Fluoride Mode of Action: Once There Was an Observant Dentist

The discovery and implementation of fluoride in the prevention of dental caries is often praised as one of the most important achievements in health care. In the early 20th century, it took 30 y to identify fluoride as the cause of enamel mottling but also of reduced caries prevalence in a population drinking water containing fluoride. Similarly, from 1960 to 1990, it took major efforts to unravel the working mode of fluoride in such detail that a rational scheme of caries prevention could be formulated. This article describes the scientific struggle leading to a consensus on the topic. For a historic purpose, the field, the actors, and their main research achievements are described. Ultimately it was generally agreed that the effect of fluoride is primarily topical by fluorides in the oral fluids rather than systemic by incorporation of fluoride in the enamel mineral crystals. Fluoride concentrations, even <1 mg/L, enhance the deposition of calcium phosphates during remineralization of enamel (and dentin). Similarly, such low levels of fluoride are effective in reducing the dissolution of the calcified tissues. This understanding has led to the development of fluoride-containing caries-preventive products that had an undisputed beneficial effect on the levels of dental caries.

Second Era of OMICS in Caries Research: Moving Past the Phase of Disillusionment

Novel approaches using OMICS techniques enable a collective assessment of multiple related biological units, including genes, gene expression, proteins, and metabolites. In the past decade, next-generation sequencing ( NGS) technologies were improved by longer sequence reads and the development of genome databases and user-friendly pipelines for data analysis, all accessible at lower cost. This has generated an outburst of high-throughput data. The application of OMICS has provided more depth to existing hypotheses as well as new insights in the etiology of dental caries. For example, the determination of complete bacterial microbiomes of oral samples rather than selected species, together with oral metatranscriptome and metabolome analyses, supports the viewpoint of dysbiosis of the supragingival biofilms. In addition, metabolome studies have been instrumental in disclosing the contributions of major pathways for central carbon and amino acid metabolisms to biofilm pH homeostasis. New, often noncultured, oral streptococci have been identified, and their phenotypic characterization has revealed candidates for probiotic therapy. Although findings from OMICS research have been greatly informative, problems related to study design, data quality, integration, and reproducibility still need to be addressed. Also, the emergence and continuous updates of these computationally demanding technologies require expertise in advanced bioinformatics for reliable interpretation of data. Despite the obstacles cited above, OMICS research is expected to encourage the discovery of novel caries biomarkers and the development of next-generation diagnostics and therapies for caries control. These observations apply equally to the study of other oral diseases.

Biofilm layers affect the treatment outcomes of NaF and Nano-hydroxyapatite

During caries formation, dental biofilms function not only as acid producers but also as reservoirs and diffusion barriers for active caries-preventive components. The aim of this study was to investigate the influence of biofilms as a stagnant layer on the efficacy of NaF and nano-hydroxyapatite (nHA). Biofilms of Streptococcus mutans C180-2 were formed on the surfaces of artificially demineralized enamel in an active attachment biofilm model. After 2 days of biofilm formation, the model was subjected to a pH-cycling schedule, together with a control group without biofilms. Specimens were treated for 5 min twice daily with water, a 10% nHA slurry, or 18.4 mM NaF. At the end of the pH-cycling period, the biofilms were removed for the determination of the viable counts, the lactic acid production, and the calcium content. The mineral changes in the demineralized enamel blocks were analyzed by transversal microradiography. No differences in the biofilm viable counts and lactic acid production were found in the different treatment groups. The mean calcium content of the biofilms in the nHA group was 60.7 ± 15.3 mmol/g wet weight, which was approximately 8-fold higher than in the other 2 groups. The application of NaF resulted in net remineralization, but in the presence of a biofilm, net demineralization was observed. In contrast, nHA treatment reduced further demineralization compared with the water treatment, but the presence of a biofilm enhanced this effect. In conclusion, the presence of biofilms clearly influenced the treatment outcomes of anticaries products. Biofilms could either enhance or impede their efficacy. This result implies that biofilms should be included in the in vitro tests for the preclinical screening of caries-protective agents.

