E.I.F. Pearce

Fluoride in Enamel Lining Pits and Fissures of the Occlusal Groove–Fossa System in Human Molar Teeth

The fluoride (F) distribution in enamel lining the occlusal fissures of human molars and premolars is difficult to investigate by normal microsampling techniques, yet this information is of importance as fissures are particularly susceptible to caries. We have used the proton probe to map the distribution of F and Ca in sections of 17 molar teeth collected from Danish and New Zealand populations. The caries status of the sectioned fissure was determined by microradiography or visually after drying. The probe scans were graphed as density images, surface plots and topographical plots. Sound enamel bordering grooves and fissures showed a high–F surface layer, in 1 sample approximately 60 μm wide near the fissure bottom but gradually widening to double this width near the fissure opening, and up to 200 μm wide in grooves, while underlying enamel had a low and almost constant F level. In this respect fissure and groove enamel resembled smooth surface enamel. The F concentration fluctuated along the surface layer, reaching maximum values ranging from 1,800 to 4,200 ppm in 5 non–carious fissures. Incipient caries in fissure enamel usually but not always resulted in an increase in F in the outer layer, the F maximum values in 5 such samples ranging from 1,900 to 7,200 ppm. F maximum values in outer enamel were higher in 7 samples showing advanced caries, 2,700–10,000 ppm. The lesion subsurface body usually showed normal F values, as did carious dentine in the advanced lesions. The variable characteristics of the outer layer in sound fissure enamel are likely to be the result of a developmental process rather than environmental influences. The fact that we normally failed to find increased F concentrations in subsurface carious fissure enamel or in underlying carious dentine suggests that F does not diffuse into the deep part of fissure lesions, and probably has minimal effect on slowing the progress of such lesions, a suggestion in accord with clinical findings on the F effect on fissure caries.

The Effect of Sucrose Application Frequency and Basal Nutrient Conditions on the Calcium and Phosphate Content of Experimental Dental Plaque

A reduced pool of calcium in dental plaque would be expected to increase the ability of plaque fluid to dissolve the underlying enamel when the pH falls during sugar exposure. We have examined the relationship between frequency of sugar application and Ca and Pi concentrations in artificial mouth plaque microcosm biofilms. Ten plaques were grown simultaneously from a human saliva inoculum using a continuous flow of simulated saliva, DMM, supplemented with either urea or glucose to modulate the resting pH. In addition the plaques received sucrose applications of varying frequency: 12-, 8-, 6-, or 4-hourly, or not at all. After 15 days the plaques were sampled by taking 4 full-thickness specimens of each, and acid-extractable Ca and Pi, and alkali-soluble protein and carbohydrate were determined. Ca and Pi concentrations were in a range comparable with those in human plaque, except in the DMM + urea plaque receiving no sucrose, when concentrations were higher. Plaque Ca concentration decreased significantly as sucrose application frequency increased. Increasing sucrose application frequency also reduced the protein, i.e. the cell biomass, content of the plaques and, in the case of DMM + urea plaques, increased the water-insoluble hexose content, presumably extracellular polysaccharide. Reduced biomass was partly due to the bulking of plaque with extracellular polysaccharide, but the marked effect of urea on polysaccharide formation is not understood. This study shows that increasing frequency of sugar application alters dental plaque by reducing its mineral protection capacity.

Notes on the dissolution of human dental enamel in dilute acid solutions at high solid/solution ratio.

The aim of the present study was to elucidate the nature of the relationship between enamel apatite and lesion fluid during demineralization. Powdered enamel in samples of 1.0 g was suspended in 3 ml of 10, 30, 50, or 70 mmol/l HCl under gentle agitation for up to 24 h at 20 degrees C. After 20 min and 24 h, pH and the concentrations of calcium and phosphate were determined and the degree of saturation with respect to various calcium phosphates calculated. The experiments were replicated 15 times using the same enamel samples. Twenty minutes after the start of dissolution, both pH and concentrations of calcium and phosphate had increased, and the solution became supersaturated with respect to hydroxyapatite, and in some runs also with respect to brushite. During the subsequent 23 h and 40 min, pH continued to increase, despite the supersaturation with respect to apatite, whilst the concentrations of calcium and phosphate decreased due to formation of apatite and, occasionally, brushite mineral. The data indicated that release of carbonate from enamel and its conversion to H2CO3 caused the increase in pH and thus, played a major role in the dissolution-reprecipitation process.

