In situ models, physico-chemical aspects

Ion release and hydroxyapatite precipitation of resin composites functionalized with two types of bioactive glass

OBJECTIVES

To prepare experimental composites with bioactive glass (BG) and investigate their release of calcium (Ca), phosphate (PO₄), and fluoride (F), as well as pH changes and apatite precipitation after immersion.

METHODS

Experimental composites were prepared with 0, 10, or 20 wt% of either BG 45S5 or a customized low-Na F-containing BG. Three commercial ion-releasing materials were used for reference. Material specimens were immersed in lactic acid (pH = 4.0) and artificial saliva (pH = 6.4). Ion concentrations (atomic absorption spectrometry for Ca, UV-vis spectrometry for PO₄, and ion-selective electrode for F) and pH were measured after 4, 8, 12, 16, 20, 24, 28, and 32 days. After immersion, composite specimens were analyzed using scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy.

RESULTS

Material-dependent concentrations of Ca, PO₄, and F were measured in the lactic acid solution, while a decrease of Ca and PO₄ concentrations was observed in artificial saliva. The uptake of ions from artificial saliva indicates their precipitation on specimen surfaces, which was supported by the results of SEM and FTIR investigations. In experimental composites functionalized with both bioactive glass types and a commercial "alkasite" material, apatite was precipitated not only in artificial saliva but also in the lactic acid solution.

CONCLUSIONS

Experimental BG-containing composites and selected commercial restorative materials demonstrated the potential for releasing multiple ion types and increasing pH.

CLINICAL SIGNIFICANCE

The observed effects can be beneficial for preventing demineralization and promoting remineralization of dental hard tissues, while apatite precipitation can additionally help in sealing marginal discontinuities.

A Potassium Based Fluorine Containing Bioactive Glass for Use as a Desensitizing Toothpaste

Potassium releasing bioactive glasses (BAGs) may offer improved relief for dentine hypersensitivity compared to conventional sodium containing BAGs by releasing K⁺ ions for nerve desensitization and occluding dentinal tubules to prevent fluid flow within dentinal tubules. Potassium oxide was substituted for sodium oxide on a molar basis in a fluoride containing BAG used in toothpastes for treating dentine hypersensitivity. The BAG powders were then immersed in an artificial saliva at pH 7 and tris buffer and the pH rise and ion release behavior were characterized by ICP-OES and ISE. The potassium and sodium containing BAGs were characterized by XRD, DSC, FTIR and NMR. Both BAGs presented amorphous diffraction patterns and the glass transition temperature of the potassium glass was higher than that of the sodium glass. The ³¹P MAS-NMR spectra indicated a peak at 2.7 ppm corresponding to apatite and a small peak at −103 ppm indicated crystallization to fluorapatite. Both BAGs dissolved and formed apatite at similar rates, although the dissolution of the potassium glass was slightly slower and it released less fluoride as a result of partial nanocrystallization to fluorapatite upon quenching. The potassium release from the potassium ions could potentially result in nerve deactivation when used in toothpastes.

Inflammatory Response Mechanisms of the Dentine-Pulp Complex and the Periapical Tissues

The macroscopic and microscopic anatomy of the oral cavity is complex and unique in the human body. Soft-tissue structures are in close interaction with mineralized bone, but also dentine, cementum and enamel of our teeth. These are exposed to intense mechanical and chemical stress as well as to dense microbiologic colonization. Teeth are susceptible to damage, most commonly to caries, where microorganisms from the oral cavity degrade the mineralized tissues of enamel and dentine and invade the soft connective tissue at the core, the dental pulp. However, the pulp is well-equipped to sense and fend off bacteria and their products and mounts various and intricate defense mechanisms. The front rank is formed by a layer of odontoblasts, which line the pulp chamber towards the dentine. These highly specialized cells not only form mineralized tissue but exert important functions as barrier cells. They recognize pathogens early in the process, secrete antibacterial compounds and neutralize bacterial toxins, initiate the immune response and alert other key players of the host defense. As bacteria get closer to the pulp, additional cell types of the pulp, including fibroblasts, stem and immune cells, but also vascular and neuronal networks, contribute with a variety of distinct defense mechanisms, and inflammatory response mechanisms are critical for tissue homeostasis. Still, without therapeutic intervention, a deep carious lesion may lead to tissue necrosis, which allows bacteria to populate the root canal system and invade the periradicular bone via the apical foramen at the root tip. The periodontal tissues and alveolar bone react to the insult with an inflammatory response, most commonly by the formation of an apical granuloma. Healing can occur after pathogen removal, which is achieved by disinfection and obturation of the pulp space by root canal treatment. This review highlights the various mechanisms of pathogen recognition and defense of dental pulp cells and periradicular tissues, explains the different cell types involved in the immune response and discusses the mechanisms of healing and repair, pointing out the close links between inflammation and regeneration as well as between inflammation and potential malignant transformation.

