gamma wHAP = 9.3 x 10(-3) J m-2. Kinetics studies of the crystallization of HAP on HAP, dentin, and enamel yielded the interfacial free-energy values, gamma wHAP = 17.1 x 10(-3) J m-2, 17.7 x 10(-3) J m-2, and 9.4 x 10(-3) J m-2, respectively, probably reflecting the interfacial energies of the deposited phases rather than those of the dental hard-tissue substrata. The lower interfacial tension values are consistent with the higher solubilities of these solid phases: logKSO = -52.0, -55 approximately 57, and -58 approximately 59, for root dentin, enamel, and HAP, respectively, expressed as an equivalent HAP ionic product. The higher interfacial free-energy is also consistent with the slower mineralization of HAP on dentin and enamel surfaces.

The kinetics of growth of calcium phosphate on seed materials prepared from human tooth enamel has been studied at 25 degrees C and pH 7.40 in stable supersaturated solutions of calcium phosphate. The grown material was characterized chemically, by specific surface area (SSA) measurements and by subsequent dissolution experiments. The marked increase in SSA in the early stages of the precipitation reaction parallel the results obtained with synthetic hydroxyapatite seed crystals and the results of the dissolution experiments indicate a chemical composition close to that of octacalcium phosphate for this intermediate phase. At low supersaturation it is possible to avoid the formation of the precursor phase.

The kinetics of growth of calcium phosphates on synthetic hydroxyapatite (HAP), human enamel, whole dentin, and human calculus has been studied at 37 C at pH values from 4.97 to 5.10 held constant by the pH-stat addition of base. Following an initial induction period, dicalcium phosphate dihydrate (DCPD) crystallization takes place on the surface of these solids in all cases, even though the solutions are highly supersaturated with respect to HAP. The second order crystal growth kinetics are characteristic of the crystallization of DCPD on pure DCPD seed material. The ability of the substrate surfaces to nucleate DCPD falls in the order, synthetic HAP greater than enamel greater than calculus greater than dentin. Nucleation of DCPD, which is completed during the initial induction period, is very sensitive to small changes in pH, from 4.99 to 5.10.

The kinetics of dissolution of powdered bovine enamel and of human enamel, both untreated and extracted with either hypochlorite or chloroform, has been studied using a constant solution composition technique in undersaturated solutions of calcium phosphate (total molar calcium concentration, TCa = 0.3 to 13.1 X 10(-3) mol L-1, total molar phosphate, Tp = 0.18 to 7.9 X 10(-3) mol L-1) at an ionic strength of 0.15 mol L-1, and pH = 4.5. The kinetic equations describing the dissolution reactions suggest a surface dislocation mechanism, and the presence of fluoride ion markedly retarded the reaction. For human enamel, a fluoride level of only 0.5 ppm reduced the rate of dissolution ten-fold. In contrast, the dissolution of hydroxyapatite, HAP, is best interpreted in terms of a polynucleation process.

Various etching methods are examined with respect to their ability to induce the surface nucleation and growth of fluorapatite from solutions of low, sustained supersaturation with respect to hydroxyapatite and fluorapatite. The constant composition kinetics method enables the rates of remineralization to be precisely determined, and the possibility of the direct growth of fluorapatite on calcium fluoride crystals in calcium phosphate solutions of low supersaturation has also been examined.

A highly reproducible seeded crystal growth method has been used to study enamel mineralization in the presence of traces of fluoride ion. The resulting increased rate of remineralization over a range of calcium phosphate supersaturation may be attributed to the formation of surface fluorapatite.

The course of enamel remineralization in aqueous solution is markedly dependent upon factors such as ionic medium supersaturation, pH, ionic strength, and the relative concentration of mineralizing surface. Although numerous attempts have been made to determine the mechanism of both the spontaneous and seeded precipitation reactions, it is only recently, through the use of a constant solution composition technique, that quantitative information can be obtained concerning the intermediate calcium phosphate phases which are formed during mineralization. Studies of surface phases in the presence of potential anti-calculus and anti-caries agents are facilitated using this approach.

The in vitro remineralization of acid-etched enamel surfaces offers an attractive model for simulating carious lesion recalcification and the repair of teeth in the oral environment. In the present work the influence of the extent of etching with hydrochloric and phosphoric acids has been examined. The kinetics of growth of hydroxyapatite as an exclusive phase on the pretreated enamel surfaces has been studied by means of a constant solution composition procedure. This method enables studies to be made at low constant supersaturations so that the stoichiometry of the precipitating phase may be defined. The very small rates of mineralization with HAP were determined with a precision hitherto unattainable. In addition, the influence of fluoride ion in enhancing the remineralization of damaged enamel has been investigated.

