Nature and mechanisms of dental fluorosis in animals

Excessive fluoride impairs autophagy flux in ameloblasts which is prevented by the autophagy activator rapamycin

Excessive fluoride intake can cause dental fluorosis during teeth development and growth. However, the mechanisms underlying fluoride-induced enamel damage are still not fully elucidated. Previously, we observed fluoride-induced autophagy in ameloblasts, but the effects of fluoride on autophagy flux in ameloblasts remain unclear. Hence, this study aimed to clarify the effects of fluoride and rapamycin, an autophagy activator, on autophagy flux in ameloblasts. This in vitro study used the murine ameloblast-derived cell line LS8. Cells were treated with different concentrations of sodium fluoride (NaF) to evaluate NaF-induced cytotoxicity. Using transmission electron microscopy, we observed an increase in the number of autophagosomes with increasing fluoride concentrations. Western blot analyses showed increases in microtubule-associated protein 1 light chain 3 (LC3) and SQSTM1 (p62) expression after NaF treatment and an increase in LC3II expression after bafilomycin A1 administration. Together with changes in RFP-GFP-LC3 lentivirus expression, this demonstrated that fluoride impaired autophagy flux. Furthermore, we evaluated whether rapamycin can alleviate fluoride-induced cytotoxicity by restoring autophagy flux. Compared to the NaF-treated group, LS8 cells cotreated with NaF and rapamycin grew considerably better and had significantly decreased p62 expression. Taken together, these data suggest that fluoride-induced impaired autophagosome degradation may damage ameloblasts. This provides experimental in vitro evidence and an explanation for the observed NaF-induced toxicity of ameloblasts. Rapamycin probably alleviates this impairment by decreasing the expression of p62, thereby preventing autophagy defects.

In vivo effect of fluoride combined with amoxicillin on enamel development in rats

Some evidence in vitro suggested that amoxicillin and fluoride could disturb the enamel mineralization. Objective: To assess the effect of amoxicillin and of the combination of amoxicillin and fluoride on enamel mineralization in rats. Methodology: In total, 40 rats were randomly assigned to four groups: control group (CG); amoxicillin group (AG - amoxicillin (500 mg/kg/day), fluoride group (FG - fluoridated water (100 ppm -221 mg F/L), and amoxicillin + fluoride group (AFG). After 60 days, the samples were collected from plasma and tibiae and analyzed for fluoride (F) concentration. The incisors were also collected to determine the severity of fluorosis using the Dental Fluorosis by Image Analysis (DFIA) software, concentration of F, measurements of enamel thickness, and hardness. The data were analyzed by ANOVA, Tukey’s post-hoc test, or Games-Howell post-hoc test (α=0.05). Results: Enamel thickness of the incisors did not differ statistically among the groups (p=0.228). Groups exposed to fluoride (AFG and FG) have higher F concentrations in plasma, bone and teeth than those not exposed to fluoride (CG and AG). The groups showed a similar behavior in the DFIA and hardness test, with the FG and AFG groups showing more severe fluorosis defects and significant lower hardness when compared with the AG and CG groups, with no difference from each other. Conclusion: The rats exposed to fluoride or fluoride + amoxicillin developed dental fluorosis, while exposure to amoxicillin alone did not lead to enamel defects.

Calcium and Vitamin D Supplementation Effectively Alleviates Dental and Skeletal Fluorosis and Retain Elemental Homeostasis in Mice

Fluoride (F) is an essential trace element, but chronic exposure beyond the permissible limit (1.5 ppm) effectuates dental and skeletal fluorosis. Although 200 million people across the world are suffering from toxic manifestations of F, till now proper treatment is not available. In this study, we assessed the effectiveness of calcium and vitamin D supplementation for alleviation of fluorosis. Swiss albino mice were divided into 6 groups; group I-control group (received drinking water ˂ 0.5 ppm F; within the permissible limit), group II-treated with 15 ppm of sodium fluoride (NaF) for 4 months, group III-treated with 15 ppm of NaF for 8 months through drinking water. Group IV-orally treated with 15 ppm NaF for 4 months, thereafter received only drinking water for next 4 months, group V-orally treated with 15 ppm NaF for 4 months, thereafter received drinking water supplemented with calcium and vitamin D (2.5-g calcium kg^-1 diet and 1000 IU vitamin D kg^-1 diet) for next 4 months, and group VI was treated with 15 ppm of NaF through drinking water as well as supplemented with calcium and vitamin D for 4 months. NaF treatment caused dental fluorosis, skeletal fluorosis, and alteration of bone's metal profile. Substitution of NaF-containing water with normal drinking water reduced the severity of fluorosis but supplementation of calcium and vitamin D effectively alleviated dental and skeletal fluorosis, reduced F deposition, and retained elemental homeostasis of the bone. Our findings strongly support that calcium and vitamin D act as redeemer of fluorosis. Graphical Abstract.