Sorptivity of water in enamel for categorizing caries lesions

OBJECTIVE

The aim of this research was to determine the association between sorptivity of water and the state of mineralization in carious enamel of different stages of severity.

METHODS

As a preliminary work, water droplets of 1.5 μL were placed on the surfaces of hydroxyapatite (HA) discs of different densities. The water droplet profile was dynamically recorded every second over a period of 10 s using a contact angle meter to determine the relationship between sorptivity and density. To measure and calculate sorptivity on enamel surfaces, varnish was painted on the labial surface of 96 extracted caries-free human teeth, leaving two 1.4 ± 0.1 mm diameter circular exposed test sites. The specimens were randomly divided into 6 groups (n = 16) and subjected to 0(G0), 7(G7), 14(G14), 21(G21), 28(G28) and 35(G35) days of pH cycling, respectively. A 0.7 μL water droplet was placed on each exposed site and Optical Coherence Tomography was used to measure its height every 10 seconds for 2 min. Sorptivity was computed by considering sorption equations and Washburn's analysis of capillary kinetics and correction for evaporation was also performed. Micro-Computed Tomography scans of the specimens were obtained and delta Z (ΔZ) is the parameter used to measure mineral loss. ΔZ at 10 μm (ΔZ₁₀) and 50 μm (ΔZ₅₀) from the surface were calculated. One-way ANOVA and Post-hoc Tukey tests were used to compare sorptivity between groups and bivariate correlations were used to analyze the association between sorptivity and ΔZ.

RESULTS

Sorptivity was found to be inversely and linearly correlated with HA density with R² value of 0.95. With enamel, there is a general trend of increase in mean sorptivity from G0 to G35, except for a decrease in G21. The same trends were observed for both ΔZ₁₀ and ΔZ₅₀. The decrease in sorptivity in G21 coincided with the presence of a surface hypermineralized layer in G21 samples. Post-hoc Tukey showed significant differences in mean sorptivity between G0 and G14, G0 and G21 as well as G14 and G21. Post-hoc Dunnett's T3 showed significant differences for ΔZ₁₀ between G0 and G14 as well as G14 and G21. Significant correlation between mean sorptivity and ΔZ₁₀ was detected with Pearson correlation coefficient of 0.461. For ΔZ₅₀, post-hoc Tukey showed significant differences between G0 and G14 but no significant difference was detected between G14 and G21. No correlations were detected between mean sorptivity and ΔZ₅₀.

SIGNIFICANCE

Sorptivity was found to be inversely and linearly correlated with HA density with R² value of 0.95. With enamel, there is a general trend of increase in mean sorptivity from G0 to G35, except for a decrease in G21. The same trends were observed for both ΔZ₁₀ and ΔZ₅₀. The decrease in sorptivity in G21 coincided with the presence of a surface hypermineralized layer in G21 samples.

