Effect of monovalent ions in glass ionomer cements on their interaction with sodium fluoride solution

The effects on surface morphology of glass ionomer cements following exposure to 0.2% NaF solution were studied. The effect of cement on the solution was also evaluated. The four cements were chosen to contain Na and F, Na alone, F alone and neither Na nor F to show any interactions produced by having the same ion in both the cement and solution. Four glass ionomer cements were formulated so that they differed only in respect of the glass component. AH2 (a glass used in dental restorative cement) contained both Na and F, MP4 (a glass used in orthopaedic cement) contained Na only, LG26 (a glass used in surgical cement) contained F only and LG30 (an experimental control glass) contained neither F nor Na. Discs of cement were set in moulds at 37 degrees C for 1 h, then matured in water for 3 d. Each test disc was then immersed in 10 ml 0.2% NaF for 24 h at 37 degrees C whereas control discs remained in water. The test and control disc surfaces were assessed qualitatively using electron microscopy and quantitatively by linear profilometry generating roughness values (Ra). Test solution pH was measured before and after cement immersion. Inspection of the electron micrographs showed considerable disruption of AH2 and LG26 test surfaces compared to their controls whereas MP4 and LG30 showed similar surfaces for test and control. Statistical analysis of the Ra values showed that AH2 and LG26 test surfaces were significantly rougher than their controls as well as LG30 and MP4 test surfaces, which were not significantly different from their controls. All NaF solutions show pH increases; those for AH2 and MP4 were significantly higher than those for LG26 and LG30. The F-containing cements were subject to surface disruption whereas F-free cements were not. The Ra values of test surfaces correlated strongly (r = 0.998) with the F uptake of the cements (data from a previous study) but it was not possible to ascribe the causality to this association. The pH changes appear to be influenced by whether or not Na is present in the cement. The resultant pH values are too near to neutral for pH alone to explain the surface disruption observed. In addition, it is concluded that the changes in OH ion concentration are too low to permit F-/OH- interchange as a possible explanation for F uptake by these cements.     

Effects of adding sodium and fluoride ions to glass ionomer on its interactions with sodium fluoride solution

This investigates the effects of the addition of Na and F ions to a glass ionomer cement in which those ions are not inherently present on its interactions with dilute (0.2%) NaF solution. Both the effect of the solution on the cement's surface morphology and the effect of the cement on the solution in terms of take up of Na+ and F- and of change in pH are to be investigated. These results are to be compared to previous results obtained with glasses which contained both, one, or neither of the ions as components of their glasses. NaF (1.3% by weight in the mixed cement) was added to the powder components of a glass ionomer based on LG30 glass (which contains Al, Si, Ca, P, and O only). Discs of cement were set in moulds at 37 degrees C for 1 h then stored in water at 37 degrees C for 3 days. Each test disc was then immersed in 10 ml 0.2% NaF solution whereas controls remained immersed in water (N = 3 for test and control). Test and control disc surfaces were assessed both qualitatively by electron microscopy and quantitatively by linear profilometry (Ra values). Potentiometry was used to measure solution pH and Na and F concentrations using a pH electrode and suitable ion selective electrodes both before and after cement immersion. The surface of test specimens was subject considerable disruption with the polysalt cement matrix being removed and residual glass particles being disclosed. The controls showed no such disruption. This effect was reflected in a significant difference of Ra. Such an effect was not shown by test and control surfaces of LG30 but a similar effect was to that shown by LG26 (which contains F as a glass component). Solution pH changed by 1 unit which was much more than the change shown by LG30 or LG26 but is similar to that of AH2 and MP4 cements which both contain Na. The Na and F uptake was much lower than for LG30 whereas that of LG26 was higher than LG30. The Na:F ratio was 0.29:1 compared to 1.26:1 for LG30 (LG26 = 1.01:1, AH2 = 1.02:1, MP4 = 1.04:1). Fluoride addition to a F-free glass ionomer renders it vulnerable to surface disruption by NaF solution showing that fluoride complexes produced in glass dissolution are not necessarily involved in this process. Sodium addition to a Na-free glass ionomer confirms the role of this cement in enhancing pH change in NaF solution. The level of uptake of F- from a NaF solution in much lower than that for the F-free glass ionomer which shows there is no direct relationship between F- uptake and surface disruption. The ratio of Na:F uptake is below 0.3:1, but the pH change is similar to cements where the ratio is close to unity which indicates that F-/OH- interchange is not a significant mechanism even when anion/cation uptake is not balanced.     