MicroRNA 224 Regulates Ion Transporter Expression in Ameloblasts To Coordinate Enamel Mineralization

Enamel mineralization is accompanied by the release of protons into the extracellular matrix, which is buffered to regulate the pH value in the local microenvironment. The present study aimed to investigate the role of microRNA 224 (miR-224) as a regulator of SLC4A4 and CFTR, encoding the key buffering ion transporters, in modulating enamel mineralization. miR-224 was significantly downregulated as ameloblasts differentiated, in parallel with upregulation of SLC4A4 and CFTR. Overexpression of miR-224 downregulated SLC4A4 and CFTR expression in cultured human epithelial cells. A microRNA luciferase assay confirmed the specific binding of miR-224 to the 3' untranslated regions (UTRs) of SLC4A4 and CFTR mRNAs, thereby inhibiting protein translation. miR-224 agomir injection in mouse neonatal incisors resulted in normal enamel length and thickness, but with disturbed organization of the prism structure and deficient crystal growth. Moreover, the enamel Ca/P ratio and microhardness were markedly reduced after miR-224 agomir administration. These results demonstrate that miR-224 plays a pivotal role in fine tuning enamel mineralization by modulating SLC4A4 and CFTR to maintain pH homeostasis and support enamel mineralization.

Ultrastructural and immunohistochemical study of the influence of fluoride excess on the development of rat incisor tooth buds

Little information is available on the pathogenesis of fluorosis during the fetal and initial postnatal period. In the present study, female rats received 0 (control), 7 or 100 ppm of sodium fluoride in drinking water, one week before breeding and throughout gestation and nursing periods. The hemimandibles of the offspring were collected at 0, 7 and 14 days of postnatal life (n = 5) and processed for morphological analyses by light and electron microscopy, immunohistochemical analysis for amelogenin and morphometric study of enamel matrix and ameloblasts of incisors. The results showed a decrease in matrix production at the secretory phase at all study periods for the 100 ppm group. In this same group, the secretory ameloblasts showed reduction of enamel matrix secretion, disorganization of mitochondrial crests, large vacuoles at the apical portion of the cytoplasm, retention of intracisternal material and dilatation of some cisterns in the rough endoplasmic reticulum. In the groups of animals aged 7 and 14 days, analysis of variance showed significant reduction (p<0.05) in cytoplasmic volume of 23.80% and 24.75%, respectively, in relation to the control group. The smooth-ended maturation ameloblasts exhibited a large number of vacuoles with electron-dense endocytic matrix, suggesting a delay in the resorption process. Immunohistochemical analysis showed no difference in the intensity and labeling pattern of the enamel matrix in any study group. Interestingly, in offspring at the age of 14 days for the 7 ppm group, there was an increase in the matrix length at the secretory phase. Therefore, part of the excessive dose of sodium fluoride given to the mother in drinking water can reach the offspring through the placenta and mother’s milk, causing morphological changes in ameloblasts and suggesting a reduction in secretion and a delay in matrix resorption.

In-office dental bleaching and enamel microabrasion for fluorosis treatment

Recently, mostly as a result of drinking water fluoridation, the number of young patients affected by fluorosis increased considerably. This study describes a minimally invasive technique, using in-office dental bleaching (35% hydrogen peroxide) and enamel microabrasion (silicon carbide and 12% hydrochloric acid) to eliminate fluorosis like stains. The association of techniques was efficient and can be recommended as a good conservative alternative for the treatment of fluorosis affected teeth.