Determination of arginine catabolism by salivary pellet

To determine the formation of ammonium from arginine by oral bacteria residing in saliva and dental plaque, an arginolytic activity assay based on the work described by Nascimento et al. [2] was developed. Following the original methodology, insufficient ammonium production could be determined.

To improve the method for our research goal, the following modifications were made to the original protocols:•

The following changes were made to the arginine catabolism assay resulting in a 1000-fold increase in sensitivity: (i) the salivary pellet was washed and concentrated five times resulting in the removal of low density compounds interfering with the assay, (ii) the pH of the Tris–maleate buffer was increased from 6.0 to 7.5 resulting in a better conversion of arginine to ammonium and (iii) the incubation time was increased to 3 h to ensure that non-responders and salivary pellets low in cell numbers could yield detectable levels of ammonium.

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Removal of a centrifuge step from the protein determination resulted in a higher protein yield improving the accuracy of the assay.

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Changing from the use of the toxic, environmentally hazardous, mercury containing Nessler's reagent to a colorimetric enzyme assay achieved a safer and greener determination of ammonium concentration.

Novel anticaries and remineralizing agents: prospects for the future

Although the use of fluorides has been successful in reducing dental caries, the need remains to develop and evaluate new approaches and promising products for caries prevention. Comprehensive caries-prevention protocols should encompass fluoride and other agents affecting the de-/remineralization balance but also antimicrobial strategies. Different from the traditional restorative approach, the current opinion is that caries should be detected and monitored in its earliest stages, when a nonsurgical reversal can still be achieved. This paradigm shift has implications for methods of caries diagnosis, the choice of preventative materials and the design of randomized clinical trials. This article summarizes the highlights of a special conference dedicated to the topic of novel anticaries and remineralizing agents (ICNARA 2), and identifies the current consensus and remaining questions on pivotal issues in this field.

Different response to amine fluoride by Streptococcus mutans and polymicrobial biofilms in a novel high-throughput active attachment model

BACKGROUND/AIMS

The antimicrobial resistance of microorganisms in biofilms and the polymicrobial interactions in these biofilms that modulate resistance require novel strategies to evaluate the efficacy of caries-preventive compounds. The current study aimed to evaluate the effects of a caries-preventive agent in Streptococcus mutans and polymicrobial biofilms.

METHODS

We developed a novel high-throughput active attachment model. The model consisted of a custom-designed lid containing glass discs that fit on top of standard 24-well plates. Biofilms were formed using either S. mutans C180-2 or saliva. At the end of biofilm formation (up to 96 h) the biofilms were treated with amine fluoride (AmF) solutions. The viability of the biofilms was determined by CFU counts, and metabolic activity was measured via lactate production.

RESULTS

The effect of AmF on the viability of the polymicrobial biofilms was significantly less than that on the S. mutans biofilms, indicating a higher resistance in the complex biofilms. Both types of biofilms became more resistant to AmF with age. The higher resistance of the polymicrobial biofilms was not reflected in metabolic activity; in dose-response experiments AmF reduced lactate production in both types of biofilms to the same extent. Moreover, the age-induced increased resistance in the polymicrobial biofilms was less pronounced in terms of the inhibition of metabolic activity.

CONCLUSIONS

This study clearly shows that when evaluating the efficacy of caries-preventive compounds it is essential to use appropriate polymicrobial biofilm models, and more importantly that efficacy needs to be judged based on the reduction of acid formation (i.e. cariogenic potential) as well as on bacterial viability.

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.

