The macroscopic and scanning electron-microscopic appearance and microhardness of the enamel, and the related histological changes in the enamel organ of erupting sheep incisors resulting from a prolonged low daily dose of fluoride

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.

Investigation on the Gradient Nanomechanical Behavior of Dental Fluorosis Enamel

This study aims to investigate the gradient nanomechanical behavior of dental fluorosis enamel and provide appropriate selection criteria for restorative materials. The nanomechanical properties of the outer, middle, and inner layers of normal tooth enamel, mild dental fluorosis enamel, and severe dental fluorosis enamel were tested by nanoindentation under an applied load of 2000 μN and holding time of 30 s. The nanotribological properties were then evaluated through nanoscratch tests under an applied load of 1000 μN. In addition, the nanotribological property of the outer layer of dental fluorosis enamel was compared with that of four restorative materials, namely, lithium disilicate glass-ceramic (IPS e.max CAD), polymer-infiltrated-ceramic network (PICN), composite resin block (Lava™ ultimate), and conventional composite resin (Fltek™ Z350XT). The nanohardness and elastic modulus of mild dental fluorosis enamel increased from the outer to the middle layers and then decreased from the middle to the inner layers. By contrast, the changed displacement, friction coefficient, and nanoscratch depth and width decreased from the outer to the middle layers and then increased from the middle to the inner layers. In severe dental fluorosis enamel, nanohardness and elastic modulus increased from the outer to the inner layers, but the changed displacement, friction coefficient, and nanoscratch depth and width decreased from the outer to the inner layers. The nanoscratch depth and width of Lava™ ultimate were similar to those of the outer layer of the mild dental fluorosis enamel. The gradient nanomechanical behavior of dental fluorosis enamel significantly differed from that of normal tooth enamel. Dental materials with a wear resistance similar to that of the opposing enamel are a good choice for restoring dental fluorosis (trial registration: WCHSIRB-D-2014-126, registered 25 December 2014).

Developmental and Post-Eruptive Defects in Molar Enamel of Free-Ranging Eastern Grey Kangaroos (Macropus giganteus) Exposed to High Environmental Levels of Fluoride

Dental fluorosis has recently been diagnosed in wild marsupials inhabiting a high-fluoride area in Victoria, Australia. Information on the histopathology of fluorotic marsupial enamel has thus far not been available. This study analyzed the developmental and post-eruptive defects in fluorotic molar enamel of eastern grey kangaroos (Macropus giganteus) from the same high-fluoride area using light microscopy and backscattered electron imaging in the scanning electron microscope. The fluorotic enamel exhibited a brownish to blackish discolouration due to post-eruptive infiltration of stains from the oral cavity and was less resistant to wear than normally mineralized enamel of kangaroos from low-fluoride areas. Developmental defects of enamel included enamel hypoplasia and a pronounced hypomineralization of the outer (sub-surface) enamel underneath a thin rim of well-mineralized surface enamel. While the hypoplastic defects denote a disturbance of ameloblast function during the secretory stage of amelogenesis, the hypomineralization is attributed to an impairment of enamel maturation. In addition to hypoplastic defects, the fluorotic molars also exhibited numerous post-eruptive enamel defects due to the flaking-off of portions of the outer, hypomineralized enamel layer during mastication. The macroscopic and histopathological lesions in fluorotic enamel of M. giganteus match those previously described for placental mammals. It is therefore concluded that there exist no principal differences in the pathogenic mechanisms of dental fluorosis between marsupial and placental mammals. The regular occurrence of hypomineralized, opaque outer enamel in the teeth of M. giganteus and other macropodids must be considered in the differential diagnosis of dental fluorosis in these species.

