Effect of fluoride on the saturation of an acetate buffer with respect to hydroxyapatite

Changing paradigms in concepts on dental caries: consequences for oral health care

Kuhn proposed in his Structure of Scientific Revolutions (1962) that the theoretical framework of a science (paradigm) determines how each generation of researchers construes a causal sequence. Paradigm change is infrequent and revolutionary; thereafter previous knowledge and ideas become partially redundant. This paper discusses two paradigms central to cariology. The first concerns the most successful caries-preventive agent: fluoride. When it was thought that fluoride had to be present during tooth mineralisation to 'improve' the biological apatite and the 'caries resistance' of the teeth, systemic fluoride administration was necessary for maximum benefit. Caries reduction therefore had to be balanced against increasing dental fluorosis. The 'caries resistance' concept was shown to be erroneous 25 years ago, but the new paradigm is not yet fully adopted in public health dentistry, so we still await real breakthroughs in more effective use of fluorides for caries prevention. The second paradigm is that caries is a transmittable, infectious disease: even one caused by specific microorganisms. This paradigm would require caries prevention by vaccination, but there is evidence that caries is not a classical infectious disease. Rather it results from an ecological shift in the tooth-surface biofilm, leading to a mineral imbalance between plaque fluid and tooth and hence net loss of tooth mineral. Therefore, caries belongs to common 'complex' or 'multifactorial' diseases, such as cancer, cardiovascular diseases, diabetes, in which many genetic, environmental and behavioural risk factors interact. The paper emphasises how these paradigm changes raise new research questions which need to be addressed to make caries prevention and treatment more cost-effective.

Effect of mineral supplements to citric acid on enamel erosion

The aim of this study was to evaluate the effect of mineral supplements to citric acid (1%; pH 2.21) on enamel erosion under controlled conditions in an artificial mouth. From each of 156 bovine incisors one polished enamel sample was prepared. The samples were divided among 13 experimental groups (n=12). In group 1 citric acid only was used (control). In groups 2-10 either calcium, phosphate or fluoride in various low concentrations was admixed to the citric acid. In groups 11-13 the citric acid was supplemented with a mixture of calcium, phosphate and fluoride. For demineralisation the specimens were rinsed with the respective solution for 1 min, immediately followed by a remineralisation period with artificial saliva (1 min). The specimens were cycled through this alternating procedure five times followed by rinsing for 8 h with artificial saliva. The de- and remineralisation cycle was repeated three times for each specimen interrupted by the 8 h-remineralisation periods. Before and after the experiments, the specimens were examined using microhardness testing (Knoop hardness) and laser profilometry. Hardness loss and enamel dissolution was significantly higher for the controls as compared to the remaining groups. Significantly lowest hardness loss for all groups was recorded for group 12 with admixture of calcium, phosphate and fluoride to citric acid. The significantly highest enamel loss was recorded for the controls compared to all other samples. Groups 3 and 4 revealed significantly lower and higher tissue loss compared to the remaining groups (2-13), respectively. The other groups did not differ significantly from each other. Modification of citric acid with calcium, phosphate and fluoride exerts a significant protective potential with respect to dental erosion. However, with the low concentrations applied enamel dissolution could not be completely prevented.

Construct and predictive validity of clinical caries diagnostic criteria assessing lesion activity

Even though there is no "gold standard" for determining caries lesion activity, it is nonetheless possible to evaluate the validity of such diagnostic measures. The aim of this study was to estimate the construct and predictive validity of caries lesion activity assessments by means of their ability to reflect known effects of fluoride on caries. A three-year trial of the effect of daily supervised brushing with fluoride toothpaste was carried out among 273 12-year-old children. All children were examined clinically according to diagnostic criteria for activity assessment. The relative risk (fluoride vs. control) for caries lesion transitions among diagnostic categories was calculated. Fluoride inhibited progression of caries at all stages of lesion formation while at the same time enhancing lesion regression. The effects were most pronounced for active non-cavitated lesions. It is concluded that the clinical diagnostic criteria have construct and predictive validity for the assessment of caries lesion activity.

