Enamel demineralization under driving forces found in dental plaque fluid

Kinetics of ion release from a conventional glass-ionomer cement

Release kinetics for sodium, silicon, aluminium, calcium and phosphorus from conventional glass-ionomer dental cement has been studied in neutral and acid conditions. Specimens (6 mm height × 4 mm diameter) were made from AquaCem (Dentsply, Konstanz, Germany), 6 per experiment. They were matured (37 °C, 1 h), then placed in 5 cm3 storage solution at 20-22 °C. In the first experiment, deionised water, changed daily for 28 days, was used. In the second, deionised water, changed monthly for 21 months, was used. In the third, lactic acid (20 mmol dm-3, pH: 2.7 ± 0.1), changed monthly for 21 months was used. After storage each solution was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Results showed that in neutral conditions, no calcium was released, but in acid, significant amounts were released. The other elements (Na, Al, Si and P) were released in neutral as well as acid conditions, with greater amounts in acid. More frequent changes of water gave greater release. In neutral conditions, release over 21 months followed the equation: [E]c = [E]1t/(t + t½) + β√t ([E]c is the cumulative release of the element). In acid conditions, this became: [E]c = [E]1t/(t + t½) + αt. Hence release of all elements was shown to occur in two steps, a rapid initial one (half-life: 12-18 h) and a longer second one. In neutral conditions, the longer step involves diffusion; in acid it involves erosion. These patterns influence the material's bioactivity.

A Novel Kinetic Method to Measure Apparent Solubility Product of Bulk Human Enamel

Introduction: Tooth enamel mineral loss is influenced by its solubility product value, which is fundamental to the understanding of de- and remineralization resulting from a carious or erosive challenge. Published pKsp values for human enamel and hydroxyapatite range from 110 to 126 suggesting a heterogeneous nature of enamel solubility. However, this range of values may also result from the variety of methods used, e.g., some authors reporting values for suspensions of enamel powder and others for bulk enamel. The aim of this study was to develop a method to measure the solubility of bulk human enamel under controlled in vitro conditions simulating demineralization behavior of enamel within the oral environment using scanning microradiography (SMR). SMR was used to monitor real-time changes in enamel demineralization rates at increasing calcium concentrations in a caries simulating demineralization solution until the concentration at which thermodynamic equilibrium between enamel and solution was achieved. Method: 2 mm thick caries free erupted human enamel slabs with the natural buccal surfaces exposed were placed in SMR cells exposed to circulating caries-simulating 2.0 L 0.1 M pH = 4.0 acetic acid, at 25°C. SMR was used to continuously measure in real-time the decrease in mineral mass during the demineralization at 5 different points from on each slab. Demineralization rates were calculated from a linear regression curve of projected mineral mass against demineralization time. Changes in the demineralization rates were monitored following a series of successive increases in calcium (and phosphate at hydroxyapatite stoichiometric ratios of Ca:P 1.67) were added to the demineralizing solution, until demineralization ceased. The pH was maintained constant throughout. Results: Demineralization halted when the calcium concentration was ~30 mM. At higher calcium concentrations, mineral deposition (remineralization) occurred. By comparison with results from speciation software calculations for the calcium phosphate ternary system, this result suggests that the bulk solubility product of enamel (pKsp_BEnamel) under the conditions used is 121. Discussion: The apparent pKsp_BEnamel under these conditions was higher than many previous reported values, and much closer to those previously reported for HAp. However, this is a bulk value, and does not reflect that enamel is a heterogeneous material, nor the influence of ionic inclusions.

Predictive research methods of enamel and dentine for initial caries detection

Currently, various research methods of enamel and dentine for precautionary diagnostics of initial caries forms are developed; however, the vast majority of these do not provide objective criteria of caries diagnostics or are very difficult to perform. Therefore, the search of diagnostics and enamel research methods, which will allow predicting caries emergence and to carry out personalised prevention of this pathology, is necessary. In this review, modern diagnostic methods that allow understanding the main aspects of caries process, assess the risk of its development, and also suggest the possibility of emergency prevention of caries progression in the nearest future are presented.

The control of phosphate ion release from ion permeable microcapsules formulated into rosin varnish and resin glaze

OBJECTIVES

The occurrence of recurrent caries at the interface of dental materials and the enamel surface is an important performance issue. The objective of this study was to investigate the most effective way to control the release rate of bioavailable phosphate ions contained in aqueous solutions within ion permeable microcapsules formulated into rosin based varnishes and resin based sealants, in order to promote remineralization.

METHODS

Microcapsules that contained aqueous solutions of K2HPO4 with concentrations from 0.8 to 7.4M were prepared. 3-50w/w% of microcapsules were loaded into both rosin and resin based dental formulations.