The Effect of Lesion Characteristics at Baseline on Subsequent De- and Remineralisation Behaviour

The aim was to study the effect of lesion characteristics at baseline on subsequent de- and remineralisation behaviour. Artificial lesions used during in vitro and intra-oral studies exhibit an increasing tendency toward net remineralisation with increasing integrated mineral loss at baseline (DeltaZ(base)). Proposed explanations include: (i) small lesions may be more vulnerable to demineralisation, and (ii) large lesions may be more difficult to remineralise. To evaluate these hypotheses, subsurface lesions were created in numerous blocks of human and bovine enamel, with a range of depths and DeltaZ(base), measured by microradiography. One group of lesions was further demineralised, a second group remineralised, and subsequently, both groups re-analysed. Under demineralising conditions, there was a marked decrease in further mineral loss with increasing DeltaZ(base). Under remineralising conditions lesions gained mineral in proportion to DeltaZ(base). The decrease in demineralisation of lesions with larger DeltaZ(base) may be partially a result of decreased intrinsic solubility through modified chemical composition, e.g. loss of magnesium, carbonate etc. The results may explain the tendency toward net remineralisation with increasing DeltaZ(base) in pH-cycling regimes.

The Effect of Adjacent Dentine Blocks on the Demineralisation and Remineralisation of Enamel in vitro

The aim was to investigate interactions between enamel and dentine at low pH under conditions simulating those at the enamel-dentine junction. Sound enamel blocks were demineralised in acid-gel systems, at pH 4.6, either in isolation, next to one, or in the middle of two, abutting dentine blocks. The gels were initially infinitely undersaturated with respect to enamel. In a second study, enamel blocks containing pre-formed lesions were placed in acid-gel systems, at pH 5.0, either in isolation or next to dentine blocks. The systems were initially either partially or infinitely undersaturated. In the partially saturated systems, calcium and phosphate concentrations were representative of plaque fluid. In the first study, demineralisation of enamel next to one dentine block was reduced in inverse proportion to the distance from the dentine. Demineralisation of enamel between two dentine blocks was retarded markedly across the whole block. In the second study, in the partially saturated systems, enamel lesions next to dentine blocks remineralised, whereas those in isolation demineralised further. We suggest that diffusion of dissolved dentine mineral over the enamel in the infinitely undersaturated system was sufficient to reduce undersaturation, thus retarding demineralisation, and that in the partially saturated systems, dentine dissolution together with the added calcium phosphate caused remineralisation of enamel lesions. Fluoride released from dissolving dentine may have augmented these effects. Different rates of demineralisation in enamel and dentine, or enamel remineralisation with concurrent dentine demineralisation, enabled by differences in their solubilities, could help explain the progression of so-called 'hidden caries'.

Effect of dentinal fluid composition on dentin demineralization in vitro

Dentin demineralization is reduced by perfusion with water. We hypothesized that a simulated dentinal fluid (SDF) that contains albumin, in addition to electrolytes, would be more effective in reducing dentin demineralization than water alone, and this effect would increase with increasing flow rate of SDF. Perfusion rate in tooth segments that carried buccal cervical dentin windows was measured in a fluid transport set-up. These windows were then demineralized under perfusion with water, or SDF at 1.47 kPa for 31 days. We analyzed integrated mineral loss and lesion depth with the use of transverse microradiography (TMR), which revealed that 38% more mineral dissolved from dentin lesions perfused with water than from those perfused with SDF. The former were also 18% deeper. Flow rate of dentinal fluid showed no correlation with demineralization. We concluded that composition of dentinal fluid is an important determinant of the rate of lesion formation and progression in dentin.

Microbiological evaluation of dental unit water systems in general dental practice in Europe

A range of opportunistic pathogens have been associated with dental unit water systems (DUWS), particularly in the biofilms that can line the tubing. This study therefore aimed to assess the microbiology of DUWS and biofilms in general dental practices across seven European countries, including the United Kingdom (UK), Ireland (IRL), Greece (GR), Spain (ES), Germany (D), Denmark (DK) and the Netherlands (NL). Water supplied by 51% of 237 dental unit water lines exceeded current American Dental Association recommendations of < or = 200 colony-forming units (CFU) ml(-1). Microbiological loading of the source waters was between 0 (Denmark, the Netherlands and Spain) and 4.67 (IRL) log CFU ml(-1); water line samples from the DUWS ranged from 1.52 (ES) to 2.79 (GR) log CFU ml(-1); and biofilm counts ranged from 1.49 (GR) to 3.22 (DK) log CFU.cm(-2). Opportunistic pathogens such as legionellae (DK and ES), including Legionella pneumophila SG1 (DK and GR), and Mycobacterium spp. (DK, NL, GR, D and ES) were recovered occasionally. Presumptive oral streptococci (ES and NL), oral anaerobes (GR), Candida spp. (UK, NL and ES) and blood (GR and IRL) were detected at relatively low frequencies, but their presence indicated a failure of the 3-in-1 antiretraction valve, leading to back siphonage of oral fluids into the water and biofilm phase. These findings confirm that a substantial proportion of DUWS have high levels of microbial contamination, irrespective of country, type of equipment and source water. The study emphasizes the need for effective mechanisms to reduce the microbial burden within DUWS, and highlights the risk of occupational exposure and cross-infection in general dental practice.

