Fluoride release from glass ionomer restorative materials and the effects of surface coating

OBJECTIVES: This in vitro study on fluoride (F) release from conventional and metal-reinforced glass ionomer investigated the following: (1) the release of F in deionized water compared to artificial saliva, (2) the effect of various surface coatings on F release, (3) the uptake of released F by hydroxyapatite, (4) the expression of the release data in a mathematical model, (5) F content in the powders and set materials, and (6) surface morphology of varnished and resin-coated specimens. METHODS: Glass ionomer Ketac-Fil (KF), Fuji II (FJ), and Ketac-Silver (KS) were mixed according to the manufacturers' instructions, and prepared into specimens of 137.8 mm2 surface area. All three specimens were suspended in 50 ml of deionized water, artificial saliva, or aqueous solution of hydroxyapatite and submitted to constant agitation at 37 degrees C. In a separate experiment, the specimens were coated with varnish or light-cured bonding resin and tested for F release in solutions similar to those for uncoated specimens. The release of F occurred for 28 days. The concentration of F was measured with F-ion-specific electrode. RESULTS: All tested products showed a strong initial rate of F release which decreased with time until it reached a relatively steady rate after two weeks. The F released from KF and FJ was comparable in both pattern and magnitude. They released approximately four times more F than KS. In all cases, the release of F in artificial saliva was significantly (p < 0.001) less than in deionized water. Surface coating the specimens significantly reduced the F release ( p < 0.05 top < 0.001, depending on the product and type of coating). The inhibitory effect of coating markedly decreased with time. Resin coating reduced F release more than varnish in KF and KS, but not for FJ. Essentially, all F released in aqueous solution was taken up by the hydroxyapatite, with FJ ranking the highest in increasing hydroxyapatite F concentration. Over the 28 days, the quantities of F released from FJ, KF, and KS were, respectively, 3.8, 2.3, and 1.0% of the total F content in the specimens. The F concentration in the set KS was 53.9 and 72.5% of that found in KF and FJ, respectively. The release data as a function of time were best described by the power curve. Micromorphological examinations revealed remnants of surface coatings on specimens after 14 days storage in artificial saliva. CONCLUSIONS: Glass ionomer cements released significantly less F in artificial saliva than in deionized water. Surface coating the specimens substantially reduced F release. These clinically relevant factors were not considered by many in vitro release studies which overestimate the F availability from glass ionomers. A recall appointment 24 h after the placement of glass ionomer restoration should be given for surface finishing.     

Changes in surface hardness of conventional restorative glass ionomer cements

The effect of a contact with an aqueous solution on the surface hardness of glass ionomers has been investigated in a few studies and for a limited number of formulations. As there is no information on the long-term changes of the surface hardness in this respect, the aim of this study was to determine these long-term changes in 10 conventionally setting glass ionomer formulations after storage in water as compared to maturation in a humid atmosphere (85% RH).

After setting for 15 min different series of glass ionomer specimens were stored at 37°C in both experimental media for 1, 7, 28 and 140 days. At the end of the specific maturation times, the mean surface hardness was determined on the basis of Knoop Hardness (KHN). Data were analyzed using ANOVA.

In a humid atmosphere the surface hardness generally increases rapidly initially, followed after 1 day by a more gradual increase. In contact with water, the hardness also increases up to one day but not to the same extent. The surface hardness then remains constant or slightly decreases. Evidence of a detrimental softening of the surface ascribed previously to the loss of matrix forming ions was not found. A surface erosion is not likely to occur.     

Fluoride uptake by glass ionomer cements: a surface analysis approach

Despite extensive research, the mechanism by which glass ionomer cements take up fluoride ions from solution remains unclear. To date, the majority of studies have concentrated on measuring the removal of ions from solution. In this study, we demonstrate the application of X-ray photoemission spectroscopy and secondary ion mass spectrometry to the surface analysis of the cements, after the introduction of fluoride either by doping or by immersion. Fluoride ion uptake from potassium fluoride solution is correlated with the formation of a surface layer which is rich in calcium as well as fluoride.     

Effect of maturation on the fluoride release of resin-modified glass ionomer and polyacid-modified composite resin cements

The effect of an early water contact on the fluoride release is studied for the resin-modified glass ionomer cements (RM-GIC) GC Lining LC, PhotacBond, Vitremer and Vitrebond and for the polyacid-modified composite resins (PAM-C) Variglass and Dyract. Six months fluoride release profiles were determined in regularly renewed water (37 degrees C), for the products directly after light curing and after 24 h maturation in a humid atmosphere (85% RH). ANOVA shows that both the short-term and the long-term fluoride release of a RM-GIC are influenced by this maturation. This indicates that direct water contact for this material should be avoided. For the RM-GIC a correlation is found between the initial fluoride release process and the long-term process. For the PAM-C materials, no differences in the fluoride release are found as a function of maturation, indicating that early water contact has no effect. The amounts of fluoride released by PAM-C are low compared to RM-GIC, which can affect their caries preventive potential. The results are explained on the basis of the setting reaction of both types of materials.     