RESULTS

The effect of initial salt solution concentration inside the microcapsules and weight percent loading of the microcapsules on release rate were contrasted. The effect of microcapsule loading was found to be highly dependent on the continuous phase. In rosin, 3-15w/w% loading resulted in rapid release of ions. Higher weight percent loadings were initially slower but resulted in sustained release of ions. In resin, 3-15w/w% formulations slowly released ions for at least 300 days, while higher loading formulations released an initial burst of ions. Initial salt solution concentration contained inside the microcapsule affected ion release rate. Initial rate of ion release was greatest at a concentration that was less than the maximum concentration studied in both continuous phases.

SIGNIFICANCE

Phosphate ion release can be controlled from resin or rosin based dental material by adjusting initial salt solution concentration in microcapsules or percent loading of microcapsules. The potential for burst release from a varnish or slow, sustained release from a sealant has been demonstrated.

Elevated Fluoride Products Enhance Remineralization of Advanced Enamel Lesions

Caries prevention might benefit from the use of toothpastes containing over 1500 ppm F. With few clinical studies available, the aim of this pH-cycling study was to investigate the dose response between 0 and 5000 ppm F of de- and remineralization of advanced (> 150 μm) enamel lesions. Treatments included sodium and amine fluoride, and a fluoride-free control. Mineral uptake and loss were assessed from solution calcium changes and microradiographs. Treatments with 5000 ppm F both significantly enhanced remineralization and inhibited demineralization when compared with treatments with 1500 ppm F. Slight differences in favor of amine fluoride over sodium fluoride were observed. The ratio of de- over remineralization rates decreased from 13.8 to 2.1 in the range 0 to 5000 ppm F. As much as 71 (6)% of the remineralized mineral was calculated to be resistant to dissolution during subsequent demineralization periods. With 5000-ppm-F treatments, more demineralizing episodes per day (10 vs. 2 for placebo) would still be repaired by remineralization.

Effect of Lesion Characteristics and Mineralising Solution Type on Enamel Remineralisation in vitro

The aim was to study the effect of lesion preparation technique and solution composition on remineralization of artificial lesions in vitro. Lesions were prepared with similar total mineral loss, but different mineral distribution, i.e., low (14.0) or high R (34.8) values. Lesions from both groups were remineralized (10 days, 37 degrees C) in two different solutions, with similar supersaturation with respect to hydroxyapatite (St), but calcium:phosphate ratios representing either hydroxyapatite stoichiometry or plaque fluid (PF). Remineralization was quantified microradiographically, mineral distribution was compared with natural white-spot lesions. Mineral loss and depth decreased significantly, and surface-zone mineral content (Zmax) increased significantly, in all lesions. Overall there was a significant relationship of decreasing remineralization with increasing Zmax, but not within either lesion type. PF was significantly more efficient than St in high-R lesions, with lesions remineralizing almost completely in PF. Remineralization was not significantly different in PF or St for low-R lesions but in high-R lesions, PF was more efficient than St, possibly through differences in relative saturations with respect to different calcium phosphates. Differences in area:solution ratios and baseline Zmax values may also have explained the different response to PF. Low-R lesions were similar to natural white-spot lesions in terms of mineral distribution, whereas high-R were not. Concluding, both lesion and remineralizing solution type had a marked influence on remineralization. It is proposed that use of low-R lesions would be more appropriate where more physiologically relevant mineral distribution is required, whereas high-R lesions would be appropriate for studying inherent remineralizing efficiency.

Effect of fluoride on artificial caries lesion progression and repair in human enamel: regulation of mineral deposition and dissolution under in vivo-like conditions

OBJECTIVE

This study was carried out to determine in vitro the effect of fluoride on (1) the demineralization of sound human enamel and (2) the progression of artificial caries-like lesions, under relevant oral conditions.

METHODS

Thin sections of sound human enamel were exposed to solutions undersaturated with respect to tooth enamel to a degree similar to that found in dental plaque fluid following sucrose exposure in vivo, containing fluoride concentrations (0-0.38ppm) found in plaque fluid. Mineral changes were monitored for 98 days, using quantitative microradiography. The effect of fluoride (1.0-25.0ppm) on the progression of artificial caries-like lesions was similarly studied.

RESULTS

Fluoride concentrations of 0.19ppm and greater were found to prevent the demineralization of sound enamel in vitro. However, significantly higher concentrations of fluoride (25.0ppm) were required to prevent further demineralization of artificial caries-like lesions. Demineralizing solutions with intermediate fluoride concentrations (2.1-10.1ppm) induced simultaneously remineralization in the outer portion of the lesion and demineralization in the inner portion. Simultaneous remineralization and demineralization were also observed in hydroxyapatite pellets.