The Role of Chronic Exposure to Amoxicillin/Clavulanic Acid on the Developmental Enamel Defects in Mice

Amoxicillin used in early childhood may be associated with enamel hypomineralization. Our aim was to assess disturbances of amelogenesis in mice lower incisors induced by chronic administration of amoxicillin/clavulanic acid (AMC). Twenty-eight C57BL/6 male mice, of similar age, randomly divided into a control and 3 treatment groups ( n = 7) received subcutaneous injection, once per day, for 60 days: 50, 100, and 150 mg/kg BW of AMC. Scanning electron microscopy/energy dispersive X-ray spectroscopy analysis in AMC treatment groups showed higher content in F and a decrease in P and Ca. Morphology changes ranged from scratched patterns, and small isolated pits-like enamel loss, to generalized demineralized enamel surface, giving a rough, foamy, scaly, or even cracked eggshell appearance to the affected areas. Histological analysis showed disturbances of maturation ameloblasts, which were less organized, with increased amounts of clear vacuoles in the cytoplasm and slightly more elongated and less condensed nucleus. Additionally, they were often detached from the enamel matrix. Transitional ameloblasts formed underlying the cysts of varied sizes. In conclusion, AMC dose-dependently affect ameloblast functions especially in the maturation phase, causing hypomineralized enamel formation with quantitative and/or qualitative defects.

Fluoride incorporation into apatite crystals delays amelogenin hydrolysis

Enamel fluorosis has been related to an increase in the amount of amelogenin in fluorosed enamel compared with normal enamel in the maturation stage. In this study we tested the hypothesis that fluoride incorporated into carbonated apatite alters amelogenin hydrolysis. Recombinant human amelogenin (rh174) was allowed to bind to 0.15 mg of carbonated hydroxyapatite (CAP) or to fluoride-containing carbonated hydroxyapatite (F-CAP) synthesized to contain 100, 1,000, or 4,000 ppm F(-). After 3 h of digestion with recombinant human matrix metalloproteinase 20 (MMP20) or kallikrein-related peptidase 4 (KLK4), bound protein was characterized by reverse-phase high-performance liquid chromatography (HPLC). Proteolytic fragments of amelogenin formed after 24h of digestion with MMP20 of KLK 4 were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The hydrolysis, by both MMP20 and KLK4, of amelogenin bound to F100-CAP was significantly reduced in a dose-dependent manner compared with the hydrolysis of amelogenin bound to CAP. After 24 h of hydrolysis, a similar number of MMP20 cleavage sites was found for amelogenin bound to CAP and amelogenin bound to F100-CAP; however, 24 fewer KLK4 cleavage sites were identified for amelogenin bound to F100-CAP than for amelogenin bound to CAP. These results suggest that the reduced hydrolysis of amelogenins in fluorosed enamel may be partially caused by the increased fluoride content in fluoride-containing apatite, contributing to the hypomineralized enamel matrix phenotype observed in fluorosed enamel.

Fate of fluoride-induced subameloblastic cysts in developing hamster molar tooth germs

White opacities and pits are developmental defects in enamel caused by high intake of fluoride (F) during amelogenesis. We tested the hypothesis that these enamel pits develop at locations where F induces the formation of sub-ameloblastic cysts. We followed the fate of these cysts during molar development over time. Mandibles from hamster pups injected with 20mg NaF/kg at postnatal day 4 were excised from 1h after injection till shortly after tooth eruption, 8 days later. Tissues were histologically processed and cysts located and measured. Cysts were formed at early secretory stage and transitional stage of amelogenesis and detected as early 1h after injection. The number of cysts increased from 1 to almost 4 per molar during the first 16h post-injection. The size of the cysts was about the same, i.e., 0.46±0.29×10(6)μm(3) at 2h and 0.50±0.35×10(7)μm(3) at 16h post-injection. By detachment of the ameloblasts the forming enamel surface below the cyst was cell-free for the first 16h post-injection. With time new ameloblasts repopulated and covered the enamel surface in the cystic area. Three days after injection all cysts had disappeared and the integrity of the ameloblastic layer restored. After eruption, white opaque areas with intact enamel surface were found occlusally at similar anatomical locations as late secretory stage cysts were seen pre-eruptively. We conclude that at this moderate F dose, the opaque sub-surface defects with intact surface enamel (white spots) are the consequence of the fluoride-induced cystic lesions formed earlier under the late secretory-transitional stage ameloblasts.