The influence of sample dimensions on fluoride ion release from a glass ionomer restorative cement.

The fluoride release from a commercial, restorative glass ionomer cement was found to be strongly dependent on sample surface area rather than volume. This was noted for disc, cylindrical- and bar-shaped specimens over periods ranging from 1 day to 3 yr. Release from all shapes of specimen followed the established pattern of an initial non-linear region followed by one where release was proportional to the square root of time. If fluoride levels in the cement matrix of specimens were artificially increased by incorporation during the mixing then the release pattern during the first few months was altered. The initial release increased for some specimen sizes and decreased for others. The dependency on surface area was greatly reduced for several months. By the time a year had elapsed the correlation between fluoride ion release and surface area had been re-established. The influence of additional fluoride during setting can therefore act to perturb the normal release pattern and may in some instances reduce the initial fluoride release. Release should be quoted in terms of, or with measurements of, the surface area of specimens under investigation.     

Spheroidization of glass powders for glass ionomer cements

Commercial angular glass powders were spheroidized using both the flame spraying and inductively coupled radio frequency plasma spraying techniques. Spherical powders with different particle size distributions were obtained after spheroidization. The effects of spherical glass powders on the mechanical properties of glass ionomer cements (GICs) were investigated. Results showed that the particle size distribution of the glass powders had a significant influence on the mechanical properties of GICs. Powders with a bimodal particle size distribution ensured a high packing density of glass ionomer cements, giving relatively high mechanical properties of GICs. GICs prepared by flame-spheroidized powders showed low strength values due to the loss of fine particles during flame spraying, leading to a low packing density and few metal ions reacting with polyacrylic acid to form cross-linking. GICs prepared by the nano-sized powders showed low strength because of the low bulk density of the nano-sized powders and hence low powder/liquid ratio of GICs.     

Effect of a neutral citrate solution on the fluoride release of resin-modified glass ionomer and polyacid-modified composite resin cements

The effect of 0.01 mol/l citrate solution at pH = 7 on the fluoride release is compared for the resin-modified glass ionomer cements (RM-GIC) GC Lining LC, PhotacBond, Vitremer and Vitrebond and for the polyacid-modified composite resins (PAM-C) Variglass and Dyract by means of the six-month fluoride release profiles at 37 degrees C. The fluoride release of both RM-GIC and PAM-C increases in the neutral citrate solution as compared to water, which can be explained by the ability of citrate to complex metal ions and hence to degrade the glass as well as the polysalt matrix of the cement. Although RM-GIC release more fluoride than PAM-C in water as well as in citrate solution, the relative increase in fluoride release upon immersion in citrate solution is most pronounced for PAM-C. Whereas for the latter citrate affects both the short-term and long-term fluoride release, for RM-GIC only the long-term fluoride release is affected. This suggests that the action of citrate increases with decreasing importance of the polysalt formation in the hardening of the material. This could be explained on the basis of the difference in the chemical properties of the cement matrix.     

Reactive fibre reinforced glass ionomer cements

The mechanical properties of glass ionomer cements used in restorative dentistry reinforced by chopped glass fibres were investigated. Reactive glass fibres with a composition in the system SiO(2)-Al(2)O(3)-CaF(2)-Na(3)AlF(6) and a thickness of 26 microm were drawn by a bushing process. The manufacturing parameters were optimized with respect to maximum strength of the glass fibre reinforced ionomer cements. Powder to liquid ratio, pre-treatment of the glass, grain size distribution and fibre volume fraction were varied. Glass fibre and cement were characterized by X-ray diffraction, transmission electron microscopy and energy dispersive spectroscopy techniques, respectively. The highest flexural strength of the reinforced cement (15.6 MPa) was found by compounding 20 vol% reactive fibres and extending the initial dry gelation period up to 30 min. Microscopic examination of the fractured cements indicated a distinct reactive layer at the fibre surface. A pronounced fibre pull out mode gives rise to an additional work-of-fracture contributed by pulling the fibres out of the fracture surface.     

Degradative analysis of glass ionomer polyelectrolyte cements

As part of a detailed investigation of the setting mechanisms of dental polyelectrolyte cements, the Al3+, Ca2+, and Na+ concentrations in the matrix phase of glass ionomer cements were measured as a function of the cements age by use of a selective degradation technique. In the early stages of cement formation, all three cations were rapidly released from the glass, and it is inferred that both Ca2+ and Al3+ are responsible for gelation. Even after 5 X 10(4) min, the reaction was incomplete and still continuing.     

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.     