A half-century of community water fluoridation in the United States: review and commentary

The nearly 50-year history of community water fluoridation is reviewed with the major emphasis on the benefits and safety of fluoridation. Other aspects of water fluoridation also described include the apparent reduction in measurable fluoridation benefits because of the abundance of other fluoride sources, the diffusion of fluoridation effects into fluoride-deficient communities, preeruptive and posteruptive effects, technical and cost aspects, sociopolitical and legal issues that affect the successful fluoridation of communities, and alternatives to community water fluoridation. The majority of studies have evaluated the effectiveness of water fluoridation on the permanent teeth of children, while there are fewer studies on deciduous teeth and in adults; the relationship between fluoride ingestion and bone health needs further clarification; the sociopolitical issues of fluoridation need to be better understood.

Dental fluorosis: chemistry and biology

This review aims at discussing the pathogenesis of enamel fluorosis in relation to a putative linkage among ameloblastic activities, secreted enamel matrix proteins and multiple proteases, growing enamel crystals, and fluid composition, including calcium and fluoride ions. Fluoride is the most important caries-preventive agent in dentistry. In the last two decades, increasing fluoride exposure in various forms and vehicles is most likely the explanation for an increase in the prevalence of mild-to-moderate forms of dental fluorosis in many communities, not the least in those in which controlled water fluoridation has been established. The effects of fluoride on enamel formation causing dental fluorosis in man are cumulative, rather than requiring a specific threshold dose, depending on the total fluoride intake from all sources and the duration of fluoride exposure. Enamel mineralization is highly sensitive to free fluoride ions, which uniquely promote the hydrolysis of acidic precursors such as octacalcium phosphate and precipitation of fluoridated apatite crystals. Once fluoride is incorporated into enamel crystals, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral. In the light of evidence obtained in human and animal studies, it is now most likely that enamel hypomineralization in fluorotic teeth is due predominantly to the aberrant effects of excess fluoride on the rates at which matrix proteins break down and/or the rates at which the by-products from this degradation are withdrawn from the maturing enamel. Any interference with enamel matrix removal could yield retarding effects on the accompanying crystal growth through the maturation stages, resulting in different magnitudes of enamel porosity at the time of tooth eruption. Currently, there is no direct proof that fluoride at micromolar levels affects proliferation and differentiation of enamel organ cells. Fluoride does not seem to affect the production and secretion of enamel matrix proteins and proteases within the dose range causing dental fluorosis in man. Most likely, the fluoride uptake interferes, indirectly, with the protease activities by decreasing free Ca(2+) concentration in the mineralizing milieu. The Ca(2+)-mediated regulation of protease activities is consistent with the in situ observations that (a) enzymatic cleavages of the amelogenins take place only at slow rates through the secretory phase with the limited calcium transport and that, (b) under normal amelogenesis, the amelogenin degradation appears to be accelerated during the transitional and early maturation stages with the increased calcium transport. 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 reduction without a concomitant risk of dental fluorosis. Further efforts and research are needed to settle the currently uncertain issues, e.g., the incidence, prevalence, and causes of dental or skeletal fluorosis in relation to all sources of fluoride and the appropriate dose levels and timing of fluoride exposure for prevention and control of dental fluorosis and caries.

Dental fluorosis: variability among different inbred mouse strains

Concurrent with the decline in dental caries has been an increase in the prevalence of dental fluorosis, a side-effect of exposure to greater than optimal levels of fluoride during amelogenesis. The mechanisms that underlie the pathogenesis of dental fluorosis are not known. We hypothesize that genetic determinants influence an individual's susceptibility or resistance to develop dental fluorosis. We tested this hypothesis using a mouse model system (continuous eruption of the incisors) where genotype, age, gender, food, housing, and drinking water fluoride level can be rigorously controlled. Examination of 12 inbred strains of mice showed differences in dental fluorosis susceptibility/resistance. The A/J mouse strain is highly susceptible, with a rapid onset and severe development of dental fluorosis compared with that in the other strains tested, whereas the 129P3/J mouse strain is least affected, with minimal dental fluorosis. These observations support the contribution of a genetic component in the pathogenesis of dental fluorosis.