Fluorides in caries prevention and control: empiricism or science

The caries-preventive effects of fluoride are beyond any reasonable doubt! Inclusion of fluoride use in caries prevention protocols has resulted in significant reduction in caries prevalence in the majority of the population. Nevertheless, even in low-caries prevalence populations up to 20% of individuals may suffer to an unacceptable degree from caries. In the history of caries research various phases can be discerned. Starting with the initial - laboratory - studies to reveal the mode of action of fluoride, attention later shifted to intra-oral studies and in situ product testing. Currently much emphasis is given to evidence-based dentistry and guidelines for clinical practice, which trend has also focussed the research on fluoride and caries. While on some topics, such as the efficacy of fluoride toothpastes, evidence is convincing, additional research is indicated to resolve remaining questions. One such question is that of high-prevalence individuals for which a comprehensive research programme focussing both on caries aetiological and behavioural aspects should be further developed. Efforts should continue to be directed at improving our understanding of fluoride, in particular on topics where success so far has failed.

Incidence of mutans streptococci and lactobacilli in oral cleft children wearing acrylic plates from shortly after birth

Children with a palatal cleft can be treated with preoperative infant orthopedics including an acrylic plate that is applied shortly after birth to obturate the cleft. It is advised to wear these plates until the 18th month of age. Such a plate, being a hard non-shedding surface, may be expected to facilitate early colonization of mutans streptococci. The first aim of the present investigation was to assess the incidence of mutans streptococci and lactobacilli in children with cleft lip and/or palate during the first 2 years of life. The second aim was to study whether preoperative orthopedics, that is, the wearing of an acrylic plate, had facilitated the establishment of mutans streptococci and lactobacilli. The third aim was to determine other factors associated with colonization of these organisms in these children. Sixty-two Caucasian Dutch children with cleft lip and/or palate participated in this study. Twenty-four of these children were treated with preoperative infant orthopedics and had been wearing an acrylic plate from within a few days after birth. At regular control visits plaque and saliva samples and samples from the surface of the acrylic plate were taken, while a dental examination was performed to document the emergence of the primary teeth, caries status, gingival condition and oral hygiene procedures. Saliva samples were also taken from the accompanying parents. At the visit at the age of 18 months, the parents were interviewed using a structured questionnaire. At this age, the prevalence of mutans streptococci and lactobacilli was compared to that in a control group of non-cleft children. The oral cleft children wearing an acrylic plate from shortly after birth were colonized earlier with mutans streptococci and lactobacilli than the non-plate oral cleft children. In the children wearing acrylic plates, the prevalence of lactobacilli decreased with age, while the prevalence of mutans streptococci increased. At the age of 18 months the prevalence of mutans streptococci was comparable in both groups of oral cleft children and in the control children. There was no relation between the numbers of mutans streptococci in the saliva of the mothers and their children. The presence of mutans streptococci in the saliva of the oral cleft children was significantly associated with between-meal snacking and with the presence of lactobacilli.