Distribution of fluoride in glass ionomer cement determined using SIMS

The uptake by glass ionomer cement of ions (particularly fluoride) from solutions in which the cements have been immersed has been extensively reported. The concentrations within the cement often greatly exceed those in the immersing solution. The distribution of these ions has not been determined. The aim of this study is to use SIMS to investigate the levels of ions within the cement at different depths below the immersed surface of the cement. K+ and F were the ions studied and uptake was into a cement containing neither K nor F (LG30) and one containing F (AH2).The surface was analysed using a Cameca ims4f instrument employing a 14.5 keV Cs+ primary ion beam. This was calibrated on cements made from a series of glasses in which fluorine content was systematically substituted for oxygen (without other elemental changes). XPS, which is very much a surface technique, was used in confirmatory role with respect to the SIMS analysis. Cement discs were made from LG30- and AH2-based cements. After maturation for 72 h these were immersed in 0.275% KF solution for 24 h. SIMS analysis indicated appreciable surface F concentration on LG30 and on AH2 an enhanced F concentration. In contrast, K was not detected on the LG30 surface and only at a low level on AH2. These results were confirmed by XPS. Using the ion beam of the SIMS to sputter away cement enabled the F depth profile on LG30 to be measured to 10 microm. Over this distance the F content drops from 6.2 mmol/g at 0.2 microm from the surface to 0.2 mmol/g at 10 microm. No K was detected down to 13 microm from the surface. From the results of this study, it can be concluded that SIMS is an appropriate tool for further investigation of the distribution of ions uptaken by glass ionomer cements.     

Effect of monovalent ions in glass ionomer on their uptake and re-release

AIMS: The study aims to directly measure uptake of Na and F ions by glass ionomer cement from dilute NaF solution and compare this with the subsequent re-release of these ions into water. In addition, the effect of the presence or absence of Na and/or F as a component of the glass is evaluated. MATERIALS AND METHODS: The four glass ionomers used differed only in glass composition; AH2 contained both Na and F, LG26 contained F, MP4 contained Na and LG30 contained neither Na nor F. Discs of cement were set in moulds at 37 degrees C for 1 h and matured in water at 37 degrees C for 3 days. Test discs were immersed in 0.2% NaF solution for 24 h, control discs in water. Discs were subsequently immersed in water which was changed regularly. Ion-selective electrode measurements (F and Na) and atomic absorption spectrometry (Na) were used to determine uptake (change in immersion solution concentration) and re-release into water. RESULTS: All cements took up large quantities of Na and F ions (range 95-336 mumol g-1). This resulted in internal ion concentrations from 16 to 56 times higher than the immersing solution. All re-release was complete within 97 days. No cement re-released more ion than taken up. Glass ionomers containing fluoride took up more Na and F than fluoride-free ones and then re-released a lower percentage of these ions. The cements all took up Na and F ions in equimolar proportions, but initially re-released more F than Na with F-free cement results tending to unity by 97 days. CONCLUSIONS: Glass ionomer cements take up Na and F ions from NaF solution in large quantities and in equimolar proportion. This is re-released either wholly or in part in 97 days by which time the release does not differ from the controls. The presence or absence of F in the cement composition markedly influences both uptake and re-release. Fluoride/hydroxyl interchange does not appear to play an important role in uptake.     