Intra-oral distribution of dental fluorosis in Newburgh and Kingston, New York

Previous studies of intra-oral distribution of dental fluorosis in low-fluoride areas reported that teeth that formed later in life were more frequently affected compared with the early-forming teeth. The steady increase of plasma fluoride with age, even under constant fluoride exposure, has been suggested as a possible mechanism for this clinical manifestation. To determine the intra-oral distribution of Dean's Index scores and the effect of fluoride exposure on early- and late-forming teeth, we analyzed data collected on 2193 seven- to 14-year-old lifelong residents of fluoridated or non-fluoridated areas. Logistic regression procedures were used to determine the effects of fluoridation, early brushing, daily supplements, and other socio-demographic variables on early- and late-forming teeth. The results show that the occurrence of very mild or greater levels of fluorosis in the upper anterior teeth was 7 to 10% in the fluoridated area and 5 to 9% in the nonfluoridated area. In the fluoridated area, the occurrence of fluorosis increased from anterior to posterior teeth. Both early- and late-forming teeth were affected by exposure to fluoridation, daily fluoride supplement use, or brushing before the age of two years. This analysis showed that the esthetic consequence of exposure to multiple sources of fluoride was less dramatic, as evidenced by the lower frequency in upper anterior teeth compared with posterior teeth. The longer maturation process of the posterior teeth and the thicker enamel appear to be the likely explanation for the higher occurrence of dental fluorosis in posterior teeth.

Dental diseases of puppies and kittens

Although there are certain differences warranting specific options therapeutically in young animals, many of the basic dental principles still apply. The key is examining the oral cavity of every patient at each visit and knowing how to recognize and manage any abnormalities encountered. Providing sound oral care from the time of a patient's first visit can figure significantly in a lifelong commitment to oral health that can have a positive impact on the patient's overall health.

The dental pathology of feral cats on Marion Island, part II: periodontitis, external odontoclastic resorption lesions and mandibular thickening

Skulls (n = 301) of adult feral cats from Marion Island were examined macroscopically. Dental calculus was found infrequently (9.0% of cats, 0.76% of teeth), unlike the hard tissue lesions of moderate and advanced periodontitis and tooth loss (presumably due to periodontitis), which were commonly seen (61.8% of cats, 14.8% of teeth). Relatively few of these abnormalities were associated with external odontoclastic resorption lesions, which affected 14.3% of cats and 1.2% of teeth-less than in most recent surveys in domestic cats. Abnormal thickening of the mandibula, found in 39.5% of specimens, was most commonly bilateral (83.3%). The lesions ranged from a focal periosteal reaction, to localized exostosis, to generalized swelling and loss of density, to grossly enlarged mandibles with increased bone density. Mandibular swelling was significantly associated with other abnormalities (periodontitis, dental fractures, external odontoclastic resorption lesions and periapical lesions), but many cases of mandibular swelling were accompanied by only minor dental defects.

Absence of detrimental effects of fluoride exposure in diabetic rats

This study is part of a comprehensive programme to investigate fluoride toxicity and the hypothesis that fluoride ingested by "medically compromised' animals will result in altered physiological function. Its objectives were to monitor fluoride retention, tissue fluoride concentrations and genetic variables in diabetic and control rats chronically exposed to fluoride, and to determine whether or not adverse effects occurred. Male, Zucker fatty diabetic rats and Zucker age-matched lean controls were fed a low-fluoride diet ( < 1.2 parts/10(6) F-) ad libitum and received 0, 5, 15 or 50 parts/10(6) fluoride in their drinking water for 3 or 6 months. Fluoride metabolic balance was determined for 4 days before the end of each study phase. Plasma and urine were analysed for biochemical markers of tissue function, and plasma, urine, faeces and tissues were analysed for fluoride. Bone marrow cells from animals killed after 6 months of treatment were examined for frequency of sister chromatid exchange, a marker of genetic damage. The diabetic rats consumed, excreted and retained significantly greater amounts of fluoride than the controls (p < 0.05). There were dose-related increases in fluoride excretion, retention and tissue concentrations in both classes of animals, which were significantly greater in the diabetic rats. In spite of greater amounts of fluoride in the tissues of diabetic animals, there was no evidence, under these experimental conditions, that any of the fluoride exposures tested caused measurable adverse effects on the physiological, biochemical or genetic variables that were monitored.