Enamel erosion by some soft drinks and orange juices relative to their pH, buffering effect and contents of calcium phosphate

The capability of a soft drink or a juice to erode dental enamel depends not only on the pH of the drink, but also on its buffering effect. As the latter is the ability of the drink to resist a change of pH it may add to the effects of the actual pH. The aim of the present study was to compare the pH and the buffering effect of various soft drinks with their erosive effects and the solubility of apatite. In 18 soft drinks, mineral waters and juices available on the Danish market, pH and the concentrations of calcium, phosphate and fluoride were determined. The buffering effect was determined by titration with NaOH. Human teeth (n = 54) covered with nail varnish except for 3x4-mm windows were exposed to 1.5 liters of the drink for either 7 days or 24 h under constant agitation. The depth of the erosions was assessed in longitudinal sections. The depth was found to vary greatly from 3 mm eroded by the most acidic drinks and fresh orange juice to only slightly affected surfaces by most of the mineral waters. The dissolution of enamel increased logarithmically inversely with the pH of the drink and parallel with the solubility of enamel apatite. Orange juice, pH 4.0, supplemented with 40 mmol/l calcium and 30 mmol/l phosphate did not erode the enamel as the calcium and phosphate saturated the drink with respect to apatite. Generally, the lower the pH the more NaOH was necessary to bring the pH to neutrality. In particular the buffering effect of the juice was high. For all drinks, no effect of their low fluoride concentrations was observed.

Physical model for lesion formation in the presence of low levels of solution fluoride

A quantitative physical model is presented for the formation of subsurface carious lesions in the presence of low levels of solution fluoride. Calculations using independently determined model parameters are in agreement with mineral density profiles measured in bovine enamel lesions. The proposed mechanism is controlled by fluoride in the following way: as fluoride diffuses into enamel, it is rapidly adsorbed to enamel crystallites, resulting in very low microenvironmental fluoride concentrations, so long as the crystals are not saturated with respect to fluoride adsorption. The result of this saturable adsorption is a widening band of fluoride-saturated crystals near the surface, beneath which the microenvironmental fluoride concentrations are negligible. In the saturated band, the microenvironmental fluoride concentration in the pore solution is high enough to suppress dissolution, while in the deeper, relatively fluoride free region, dissolution can occur. In addition to predicting observed mineral density profiles, the model also predicts the demarcation in solution conditions between the regime where subsurface lesion formation occurs and that where the dissolution pattern is that of surface erosion; and the lack of insensitivity of dissolution rate to hydrodynamics in the presence of low levels of fluoride, as contrasted to the square root of stirring rate dependency observed in the absence of fluoride.

Dental tissue effects of fluoride

It is now well-established that a linear relationship exists between fluoride dose and enamel fluorosis in human populations. With increasing severity, the subsurface enamel all along the tooth becomes increasingly porous (hypomineralized), and the lesion extends toward the inner enamel. In dentin, hypomineralization results in an enhancement of the incremental lines. After eruption, the more severe forms are subject to extensive mechanical breakdown of the surface. The continuum of fluoride-induced changes can best be classified by the TF index, which reflects, on an ordinal scale, the histopathological features and increases in enamel fluoride concentrations. Human and animal studies have shown that it is possible to develop dental fluorosis by exposure during enamel maturation alone. It is less apparent whether an effect of fluoride on the stage of enamel matrix secretion, alone, is able to produce changes in enamel similar to those described as dental fluorosis in man. The clinical concept of post-eruptive maturation of erupting sound human enamel, resulting in fluoride uptake, most likely reflects subclinical caries. Incorporation of fluoride into enamel is principally possible only as a result of concomitant enamel dissolution (caries lesion development). At higher fluoride concentrations, calcium-fluoride-like material may form, although the formation, identification, and dissolution of this compound are far from resolved. It is concluded that dental fluorosis is a sensitive way of recording past fluoride exposure because, so far, no other agent or condition in man is known to create changes within the dentition similar to those induced by fluoride. Since the predominant cariostatic effect of fluoride is not due to its uptake by the enamel during tooth development, it is possible to obtain extensive caries reductions without a concomitant risk of dental fluorosis.

Enamel dissolution in relation to fluoride concentrations in the fluid of dental plaque-like layers of precultured Streptococcus sobrinus