Glass ionomer cement: evidence pointing to fluorine release in the form of monofluorophosphate in addition to fluoride ion

The fluoride ion released from glass ionomer cements into water is reportedly, in part, complexed with other elements present in the cement. When measured using ion selective electrode potentiometry (ISE) a decomplexant TISAB IV (T) is used to convert all fluoride to F- ion which the ISE can detect. In this study, an additional decomplexing procedure (H) designed to hydrolyse fluorine covalently bonded to phosphorus in the monofluorophosphate (MFP) ion into F- was also used. The soluble products from three glass ionomers were analysed by both techniques (H & T). Five 1 x 10 mm discs were each immersed in 10 ml of de-ionised water. This was changed and 4 ml analysed by T and 4 ml by H at 1, 2, 3, 6, 10, 13, 17, 21, 24, 28, and 31 days. H was greater than T for 161 of the 165 pairs ( chi2=74.7, p=<0.001 ). The total cumulative F release H (in micromol/g cement) at 31 days for AH2 was 122.3, s.d. 30.8; LG26 44.0, s.d. 1.55; LG30 10.0, s.d. 3.15 as compared T results of 100.1, s.d. 31.1; 30.3, s.d. 1.92; 3.7, s.d. 1.36, respectively. In all three cases the H was significantly greater than T (matched pair 't' test with p=0.01 or less). H-T was show to have a very strong associative relationship with t1/2 (R2=0.98 or greater p<0.001 ). Evaluating the ratio of P:F in the cements in comparison with the ratio of additional F measured by H to that measured by T produced a relationship log[(H-T)/T]=0.28 x log[P/F]-0.45 with R2=0.999. It is concluded that glass ionomers release more fluorine than is detected by ISE using TISAB IV. If this F is in the form of MFP this may be released more completely into saliva than F as F-, release of which is substantially reduced by Ca2+, since calcium monofluorophosphate is more soluble than CaF2.     

The effects of adding fluoride compounds to a fluoride-free glass ionomer cement on subsequent fluoride and sodium release

Studies have shown that ions in a glass ionomer matrix are 1-10% of the amounts present in the original glass. To measure more precisely the release from a cement matrix, known amounts of ions were added to LG30 glass which was fluoride and sodium-free. Cement without additions acted as the control. 1.4-1.6% of each of sodium, calcium and aluminum fluorides were added to three portions of control blend. The sodium and fluoride release into deionised water from five discs of each cements blend was measured for 8 months. This represented complete release for sodium but not for fluoride. Traces of fluoride and sodium in the glass produced low but measurable amounts indicating about a third of the fluoride and substantially all sodium present in LG30 was released. The addition of calcium fluoride had no significant effect on sodium or fluoride release and aluminium fluoride minimal effects. Adding sodium fluoride significantly enhanced release of both ions although fluoride release was less than from a glass containing 5% fluoride. Only small proportions of the additions, 2-5% of the fluoride and 13% of sodium, were released. Sodium and fluoride appeared to be released independently. For LG30 cements additives were poor at supplying extra ions.     