The impact of fluoride on ameloblasts and the mechanisms of enamel fluorosis

Intake of excess amounts of fluoride during tooth development cause enamel fluorosis, a developmental disturbance that makes enamel more porous. In mild fluorosis, there are white opaque striations across the enamel surface, whereas in more severe cases, the porous regions increase in size, with enamel pitting, and secondary discoloration of the enamel surface. The effects of fluoride on enamel formation suggest that fluoride affects the enamel-forming cells, the ameloblasts. Studies investigating the effects of fluoride on ameloblasts and the mechanisms of fluorosis are based on in vitro cultures as well as animal models. The use of these model systems requires a biologically relevant fluoride dose, and must be carefully interpreted in relation to human tooth formation. Based on these studies, we propose that fluoride can directly affect the ameloblasts, particularly at high fluoride levels, while at lower fluoride levels, the ameloblasts may respond to local effects of fluoride on the mineralizing matrix. A new working model is presented, focused on the assumption that fluoride increases the rate of mineral formation, resulting in a greater release of protons into the forming enamel matrix.

Tooth displacement and root dilaceration after trauma to primary predecessor: an evaluation by computed tomography

This case report illustrates the relationship between trauma to a deciduous tooth and the consequences to the permanent dentition. The patient suffered an injury to the central incisors at the age of four with resultant pulpal necrosis. The permanent dentition was affected by displacement from its original position and a severe dilaceration at the crown/root junction.

Spatial distribution of dental fluorosis in roe deer (Capreolus capreolus) from North Bohemia (Czech Republic) and its relationships with environmental factors

We assessed the spatial variation of fluoride load on the local ecosystem in the Czech part of the Ore Mountains region and its southern foothills - a heavily polluted part of Europe's "Black Triangle" region. Dental fluorosis in roe deer (Capreolus capreolus) served as a biomarker of fluoride exposure and thus as an indicator of environmental pollution by fluoride. The mean dental lesion index of fluorosis (DLI) calculated from the analyzed mandibles of wild roe deer (>or=2 years of age) was assigned to the hunting ground from which the specimens originated and classified into one of the eight fluorosis categories. Environmental factors potentially related to dental fluorosis (atmospheric deposition of sulfur, concentration of fluoride in and pH of surface waters, geomorphologic features, bedrock and soil type, and vegetation cover), which were represented in the study by GIS layers, were examined to explain the distribution pattern and severity of fluorosis in the roe deer. The study revealed that 75.5% of 616 analyzed mandibles showed dental fluorosis to different extent, with individual DLIs ranging from 0 to 21. The spatial pattern of marked fluorosis on the Czech side continues that found in a previous study on the German side of the Ore Mountains. Together they create a landscape island around several thermal power plants in the region. General Linear Model (GLM) analyses revealed significant relationships between degree of forest damage, soil type, and atmospheric sulfur deposition from air pollution and dental fluorosis, expressed as mean DLI in the roe deer.

Timing of fluoride intake in relation to development of fluorosis on maxillary central incisors

OBJECTIVES

Several studies have focused on the timing of fluoride intake relative to the development of dental fluorosis. This study reports the relationships of fluoride intake during the first 48 months of life with fluorosis on early-erupting permanent teeth.

METHODS

Subjects were followed from birth to 48 months with questionnaires every 3-4 months. Questionnaires gathered data on intakes from water, diet, supplements, and dentifrice to estimate total fluoride intake. Early-erupting permanent teeth of 579 subjects were assessed for fluorosis using the Fluorosis Risk Index (FRI) at approximately age 9. Fluorosis cases were defined as having FRI definitive or severe fluorosis on both maxillary central incisors. Individuals with FRI questionable fluorosis were excluded. The importance of fluoride intake during different time periods was assessed using t-tests and logistic regression.