Two experiments were conducted to find out what concentration of fluoride (F), when added as NaF to the fluid of an in vitro caries model, would be sufficient for full protection of the underlying enamel during a long (20 or 40 h) fall in 'plaque' pH. An existing caries model with bovine enamel and Streptococcus sobrinus 'plaque' was employed, while the fluid phase was initially either fully or partially saturated with tricalcium phosphate. In the model with fully saturated and more strongly buffered fluid, a 10 parts/10(6) addition of F to the fluid phase caused no inhibition of the pH fall. In the model with partially saturated and weakly buffered fluid, prevention of the pH fall increased along with increase of added F from 2 to 20 parts/10(6). In each model, the 10 parts/10(6) addition of F was associated with complete prevention of the increase of calcium (Ca) in the fluid phase. In the partially saturated, weakly buffered model, findings on the inorganic P of the fluid phase agreed with findings on Ca. The findings for microhardness of the enamel surface indicated a complete prevention of the enamel softening through the 10 parts/10(6) F additive. Enamel F content was elevated by the fermentation, and even more so when F was added with sugar to the fluid phase. The Ca content of Strep. sobrinus cells was reduced when incubated with sucrose for 40 h and, especially, with sucrose and F.(ABSTRACT TRUNCATED AT 250 WORDS)

A study of the relationship between trace elements in saliva and dental caries in children

About 8 ml of saliva were collected from each of 272 children, living in rural areas in North India, equally distributed into age groups 4-7 and 12-16 yr and stratified into low, moderate and high caries groups. The salivas were analysed for Zn, Cu, Fe, Mn by atomic absorption spectrophotometry and for F with a fluoride-sensitive electrode. The elements Cu and F had a consistent, inverse relationship with caries experience. The concentration of Zn, Fe and Mn in saliva did not have any consistent relationship with caries experience.

Release of fluoride from conventional and metal-reinforced glass-ionomer cements

Fluoride was extracted by 1 or 50 ml deionized water from 0.3 ml samples of two conventional and three metal-reinforced glass-ionomer cements. The fluoride concentration was determined by electrode. It was found that the fluoride concentration around the glass-ionomer cements ranged 2-3 ppm after 15 min and 3-5 ppm after 45 min. The maximum fluoride concentration attained within 24 h varied from 15-21 ppm, while the total amount of fluoride released during 100 days varied from about 2 mg fluoride to 12 mg fluoride per ml glass-ionomer cement. An alloy reinforced cement and its conventional counterpart released the highest amount of fluoride whereas a glass-cermet type released the lowest amount. Within the premises of this study it was concluded that composition and handling of the cements are of importance for the release of fluoride, and that a caries inhibitory fluoride concentration can be established in the surroundings of all of the cements.

Physicochemical perspectives on the cariostatic mechanisms of systemic and topical fluorides

The cariostatic mechanisms of systemic and topical fluoride agents are considered in light of a previously proposed model for the caries process. It is noted that fluoride, in principle, can induce a cariostatic effect: by (1) reducing enamel solubility when incorporated into the mineral structure; (2) by fostering the remineralization of incipient enamel lesions and the deposition of fluoridated phases (within dental plaque) which provide a source of mineral ions (Ca, P, F) under acidic conditions; and (3) by reducing the net rate of transport of matter out of the enamel surface, under acidic conditions, by inducing the reprecipitation of fluoridated hydroxyapatite phases within enamel. It is also noted that relatively high (approximately 12,000 ppm) concentrations of professionally applied topical fluorides significantly reduce the rate of acid production of cariogenic organisms (e.g., S. mutans). Based on an analysis of available clinical and laboratory data, it is concluded that the benefits provided by water fluoridation result, to a large degree, from a topical effect. It is suggested that clinical procedures be developed to establish and maintain low levels of free fluoride in plaque fluid. It appears that this will require frequent exposure to topical fluorides.

Chemical events during tooth dissolution

Information on chemical changes during enamel dissolution has been collected from investigations on hydroxyapatite solubility, enamel solubility, artificial lesion formation, and natural caries. Although hydroxyapatite and enamel will ultimately dissolve in acid or during caries, compositional changes also occur. Most notably, there is a preferential dissolution of calcium, both from hydroxyapatite and from enamel, and of carbonate and magnesium from enamel. Root dentin yields substantial amounts of magnesium on acid attack. Fluoride may be involved in surface zone formation during attack, but an additional theory of coupled diffusion is described. Calcium-deficient mineral is produced during an acid attack, and this has lattice parameters and solubility behavior different from those of stoichiometric material. The interaction of fluoride produces a more stable lattice, resisting dissolution and favoring accretion, and tending to counteract the effects of carbonate and magnesium in forming mineral. The provision of fluoride, albeit at low levels, in plaque fluid is seen as being important in maintaining the net integrity of the tooth. More information is also needed on the role of the organic phase in tooth structures during caries and acid attack.