Kinetics of fluoride release from zinc oxide-based cements

Considerable attention has been given to the release of the cariostatic fluoride ion from glass-based dental cements (dental silicate and glass ionomer). In these, the total available fluoride content is not precisely known since fluorine is distributed between the cross-linked aqueous salt matrix, partially dissolved glass, and undissolved glass. In analogous cements based on zinc oxide the fluoride is added as highly soluble SnF2. The object of this study is to compare the F- ion release profiles of commercial zinc polycarboxylate and zinc phosphate containing 4.4 and 3.6% SnF2, respectively. Mixed cements were clamped in split ring moulds to produce discs of 10 mm x 1 mm after storage at 37 degrees C for 1 h. Each was weighed and immersed in 10 ml of deionised water. When this changed, at 13 time intervals up to 98 days, the fluoride content was measured using an ion selective electrode. The mean (N = 3) values obtained were expressed cumulatively [F] in micromol F ion/g cement. The total [F] released was 111 for the zinc polycarboxylate and 286 for zinc phosphate compared with total F in the cements of 561 and 464, respectively. When the cumulative [F] was plotted versus t(1/2) close associations were found for both cements. For the polycarboxylate the regression line [F] = 10.6t(1/2) + 9.9 fitted well over the whole 98 days (R = 0.997). For the phosphate a better fit regression line was obtained using results up to 32 days only; [F] = 36.8t(1/2) - 8.4 (R = 0.999). For t > 32 days results increasingly deviated from this line. These results fitted a regression line of the form [F] = 81.7log(e) t - 87.3 (R = 0.9997). Comparisons are made with data from previous authors both for zinc phosphate cement and glass-based cements and with diffusion theory of F ion release. It is concluded that zinc-based cements provide some indications of how glass-based cements may behave over long periods of release and that zinc phosphate is the material of clinical choice for orthodontic cementation if maximal fluoride release is the prime criterion.     

The glass ionomer cement: the sources of soluble fluoride

This study aimed to investigate certain processes of fluoride production which enable glass ionomer cements to leach fluoride. Two fluoroaluminosilicate glasses, G338 and LG26 were used. The free and total fluoride which could be dissolved from the glasses was measured, before and after acetic acid washing. Both glasses contained appreciable amounts of soluble fluoride prior to any acid treatment. The latter process reduced the amount to some 75% of the original levels. Replacing the customary polymeric acid with propionic acid produced a cement which disintegrated in water allowing the amount of fluoride generated by the cement forming process to be measured. Cement production increased soluble fluoride by a further 3%. Both glasses behaved similarly when undergoing the various processes. G338 produced significantly greater quantities of fluoride, of the order of 10, compared with LG26 although containing only three times the amount of fluoride in the glass formula. A substantial proportion, over half, of the total fluoride was complexed especially after contact with cement and when G338 was used. During the period of the experiment, 21 days, total fluoride release did not seem to depend on the square root of time.     

A preliminary study of the effect of glass-ionomer and related dental cements on the pH of lactic acid storage solutions

Glass-ionomer cements, both self-hardening and resin-modified, have been shown to increase the pH of lactic acid solutions in which they are stored. Similar results have been obtained for a zinc phosphate and a zinc polycarboxylate cement. The pH was increased over a period of 7 days by between 1.54 and 2.65 pH units from an initial value of pH of 2.60, depending on the cement. It is concluded that, as a result of this ability to neutralize surrounding aqueous solutions, these materials may have the beneficial effect in vivo of inhibiting caries development. In the case of glass-ionomers, this mechanism might complement that of fluoride release.     

Potassium ion release from a glass ionomer cement matrix

Glass ionomer was mixed with water alone and with water containing two levels of potassium ion, added as potassium chloride. The dimensions of the specimens varied; discs, cylinders and a rectangular bar were used. Release of potassium was measured until it ceased. A little potassium was released from cement mixed with water but could be accounted for by traces present in the glass. Release took 2 months for the lower level and 1.5 years for the higher level of addition. For some specimen groups not all the potassium added was subsequently released. Potassium release tended towards the pattern found for fluoride release naturally occurring from glass ionomer cements in being proportional to t 1/2 but was less certain at the lower addition. Release was not related to the dimensions of the sample as has been shown for release of intrinsic fluoride.     

