Fluoride Release from Glass Ionomer Cement and Resin-modified Glass Ionomer Cement Materials under Conditions Mimicking the Caries Process
The anticaries potential of restorative ionomeric materials should be evaluated under a pH-cycling regime that simulates the caries process of demineralization and remineralization. Ten glass ionomer cement (GIC) materials and five resin-modified glass ionomer cement (RMGIC) materials were evaluated. A resin composite was used as a negative control. Six discs of each material were immersed for 6 and 18 hours each day in demineralizing (De-) and remineralizing (Re-) solutions, respectively. The solutions were changed daily over 12 days, during which the fluoride concentration was determined using an ion-specific electrode. The results were expressed as (1) the daily fluoride concentration in the Deand Re- solutions (μg F/ml), (2) the amount of fluoride released daily in the De- + Re- solution per area of specimens (μg F/cm2/day), and (3) the cumulative release over the 12-day period (μg F/cm2). During the first days, all materials showed a surge in fluoride release, followed by a gradual decline; however, three distinct patterns were observed, specifically: (1) greater fluoride release in the De- solution compared to the Resolution during the study period; (2) an initial higher release in De- solution; and (3) a similar release in both solutions over the whole period. The materials differed statistically (p<0.05) with respect to daily and cumulative fluoride release. One GIC (Maxxion R) and one RMGIC (Resiglass R) had the highest and lowest ability to release fluoride, respectively. In conclusion, the GICs and RMGICs evaluated exhibited distinct qualitative and quantitative patterns of fluoride release under conditions simulating the caries process, which might reflect their anticaries potential.SUMMARY
INTRODUCTION
Despite efforts to control dental caries, 2.5 billion people have untreated caries lesions.1 Glass ionomer cement (GIC) and resin-modified glass ionomer cement (RMGIC) are biocompatible restorative dental materials that also release fluoride.2 Fluoride release is considered an important property of GIC and RMGIC3 because it could prevent caries lesions in enamel and dentin adjacent to restoration areas, something shown by in situ studies.4–6 The anticaries effect of these materials is facilitated because they release fluoride constantly in the mouth over long time frames and the mechanism of action involved has been discussed.7
Fluoride release from GIC and RMGIC probably mimics the known effect of fluoride on the caries process that occurs in the oral cavity.7 Thus, the in vitro evaluation of this process could provide insights into the mechanism of how these materials contribute towards controlling caries.7–10 At present, several GIC and RMGIC materials are available on the market; however, their anticaries potential, in terms of their ability to release fluoride, has only been evaluated in conditions that do not mimic the caries process.11–18 In fact, just one evaluation has been made under conditions mimicking the caries process.19In vitro models could be used to simulate what happens in the oral cavity; however, it is important to use appropriate media to evaluate the release of fluoride from dental materials under laboratory conditions. Furthermore, the amount of fluoride released by different GIC and RMGIC materials changes with the media being used, which might lead to misleading interpretations of their anticaries potential.7
Here, we hypothesized that different types of ionomeric materials (ie, GICs and RMGICs) would present different (quantitative and/or qualitative) patterns of fluoride release when tested under conditions modeling the caries process.
METHODS AND MATERIALS
Experimental Design
Fifteen restorative glass-ionomer cement materials (10 GICs and five RMGICs), found in the market place, were used (Table 1). The fluoride release of these materials was evaluated in demineralizing (De-) and remineralizing (Re-) solutions (pH-cycling regime).9,10 A resin composite was used as the negative control. The model used here simulated a high cariogenic challenge,20 and it has been used to evaluate in vitro how GIC affects enamel demineralization adjacent to a restoration.21
Disk-shaped specimens (n=6) were immersed daily in De- and Re- solutions for 6 and 18 hours, respectively. These solutions were changed daily over a 12-day period. Fluoride concentrations in the De- and Resolutions were analyzed with a fluoride ion-specific electrode (FI-SE). Fluoride concentrations in the Deand Re- solutions for each material were compared descriptively. The amount (μg) of fluoride released daily in the De- and Re- solutions was summed. The materials were statistically compared using μg F/cm2 per day based on (1) the 7th day, when release had stabilized, and (2) the cumulative release over the 12-day period. The materials were coded, which allowed for blind analysis with FI-SE.