Investigation of Factors Influencing Stain Formation Utilizing an in Situ Model

In order to understand the factors of extrinsic stain formation more fully, we have developed an in situ stain model. This consists of polished bovine enamel blocks attached to partial or full dentures worn by adult volunteers for 24 h per day. The dentures were cleaned twice daily with a commercial dentifrice and toothbrush, with care taken to avoid brushing the inserts. A Minolta CR321 Chroma Meter in the L*a*b* mode was used for taking reflectance measurements of the stain formed on the enamel inserts. From these values, changes in the color of the inserts were calculated and the level of stain determined. In general, the stain formed on the enamel inserts was yellow and increased in intensity and darkness with time. The enamel inserts with the largest stain increases were from smokers rather than non-smokers. No correlation was observed between amount of stain and quantity of tea and coffee consumed. When the effects of surface roughness on in situ stain formation were considered, the major variable in this study was found to be the location of the enamel insert in the denture rather than the surface roughness.

Effects of dentifrices differing in fluoride compounds on artificial enamel caries lesions in vitro

The aim of this study was to compare the caries-preventive effect of a stabilized stannous fluoride/sodium fluoride dentifrice containing sodium hexametaphosphate with those of a regular, solely sodium fluoride-containing and amine fluoride-containing dentifrice on pre-demineralized bovine enamel specimens using a pH-cycling model. Bovine enamel specimens with two artificial lesions each were prepared. Baseline mineral loss of both lesions was analyzed using transversal microradiography (TMR). Eighty-five specimens with a mean (SD) baseline mineral loss of 3393 (683) vol% × µm were selected and randomly allocated to five groups (n = 13/15). Treatments during pH-cycling (28 days and 2 × 20 min demineralization/day) were: brushing twice daily with slurries of AmF (1400 ppm F^-), NaF (1450 ppm F^-), SnF₂/NaF (1100 ppm F^-/350 ppm F^-), and fluoride-free (FF) dentifrices or they were immersed in distilled water and remained unbrushed (NB). Subsequently, from each specimen one lesion was covered with acid-resistant varnish, while the remaining lesion was demineralized for another 14 days. Differences in integrated mineral loss (∆∆Z) were calculated between values before and after pH-cycling (∆∆Z _E1) as well as before pH-cycling and after second demineralization (∆∆Z _E2) using TMR. Treatments AmF and NaF induced a significantly higher mineral gain (∆∆Z _E1/∆∆Z _E2) compared to treatments FF and NB (p < 0.05; ANOVA test). Except for treatments AmF and NaF no significant differences in mineral loss between before and after pH-cycling could be observed (p < 0.05; t test) [∆∆Z _E1: AmF:1563 (767); NaF:1222 (1246); SnF₂/NaF:258 (1259); FF:-52 (1223); NB:-151 (834)]. Both dentifrices with either AmF or NaF promoted remineralization, whereas SnF₂/NaF dentifrice did not promote remineralization in a biofilm-free pH-cycling model.

In situ remineralisation response of different artificial caries-like enamel lesions to home-care and professional fluoride treatments

Background

Artificial lesions produced by different protocols might directly influence the response to different remineralising treatments. This study compared the response of different artificial caries-like enamel lesions to home-care and professional fluoride based-remineralising treatments in situ.

Methods

The tested demineralising protocols were methylcellulose- MC gel, polyacrylic acid - PA gel, tetraethyl methylene diphosphanate - TEMDP solution, and acetate- Buffer solution. The lesions were remineralised using an in situ model, following a crossover and double blind design. Twelve subjects wore intra-oral appliances during 3 phases (3 d each): control (C) (saliva); home-care F⁻ treatment (FD) (1,100 ppm F⁻ dentifrice, 2x1 min/day); and professional (FVD) (22,600 ppm F⁻ varnish) plus FD. The de-remineralisation was measured by transverse microradiography-TMR and hardness (surface hardness/cross-sectional hardness, SH/CSH, respectively).

Results

For SH, lesions produced by PA gel were the only one showing significant differences among the remineralising treatments (C x FD x FVD); while the TEMDP lesion were not responsive to any fluoride treatment (for both SH/CSH). For TMR, there were no differences among the remineralising treatments, regardless of the type of lesion. Generally, the most responsive lesions to fluoride were the less demineralised lesions (considering hardness: PA gel and Buffer).