Chemical events during tooth dissolution

The aims of the present survey were to describe some aspects of (a) enamel solubility in relation to pH and to the fluoride concentration, (b) chemical changes which occur during enamel dissolution, and (c) enamel resistance with an attempt to relate chemical events to clinical observations. The solubility of enamel powder increases dramatically with a decrease of pH. As long as the fluoride concentration in solution is high, the amount of enamel dissolved is low. When fluoride occurs only in the solid phase, the solubility is only slightly affected. Two clinically distinct lesions have been described in dental enamel: the erosion lesion, characterized by a dissolution of enamel from the surface; and the caries lesion, in which the enamel surface layer, accumulating fluoride, remains relatively intact, while the subsurface enamel dissolves. It was concluded that any dissolution of enamel is caused by an undersaturation with respect to enamel apatite. When whole enamel dissolves, the saturation with respect to fluorapatite determines whether a well-mineralized surface layer remains, i.e., whether the dissolution results in an erosion or a caries lesion. The degree of supersaturation with respect to fluorapatite determines the uptake of fluoride in the surface layer and its mineral content. It was observed that different areas of the enamel surface possess different degrees of resistance. A satisfactory explanation for the resistance has not been given.

The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6-7.6 at 20 degrees C

Samples of 200 mg powdered enamel apatite were suspended in 10 ml of aqueous solutions made from phosphoric acid and calcium hydroxide for 60 days at 20 degrees C. Calcium and phosphate concentrations were determined by spectroscopy; pH and the concentrations of fluoride and carbonate by electrometry. The solubility product of hydroxyapatite varied linearly from 10-56.9 at pH 4.6 to 10-52.8 at pH 7.6. The carbonate concentration in the aqueous phase was too low to account for calcium carbonate complex formation of significance to the solubility product. After the equilibration, X-ray diffraction analysis of the powder showed that the salt was unchanged pure apatite with no trace of a transformation to brushite or any other calcium phosphate. A re-equilibration of some of the powder samples for another 60 days produced similar solubility products. Thus the change of the solubility product was most likely attributable to a pH-induced change of the ionic composition of the enamel crystal surface.

Solubility, unit cell dimensions and crystallinity of fluoridated human dental enamel

The aim was to describe the dimensions of the fluorhydroxyapatite unit cell and the solubility of fluorhydroxyapatite as a function of the fluoride concentration in the apatitic lattice. Various amounts of powdered human dental enamel were suspended in a pH 4.5 acetate buffer containing fluoride. The suspensions were equilibrated for 365 days at 20 degrees C. After equilibration the fluoride concentration in the apatite varied from 0.003 to 0.89 (mole fraction). X-ray diffraction showed that the length of the alpha-axis decreased with increasing fluoride concentration, whilst a broadening of the (300) reflection indicated the presence of mixtures of various fluorhydroxyapatites. The solubility of the apatite after the long equilibration decreased slightly with increasing fluoride concentration in the solid. In a few samples the fluoride concentration in the aqueous phase was still discernible, indicating that even after 365 days, equilibrium had not been attained. In these samples the amount of dissolved apatite was extraordinarily low.

Demineralization and remineralization around orthodontic appliances: an in vivo study

The presence of clinically detectable areas of decalcification (observable as whitened areas) following the removal of orthodontic appliances is well recognized. The aim of the present study was to determine quantitatively the amount of demineralization and the ability of commercially available products to inhibit or reverse orthodontically related demineralization. Twenty orthodontic patients scheduled to have premolars extracted were randomly divided into four groups--one control and three test groups. The extracted premolars (numbering 58) were bracketed using an acid-etch composite system; each patient was given precise oral hygiene instructions and supplied with a sodium fluoride (1,100 ppm fluoride) dentifrice and an orthodontic toothbrush. The control group brushed only with the supplied dentifrice. In addition to brushing with the dentifrice, those in test group I rinsed once each night with a sodium fluoride (0.05%) mouthrinse; group II received a weekly topical APF treatment (1.2% fluoride); and Group III received a weekly topical APF treatment and rinsed once each night with the sodium fluoride mouthrinse. All premolars were extracted after 1 calendar month. Mineral profiles were determined on cross-sectioned teeth 50 to 75 micron occlusal and cervical to the brackets, directly underneath the brackets, and 500 micron away from the brackets. The control teeth (dentifrice only) demonstrated up to 15% demineralization to a depth of 50 micron. All of the test teeth produced rehardening and/or inhibition of demineralization (P less than 0.01). Those in test group III showed a particularly hard outer layer. The study demonstrated that measurable demineralization occurred around orthodontic appliances after only 1 month and this demineralization can be completely inhibited and/or reversed by the use of commercially available fluoride products.