Modification of amino acid residues in carious dentin matrix

The Maillard reaction between sugar and protein has been postulated as the cause for the browning and arrestment of caries lesions. This reaction has been implicated as the cause for decreased degradability of collagen in vivo. The aim of the present study was to verify the occurrence of the reaction in vivo. Carious and sound dentin samples were taken from extracted human teeth and analyzed for the fluorescence characteristic of the Maillard reaction and oxidation and, by HPLC, for Maillard products. In addition, physiological cross-links were analyzed by HPLC. Oxidation- and Maillard reaction-related fluorescence increased in collagenase digests from carious dentin. Advanced Maillard products (carboxymethyllysine and pentosidine) increased, whereas furosine, a marker for the initial reaction, was not observed consistently. This implies no direct addition of sugars to protein, but rather the addi-tion of smaller metabolites and glycoxidation products. In addition, the physiological cross-links hydroxylysinonorleucine and dihydroxylysinonorleucine decreased in carious dentin. Also for hydroxylysylpyridinoline, a decrease was observed, but not consistently. In conclusion, the caries process modifies amino acids in dentin collagen, which can lead to increased resistance against proteolysis and ultimately to caries arrestment.

The Maillard reaction in demineralized dentin in vitro

The Maillard reaction between carbohydrate and protein has been proposed as a cause of the browning of carious lesions. The aim of the present investigation was to determine the occurrence of this reaction in bovine dentin collagen in vitro and to establish the effect of the reaction on the proteolytic degradation of bovine dentin collagen in vitro. Slices of demineralized bovine dentin were incubated with 0.2 M glucose or buffer for 10 weeks at 37 degrees C. The formation of initial (furosine) and advanced (pentosidine) products of the Maillard reaction in dentin exposed to glucose was confirmed by HPLC. After reduction with NaBH4 to prevent intermediate Maillard products from further reaction, slices were either degraded with collagenase for fluorescence measurement or incubated with trypsin or pepsin to assess enzymatic degradation. Fluorescence characteristic for the Maillard reaction increased in glucose-exposed slices. Degradation of collagen by pepsin, but not by trypsin, was greatly depressed following glucose pretreatment. This may indicate an altered sensitivity to proteolytic degradation; the Maillard reaction thus has a potential role in caries arrestment.

The influence of the organic matrix on demineralization of bovine root dentin in vitro

The effect of matrix degradation on the rate of demineralization of dentin lesions was investigated. It was hypothesized that the demineralized matrix would inhibit the demineralization of the underlying mineralized dentin. Bovine root dentin specimens were alternately demineralized and incubated with either a bacterial collagenase or buffer (control). The demineralization was carried out under various conditions: Acetic acid solutions were used to form incipient and advanced erosive lesions, and lactic acid solutions containing a bisphosphonate were used to form incipient subsurface lesions. Under all conditions, the demineralization was found to be accelerated when the matrix was degraded by collagenase. This increase was more pronounced in advanced erosive lesions than in incipient lesions. Microscopic examination of collagenase-treated specimens revealed that the matrix of erosive lesions contained several layers of differently affected matrices, whereas the matrix of subsurface lesions appeared to be equally affected throughout the lesion. In conclusion, the matrix degradation was different in erosive and subsurface lesions but promoted the demineralization in both types of lesions.

Calcification of a cariogenic Streptococcus and of Corynebacterium (Bacterionema) matruchotii

The main aim of this investigation was to challenge the idea that cariogenic streptococci do not calcify. Calcium uptake of calcification of Streptococcus mutans C180-2, proven to be an acidogenic and cariogenic strain, was compared with calcium uptake and calcification of Corynebacterium (Bacterionema) matruchotii, known as a ready calcifier. Bacteria were grown on Brain Heart Infusion Agar (BHIA) and on well-buffered semi-synthetic E-agar, both containing 1.4 mmol/L calcium, 2 g/L glucose, initial pH 7.4. Calcium uptake from BHIA by C. matruchotii (25 mmol Ca/kg wet bacterial cell mass), but not by S. mutans, was found. Grown as a plaque-like lawn on E-agar, the S. mutans cell mass concentrated calcium to 63 +/- 11 mmol/kg compared with 145 +/- 61 mmol/kg in C. matruchotii. X-ray diffraction confirmed the presence of crystalline apatite in the bacterial cell masses. Electron microscopy revealed crystals and mineralized deposits in both organisms. Heavy calcifications in some cells of S. mutans were seen. Calcification was partly inhibited by magnesium ion and by methanehydroxybisphosphonate. S. sobrinus 6715, as well as freshly isolated S. mutans and S. sobrinus from patients, concentrated very large quantities of calcium, up to 500-fold from the medium, when maintained for several weeks on E-agar of initial pH 7.6. Our observations widen the view on acidogenic bacteria as mineralization agents and support the notion that members of the mutans group of streptococci may be involved in events that trigger heavy intracellular calcifications and, possibly, dental calculus formation.