Conclusions

The type of lesion has influence on the surface remineralisation degree induced by home-care and professional fluoride treatments using this in situ model.

The effect of various model parameters on enamel caries lesions in a dose-response model in situ

OBJECTIVES

The aim of this exploratory double-blinded, randomized, cross-over, in situ study was to compare the effects of various model parameters ('intervention', 'brushing', 'position') on enamel caries lesions in a dose-response model.

METHODS

In each of four experimental legs of four weeks 16 participants wore intraoral mandibular appliances with four 'plaque-retaining' and four 'easily cleanable' positioned pre-demineralized bovine enamel specimens in the vestibular flanges mimicking proximal and buccal surfaces, respectively (n=512). The four randomly allocated interventions (either application only or brushing) included the following dentifrices: AlF3 1360ppmF(-)+chlorhexidine 0.05% (Lacalut aktiv, LA1360), NaF 1,450ppmF(-)(Blend-a-Med ProExpert), NaF 500ppm F(-) and 0ppm F(-) as negative control (NC) (both experimental, based on Blend-a-Med ProExpert).

RESULTS

Differences in integrated mineral loss (ΔΔZ) and lesion depth (ΔLD) were calculated between values before and after the in situ period using transversal microradiography. Significant differences for ΔΔZ [adjusted mean (95% CI))] were found between NC, NaF500 and LA1360for both 'plaque-retaining' [-1830 (-2371;1289); -986 (-1530;442); -2 (-548;544)vol%×μm] as well as 'easily cleanable' specimens [-399 (-682; -116); -391 (-672; -110); -16 (-302;270)vol%×μm]. Values for NaF1450 revealed a similar dose-response as LA1360.Values for LA1360 and NaF1450 did not differ significantly (p>0.05; ANCOVA).

CONCLUSION/CLINICAL SIGNIFICANCE

The design of the present in situ study was able to reveal a fluoride dose-response to hamper further demineralization of enamel specimens for 'easily cleanable' and 'plaque-retaining' sites being brushed or not. Particularly 'plaque-retaining' sites seem to be recommendable for measuring potential anticaries efficacy in situ.

Molecular and Structural Evaluation of Dentin Caries-Like Lesions Produced by Different Artificial Models

This study evaluated structural and molecular issues of dentin caries-like lesions produced by different artificial models (ACL) compared with natural caries lesions (NCL). One hundred twenty-four sound occlusal dentin blocks and 47 carious blocks were obtained and surface hardness was analyzed (SH1). They were assigned to groups according to ACL: GB: Biological; GC: Chemical; GIS: In situ; GNC: natural caries (control). Blocks from groups 1, 2 and 3 were submitted to caries lesion induction. NCL and ACL blocks were submitted to surface hardness (SH 2), FT-Raman and µEDXRF analysis. All blocks were longitudinally sectioned and one of the halves was submitted to cross-sectional hardness (CSH) and the other to SEM analysis. SH1 and SH2 data were submitted to t test (unpaired and paired, respectively), CSH and SEM data to two-way and one-way ANOVA respectively, and Tukey and t tests, respectively (p<0.05). Data from FT-Raman/µEDXRF were submitted to one-way ANOVA and Dunnett multiple-comparisons test (a=0.05). GB and GNC showed lowest SH2 values that were significantly different from GC and GIS. Regarding CSH, GB and GNC showed no significant difference between them. SEM showed similar caries lesion depth for GB and GNC, being significantly higher than for GC and GIS. µEDXRF showed similar values of calcium and phosphate for GB and GNC; GNC values were significantly different from GIS. No significant difference was found among the groups concerning phosphate, carbonate and CH bonds values. For collagen type I, GC values were significantly different compared to other groups. It may be concluded that caries-like lesions produced by GB were the closest model to NCL.

The effects of fruit smoothies on enamel erosion

OBJECTIVES

This prospective, randomised in vitro study was to investigate the pH and titratable acidity of fruit smoothie drinks and to assess the effect of these drinks on enamel erosion.