Defluoridation of Drinking Water by Boiling with Brushite and Calcite

Existing methods for defluoridating drinking water involve expensive high technology or are slow, inefficient and/or unhygienic. A new method is now suggested, encompassing brushite and calcite suspension followed by boiling. Our aim was to examine the efficiency of the method and the chemical reactions involved. Brushite, 0.3–0.5 g, and an equal weight of calcite were suspended in 1 litre water containing 5–20 ppm fluoride. The suspensions were boiled in an electric kettle, left to cool and the calcium salts to sediment. Solution ion concentrations were determined and sediments were examined by X-ray diffraction. In distilled water initially containing 5, 10 and 20 ppm fluoride the concentration was reduced to 0.06, 0.4 and 5.9 ppm, respectively. Using Aarhus tap water which contained 2.6 mmol/l calcium the final concentrations were 1.2, 2.5 and 7.7 ppm, respectively, and runs without calcite gave results similar to those with calcite. Without boiling the fluoride concentration remained unaltered, as did the brushite and calcite salts, despite occasional agitation by hand. All solutions were supersaturated with respect to fluorapatite and hydroxyapatite and close to saturation with respect to brushite. Boiling produced well-crystallised apatite and traces of calcite, while boiling of brushite alone left a poorly crystallised apatite. We conclude that boiling a brushite/calcite suspension rapidly converts the two salts to apatite which incorporates fluoride if present in solution, and that this process may be exploited to defluoridate drinking water.

Fluoride Distribution in Sound and Carious Root Tissues of Human Teeth

Proton probe analysis has been used to provide for the first time quantitative F concentration data in carious root tissues from subjects consuming water containing 1 ppm F. In small lesions at the neck of the tooth with minimal tissue loss the F concentration was significantly higher at the outer lesion edge than at the outer edge of adjacent sound root tissue. In one sample with high F values the lesion edge had 19,000 ppm F and the adjacent sound root surface 5,400 ppm F (μg F/g apatite). In large lesions with extensive cavitation F was again concentrated in the outer edge of the lesion and was significantly higher (1,800–4,100 ppm) than in adjacent sound inner dentine (190–290 ppm). Fluoride concentrations varied markedly along the outer edge of both normal and carious root tissues. Fluoride increase at the lesion edge is not an effect of tissue shrinkage but probably a result of remineralisation events during caries. This additional F may be expected to increase tissue resistance to further acid attacks.

Fluoride content of the enamel and dentine of human premolars prior to and following the introduction of fluoridation in New Zealand.

The fluoride content of the enamel and dentine of premolars was used as a determinant of the availability of ingested fluoride in New Zealand prior to and following the introduction of water fluoridation 40 years ago. Premolar teeth, which developed during the periods (PRE and POST respectively) under study, were selected from teeth extracted from 12 to 14-year-old children resident in different geographic areas in the country. The fluoride content, determined by multiple proton microprobe analyses, of surface enamel, deep enamel, and dentine, were for PRE teeth 440, 65 and 115, respectively. For POST teeth the mean values were significantly (p<0.001) higher, by 69, 29 and 102% respectively. The relevance of the change in fluoride content was assessed by comparison with published reports on the fluoride content of teeth developed in communities exposed to low (<0.5 ppm), optimal (1-2 ppm) and high (>3 ppm) naturally occurring fluoride levels in drinking water. The PRE teeth had a fluoride content associated with a low fluoride exposure and POST teeth with optimal fluoride exposure during tooth development. It was concluded that fluoride availability in New Zealand teeth had increased over the past 30 years but this increase is compatible with exposure of the community to optimal rather than excessive levels of ingested fluoride.

Stoichiometry of Fluoride Release from Fluorhydroxyapatite during Acid Dissolution

Release of F from fluorhydroxyapatite (FHAp) during acid dissolution was studied to validate the use of this mineral as a plaque reservoir of F. FHAp minerals having a wide range of F concentrations were synthesised by aqueous precipitation, and samples repeatedly exposed to 50 mM lactic acid solution, pH 4.5, or similar lactic/acetic/formic acid mixtures, until dissolution was complete. While the Ca/P ratio in solution remained relatively constant and close to the ratio in the solid, the solution F/Ca ratio invariably changed during dissolution. During initial stages the F/Ca solution ratio was lower than in the solid but rose to reach a plateau higher than in the solid as dissolution progressed, an effect that was more pronounced with low-F FHAp. With these minerals the plateau F/Ca level never reached 0.2, suggesting that a F-enriched FHAp rather than pure fluorapatite precipitates during dissolution. It is concluded that a high-F FHAp mineral would best serve as an apatitic plaque reservoir of F.