Solubilization of dentin matrix collagen in situ

The effects of the oral environment on dentin matrix collagen were studied. In the partial prostheses of 12 participants, two completely demineralized dentin specimens were mounted covered by a Dacron gauze. After an experimental period of seven weeks, the specimens were transferred to a trypsin-containing buffer for determination of the amount of denatured collagen. Subsequently, the specimens were incubated with a bacterial collagenase for assessment of the amount of collagen. After the intra-oral exposure, there was a collagen loss varying between 1 and 47 wt%. This variation might be due to differences in proteolytic activity of the colonizing microflora. After exposure to the oral environment, only about 0.5 wt% of the available collagen was trypsin-degradable. This indicates a rapid solubilization of the denatured collagen from the specimens into the oral cavity. A separate group of specimens was examined by light microscopy and transmission electron microscopy. Various degrees of breakdown could be discerned. Some experimental specimens showed loss of surface integrity and tubules heavily infected with different types of micro-organisms. The lumens of the tubules were enlarged, sometimes creating caverns as a result of the loss of the intertubular collagenous matrix.

Preventive measures and caries progression: an in vitro study on fissures and smooth surfaces of human molars

The aim of this study was to compare pit-and fissure sealants with fluoride treatments in their effectiveness to inhibit or reduce the progression of enamel lesions, particularly in the fissure region. The effects of fissure probing as used in diagnosis were compared with in the same protocol. Using a combination of image analysis of X-ray pictures of full crowns and a light microscopic evaluation of cross-sections provided information on the three dimensional spread of enamel demineralization. Pit-and-fissure sealants were superior in protecting enamel against progression of demineralization, as long as the walls of the fissures were well covered. The protection against demineralization offered by fluoride was mainly restricted to the smooth enamel surfaces; in fissures no protection could be measured. Surface defects of the enamel caused by probing or abrasion aided the progress of the lesion.

Silica-induced precipitation of calcium phosphate in the presence of inhibitors of hydroxyapatite formation

The promotion and the inhibition of hydroxyapatite formation by various substances were determined by measurement of the induction time of spontaneous precipitation (ti) from supersaturated solutions. Silica was found to decrease ti in Hepes-buffered (pH 7.2) supersaturated solutions with a wide range of calcium-to-phosphate ratios and concentrations. Also, in suspensions of the oral bacteria S. mutans or C. matruchotii in 1 mmol/L calcium, 7.5 mmol/L phosphate, and 50 mmol/L Hepes (pH 7.2), silica was capable of stimulating precipitation. Macromolecules derived from these bacteria by freezing and thawing appeared to be strong inhibitors of calcium phosphate precipitation. In the presence of silica, the effects of these bacterial inhibitors could be partially overcome, which supports the idea that silica in dental plaque is a promoter of calculus formation. In contrast, inhibition of calcium phosphate precipitation by a low-molecular-weight inhibitor, pyrophosphate, could not be counteracted by silica.