Refining the estimate of the critical period for susceptibility to enamel fluorosis in human maxillary central incisors

PURPOSE

The purpose of this study was to determine an improved estimate of the critical period for susceptibility to fluorosis in human maxillary central incisors.

METHODS

The fluorosis score, S, of the incisal (I), middle (M), and cervical (C) third divisions on the labial surface of right maxillary central incisors of subjects (a representative sample of 1,085 Hong Kong Chinese children aged 7 to 12 years surveyed in 1986) was determined according to the Chronological Fluorosis Assessment (CFA) Index. Subject data were grouped by month of birth relative to June 1978, when the designated concentration for waterborne fluoride in the community water supply was reduced from 1.0 to 0.7 mg/L. The analytical task was one of finding the correlation maximum in a system of 12 variates and adjustable parameters including the waterborne fluoride concentration, [F-], and the fluorosis score.

RESULTS

The main findings were: (1) the peak correlation of S vs [F-] for the male incisal third centered at 17.5 months after birth; (2) and six months later for females; (3) the correlation of S with [F-] is maximized for S(M) following S(I) by two months, and S(C) following S(M) by a further two months for both males and females; and (4) the critical period for exposure to fluoride is of about four months' duration for each third.

CONCLUSIONS

The maxillary central incisor, as a whole, appears most at risk to fluorosis from dietary fluoride between age 15 and 24 months for males and between 21 and 30 months for females.

Proton microprobe assessment of the distribution of fluoride in the enamel and dentine of developing central incisors of sheep and changes induced by daily fluoride supplements

Ten sheep were given 0.5 mg fluoride (F) and 10 sheep 0.2 mg F/kg body wt orally for periods of 1-6 months while 8 sheep received no additional F. One incisor from each sheep was sectioned longitudinally in the midline and, using the proton microprobe, multiple scans for calcium and F were made across the enamel and dentine. F was determined by proton-induced gamma-ray emission and calcium by X-ray emission. Tooth length and hence the stage of ameloblast activity for each of the 28 teeth at the start of the experiment was determined using a tetracycline marker. In addition, the stage of enamel development of the eight control teeth (no dietary F) at the time of their extraction was assessed from their macroscopic appearance. Continuous changes in F levels occurred in both enamel and dentine throughout tooth development and also in the mature enamel and associated dentine after ameloblast regression. All scans for all stages of tooth development and all F treatments showed a high F concentration at the enamel surface. Early in the secretory phase, a wide-based F peak occupied the entire width of the enamel with a similar F peak in the dentine. In the control teeth, no consistent increase in F concentration occurred at the enamel surface during later development. When F supplements were started early in the maturation phase an increase in F concentration only at the enamel surface was recorded. When F supplements were also given during the secretory phase, higher F concentrations were recorded not only at the enamel surface but also for the inner enamel and dentine plateau. These findings, based on a small number of sheep, indicate that further research is needed to clarify the method and control of F uptake and to determine the changes in these processes during the different stages of tooth development.

Effect of aging on animal response to chronic fluoride exposure

This study was conducted to test the hypothesis that physiological changes which occur during aging increase the biological impact of fluoride and reduce the threshold of safe fluoride exposure. Four groups of rats were fed a low-fluoride diet (< 1.2 ppm) ad libitum and received 0, 5, 15, or 50 ppm fluoride in their drinking water. Animals were killed after three, six, 12, or 18 months of treatment. Blood and urine were monitored for biochemical markers of tissue function, and plasma, urine, feces, and representative tissues were analyzed for fluoride. In addition, bone marrow cells from animals killed after 18 months of treatment were examined for frequency of sister chromatid exchange (SCE), a marker of genetic damage. Study results indicated that, within treatment groups, fluoride intake, excretion, and retention did not change significantly between three and 18 months. Fluoride concentration in soft tissues did not change with treatment duration in the fluoride-treated animals. Mineralized tissue fluoride concentration and the total fluoride in the carcasses increased continually as the animals aged. In spite of significant, dose-related differences in tissue fluoride levels which occurred in all age groups in this study, there were no indications that increased fluoride in the tissues caused any adverse physiological or genotoxic effects. None of the monitored clinical "wellness" markers of tissue integrity and function was altered by fluoride in a clinically significant manner. Therefore, there was no evidence from this study that aging reduces the threshold of safe chronic fluoride exposure.