Toughening of dental glass ionomer cements with reactive glass fibres

A fibre reinforced glass ionomer cement (FRGIC) for dental applications was loaded with 20 vol% short fibres (430 microm) with a glass composition in the system SiO(2)-Al(2)O(3)-CaF(2)-Na(3)AlF(6). The fracture toughness and the total energy release rate were examined. A 20% anisotropic fibre alignment was observed, perpendicular to the loading direction. An increase of fracture toughness of 140% and of total energy release rate of 440% was achieved compared to the unreinforced glass ionomer cement. Matrix-fibre interface reaction is supposed to exert the major influence on mechanical behaviour of FRGIC by controlling fibre pull-out and thus the total energy release rate.     

Long-term flexural strength of glass ionomer cements.

The flexural strength of five glass ionomers was measured at intervals ranging from 1 h to 3 months after mixing. The pattern of change in flexural strength is material-specific with two water-activated materials having significantly higher long-term flexural strengths than the others. This may be attributable to the different polyacrylic acids used or to changes in the glass formation in the different materials.     

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.     

Surface roughening of glass ionomer cements by neutral NaF solutions

The objective of this study was to investigate the effect of repeated applications of a neutral NaF solution on the surface roughness of four conventional glass ionomer cements (GIC) (ChemFil Superior encapsulated, Fuji Cap II, Ketac-Fil and Hi Dense), three resin-modified (RM-) GIC (Fuji II LC encapsulated, Photac-Fil and Vitremer) and one polyacid-modified composite resin (PAM-C) (Dyract). Matured specimens were four times alternately eluted in water and exposed to 2% neutral NaF aqueous solutions for 1h. Control specimens were only subjected to elution in water for the same time period. After the treatment the surface roughness R(a) was determined using non-contact surface profilometry and selected samples were examined with SEM. Except for the PAM-C, R(a) increased drastically for the fluoride-treated samples compared to water-stored samples, the effect being most pronounced for the GIC. Surface roughening apparently is caused by a progressive disintegration or chemical erosion of the polysalt matrix of (RM-)GIC.     

Fracture properties of composite and glass ionomer dental restorative materials

The double-torsion technique has been used to determine critical stress intensity factor (K1c) values for a range of composites and glass ionomer cements used in restorative dentistry. From these values and determined elastic modulus (E) and tensile strength (sigma T) values, two other fracture properties: G1c, the critical strain energy release rate and a0, the inherent flaw size, have been evaluated. Glass ionomers have low K1c and high a0 values, coarse particle composites high K1c and high a0 values, microfine materials low K1c and low a0 values and fine particle composites medium to high K1c values and medium to low a0 values. Light curing materials have K1c values according to their group but tend to have lower a0 values than their chemically curing analogues. Glass ionomers have very low G1c values; however, there is no significant difference between the G1c values of the composite groups. The K1c and a0 values have been used to predict relative performance of the materials in highly stressed restorations and wear.     

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.     

Acid-base complex reactions in resin-modified and conventional glass ionomer cements.

Resin-modified glass ionomer cements are a recent development in which the desirable properties of glass ionomer cements and resin-composites are combined. The presence of resin may, however, retard the acid-base reaction of the ionomer component. This has led to a debate regarding the classification of resin modified materials as true glass ionomer cements, and the actual duration of the acid-base reaction after initial setting via light polymerization has taken place. To investigate this issue, a novel method employing FT-IR spectrophotometry was used to monitor the acid-base complexation reaction in a resin-modified glass ionomer cement (Fuji II LC) and a conventional, chemically cured cement (Fuji II Cap). This method involved subtraction of the background "resin" spectrum from the Fuji II LC spectra and subsequent application of a baseline to obtain the plot of absorbance area ratio in the range of 1685 to 1510 cm(-1) (complexed carboxyls) to that in the range of 1750 to 1685 cm(-1) (free carboxyls). This study demonstrates evidence of a delayed acid-base reaction for the resin-modified cement, which levels off after 168 hours of cement mixing. In contrast, the complexation reaction of the conventional glass ionomer cement was essentially complete after 24 hours.     

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.     

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.     

Chlorhexidine release from an experimental glass ionomer cement

Glass ionomer cements (GIC) can potentially be used as matrices for the slow release of active species, as has been shown previously for fluoride ions. This study investigated the use of an experimental GIC as a carrier for the release of chlorhexidine acetate (CHA) at included concentrations ranging from 0.5% to 13.0% of CHA by weight. Release into water was examined using high-performance liquid chromatography. All measurable chlorhexidine was released within 22 h1/2, however this was less than 10% of the total mass incorporated in the specimens. An increased percentage of CHA incorporated into the powder gave an increased release into the surrounding water. The bulk of the CHA was retained within the cement. For comparison, the surface chemistry of a CHA-containing GIC was examined using X-ray photoelectron spectroscopy before and after prolonged immersion in water. This confirmed retention of a large amount of CHA. Spectra after leaching appeared very similar to those from a CHA-free GIC after immersion in a CHA solution. In order to explore the effect of CHA-inclusion on the cement properties, compressive strengths, working and setting times were also measured. In general, compressive strengths were found to be decreased in direct proportion to the quantity of CHA added, while working and setting times increased.