A polarized light and scanning electron microscope study of the effect of Duraphat treatment on in vivo caries

The present study was carried out to examine the effect of application of a fluoride varnish (Duraphat) on experimentally produced caries in vivo. After a 4-wk period of cariogenic challenge on homologous pairs of premolars, Duraphat was applied on one of the teeth while the other was extracted and saved as a control. The Duraphat treated teeth remained in a cariogenic environment for a new period of 2 wk prior to extraction. All the control teeth displayed the classical characteristics of active incipient subsurface carious lesions. The Duraphat treated lesions showed an increase in surface porosity relative to their controls while the subsurface porosity was dramatically reduced. When examined in the SEM, the corresponding surface appeared smooth with a leveling out of the original surface irregularities. High power microscopy disclosed, however, a distinct network of intercrystalline spaces. Application of Duraphat prior to reestablishment of cariogenic challenge apparently gives rise to a marked redistribution of minerals in active initial carious lesions.

Effect of low levels of fluoride in solution on enamel demineralization in vitro

The effect of low levels of fluoride in solution on in vitro enamel demineralization has been studied. Extracted human teeth were exposed to 0.1 M lactate solutions (at pH 4.3) which were partially saturated with respect to enamel mineral and which contained between 0.004 and 1 ppm fluoride. Enamel demineralization was monitored using SEM and polarized light microscopy. It was observed that, in the absence of fluoride, rapid enamel demineralization occurred, resulting in the formation of cavitations within 72 hours. Using the same demineralizing medium containing as little as 0.024 and 0.054 ppm fluoride, we observed a remarkable protection of the enamel surface. Subsurface enamel demineralization was, however, observed under these conditions as well as in a solution containing 0.154 ppm fluoride. When the demineralizing solution containing 1 ppm fluoride was used, no mineral loss was detected. The observed inhibition of enamel demineralization was also shown to be associated with a significant uptake of fluoride by the enamel mineral. These observations have been noted to correlate with an increase in solution supersaturation with respect to fluoridated apatitic species. The results obtained were found to be consistent with a proposed hypothesis that the net rate of enamel demineralization will be reduced in a demineralizing medium supersaturated with respect to less soluble fluoridated phases, due to the enhancement of the precipitation rate of fluoridated apatitic phases relative to the rate of dissolution of the original enamel surface.

Solubility study of the initial formation of calcium orthophosphates from aqueous solutions at pH 5-10

In unsaturated stock solutions with various concentrations of calcium and phosphate and pH of 4-5, supersaturation with respect to brushite, hydroxy- and fluorapatite, octacalcium phosphate and amorphous tricalcium phosphate was established at 20 degrees C by increasing pH in two series of portions, one containing low concentrations of fluoride, one fluoride-free. The precipitate as identified by X-ray diffraction was compared to the ion-activity products in the liquid. Brushite and amorphous tricalcium phosphate were formed at low supersaturation with respect to the salt in question; apatite formation required a high degree of supersaturation. Octacalcium phosphate was not observed presumably due to moderate supersaturations with respect to this salt. Fluoride had no effect on brushite and tricalcium-phosphate formation; only apatite was influenced slightly through a fluorapatite precipitation. The salts were often formed in mutual competition.

Fluoride release from a fluoride-containing amalgam in vivo

Six test subjects using a low fluoride diet collected saliva samples each morning for 1 wk to determine the normal fluoride concentration. During the following 8 wk an acrylic plate, containing amalgam "fillings" corresponding to 6 MOD amalgams in premolars, was worn each night. Saliva samples were collected every morning and analyzed with a fluoride-ion electrode. Normal fluoride values ranged from 1.22 microM to 0.57 microM. The first days after insertion of the plates the fluoride concentration ranged from 91.06 microM to 12.26 microM in the subjects. The following 25-30 days there was an exponential decline of the fluoride concentration in saliva with a half-life of 6 to 7 days. The fluoride level was significantly higher (P less than 0.01) than basal levels in all subjects during this period. The following 20-25 days the fluoride level decreased more slowly, approaching the basal level. One subject had a significantly higher fluoride level for all 8 wk. Since the saliva fluoride concentrations registered seem to be sufficient to enhance remineralization, it is concluded that restorations with this material may have a favorable effect not only on secondary caries, but on any initial demineralization in the mouth.