RESULTS

One hundred and thirty-nine (24%) subjects had fluorosis on both maxillary central incisors. Mean fluoride intake per unit body weight (bw) ranged from 0.040 to 0.057 mg/kg bw, with higher intake during earlier time periods and relative stability after 16 months. In bivariate analyses, fluoride intakes during each of the first 4 years were individually significantly related to fluorosis on maxillary central incisors, with the first year most important (P < 0.01), followed by the second (P < 0.01), third (P < 0.01), and fourth year (P = 0.03). Multivariable logistic regression analyses showed that, after controlling only for the first year, the later years individually were still statistically significant. When all four time periods were in the model, the first (P < 0.01) and second years (P = 0.04) were still significant, but the third (P = 0.32) and fourth (P = 0.82) were not.

CONCLUSIONS

The first two years of life were most important to fluorosis development in permanent maxillary central incisors; however, this study also suggests the importance of other individual years.

Reconstructing impairment of secretory ameloblast function in porcine teeth by analysis of morphological alterations in dental enamel

We studied the relationship between the macroscopic appearance of hypoplastic defects in the dental enamel of wild boar and domestic pigs, and microstructural enamel changes, at both the light and the scanning electron microscopic levels. Deviations from normal enamel microstructure were used to reconstruct the functional and related morphological changes of the secretory ameloblasts caused by the action of stress factors during amelogenesis. The deduced reaction pattern of the secretory ameloblasts can be grouped in a sequence of increasingly severe impairments of cell function. The reactions ranged from a slight enhancement of the periodicity of enamel matrix secretion, over a temporary reduction in the amount of secreted enamel matrix, with reduction of the distal portion of the Tomes' process, to either a temporary or a definite cessation of matrix formation. The results demonstrate that analysis of structural changes in dental enamel allows a detailed reconstruction of the reaction of secretory ameloblasts to stress events, enabling an assessment of duration and intensity of these events. Analysing the deviations from normal enamel microstructure provides a deeper insight into the cellular changes underlying the formation of hypoplastic enamel defects than can be achieved by mere inspection of tooth surface characteristics alone.

Fluoride Intake Levels in Relation to Fluorosis Development in Permanent Maxillary Central Incisors and First Molars

Gaps remain in our knowledge about the levels of fluoride intake that cause dental fluorosis. The purpose of this study was to report the fluorosis prevalence by levels of estimated fluoride intake in an effort to understand the importance of different levels of daily fluoride intake. As part of the longitudinal Iowa Fluoride Study, subjects were followed from birth to 36 months with questionnaires every 3-4 months to gather information on fluoride intake from various sources. Daily fluoride intake in mg per kg body weight (BW) was estimated from water, beverages and selected foods, fluoride supplements and dentifrice. Six hundred and twenty-eight subjects were examined for fluorosis on permanent incisors and first molars at about age 9 by two calibrated examiners using the Fluorosis Risk Index categories. Fluorosis prevalence rates were determined separately for maxillary central incisors and first molars by levels of estimated fluoride intake. There were significant positive associations between fluorosis prevalence and levels of fluoride intake. Cumulatively from birth to 36 months, average daily intake of 0.04 mg F/kg BW or less carried relatively low risk for fluorosis (12.9% for maxillary central incisors, 6.8% for first molars). Average daily intake of 0.04-0.06 mg F/kg BW showed a significantly elevated risk for fluorosis (23.0% for maxillary central incisors, 14.5% for first molars), while fluorosis risk was even higher for average intake above 0.06 mg F/kg BW (38.0% for maxillary central incisors, 32.4% for first molars). The study suggests that fluorosis prevalence is related to elevated fluoride intake when averaged over the first 3 years of life, but is even more strongly related to fluoride intake that is elevated for all of the first 3 years of life.