Antimicrobial potency of alkali ion substituted calcium phosphate cements

Potassium and sodium containing nanoapatite cements were produced by the reaction of mechanically activated CaNaPO(4) (CSP), CaKPO(4) (CPP) and Ca(2)KNa(PO(4))(2) (CPCP) with a 2.5% Na(2)HPO(4) solution. The cements exhibited clinically acceptable setting times of approximately 5 min and compressive strengths of 5-10 MPa. The antimicrobial properties of the cements were tested with the agar diffusion test using Streptococcus salvarius, Staphylococcus epidermis and Candida albicans. All types of alkali ion containing cements showed a significantly higher antimicrobial potency with inhibition zones of approx. 4-11 mm than a commercial calcium hydroxide cement which resulted in small inhibition zones around the cement samples of a maximum of 1.5 mm. The antimicrobial properties of all the cements were not found to diminish even after longer incubation times. This behaviour was attributed to the formation of soluble alkaline metal phosphates during setting which increased the pH value in the agar gel around the alkali containing calcium phosphate cement to 8.5-10.7 compared to 6.5-8.0 for the Ca(OH)(2) product. The high antimicrobial potency of alkali-calcium phosphate cements may find an application in dentistry as pulp capping agents, root fillers or cavity liners.     

Fluoride release profiles of mature restorative glass ionomer cements after fluoride application

This study investigates the fluoridation of four conventional glass ionomer cements (GIC) (ChemFil Superior encapsulated, Fuji Cap II, Ketac-Fil and Hi Dense) and three resin-modified GIC (RM-GIC) (Fuji II LC encapsulated, Photac-Fil and Vitremer). The fluoride release of matured restorative GIC was measured as a function of time, after four repeated fluoridations in a 2% NaF aqueous solution for 1 h. This release was corrected for the intrinsic release as determined with a control group. It was demonstrated that application of fluoride is capable of recharging GIC but the subsequent high fluoride release only lasts for one or a few days. Moreover, the fluoride release behaviour depends on the cement formulation. Comparable to the intrinsic release, the net fluoride release after fluoridation is composed of a short- and a long-term process, the former being predominant after fluoridation. The total amount of fluoride released according to the short-term process increases with consecutive fluoridations. This is especially pronounced for the RM-GIC, who exhibit a relatively slow release after fluoridation as compared to the conventional GIC. An explanation for these results is suggested on the basis of the physicochemistry of the setting reaction of the cements and of the fluoridation process.     

Effects of incorporation of nano-fluorapatite or nano-fluorohydroxyapatite on a resin-modified glass ionomer cement

This study aimed to investigate the fluoride release properties and the effect on bond strength of two experimental adhesive cements. Synthesized particles of nano-fluorapatite (nano-FA) or nano-fluorohydroxyapatite (nano-FHA) were incorporated into a resin-modified glass ionomer cement (Fuji Ortho LC) and characterized using X-ray diffraction and scanning electron microscopy. Blocks with six different concentrations of nano-FA or nano-FHA were manufactured and their fluoride release properties evaluated by ultraviolet spectrophotometry. The unaltered glass ionomer cement Fuji Ortho LC (GC, control) and the two experimental cements with the highest fluoride release capacities (nano-FA+Fuji Ortho LC (GFA) and nano-FHA+Fuji Ortho LC (GFHA)) were used to bond composite blocks and orthodontic brackets to human enamel. After 24 h water storage all specimens were debonded, measuring the micro-tensile bond strength (μTBS) and the shear bond strength (SBS), respectively. The optimal concentration of added nano-FA and nano-FHA for maximum fluoride release was 25 wt.%, which nearly tripled fluoride release after 70 days compared with the control group. GC exhibited a significantly higher SBS than GFHA/GFA, with GFHA and GFA not differing significantly (P>0.05). The μTBS of GC and GFA were significantly higher than that of GFHA (P≤0.05). The results seem to indicate that the fluoride release properties of Fuji Ortho LC are improved by incorporating nano-FA or nano-FHA, simultaneously maintaining a clinically sufficient bond strength when nano-FA was added.     