A comparison of some effects of fluoride on apatite formation in vitro and in vivo

In a liquid (22 degrees C) saturated with and in contact with powdered bone apatite, the fluoride ion activity was adjusted to 1-10 parts/10(6). Due to the fluorapatite (FAP) supersaturation produced hereby, a rapid formation of this salt occurred leading to a decrease of the concentration of the ions involved. When no more fluoride was available in the liquid, the concentrations of calcium and phosphate increased again, due to dissolution of presumably hydroxyapatite (HAP). Fifty four rats were given either 10, 20, or 40 mg NaF per kg body weight intraperitoneally. The animals were sacrificed from 5 min to 96 h after the injection. The fluoride concentration in plasma increased to a peak, after which it decreased. Plasma calcium decreased and remained low until the fluoride had attained normal levels. The intensity of mineralization of the growing dental hard tissue was monitored on microradiographs. Corresponding to the plasma fluoride peak and the decrease of plasma calcium, a hypermineralized layer was formed while a hypomineralized zone was formed during plasma calcium increase after disappearance of fluoride. Similarities and dissimilarities between the in vitro and the in vivo experiments are discussed.

Effect of chlorhexidine-fluoride applications without and with Sr and Zn on caries, plaque, and gingiva in rats

Molar teeth of four groups of Osborne-Mendel rats, kept on a cariogenic diet, were treated once per day for 48 days with either distilled water (solution C), a solution containing 0.05% chlorhexidine gluconate and 0.04% NaF, pH 5.8 (solution CXF), CXF plus 1000 ppm Sr (solution CXFS), or CXFS plus 1000 ppm Zn (solution CXFSZ). Caries of the hemi-sectioned lower molars was scored according to grades of severity, and the extent of the buccal plaque was estimated. Inflammatory cell infiltration, vascularity, and thickness of the oral mucosa were estimated from stained sections of the buccal oral mucosa to detect possible adverse effects of the treatments. Fissure caries lesions with involvement of the dentin were reduced by 57% (p less than 0.01) with the CXF treatment and by 68% (p less than 0.001) with the CXFS treatment. The CXFSZ treatment reduced the fissure caries least (44%, p less than 0.05), but it appreciably reduced the extent of plaque. The mucosal changes were greatest in the group receiving the CXFSZ treatment, but they were generally small and at a non-pathological level. Supplementation of the CXF solution with Sr thus appeared beneficial for caries reduction.

Alkali soluble and alkali insoluble fluoride retention in demineralized enamel in vivo

Demineralization of enamel was induced by applying orthodontic bands in 15 patients having two of their premolars extracted for orthodontic reasons. After a 4-wk caries induction period, eight patients were instructed to rinse their teeth once daily with a neutral 0.05% NaF solution, whereas seven patients received a single treatment with a neutral 2% NaF solution. The bands were reattached to the teeth during the fluoride treatment period and the teeth were extracted after two more weeks. Three consecutive enamel layers were etched off, and a significant uptake of fluoride in all three layers were found in both treatment groups. A larger part of the deposited fluoride was retained in an alkali insoluble form (i.e. fluorapatite) compared with previous studies of sound enamel. It is suggested that the chemical conditions in the cariogenic milieu were favorable for transformation of the fluoride into a stable apatite structure.

Rational use of fluorides in caries prevention. A concept based on possible cariostatic mechanisms

The aim of this review has been to suggest that future fluoride preventive programs should be based on what is at present known about the possible cariostatic mechanisms of fluoride. The relative role of enamel fluoride in caries prevention has become increasingly questioned during the last two decades. No significant relationship has been demonstrated between caries experience of the individual and fluoride content of the enamel. Furthermore, the fluoride content in surface enamel between teeth developed in low and "optimal" fluoride areas is too small to explain any significant effect on dissolution rate of the enamel. As the major explanation for the cariostatic effect of fluoride must therefore be sought in its local effect on the oral environment, the possible effects on plaque colonization, composition and metabolic activities are discussed. The effect of even low concentrations of fluoride in the liquid phase on enamel dissolution is described, and it is concluded that the major cariostatic effect of water fluoridation, fluoride tooth paste and mouth rinses can probably be ascribed to regular increases in fluoride ion activity in the oral fluids. The effect of high concentrations of topical fluoride solutions is thought to be a result of a slow dissolution of calcium fluoride deposited in initial caries lesions, whereby an increased fluoride ion concentration is maintained locally for longer periods of time. Finally, the clinical consequences of these considerations are described.