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.

Disturbed enamel formation in wild boars (Sus scrofa L.) from fluoride polluted areas in Central Europe

The pathological alterations of enamel structure in the teeth of wild boars from fluoride polluted areas in N-Bohemia (Czech Republic) and S-Saxony (Germany) were studied on a macroscopic and a microscopic level. Mandibular bone fluoride concentration (mg F(-)/kg, dry wt; mean +/-SD, individuals <24 months of age) in the specimens from N-Bohemia (754.3+/-149.6) and S-Saxony (490.8+/-135.1) was significantly higher than that of controls (free of dental fluorosis), originating from the western part of Germany (304.7+/-91.0). Fluoride content in bulk enamel (mg F(-)/kg, ash wt) of fluorotic permanent teeth from N-Bohemia (382.1+/-165.2) and S-Saxony (125.0+/-38.3) was likewise significantly increased over that of non-fluorotic control teeth from W-Germany (33.6+/-26.7). Macroscopically, fluorosed wild boar enamel exhibited opacity and discoloration of varying extent, accentuated perikymata as well as hypoplastic and posteruptive surface defects. Microradiographic and scanning electron microscopic analyses revealed enamel subsurface hypomineralization, accentuated Retzius lines and occurrence of broad, hypomineralized incremental bands of abnormal structure underlying hypoplastic enamel surface defects. The presence of zones of aprismatic enamel was associated with these bands. Incremental bands with altered enamel structure and enamel surface hypoplasias, both denoting a severe disturbance during the secretory stage of amelogenesis, have previously been observed in rodents following acute parenteral fluoride dosing. It is concluded that in the chronically fluoride exposed wild boars periods of especially elevated plasma fluoride levels exerted an acute toxic effect on the secretory ameloblasts. A feature not previously reported from fluorosed enamel was the occurrence of canal-like structures that originated at the broad incremental bands and extended into the external enamel. The presence of these canals presumably results from a delay in the resumption of secretory activity by groups of ameloblasts following a fluoride insult. Based on experimental evidence in domestic pigs and in sheep, the overall subsurface hypomineralization of fluorosed wild boar enamel is attributed to a disturbance of enamel maturation. The distribution of fluorotic enamel changes within the dentition of the wild boars could be related to the developmental sequence of tooth formation in the species. Teeth whose crown formation took place prenatally (deciduous teeth) or largely pre-weaning (permanent first molars) exhibited no or only moderate fluorotic enamel alterations. Based on the extension of enamel surface hypoplasias along the coronoapical axes of the tooth crowns, the timing of excess fluoride exposure that caused a marked disruption of enamel matrix secretion was estimated in specimens with a known date of death. The results indicate that the wild boars had been exposed to a particularly severe fluoride impact during autumn and winter of their first year of life.

Mechanism and timing of fluoride effects on developing enamel

Fluoride appears to specifically interact with mineralizing tissues, causing an alteration of the mineralization process. In enamel, fluorosis results in a subsurface hypomineralization. This hypomineralized enamel appears to be directly related to a delay in the removal of amelogenins at the early-maturation stage of enamel formation. The specific cause for this delay is not known, although existing evidence points to reduced proteolytic activity of proteinases that hydrolyze amelogenin. This delay in hydrolysis of amelogenins could be due to a direct effect of fluoride on proteinase secretion or proteolytic activity, or to a reduced effectiveness of the proteinase due to other changes in the protein or mineral of the fluorosed enamel matrix. The formation of dental fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The early-maturation stage of enamel formation appears to be particularly sensitive to the effects of fluoride on enamel formation. Although the risk of enamel fluorosis is minimal with exposure only during the secretory stage, this risk is greatest when exposure occurs in both secretory and maturation stages of enamel formation. The risk of fluorosis appears to be best related to the total cumulative fluoride exposure to the developing dentition.