Shear bond strength between titanium alloys and composite resin: sandblasting versus fluoride-gel treatment

The aim of this study was to investigate the effect of fluoride gel treatment on the bond strength between titanium alloys and composite resin, and the effect of NaF solution on the bond strength of titanium alloys. Five titanium alloys and one Co-Cr-Mo alloy were tested. Surface of the alloys were treated with three different methods; SiC polishing paper (No. 2000), sandblasting (50-microm Al2O3), and commercially available acidulated phosphate fluoride gel (F-=1.23%, pH 3.0). After treatment, surfaces of alloy were analyzed by SEM/EDXA. A cylindrical gelatin capsule was filled with a light-curable composite resin. The composite resin capsule was placed on the alloy surface after the application of bonding agent, and the composite resin was light cured for 30 s in four different directions. Shear bond strength was measured with the use of an Instron. Fluoride gel did not affect the surface properties of Co-Cr-Mo alloy and Ni-Ti alloy, but other titanium alloys were strongly affected. Alloys treated with the fluoride gel showed similar bond strengths to the alloys treated with sandblasting. Shear bond strength did not show a significant difference (p<0.05) regardless of treatment time (5, 10, and 20 min) of fluoride gel. After the ultrasonic cleaning subsequent to the fluoride-gel treatment, residues of fluoride ion or any other titanium-fluoride complexes were not detected. NaF solution did not reduce the shear bond strength of titanium alloys. To enhance the bond strength of composite resin to titanium alloys, fluoride-gel treatment may be used as an alternative technique to the sandblasting treatment.     

In vitro investigation of aluminum and fluoride release from compomers,conventional and resin-modified glass-ionomer cements: A standardized approach

The amount of fluoride release from dental cements necessary for an anticariogenic effect is not established; moreover, the possible toxic effects due to high fluoride and aluminum release are not well known and the results are still controversial. The aim of our study was to evaluate fluoride (F) and aluminum (Al) release from dental cements using a 'standardized approach' according to the end-use of the materials, i.e. biocompatibility testing. Two polyacid-modified resin composites of recent application, commonly called compomers (Dyract® and Dyract Cem®), were compared with two conventional acid-based (Fuji I?, Ketac-Cem®) and two resin-modified (Vitremer?, Vitrebond?) glass-ionomer cements (GICs). All types of cement are used in dentistry and are commercially available. Extracts of the cements into minimum essential medium, after setting over a 1-h (group A) and 1-week (group B) period, were performed. The extraction conditions were rigorously standardized. Mean values +/- standard deviation of F- and Al-levels in such extracts were measured and were expressed as μg g^-1 (micrograms of ions per gram of cement). A great difference in the amount of ion release, both F and Al, was shown among the tested materials. The GICs, as well as Ketac-Cem®, released more F and Al than the compomers. All of the materials released the greatest proportion of ions when the extraction was performed in the first hour after mixing (group A). Al- and F-values showed a highly significant positive correlation, independently from the curing time. We conclude that the biological assessment of dental cements can be performed only if a preevaluation of the leachables is obtained by applying a standardized protocol which allows a useful comparison between the different materials.     

Buffering and ion-release by a glass-ionomer cement under near-neutral and acidic conditions

Specimens of an experimental glass-ionomer cement were stored in water (initial pH 5.9) and aqueous lactic acid (initial pH 2.7) for storage periods of 1 week up to 6 weeks. Change in mass, solution pH and fluoride release were measured at weekly intervals, and other ions were determined at weeks 1, 2, 4 and 6. In water, cements raised the pH consistently to 6.7-6.9 from weeks 2 to 6, but this did not correspond to uniform amounts of ions released, nor even to consistent mole ratios of ions in solution. Similarly, in lactic acid, pH was raised to between 3.6 and 4.5, but without a consistent concentration of ions in solution. In near neutral conditions, calcium was found to be virtually insoluble and remained within the cement, whereas reasonable amounts of sodium, aluminium, phosphorus, silicon and fluoride were released at all time intervals, with downward trends over time. In acidic conditions, considerable amounts of calcium were released over time, and amounts of calcium, aluminium, phosphorus and silicon increased with time, reaching a maximum in week 4. This suggests that as maturation proceeds, there is an increase in the acid-soluble fraction of the cement containing these elements. Fluoride release was found to be as previously reported, i.e. greater amounts in the early stages of the experiment, and with a gradual decline, and with greater amounts in acid than in water. Determination of fluoride with and without the decomplexing reagent TISAB showed that 70-75% of the total fluoride was released in "free" form in water for most weeks, whereas in acid, it declined sharply and by week 6, an estimated total of 96% of the fluoride released was complexed.     