Dental fluorosis: its use as a biomarker

Several epidemiological studies, beginning with those of Dean and co-workers in the 1940's, clearly demonstrate the relationship between dental fluorosis in humans and the level of fluoride in water supplies. These studies and others have shown that, in a population, there is a direct relationship among the degree of enamel fluorosis, plasma and bone fluoride levels, and the concentration of fluoride in drinking water. However, dental fluorosis is a reflection of fluoride exposure only during the time of enamel formation, somewhat limiting its use as a biomarker. In addition, the degree of fluorosis is dependent not only on the total fluoride dose, but also on the timing and duration of fluoride exposure. At the level of an individual response to fluoride exposure, factors such as body weight, activity level, nutritional factors, and the rate of skeletal growth and remodeling are also important. These variables, along with an individual variability in response to similar doses of fluoride, indicate that enamel fluorosis cannot be used as a biological marker of the level of fluoride exposure for an individual.

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.

Effects of chronic fluoride exposure on morphometric parameters defining the stages of amelogenesis and ameloblast modulation in rat incisors

The response of ameloblasts to long-term (6 weeks) exposure to 100 ppm fluoride was examined in continuously erupting mandibular incisors of female Sprague-Dawley rats as compared to control rats receiving a similar diet (Teklad L-356) but no sodium fluoride in their drinking water. After treatment, animals from both groups were perfused intravascularly with glutaraldehyde, and the incisors were removed and processed for light microscope morphometric analyses directly from 1 microns thick Epon sections. Other animals were injected intravenously with calcein (green fluorescence) followed 4 hours later by xylenol orange (red fluorescence) in order to reveal smooth-ended ameloblast modulation bands and thereby allow quantification of parameters related to the creation and movement of modulation waves within the maturation zone of these teeth. The results indicated that rat incisors expressed four major changes in normal amelogenesis which could be attributed to the chronic fluoride treatment. First, ameloblasts produced a thinner than normal enamel layer by the time they completed the secretory stage and entered the maturation stage of amelogenesis. Second, enamel organ cells within the maturation zone, especially those from the papillary layer, were shorter in height than normal. Third, ameloblasts related to maturing enamel in areas where it was partially soluble and/or fully soluble in EDTA modulated at a rate that was much slower than normal. In some locations ameloblasts remained ruffle-ended for as much as 30% longer than normal per cycle. This upset the usual pattern such that fewer total modulation cycles were completed per unit time by these ameloblasts. Fourth, enamel proteins were lost from the maturing enamel layer at a rate that was about 40% slower than normal. The data suggested that ameloblasts detected the delay in the extracellular breakdown and/or loss of enamel proteins and they responded by remaining ruffle-ended for longer intervals than usual (positive feedback).

Biological mechanisms of fluorosis and level and timing of systemic exposure to fluoride with respect to fluorosis

Enamel fluorosis can occur following either an acute or chronic exposure to fluoride during tooth formation. Fluorosed enamel is characterized by a retention of amelogenins in the early-maturation stage, and by the formation of a more porous enamel with a subsurface hypomineralization. The mechanisms by which fluoride affects enamel development include specific effects on both the ameloblasts and on the developing enamel matrix. Maturation-stage ameloblast modulation is more rapid in fluorosed enamel as compared with control enamel, and proteolytic activity in fluorosed early-maturation enamel is reduced as compared with controls. Secretory enamel appears to be more susceptible to the effects of fluoride following acute fluoride exposure, such as may occur with the use of fluoride supplements. However, both human and animal studies show that the transition/early-maturation stage of enamel formation is most susceptible to the effects of chronic fluoride ingestion at above-optimal levels of fluoride in drinking water.