Biological mechanisms of dental fluorosis relevant to the use of fluoride supplements

Fluorosis occurs when fluoride interacts with mineralizing tissues, causing alterations in the mineralization process. In dental enamel, fluorosis causes subsurface hypomineralizations or porosity, which extend toward the dentinal-enamel junction as severity increases. This subsurface porosity is most likely caused by a delay in the hydrolysis and removal of enamel proteins, particularly amelogenins, as the enamel matures. This delay could be due to the direct effect of fluoride on the ameloblasts or to an interaction of fluoride with the proteins or proteinases in the mineralizing matrix. The specific mechanisms by which fluoride causes the changes leading to enamel fluorosis are not well defined; though the early-maturation stage of enamel formation appears to be particularly sensitive to fluoride exposure. The development of fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The risk of enamel fluorosis is lowest when exposure takes place only during the secretory stage, but highest when exposure occurs in both secretory and maturation stages. The incidence of dental fluorosis is best correlated with the total cumulative fluoride exposure to the developing dentition. Fluoride supplements can contribute to the total fluoride exposure of children, and if the total fluoride exposure to the developing teeth is excessive, fluorosis will result.

A quantitative backscattered electron imaging study of hypomineralization and hypoplasia in fluorosed dental enamel of deer

Mineral content and distribution of fluorosed and unfluorosed (control) dental enamel of roe deer and red deer cheek teeth were analyzed using digital backscattered electron (BSE) imaging of PMMA-embedded specimens. Compared to the controls, the fluorosed enamel exhibited various aberrations resulting from a fluoride-induced disturbance of the processes involved in enamel formation. Thus, the presence of surface hypoplasias and an enhancement of the incremental pattern in the fluorosed enamel are evidence of a fluoride impact on the secretory ameloblasts, whereas a (subsurface) hypomineralization of different depth and extent is indicative of a fluoride effect on the maturation stage of amelogenesis. The marked variation in the severity of enamel hypomineralization seen along the coronocervical axis of a specimen pointed to a fluoride impact of varying intensity during this period of tooth development. Our observations further indicated that, in some locations, ameloblasts severely affected by fluoride during enamel matrix formation were able to recover from this insult and to function quite normally during the maturation stage of amelogenesis. A major advantage of the BSE imaging technique used in the present study over other methods is that it allows for a combination of micromorphological information with quantitative data on the mineralization of the analyzed tissue, which proved to be very useful for the characterization of fluoride-induced changes in dental enamel.

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.

Enamel formation and the effects of fluoride

The exact biochemical events which result in enamel lesions from excess fluoride ingestion are still unknown. A number of effects of fluoride on enamel organs and on the enamel matrix components of developing teeth are, however, known. These are briefly reviewed, making reference to more recent studies. Two major influences of chronic, low-level fluoride exposure are proposed: fluoride interferes with the processes responsible for the efficient removal of organic matrix components, resulting in protein retention and disorganized enamel crystal formation, or fluoride disrupts the activities of the enamel organ cells which indirectly interferes with normal crystal formation.

Strategies for improving the assessment of dental fluorosis: focus on chemical and biochemical aspects