METHOD

Fifty enamel slabs were divided into five groups which were allocated to the sample solutions groups: Innocent(®) smoothie strawberries and bananas (SB), Innocent(®) smoothie mangoes and passion fruit (MP) and Diet Coke. Distilled deionised water (DD) was used as negative control and citric acid 0.3% as positive control. All the slabs were subjected to a 21-day pH cycling regime involving 2 min of immersions, five times a day with appropriate remineralization periods in between. Measurement of surface loss was assessed using profilometry. Independent sample t tests were used to compare mean.

RESULTS

The titratable acidity for both test smoothies were 3.5-4 times more than that needed to neutralise Diet Coke and citric acid 0.3%. The pH of SB, MP smoothie and Diet Coke was found to be 3.73, 3.59 and 2.95, respectively. MP smoothie caused the greatest amount of surface loss followed by Diet Coke. Both smoothies were found to cause significant surface loss. MP smoothie resulted in significantly higher surface loss compared with MB smoothie and citric acid 3 %.

CONCLUSION

The smoothies tested were acidic and had high titratable acidity. They produced a significant erosion of enamel in vitro. The results of this study suggest that there should be increased awareness of the erosive effects of smoothies especially as their consumption seems to be on the increase.

Different protocols to produce artificial dentine carious lesions in vitro and in situ: hardness and mineral content correlation

This study compared dentine demineralization induced by in vitro and in situ models, and correlated dentine surface hardness (SH), cross-sectional hardness (CSH) and mineral content by transverse microradiography (TMR). Bovine dentine specimens (n = 15/group) were demineralized in vitro with the following: MC gel (6% carboxymethylcellulose gel and 0.1 M lactic acid, pH 5.0, 14 days); buffer I (0.05 M acetic acid solution with calcium, phosphate and fluoride, pH 4.5, 7 days); buffer II (0.05 M acetic acid solution with calcium and phosphate, pH 5.0, 7 days), and TEMDP (0.05 M lactic acid with calcium, phosphate and tetraethyl methyl diphosphonate, pH 5.0, 7 days). In an in situ study, 11 volunteers wore palatal appliances containing 2 bovine dentine specimens, protected with a plastic mesh to allow biofilm development. The volunteers dripped a 20% sucrose solution on each specimen 4 times a day for 14 days. In vitro and in situ lesions were analyzed using TMR and statistically compared by ANOVA. TMR and CSH/SH were submitted to regression and correlation analysis (p < 0.05). The in situ model produced a deep lesion with a high R value, but with a thin surface layer. Regarding the in vitro models, MC gel produced only a shallow lesion, while buffers I and II as well as TEMDP induced a pronounced subsurface lesion with deep demineralization. The relationship between CSH and TMR was weak and not linear. The artificial dentine carious lesions induced by the different models differed significantly, which in turn might influence further de- and remineralization processes. Hardness analysis should not be interpreted with respect to dentine mineral loss.

Effect of a 5000 ppm fluoride toothpaste and a 250 ppm fluoride mouth rinse on the demineralisation of dentin surfaces

Background

The purpose of this study was to test the null hypothesis that there is no difference between the effect of (1) a 5000 ppm fluoride toothpaste, and (2) a 250 ppm fluoride mouth rinse on demineralized human dentin surfaces, against the alternative hypothesis of a difference.

Findings

Dentin specimens were obtained from the cervical regions of 45 extracted human third molars. Half the surface of each specimen was sealed with a self-etching adhesive system and served as the reference surface. The dentin specimens were randomly assigned to one of the three groups, 5000 ppm fluoride toothpaste (Duraphat), 250 ppm fluoride mouth rinse (Meridol) and distilled water (negative control).

An intraoral appliance was made for one volunteer. In each test cycle, 15 specimens were inserted in the appliance and worn for 24 hours a day, over a period of three weeks.

Once daily, the appliance was immersed in the agent being tested; either toothpaste slurry, mouth rinse or distilled water for 60 seconds.

Demineralization was assessed in terms of lesion depth (μm) and mineral loss (vol. % × μm) by transversal microradiography. Data analysis was accomplished using Kolmogorov-Smirnov test and ANOVA (SPSS 12.0).