CONCLUSIONS

Our results show that the observed effect of fluoride on enamel demineralization is not solely a function of bulk solution properties, but also depends on the caries-status of the enamel surface. A mechanistic model presented indicates that, in comparison to sound enamel surfaces, higher concentrations of fluoride are required to prevent the progression of artificial caries-like lesions under in vivo-like conditions since the diffusion of mineral ions that promote remineralization is rate-limiting.

The Effect of Fluoride at Plaque Fluid Concentrations on Enamel De- and Remineralisation at Low pH

The aim was to study the effect of fluoride, at concentrations typical of plaque fluid, on de- and remineralisation of subsurface lesions at low pH. Artificial lesions in human enamel were microradiographed to quantify mineral loss and placed in acid-gel systems at pH 4.8, 5.0 and 5.2. Calcium and phosphate were added to give initial Ca and Pi concentrations of either 4.1 and 8.0 mM, or 4.7 and 9.7 mM, at each pH value. Further, at each pH and combination of Ca and Pi, fluoride was added to the gels to give initial concentrations of 1, 2 or 5 ppm, with a non-fluoride control group. The lesions were removed after 10 days and change in mineral content quantified. Those in the non-fluoride control groups had demineralised further. Those exposed to fluoride had remineralised, the amount increasing with increasing fluoride concentration, up to a maximum value of approximately 75%. Calcium activity in the gels was reduced significantly, to levels similar to those reported for plaque fluid at low pH. Fluoride activity was also reduced, though to a lesser extent. These findings contrast with those from studies which have simulated conditions on smooth surface sites and which used experimental solutions composed to reflect salivary fluoride concentrations, where net demineralisation was observed at low pH. This reflects the need for further study of de- and remineralisation under plaque-fluid conditions. In conclusion, subsurface lesions were remineralised at low pH by fluoride at concentrations found in plaque fluid during a cariogenic challenge.

Human enamel erosion in constant composition citric acid solutions as a function of degree of saturation with respect to hydroxyapatite

The objective of this study was to investigate human enamel erosion under constant composition conditions, as a function of solution degree of saturation (DS) with respect to hydroxyapatite. The experimental conditions were relevant to the initial stages of enamel erosion by soft drinks. Nanoindentation was used to compare enamel surface softening caused by a control mineral water and two citric acid solutions with DS = 0.000 and DS = 0.032, both having pH 3.30. Enamel hardness and reduced elastic modulus were measured after 0, 30, 60, 120, 300 and 600 s exposure. A statistically significant change in enamel hardness was detected after 30 s exposure to both citric acid solutions, indicating that nanoindentation is extremely sensitive to the initial stages of erosion. There was a statistically significant difference between the mechanical properties of enamel exposed to the two citric acid solutions after 30, 60 and 120 s. At these times, the solution with DS = 0.000 caused twice as much enamel softening as that with DS = 0.032. This demonstrates that it may be possible to design a soft drink with a low erosive potential and a good taste by a small change in DS, at a typical drink pH.

Solubility properties of human tooth mineral and pathogenesis of dental caries

Dental research over the last century has advanced our understanding of the etiology and pathogenesis of caries lesions. Increasing knowledge of the dynamic demineralization/remineralization processes has led to the current consensus that bacteria-mediated tooth destruction can be arrested or even to some degree reversed by adopting fluoride and other preventive measures without using restorative materials. Our experimental approach provided new insight into the stoichiometries and solubility properties of human enamel and dentin mineral. The determination of the solubility product constant on the basis of the stoichiometric model (Ca)5.x(Mg)q(Na)u(HPO4)v(CO3)w(PO4)3.y(OH,F)1.z, verifies the difference in their solubility properties, supporting the phase transformation between tooth mineral and calcium phosphates in a wide range of fluid compositions as found in the oral environment. Further refinement of the stoichiometry and solubility parameters is essential to assess quantitatively the driving force for de- and remineralization of enamel and dentin in the oral fluid environment. Prediction of the effects of a combination of inhibitors and accelerator(s) on remineralization kinetics is also required. In order to develop devices efficient for optimizing remineralization in the lesion body, it is a critical question how, and to what extent, fluoride can compensate for the activity of any inhibitors in the mineralizing media.

Enamel dissolution in citric acid as a function of calcium and phosphate concentrations and degree of saturation with respect to hydroxyapatite

The aim of this study was to investigate enamel dissolution in citric acid solutions as a function of solution calcium and phosphate concentrations and degree of saturation with respect to hydroxyapatite (DSHA). The primary relevance of the study is the development of soft drinks with reduced erosive potential. Nanoindentation was used to investigate changes in the hardness of polished human enamel surfaces after 120 s and 300 s exposure to solutions with pH 3.30 and a range of calcium and phosphate concentrations. All solutions were undersaturated with respect to hydroxyapatite, with 0.000 < or = DSHA < or = 0.295. A complex dependence of enamel softening on calcium concentration was observed. Substantial enamel softening occurred in solutions with calcium concentrations equal to or less than 120 mm (DSHA approximately 0.104), but there was little or no statistically significant softening of the enamel for calcium concentrations over 120 mm. This condition may be applicable to soft drink formulation. Furthermore, solutions with DSHA = 0.101 and different calcium/phosphate ratios resulted in different degrees of softening of the enamel. Hence, contrary to assumptions made in many models, enamel dissolution is not simply a function of DSHA, and individual calcium and phosphate concentrations are critical.