The level and timing of systemic exposure to fluoride with respect to caries resistance

Material related to water fluoridation and fluoride availability has been examined for changing patterns of fluoride intake by humans over recent years. The difficulty of separating systemic and topical fluoride action from water and foods from that found in fluoride agents used in preventive treatment programs is discussed. Recommendations are made that water fluoridation is a well-proven program and should be continued, but that fluoride supplementation programs under the control of the individual should be carefully evaluated and, if used, should be restricted to periods after the secretory stage of enamel formation is normally finished for anterior teeth, with supplements commenced during the pre-eruption maturation period and continuing until permanent tooth eruption is complete. Although supplements can also give a topical source to the teeth, depending on the way the supplement is consumed and the duration of time it is available in the mouth, other methods of topical fluoride provision are endorsed, and these should continue through life. It is suggested, in respect of epidemiology studies, that data collected before the availability of fluoride dentifrices in 1971 should be re-examined.

The comparative sensitivity of intra-oral, in vitro, and animal models in the 'profile' evaluation of topical fluorides

The development of predictive and rapid methods for the assessment of the anticaries activity of topical fluorides has been a long-standing objective of caries researchers. These methods can provide useful benefits in a number of applications, ranging from the identification of novel agents to progress into clinical testing to the regulatory screening of commercial product variations. In the latter applications, combinations of test methods (so-called profiles) are used by manufacturers to prove that changes in formulations do not alter the efficacy of the products. Historically, combinations of in vitro and animal models have been used for basic research as well as for profile testing purposes; however, in recent years, the use of intra-oral or in situ models has increased. In this paper, in vitro, animal, and in situ methods are reviewed in terms of the historical basis for their development, protocols currently used in testing, and the primary advantages and limitations of each as applied to 'profile' applications. Recommendations are provided concerning circumstances for the appropriate use of modern test methods in formulation screening.

Dental fluorosis in perspective

The relationship between fluoride in drinking water, caries and fluorosis was established 50 years ago. But additional sources that duplicate or expand fluoride benefits may not carry the same low risk for fluorosis and should be monitored.

The severity of dental fluorosis in children exposed to water with a high fluoride content for various periods of time

Inhabitants of the Ikeno district of Japan were accidentally exposed to drinking water containing 7.8 ppm fluoride (F) for 12 years, after which water with 0.2 ppm F was substituted. Dental examinations of local inhabitants revealed that only children aged seven years or less at the introduction or aged 11 months or more at the removal of the high-F water had fluorosis. Regular inspections were made of the 86 children between those age limits. The severity of fluorosis in three tooth types (first permanent molars, upper central incisors, and first premolars) was assessed and related to the period of use of the high-F water. Continuous exposure throughout tooth development resulted in severe changes in all three tooth types. With limited exposure, the age at the beginning and at the end was an important factor in determining the severity of the fluorosis. The pattern of change from normal to severe fluorosis differed in the three tooth types, influenced by their respective times of formation. Two 'at-risk' periods for the production of moderate or severe fluorosis were evident. One started at birth and ended early in tooth development, while the other started later and ended at eruption. The duration of F exposure, although determining the initial degree of fluorosis, did not influence the rate of post-eruptive enamel loss.

Relationship of total fluoride intake to beneficial effects and enamel fluorosis

Recent studies indicate that the prevalence of very mild to moderate dental fluorosis, as classified by Dean, has increased relative to that found in earlier investigations. To date, fluoridated water, fluoride supplements, the diet, fluoride dentifrices, and other topical fluoride applications have been identified as sources of systemic fluoride. Recent evidence suggests that there is a strong association between mild to moderate enamel fluorosis and the use of fluoride supplements during early childhood, and that the presently recommended supplementation schedule for U.S. children above the age of 2 years may be too high. Evidence also suggests that there is a strong association between fluoride dentifrice use during early childhood and enamel fluorosis in fluoridated populations. These findings support the need for a careful review of existing supplementation schedules and early oral hygiene practices. There is a pressing need for additional analytical epidemiological studies to confirm existing findings and to determine whether other fluoride sources may be associated with enamel fluorosis. Further, since exposure to combinations of individual risk factors has been shown to carry more than merely an additive increase in the risk of fluorosis, these studies must be multifactorial in design. There is also a need for more fluorosis prevalence and severity data to be gathered, so that the development of enamel fluorosis as a public health problem can be assessed, and so that the success of measures implemented to maximize efficacy while minimizing unwanted side-effects can be monitored.