The Effect of pH, Temperature and Plaque Thickness on the Hydrolysis of Monofluorophosphate in Experimental Dental Plaque

Monofluorophosphate (MFP), an anti-caries agent commonly used in toothpaste, is known to be degraded to fluoride and orthophosphate by bacterial phosphatases in dental plaque. We have examined the effect of pH, temperature, plaque thickness and some ions on this process. Both natural plaque and artificial microcosm plaque incubated with purified MFP at pH 4–10 showed an optimum pH of ∼8 for hydrolysis. Diffusion and concomitant hydrolysis were examined in an apparatus in which artificial plaque was held between rigid membranes separating two chambers. When MFP diffused through a plaque of 0.51-mm thickness over 4 h it was almost completely hydrolysed at pH 8, but hydrolysis on diffusion decreased as the pH deviated from 8. MFP in toothpaste extract showed a similar pH susceptibility to hydrolysis, according to the inherent pH of the toothpaste. Hydrolysis of MFP in the toothpaste was reduced by no more than 10% when compared with a matched-pH control, suggesting that other toothpaste ingredients had no major influence on hydrolysis. Transport was slower and hydrolysis at pH 6 more complete the thicker the plaque, but hydrolysis was not significantly slower at 23°C than at 37°C. The addition of various potential activating or inhibiting ions at 0.1 and 1.0 mmol/l had small and non-significant effects on hydrolysis. The results suggest that MFP toothpaste should be formulated and used to maximise enzymic hydrolysis of this complex anion, and that plaque pH control is probably the most important factor.

The influence of salivary variables on fluoride retention in dental plaque exposed to a mineral-enriching solution

This study was carried out to examine interindividual differences in salivary variables related to plaque accumulation and to estimate their influence on the fluoride retention in plaque in vivo by a mineral-enriching solution. Two saliva samples were taken from 10 subjects, once after brushing and once after 24 h without brushing. Calcium, phosphate and monofluorophosphatase (MFPase) activity in the saliva samples were determined. The salivary flow rate and the debris index were also recorded. After plaque had formed over 3 days within in situ plaque-generating devices, subjects were instructed to rinse with a mineral-enriching mouthrinse three times a day on 4 consecutive days. Plaque exposed to distilled water plus flavoring agents served as a control. Fluoride-free dentifrice was used during the experimental period. Twenty-four hours after the last rinsing, the samples were removed from the mouth, and fluoride and mineral distributions in plaque analyzed using a method previously reported by the authors. Salivary flow, MFPase activity and calcium concentration in saliva were significantly higher after 24 h of plaque accumulation. Rinsing with the mineral-enriching solution produced retention of fluoride and phosphate in the outer and middle layers of plaque. Salivary calcium concentration had a direct effect on fluoride uptake in plaque, but no obvious relationship was found between other salivary variables and the plaque fluoride retention. The salivary calcium effect may be due to enhanced bacterial cell wall binding of fluoride via calcium bridging.

Defluoridation of Drinking Water by Co-Precipitation with Apatite

The two-step brushite saturation, apatite precipitation method which has previously been shown to be effective for the partial defluoridation of water containing 10 ppm F has now been tested at lower concentrations. The method also reduced F concentrations in the range 1-5 ppm, but was less efficient at lower F levels. Although co-precipitation as fluorapatite is the intended mechanism here, experiments indicated that F adsorption on the seed material added to the defluoridation effect. When the contribution of co-precipitation was isolated it was found that this mechanism could, on its own, account for a reduction of 2.2-2.6 ppm F in water initially containing 3-5 ppm F. A computer program which determines equilibrium F concentrations when supersaturated solutions are induced to precipitate fluorapatite was used to estimate the maximum possible F removal from water by co-precipitation. Using saturation with brushite as a starting point for precipitation, F reductions similar to those found experimentally were predicted. Computer simulations also suggest that any desired reduction in F concentration may be achieved by adding sufficient Ca and phosphate and/or by raising the initial pH.