In vitro demineralization of enamel by F-sensitive and F-resistant mutans streptococci in the presence of 0, 0.05, or 0.5 mmol/L NaF

Lactate production and accompanying enamel demineralization by fluoride-sensitive and fluoride-resistant mutans streptococci were studied in an in vitro demineralization model in the presence of 0, 0.05, or 0.5 mmol/L NaF. The fluoride-resistant strains were derived from laboratory strains or were recently isolated strains from xerostomic patients on high-dose fluoride therapy. The demineralization model was composed of a cell suspension in a glucose-agarose gel overlying a bovine enamel block. Lactate and calcium content of the agarose were determined after 22-hour incubations at 37 degrees C. Fluoride-resistant variants of Streptococcus sobrinus 6715-15 produced less lactate and caused less demineralization than did the parent strain even in the presence of fluoride. On the other hand, fluoride-resistant variants of Streptococcus mutans C180-2 and of S. mutans GS-5 produced more acid and caused greater demineralization than did their respective parent strains, both in the absence and presence of fluoride. Two recently isolated fluoride-resistant S. mutans strains produced more lactate and demineralized enamel more than did two recently isolated S. mutans strains from normal human subjects, both in the presence of 0 and 0.05 mmol/L NaF. It is concluded that adaptation to fluoride resistance does not invariably reduce the cariogenicity of mutans streptococci nor the effectiveness of fluoride in preventing demineralization.

A quantitative analysis of mineral loss and shrinkage of in vitro demineralized human root surfaces

Demineralization of dentin specimens proceeds at a faster rate than that of enamel. Although this is generally accepted, a quantification of the rate of formation of root lesions is hampered by the shrinkage of the lesions when these are dried prior to microradiographic analysis. This leads to a significant underestimation of the lesion depth and total mineral loss. The aim of this paper was to quantitate the rate of mineral loss during root lesion formation in vitro and to determine the shrinkage of root specimens as a result of drying. Unerupted roots of human teeth were subjected to a demineralizing system of 0.1 mol/L lactate buffer (pH = 4.8) with 0.2 mmol/L methanehydroxydiphosphonate during four, 11, 22, and 44 days. The root lesions were assessed by quantitative microradiography. The demineralizing solutions were analyzed to determine the amounts of root tissue dissolved. A comparison of these two sets of data showed that, with the demineralizing system used, root lesions may shrink up to 62%. Fixation of the specimens in fixative did not affect this shrinkage. Chemical analysis showed that mineral loss proceeded linearly with time. From the data-sets of this study, a model was developed to compensate for the shrinkage in the dentin specimens. In this way, it was possible to calculate the lesion depth at four demineralization times as being 130, 220, 320, and 530 microns, respectively. These values were in agreement with a microscopic determination of the lesion depth.

Induction of calcium phosphate precipitation by titanium dioxide

Titanium powder and various titanium dioxides were tested for their capacity to reduce the induction time for calcium phosphate precipitation from supersaturated solutions. Only after a pretreatment aimed at increasing its oxide surface layer was titanium powder found to accelerate the precipitation from solutions containing 2 mmol/L CaCl2, 2 mmol/L KH2PO4, 50 mmol/L Hepes, pH 7.2, and to induce precipitation from metastable solutions containing 1.2 mmol/L CaCl2, 1.2 mmol/L KH2PO4, 50 mmol/L Hepes, pH 7.2, at 37 degrees C. Even stronger effects were found when suspensions of the titanium dioxides anatase or rutile (10-50 micrograms/mL) were added to these solutions. TiO2 appeared to serve as a reactive substrate for secondary nucleation at a wide range of calcium-to-phosphate ratios and concentrations, even in the presence of 40 mg/mL bovine serum albumin, which completely inhibited precipitation in control incubations. These results suggest that the oxide surface layer of titanium implants may induce calcium phosphate precipitation in the metal-to-bone interface, which may play a role in the integration of such implants in bone.

Acidogenesis in relation to fluoride resistance of Streptococcus mutans

The velocity of acid production (Vap) of Streptococcus mutans C180-2 and of 2 fluoride-resistant mutant strains S. mutans C180-2FR and S. mutans C180-2MFR was examined in vitro at pH values between 7.0 and 4.5. The Vap of the fluoride-resistant mutants was lower than the Vap of the parent strain at pH greater than or equal to 6.0. At pH 5.5 and 5.0, the Vap of the mutant strains was higher than the Vap of the parent strain, whereas at pH 4.5 no significant differences were observed between the Vap of the 3 strains. The fluoride sensitivity of all 3 strains was amplified by a low pH environment. The fluoride concentration necessary to inhibit the acid production of the parent strain completely was 27 mM at pH 7 and 0.1 mM at pH 4.5. For the 2 mutants, the corresponding concentrations were 65 mM at pH 7 and 0.5-0.7 mM at pH 4.5. The results suggest that, if S. mutans acquires fluoride resistance in vivo, the rate of acid production in dental plaque may be decreased at pH greater than or equal to 6, but increased at lower pH levels. Low concentrations of fluoride inhibit acid production less effectively.