In vitro investigation of aluminum and fluoride release from compomers, conventional and resin-modified glass-ionomer cements: a standardized approach

The amount of fluoride release from dental cements necessary for an anticariogenic effect is not established: moreover, the possible toxic effects due to high fluoride and aluminum release are not well known and the results are still controversial. The aim of our study was to evaluate fluoride (F) and aluminum (Al) release from dental cements using a 'standardized approach' according to the end-use of the materials, i.e. biocompatibility testing. Two polyacid-modified resin composites of recent application, commonly called compomers (Dyract and Dyract Cem), were compared with two conventional acid-based (Fuji I, Ketac-Cem) and two resin-modified (Vitremer, Vitrebond) glass-ionomer cements (GICs). All types of cement are used in dentistry and are commercially available. Extracts of the cements into minimum essential medium, after setting over a 1-h (group A) and 1-week (group B) period, were performed. The extraction conditions were rigorously standardized. Mean values +/- standard deviation of F- and Al-levels in such extracts were measured and were expressed as microg g(-1) (micrograms of ions per gram of cement). A great difference in the amount of ion release, both F and Al, was shown among the tested materials. The GICs, as well as Ketac-Cem, released more F and Al than the compomers. All of the materials released the greatest proportion of ions when the extraction was performed in the first hour after mixing (group A). Al- and F-values showed a highly significant positive correlation, independently from the curing time. We conclude that the biological assessment of dental cements can be performed only if a pre-evaluation of the leachables is obtained by applying a standardized protocol which allows a useful comparison between the different materials.     

Influence of pH and fluoride on properties of an oral strain of Lactobacillus casei grown in continuous culture.

A freshly isolated oral strain, Lactobacillus casei RB1014, was grown in continuous culture to compare the effects of pH and fluoride on growth and metabolism. The cells were grown at pH 7.0 to 3.2 in the absence of fluoride and from pH 7.0 to 5.4 with 20 mM NaF. Cell numbers varied from 3 X 10(9) to 30 X 10(9)/ml on blood agar during alterations in the growth pH from 7.0 to 4.27. Only when the culture was stressed by lowering the pH to 3.2 were cell numbers drastically reduced. Cells growing at pH 7.0 without fluoride were unable to grow when plated on fluoride agar (10.5 mM) at pH 5.5; however, when the growth pH was allowed to decrease to 4.94, cells grew on the fluoride plates in numbers equal to those growing on blood agar. This fluoride tolerance trait appeared rapidly once pH control was removed and was lost when the culture was returned to pH 7.0. The addition of 20 mM NaF to the culture medium did not adversely affect growth, provided that the pH was maintained at 6.0 or above; cells tolerant to 10.5 and 16 mM NaF appeared on pH 5.5 plates during this phase. In cells removed from the chemostat throughout the experiment and incubated at the pH of growth in a pH stat, glycolytic activity was optimum at pH 5.5 in the absence of NaF. Fluoride stimulated glycolytic activity by cells incubated at pH 7.0 and by cells growing with 20 mM NaF, provided that the pH of growth remained at or above 6.0. A more detailed examination of the adaptation to fluoride tolerance during shifts to acidic pH values revealed that cells capable of growth on acidic fluoride agar plates appeared within 2 h of the start of the fall in pH of the chemostat culture. Estimation of the intracellular pH during the period of the initial pH fall revealed that the intracellular pH was identical to the extracellular pH (i.e., no pH gradient [delta pH]), indicating that fluoride would not be transported into the cells to inhibit metabolism. However, once the pH of the medium was stabilized, delta pHs were generated, with the delta pH increasing as the pH declined. The inhibition of glycolysis by fluoride increased in proportion to the delta pH. Cells grown at pH 5.5 generated larger delta pHs than did cells grown at pH 7.0, although the values were normally small (approximately 0.9 U). The data suggest that the inherent fluoride tolerance of L. casei RB1014 was associated with relatively small delta pHs.     