In order to assess fluoride accumulation and effects in developing dental tissues, one must determine the concentration profile of fluoride in the tissue and to assess separately the labile (i.e., free ions in fluid and ions associated with organic matter) and stable (i.e., incorporated into apatite lattice) pools of fluoride. Free fluoride ions in the mineralizing milieu markedly affect the driving force for precipitation and, as a result, the nature of precipitating crystals. The fluoride incorporated into the crystalline lattice increases the stability of the formed mineral. Improvement in the understanding of the mechanism of dental fluorosis requires more comprehensive information about the effects of fluoride on the ionic composition of the fluid phase, the nature of the initially precipitating mineral(s), the interactions between crystals and matrix proteins, and the enzymatic degradation of the proteins. Recent observations relevant to the role of fluoride in enamel formation include: (1) that there are threshold concentrations of fluoride below which the precipitation and hydrolysis of thin-platy octacalcium phosphate is facilitated but beyond which de novo apatite precipitation prevails; (2) that the presence of fluoride in the mineralizing milieu most likely affects the steady-state concentrations of mineral lattice ions; (3) that incorporation of fluoride into the stable pool is retarded by the presence of matrix proteins, particularly amelogenins, which inhibit the growth of apatite crystals; (4) that increasing the degree of fluoridation of apatite crystals enhances the adsorption of amelogenins onto the crystal surface, and (5) that amelogenins pre-adsorbed onto apatite crystals are more resistant to enzymatic cleavages by trypsin (used as a prototype of amelogeninases).

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.

Fluoride-induced developmental changes in enamel and dentine of European roe deer (Capreolus capreolus L.) as a result of environmental pollution

Using macroscopic, microradiographic and scanning electron-microscopic methods, the effects of increased fluoride exposure on enamel and dentine formation were studied in fluorosed mandibular premolars and molars of roe deer from the heavily industrialized Ruhr area, Germany. Macroscopically, fluorosed teeth were characterized by opaque and stained enamel and in more severe cases also by enamel surface lesions, reduction or loss of enamel ridges on their occlusal surfaces and increased wear. Microradiographically, fluorosed enamel exhibited different degrees of subsurface hypomineralization, in part apparently indicating a fluoride effect during enamel maturation. In some specimens, a pronounced but varying enhancement of the pattern of Retzius lines was observed throughout the enamel, denoting strongly intermittent fluoride exposure during enamel matrix secretion. This variation in exposure was also reflected histologically in dentine, by bands of interglobular dentine and marked accentuation of incremental lines. Microradiography of sections through enamel surface hypoplastic lesions showed the enamel forming the bottom and partly also the walls of the lesions to be highly mineralized. Scanning electron microscopy showed that the outer enamel along the more pronounced hypoplastic lesions consisted of stacked, thin layers of 'aprismatic' enamel, indicating that the ameloblasts in these areas had lost the distal (rod-forming) regions of their Tomes' processes. These observations demonstrate that the origin of enamel hypoplasias in deer clearly differs from that in rodents, where fluoride induces the formation of subameloblastic cysts. The differences in the degree of fluorotic alteration between the teeth of a single tooth row could be related to the developmental sequence of the dentition in roe deer. The roe deer is thus considered to be a very sensitive and useful bioindicator of environmental pollution by fluorides.

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 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.

Nature and mechanisms of dental fluorosis in animals

Recent studies of dental fluorosis in animals have been reviewed. The aim was to describe studies which have provided information which may be relevant to explaining the mechanisms involved in human dental fluorosis. Studies on rats, sheep, and pigs have provided details of dosage regimens which produce lesions which resemble those described in human fluorosis. In the pig and rat, the plasma fluoride concentrations associated with these dental lesions are of the same order of magnitude as those which may occur in man. Three different kinds of studies in different species have shown that fluoride affects processes occurring during enamel maturation. One study on rats has indicated that fluoride may reduce matrix removal during maturation by an effect on enamel proteases. Many studies have demonstrated accumulation of fluoride in secretory enamel and that fluoride concentrations in maturation enamel are lower than in secretory enamel. This phenomenon had previously been explained by the binding of fluoride to enamel proteins, but recent studies indicate that such binding does not occur. The hypothesis that enamel fluorosis might be caused by general effects of fluoride on calcium metabolism has not been supported by more recent studies. It was concluded that, although the mechanisms involved in dental fluorosis remain obscure, recent animal studies do seem to have provided new information which may prove to be important for our understanding of mechanisms whereby fluoride causes dental fluorosis in man.