In vitro studies on the effects of fluoride on de- and remineralization

The recent literature extensively describes the role of ambient fluoride in the de- and remineralization of dental enamel. Fluoride in sub-ppm concentrations is effective in promoting mineral deposition and inhibiting mineral dissolution. The latter phenomenon is most likely attributable to the concomitant precipitation of a fluoride-rich mineral phase which inhibits further dissolution. These fundamental processes result in an inhibition of enamel demineralization and an enhancement of enamel lesion remineralization. Alternatively, fluoride may also induce the 'arrestment' of enamel lesions. For the in vivo patterns and the effects of caries-preventive substances to be studied, de- and remineralization can best be examined with a pH-cycling system in which the pH depressions occurring in the oral environment are mimicked in a laboratory model. Such an approach has proved useful in developing optimal fluoride schemes which can be tested in animal, intraoral, and clinical studies.

Intra-oral retention of fluoride by bovine enamel from amine fluoride toothpaste and 0.4% amine fluoride liquid application

The purpose of this study was to compare in vivo fluoride accumulation in enamel and in enamel lesions from a single topical fluoride application and daily toothbrushing with a fluoride dentifrice. Amine fluoride preparations were used for both products. Intra-oral appliances with bovine enamel specimens were worn by volunteers during a period of seven weeks. After this period, the specimens were analyzed for fluoride uptake and change in demineralization susceptibility. The results demonstrated that lesions had a high fluoride uptake capacity. Fluoride content values increased by 25-30 micrograms/cm2 during a single topical application, as compared with 10-15 micrograms/cm2 during seven weeks of toothbrushing. About half the fluoride acquired as a result of topical treatment was lost during subsequent exposure to the oral fluids when no further fluoride supplementation was given. The uptake of fluoride by sound enamel was comparatively small, regardless of the use of fluoride dentifrice or application. The presence of mature plaque at the time of fluoride application did not affect the amounts of fluoride delivered. Acid susceptibility tests showed that the enamel solubility exhibited a negative correlation with fluoride content of the specimens.

In vivo bovine enamel remineralization and fluoride uptake from two dentifrices containing different fluoride concentrations

In this study, the differences in lesion remineralization and fluoride uptake after brushing with a 300 ppm F or a 1000 ppm F dentifrice (as NaF) were investigated. Twenty volunteers with partial dentures in their lower jaws were divided into two groups. Bovine enamel slabs with artificial lesions were mounted in the dentures. After a test period of six weeks, the slabs were taken out and analyzed. There was no statistically significant difference between the two groups, in F-uptake and susceptibility of the enamel to demineralization, although the findings favored the 1000 ppm F toothpaste. The microradiograms in the 300 ppm F group showed lesions more pronounced than those in the 1000 ppm group. The present study cannot support the presumption that the fluoride content of regular NaF dentifrices can be lowered to 300 ppm F without undesirable effects on lesion arrest and remineralization.

Polymer adhesion to enamel. The influence of viscosity and penetration

For a four-component acrylate adhesive system the influence of viscosity and penetration on adhesion to enamel was investigated. While low viscous monomer adhesives yield both inter- and intraprismatic penetration in enamel after polymerization, highly viscous adhesives result in the former type only. This difference in penetration of the adhesive in enamel is reflected quantitatively in the tensile strength values. Scanning electron microscopic measurements on the tag formation under various circumstances have been carried out. The SEM experiments show, e.g. that at polymerization times less than 1-5-2 min (a clinical necessity) polymer-tags were usually fractured from the bulk material due to stresses generated.