Hydrolysis of alpha-tricalcium phosphate in NaF solutions

Controlling the incorporation of Na and fluoride in apatite-based dental restoratives and analogs of bone formed under physiologic conditions may be desirable in controlling their long-term in vivo responses. Formation of these analogs was investigated. Hydrolysis of alpha-tricalcium phosphate (alpha-TCP) was carried out at physiologic temperature in NaF solutions and the solids product(s) were analyzed. alpha-TCP hydrolysis was complete within 48 h and the only solid products identified were apatites. Changes in solution chemistry were determined at 48 h and 4 weeks. Na and Ca concentrations were determined by spectrometric methods, P and F concentrations were determined by ion chromatography; pH values were obtained. The extent of fluoride and sodium incorporation in the apatites formed varied depending on the NaF concentration of the solution. Both increased with increasing NaF concentration. Apatite compositions ranged from Ca9.06(HPO4)0.94(PO4)5.06OH1.06 for hydrolysis in water to Ca9.28Na0.56(HPO4)(x)(PO4)(6-x)F0.920Hy for hydrolysis in 0.1 M NaF solution. The proportions of PO4, HPO4 and OH in Na- and F-containing apatites are indeterminate but obey the equation (y = constant+x). The (Ca+Na)/P ratio of the apatite formed by alpha-TCP hydrolysis in 0.1M NaF approaches 1.67 indicating the vacant Ca sites become almost completely filled by the Na ions. The surface areas of these apatites initially remain constant with increasing NaF concentrations; at elevated concentrations apatite surface areas decrease with increasing NaF concentration. This is consistent with the removal of Na from solution by its incorporation in the apatites, not by its adsorption onto their surfaces.     

Long-term effects of four extraction media on the fluoride release from four polyacid-modified composite resins (compomers) and one resin-modified glass-ionomer cement.

It was the aim of the present experiments to evaluate the fluoride leaching of four compomers and one resin-modified glass-ionomer cement (gic) into aqueous media over a 1-year period. Various extraction/equilibrium solutions were applied to simulate important intraoral parameters. Specimens of Dyract, Compoglass F, F2000, one experimental compomer, and the resin-modified gic Fuji II LC were stored for 366 days in distilled water (I), acidic medium (pH 4.2) (II), neutral medium (pH 7.0) (III), or solution III supplemented with 1.6 u/mL porcine liver esterase (IV). Equilibrium media were changed and fluoride concentration was measured every 48 h (first 30 d), thereafter each week (twice), then every 14 d (three times), and finally every 28 d up to a total period of 1 year. Data were statistically analyzed for significant differences by means of Scheffe' tests (p < 0.05). The gic and the compomers leached significantly more fluoride into the acidic solution in the initial phase compared to the other media (p < 0.05). Cumulative fluoride release from all materials (except experimental compomer) was elevated because of esterase activity (p <0.05). Measurable but low quantities of fluoride were segregated into all media during the 1-year period. But no significantly different long-term fluoride release into the various media from the investigated materials was found. The data indicate that plaque-associated organic acids or salivary hydrolases may increase initial fluoride release from resin-modified gics or compomers in the oral cavity. Because long-term fluoride release from all materials decreased to low concentrations within a 30-day period, their caries preventive effect remains questionable.