Statistically significant differences for mineral loss and lesion depth were found between the toothpaste and the mouth rinse as well as between the toothpaste and the control group, but not between the mouth rinse and the control group.

Conclusion

Within the limitations of this study, the results suggest that treatment of demineralised dentin with a toothpaste containing 5000 ppm fluoride may considerably reduce mineral loss and lesion depth on exposed dentin.

Remineralization of deep enamel dentine caries lesions

Enamel remineralization is generally studied in superficial (up to 100 mum) lesions, but in vivo caries lesions may be tenfold deeper. This article addresses the question whether deep lesions, and extending into dentine, can be remineralized under optimal conditions and if this process is influenced by agents affecting calcium phosphate precipitation and dissolution. Lesions through enamel into dentine were first formed in thin sections and then continuously remineralized for periods up to 200 days. With longitudinal assessment by transversal microradiography it was showed that remineralization throughout the depth of the lesion and into the dentine was possible, although this process is very slow. Fluoride and bisphosphonate treatments affected mainly the deposition in the outer enamel. Although it was assumed that this would affect the diffusion of ions to deeper layers, the treatments had no impact on remineralization in the inner enamel or dentinal parts of the lesions. These findings are discussed with relevant theoretical considerations, and in their possible clinical implications.

The anti-caries efficacy of calcium carbonate-based fluoride toothpastes

AIM

To summarise clinical support for the anti-caries efficacy of fluoride toothpastes containing sodium monofluorophosphate (SMFP) and to discuss the possible means by which the abrasive particles in calcium carbonate-based SMFP toothpastes might complement and/or enhance fluoride efficacy.

BACKGROUND

The anti-caries efficacy of fluoride has been proven beyond any reasonable doubt, and the efficacy of SMFP, when incorporated into a variety of compatible toothpaste formulations, has been established in numerous clinical trials. Calcium carbonate-based toothpastes may also influence caries by effecting an increase in plaque calcium levels; an inverse relationship between plaque calcium and caries is well-established. It has also been reported that plaque fluoride levels are dependent on plaque calcium levels. Hence elevated plaque calcium resulting from the use of calcium carbonate-based toothpastes has the potential to elevate plaque fluoride, itself linked to reduced caries experience. It has been shown that calcium carbonate particles are retained by plaque and this may also influence caries by neutralising harmful plaque acids and concurrently liberating calcium.

CONCLUSION

Fluoride delivered from calcium carbonate-based SMFP toothpastes is an effective means of reducing caries. Further, calcium carbonate may confer additional benefits through elevation of oral calcium levels and neutralisation of plaque-acids.

An in situ model to study the toothpaste abrasion of enamel

OBJECTIVES

In order to understand the clinical relevance of dentifrice abrasivity on the dentition in vivo, an in situ enamel wear model has been developed.

MATERIALS AND METHODS

Polished human enamel blocks were indented with a Knoop diamond, attached to dentures and worn by adult volunteers for 24 h per day. The blocks were brushed for 30 s, twice per day with dentifrices of known relative dentine abrasivity (RDA) and relative enamel abrasivity (REA). The dentifrices used were either dentifrice A (RDA=85, REA=3.4), dentifrice B (RDA=189, REA=2.0) or dentifrice C (RDA=132, REA=42.7). After 28 days, the blocks were removed and the geometry of each Knoop indent was remeasured. From the baseline and post-treatment values of indent length, the amount of enamel wear was calculated from the change in the indent depth.

RESULTS

The median values for enamel wear of dentifrices A, B and C were -0.02, 0.01 and -0.48 microm, respectively. The differences between dentifrice C and dentifrices A and B were of statistical significance.

CONCLUSION

This study has demonstrated the usefulness of an in situ technique for investigating the relationship between the abrasivity of a dentifrice in vitro and the wear of enamel in situ.