Enamel dissolution as a function of solution degree of saturation with respect to hydroxyapatite: a nanoindentation study

The objective of this study was to investigate human enamel dissolution as a function of degree of saturation (DS) of the surrounding solution with respect to hydroxyapatite. Nanoindentation was used to compare changes in enamel nanomechanical properties due to dissolution by two solutions. Citric acid solution (DS=0.000, pH 3.30) and citric acid solution containing calcium (299 mg/l) and phosphate (54.0 mg/l) (DS=0.032, pH 3.30) were compared with a control mineral water (DS=0.673, pH 7.48). Exposure times were 0, 120, 300, 600, 900, and 1200 s. Compared to untreated enamel, there was a statistically significant change in enamel hardness after 120 s exposure to both citric acid solutions, and in elastic modulus after 300 s exposure. The rate of change of both variables decreased with exposure time. This suggests that dissolution rate is diffusion-limited under these conditions, in agreement with previous studies. There was no statistically significant difference between the hardness or elastic modulus of enamel exposed to the two citric acid solutions at any time. This may be due to a change in solution composition during contact with the enamel.

Association of caries activity with the composition of dental plaque fluid

This study tests the hypothesis that caries activity is associated with lower degrees of saturation with respect to enamel mineral in dental plaque fluid following sucrose exposure. Plaque fluids were obtained from caries-free, caries-positive, and caries-active subjects. Samples were collected before and at 3 and 7 min after a sucrose rinse on consecutive weeks and analyzed for organic acids, inorganic ions, pH, calcium activity, and, in selected samples, total protein. After sucrose, pH values were significantly lower in the caries-active group in comparison with the caries-free and caries-positive groups. Total and free calcium concentrations increased with decreasing pH, with free calcium being about one-third of total calcium. The caries-active group exhibited significantly lower degrees of saturation with respect to enamel mineral, after sucrose, and had significantly higher mutans streptococci levels in plaque than did the caries-free samples. Thus, saturation levels in post-sucrose plaque fluids reflect the cariogenic potential of dental plaque.

Critical pH in resting and stimulated whole saliva in groups of children and adults

OBJECTIVE

The concentrations of calcium and phosphate in saliva have significant influence on the protective mechanisms of dental hard tissues within the oral environment. A lower calcium concentration means: 1) a lower thermodynamic driving force for hydroxyapatite precipitation at normal oral pH; 2) a higher driving force for hydroxyapatite dissolution at low pH; 3) a lower critical pH. The aims of this study were to: 1. determine the calcium and phosphate concentrations 2. calculate the critical pH for enamel and 3. determine the driving forces for demineralization and remineralization in a group of children and adults.

METHODS

In this comparative study, calcium and phosphate concentrations were measured in the resting and stimulated saliva of child and adult volunteers using a spectrophotometric system used in routine blood analysis. Salivary flow rates were also measured in each group.

RESULTS

The calcium concentrations were lower in children than adults, but the phosphate concentrations were not significantly different. The critical pH was significantly higher for children than adults in both resting and stimulated saliva. Therefore, the thermodynamic driving forces for; (1) demineralisation at low oral pH, is greater, and (2) for remineralisation at normal oral pH, is lower, in children compared to adults.

CONCLUSION

The results of this study show that from thermodynamic considerations alone, there is increased risk of demineralization in children compared with adults.

Remineralization of caries lesions extending into dentin

Remineralization is one aspect of the overall process of tooth decay. However, it is primarily studied in shallow lesions. The aim of this study was to explore whether caries lesions in enamel and extending into the dentin can be remineralized. A single-section model was developed for the longitudinal and non-destructive monitoring of changes in enamel and dentin. Lesions at least 200 microm into dentin were formed in undersaturated acetate buffers. Next, the lesions were divided into groups (three treatment and one control) and remineralized. The treatments were: weekly immersion in 1,000 ppm fluoride, single treatment with methanehydroxybisphosphonate, and a constant level of 1 ppm fluoride. De- and remineralization was assessed by transverse microradiography. Remineralization was observed in enamel, but also in dentin, indicating that, deep into dentin, the pores become supersaturated to apatite formation. Treatments affected remineralization only in the outer part of enamel. Both findings are explained by a relatively fast diffusion of mineral ions, with precipitation being rate-limiting. The results suggest that dentin remineralization, underneath enamel, can be achieved and could possibly be used in clinical treatment strategies.