Differential diagnosis of dental fluorosis

Differentiating between fluorotic and non-fluorotic defects of dental enamel is an important diagnostic decision in epidemiology and public health dentistry. The commonly accepted diagnostic criteria for fluorosis discriminate between non-discrete symmetrical and asymmetrical distributions of opacities of dental enamel. These criteria appear to identify most cases of dental fluorosis. However, it is not yet confirmed that the pattern and distribution of dental fluorosis are a unique phenomenon. Metabolic, physiological, other trace elements, and malnutrition have been reported to induce bilateral symmetrical developmental enamel opacities. Misdiagnosis of non-fluoride-induced opacities remains a possibility. Reports of unexpectedly high population prevalence and individual cases of fluorosis, where such diagnoses are incompatible with the known fluoride history, indicate the need for a more precise definition and diagnosis of dental fluorosis. A more discriminating diagnostic procedure is recommended. This calls for a positive identification of the levels of fluoride available to communities and individuals before a diagnosis of fluorosis is confirmed. We believe a more critical approach to the diagnosis of fluorosis will be helpful in the rational use and control of fluorides for dental health, and in the identification of factors associated with inducing developmental defects of enamel.

Developmental defects of enamel--historical and present-day perspectives of their pathogenesis

Developmental defects of enamel are visible deviations from the normal translucent appearance of tooth enamel resulting from enamel organ dysfunction. In the past, information about the activities of the ameloblasts has determined the terminology used to describe the lesions. Advances in our understanding of the complicated secretory and maturation phases of amelogenesis have required a re-appraisal of the concepts of defect formation. The phase of ameloblast activity, the duration of the disturbance, and its severity leading to temporary or permanent inactivity of the cells determine the appearance of the three common types of lesions--hypoplasia, and diffuse and demarcated opacities.

Macroscopic and scanning electron microscopic appearance and hardness values of developmental defects in human permanent tooth enamel

Defects present in 12 human permanent teeth were classified on the basis of their macroscopic appearance as hypoplasia (three teeth), diffuse opacities (three teeth), white demarcated opacities (one tooth but two defects), or yellow demarcated opacities (five teeth but six defects). The hardness values and SEM appearance of the defective enamel were determined after the teeth were sectioned through the lesion(s) and were distinctive for each type of defect. The thin enamel of the hypoplastic lesions was either opaque (with reduced hardness values) or translucent (with near-normal hardness values and sometimes a change in prism orientation external to an incremental line). The enamel of the diffuse and demarcated opacities was of normal thickness. The changes in the macroscopic and SEM appearance, and the reduced hardness values of the diffuse patchy opacities, were restricted to the outer 150 microns of the enamel. The demarcated opacities varied in position and depth, and in places had a clearly marked boundary with the adjacent normal enamel. Hardness values were related to color change, with yellow lesions being softer than white. Although prism direction was normal within demarcated opacities, prism outlines were less distinct. The findings suggest that temporary and permanent dysfunction of ameloblasts can occur in both secretory and maturation phases, influencing the final appearance of the lesion.

Proton microprobe determination of fluorine profiles in the enamel and dentine of erupting incisors from sheep given low and high daily doses of fluoride

Developmental defects in incisors were induced by daily oral ingestion of sodium fluoride solutions. Teeth extracted at eruption from sheep that had been subjected to four different fluoride regimens--0.2 or 0.5 mg F/kg body weight daily for 6 months, 2 or 6 mg F/kg body weight daily for 21 days--were analysed for fluorine by gamma emission using a proton microprobe. Calcium and zinc profiles were also measured using proton-induced X-ray emission. Diffuse opacities, similar in appearance to mild human fluorosis, were produced by the first two regimens, whereas the last two produced hypoplastic lesions. Different distributions of fluoride were found in the unerupted enamel and dentine, and these patterns reflected variations in both the duration and concentration of the fluoride dose used to induce the fluorotic lesions.