Modern methods for assessing the cariogenic and erosive potential of foods

Assessment of the acidogenic and cariogenic erosive potential of foods requires the use of reproducible and standardised methods. The methodology laid down at the San Antonio conference in 1985 remain today the basis for research on and the testing of foods. The 1999 workshop has updated the appropriate methodology and introduced guidelines on the testing of erosive potential of foods. It is to be hoped that researchers wishing to evaluate foods and their effect on the teeth will continue to use these methods.

Food starches and dental caries

Sucrose and starches are the predominant dietary carbohydrates in modern societies. While the causal relationship between sucrose and dental caries development is indisputable, the relationship between food starch and dental caries continues to be debated and is the topic of this review. The current view of dental caries etiology suggests that in-depth evaluation of the starch-caries relationship requires the consideration of several critical cariogenic determinants: (1) the intensity (i.e., the amount and frequency) of exposure of tooth surfaces to both sugars and starches, (2) the bioavailability of the starches, (3) the nature of the microbial flora of dental plaque, (4) the pH-lowering capacity of dental plaque, and (5) the flow rate of saliva. Studies of caries in animals, human plaque pH response, and enamel/dentin demineralization leave no doubt that processed food starches in modern human diets possess a significant cariogenic potential. However, the available studies with humans do not provide unequivocal data on their actual cariogenicity. In this regard, we found it helpful to distinguish between two types of situations. The first, exemplified by our forebears, people in developing countries, and special subject groups in more modern countries, is characterized by starch consumption in combination with a low sugar intake, an eating frequency which is essentially limited to two or three meals per day, and a low-to-negligible caries activity. The second, exemplified by people in the more modern societies, e.g., urban populations, is characterized by starch consumption in combination with significantly increased sugar consumption, an eating frequency of three or more times per day, and a significantly elevated caries activity. It is in the first situation that food starches do not appear to be particularly caries-inducive. However, their contribution to caries development in the second situation is uncertain and requires further clarification. Although food starches do not appear to be particularly caries inducive in the first situation, the possibility cannot be excluded that they contribute significantly to caries activity in modern human populations. The commonly used term "dietary starch content" is misleading, since it represents a large array of single manufactured and processed foods of widely varying composition and potential cariogenicity. Hence, increased focus on the cariogenicity of single starchy foods is warranted. Other aspects of starchy foods consumption, deserving greater attention, include the bioavailability of starches in processed foods, their retentive properties, also in relation to sugars present (starches as co-cariogens), their consumption frequency, the effect of hyposalivation on their cariogenicity, and their impact on root caries. The starch-caries issue is a very complex problem, and much remains uncertain. More focused studies are needed. At present, it appears premature to consider or promote food starches in modern diets as safe for teeth.

Current concepts on the theories of the mechanism of action of fluoride

The caries-preventive effect of fluoride is mainly attributed to the effects on demineralization/remineralization at the tooth oral fluids interface. Sub ppm levels of fluoride in saliva are effective in shifting the balance from demineralization, leading to caries, to remineralization. This is attributed to the fluoride-enhanced precipitation of calcium phosphates, and the formation of fluorhydroxyapatite in the dental tissues. Low fluoride levels are found in saliva after toothbrushing with fluoride containing dentifrices. Similar concentrations are ineffective in interfering with processes of growth and metabolism of bacteria, and also do not result in a significantly reduced dissolution of tooth mineral as a result of (firmly bound) fluoride incorporation. Comparative studies of fluoride efficacy have shown that higher concentrations in solution are needed in pH-cycling studies of dentine than in enamel to maintain the mineral balance or to induce remineralization. This is attributed to the greater solubility of the dentine and the smaller size of the dentine crystallites compared to enamel. Fluoride slow-release devices, in the form of fluoride-releasing restorative materials, may serve to increase the fluoride levels in saliva and plaque to levels at which caries can be prevented, also in high-risk patients. Research questions for the next millennium and future perspectives for fluoride applications should be found in the retention and slow release of fluoride after various combinations of fluoride treatment, the combination of fluoride and anti-microbial treatment, the individualization of caries prevention, and the combination of preventive schemes with new developments in caries diagnosis.