It is now well-established that kinetic aspects as well as considerations based solely on solubilities and thermodynamic driving forces should be taken into account while one is attempting to understand the mechanism of dental caries. In the present study, kinetic comparisons of the dissolution of hydroxyapatite, carbonated apatite, and ground human dental enamel have been made in order that the appropriateness of these synthetic phases as enamel dissolution models can be assessed. Specific additives used to form intact surface layers in vitro have also been investigated. An interesting phenomenon related to surface-controlled dissolution has been revealed. During Constant Composition experiments, the dissolution rates for all the systems decrease markedly as the reaction proceeds. Further tests with fresh crystals suggest that micro-impurities, in addition to microstructural changes of the dissolving surfaces, may play a role in the case of hydroxyapatite but do not influence the dissolution of carbonated apatite. Kinetic results for ground human enamel indicate the release of dissolution poisons. Nevertheless, the results confirm expectations that carbonated apatite may be a better model for enamel than near-stoichiometric synthetic hydroxyapatite. [ABSTRACT FROM AUTHOR]

Historically, there has been considerable debate concerning the roles of loosely bound (calcium fluoride) and firmly bound (fluorapatite) fluoride for caries prevention. Research now shows that fluorapatite (FAP) is a finite reaction product of enamel/apatite fluoridation with or without CaF2 formation, suggesting that CaF 2 always be considered as a supplement to, rather than a substitute for, FAP formation. In the presence of low levels of fluoride in the solution phase, the crystallization of hydroxyapatite is enhanced, while the corresponding dissolution is retarded. Fluoride in the bulk FAP or CaF2 solid phase, in contrast, has limited impact on crystal growth or dissolution kinetics. Both FAP and CaF2 can provide F to the solution phase to enhance remineralization and retard demineralization of enamel HAP crystallites. The FAP provides most of this F under low pH conditions, while CaF2 provides F at neutral or lower pH. The reactivity of fluoride on sound and carious enamel differs significantly. Carious enamel acquires more fluoride, acquires it more quickly, and itself acts as a source of retained fluoride in comparison with the more limited reactivity of sound enamel. Overall, the most important question concerning fluoride reactivity relates to its efficiency in enhancing remineralization or retarding demineralization processes. This is influenced not only by the reaction products, e.g., loosely or firmly bound fluoride, but also by the nature of the enamel substrate and frequency of application of the topical fluoridating agent. Inasmuch as the reactivity of bulk HAP is dominated by surface layers of FAP material, the debate over usefulness of various fluoride reaction products solely on a chemical level is no longer critical. Instead, all factors influencing the efficiency of a fluoridating regimen must be considered in the development of improved systems for caries prevention. [ABSTRACT FROM PUBLISHER]

The kinetics of dissolution of hydroxyapatite (HAP) and human enamel (HE) powders in lactate buffers was investigated by means of a Constant Composition technique. The rates were interpreted in terms of a surface-controlled dissolution mechanism. It was shown that the overall dissolution of both HAP and HE crystallites in lactate buffers resulted from an interplay of the calcium chelating action of lactate ion, its interaction with surface calcium sites, and the retardation effects of both ionized (L-) and un-ionized (HL) lactic acid due to adsorption at phosphate surface sites. The presence of lactate in demineralizing solutions also influenced the "acquired crystallite resistance" for dissolution, and the rates decreased as the reaction proceeded. The influence of surface protective agents [methanehydroxydiphosphonate (MHDP) and polyacrylic acid (PAA)] on HAP and HE dissolution rates was also extensively studied in partially saturated lactate buffer (0.05 mol L-1). Although the overall kinetic effects of both additives appeared to be similar, there were considerable mechanistic differences. MHDP might interact with lactate by competition for surface calcium sites. In contrast, PAA appeared to act independently of lactate under the experimental condition used. This would tend to support the recommendation of White (1987), that PAA was more suitable as a surface protective agent for artificial caries lesion preparation. [ABSTRACT FROM AUTHOR]

Artificial white spot lesions have been prepared on bovine enamel surfaces by the controlled addition of lactate buffer containing methylhydroxydiphosphonate ions. The rates of remineralization have been measured using a constant corn position method in solutions of calcium phosphate of low supersaturation. Deposition of new hydroxyapatite solid phase takes place exclusively within the white spot lesion at a rate appreciably slower than that of an acid-etched enamel surface, probably due to the presence of methylhydroxydiphosphonate located within the intact surface region of the lesion. [ABSTRACT FROM AUTHOR]

The prospect of restoration of damaged dental tissues has recently attracted great interest. An understanding of the remineralization of human dentin in vitro was attempted by using supersaturated calcium phosphate solutions at sustained supersaturation by means of a constant solution composition method. The direct growth of HAP from solutions of low supersaturation on powdered whole human dentin was confirmed. Moreover, it was found that the kinetics were similar to those of the seeded growth at synthetic HAP. A high apparent activation energy pointed to a surface-controlled mechanism. [ABSTRACT FROM AUTHOR]