Preparation of Specimens
Six cylindrical specimens (7.8 mm in diameter and 2.2 mm in thickness) were prepared at room temperature (23°C±2°C) and (50%±20%) relative humidity, according to ISO 9917-122 and 9917-223 recommendations and the manufacturer’s instructions. Materials were mixed and placed in Teflon molds using a Centrix system (Nova DFL, Rio de Janeiro, Brazil). Conventional glass ionomers were allowed to set under pressure between a polyester strip and glass plate for 15 minutes. Resin-modified glass ionomers and resin composite were placed on a glass plate between polyester strips and were light-activated on both the upper and lower surfaces. One multipeak light curing unit (Valo Cordless, Ultradent Products Inc., South Jordan, UT, USA), with a 9.4 mm active diameter and 1477 mW/cm2 irradiance, was used for all light curing procedures. The light curing unit was positioned 1 mm above the surface strip, and it was activated for 20 or 40 seconds, according to the manufacturer’s specifications. No protection was applied to the surface of the specimens. After hardening, specimens were stored at 37°C and 100% relative humidity for 24 hours,9,10 and excess material was removed with a scalpel blade, coarse/medium Opti-Disc™ contouring discs (KaVo Dental, Brazil), and a low-speed handpiece.
The pH Cycling Regime
The specimens were immersed separately in polyethylene tubes containing 1.0 mL De- solution (50 mM acetate buffer [pH 5.0] containing Ca 1.12 mM and Pi 0.73 mM). This solution was 50% undersaturated when compared to human enamel/dentin (see References 24–26 for details on preparation). After 6 hours, a 0.1 ml volume of TISAB III (Orion 940911, Thermo Fisher Scientific, Cambridge, MA, USA) was added to the tubes. The specimens were then removed, washed with purified water, dried with laboratory absorbent tissue, and transferred to another tube containing 1.0 mL Re- solution (Tris buffer 0.1 M [pH 7.0] containing Ca 1.5 mM, Pi 0.9 mM, and KCl 150 mM).21 After 18 hours of immersion in Re- solution, the same procedure as described for the De- solution was repeated. The tubes were maintained at 37°C under agitation (90 rpm) over the 12-day study period. After each daily cycle, fresh De- and Re- solutions were used. TISAB III was used to buffer the De- and Resolutions.10 The sources of Ca ions and Pi ion salts used to prepare the De- and Re- solutions were CaCl2 and KH2PO4, respectively, and the pH was adjusted.
Determination of Fluoride
Fluoride concentration in the De- and Re- solutions was recorded daily using FI-SE (Thermo Scientific Orion 96-09, Orion Research Incorporated, Cambridge, MA, USA) coupled to the ion-analyzer Star A214 (Thermo Scientific Orion). The electrode was calibrated with the standards of fluoride solutions.10 The fluoride concentration was determined by performing a linear regression analysis of the logarithm of fluoride concentrations of the standards with the respective mV readings (r2=0.9999) on an Excel spreadsheet (Microsoft, Redmond, WA, USA). The results obtained from the De- and Re- solutions were expressed in μg F/ml. The amount of fluoride released by the materials each day in the De- + Re- solutions, the amount of fluoride on day 7 after the stabilization of all materials, and the cumulative amount of the 12-day period was expressed as μg F/cm2 of the exposed surface area of the specimens.
Statistical Analyses
The daily fluoride concentrations of the De- and Resolutions were analyzed descriptively. Data of daily fluoride release in the De- + Re- solutions was adjusted to a log-normal distribution. Three outliers were removed, and the remaining data were statistically analyzed by repeated measures for differences between the materials. Data on the cumulative difference of fluoride released by the materials evaluated over the 12-day period were log-transformed. One outlier was removed to homogenize the variance and normally distribute the measurements. All data were analyzed by analysis of variance (ANOVA), followed by Tukey’s test (α=0.05), using SAS software 8.01 (SAS Institute Inc., Cary, NC, USA).
RESULTS
Qualitative Evaluation of Fluoride Release in De- and Re-solutions
The general pattern of daily fluoride release by the ionomeric materials in De- (Figure 1A) and Re- (Figure 1B) solutions over the 12-day period involved an early surge of fluoride release on the first five days, followed by a gradual decline, until stabilization throughout the remaining period. In general, more fluoride was released by the De- solution compared to the Re- solution.
Citation: Operative Dentistry 46, 4; 10.2341/19-296-L
However, when the pattern of fluoride release from each material in the De- and Re- solutions was compared, we detected three distinct patterns (Figure 2A, 2B, and 2C). Three GICs (Maxxion R, Vitro Fil R, and Ionglass R) exhibited pattern A of fluoride release, with more fluoride being released in the De- solution compared to the Re- solution over the 12 days, while none of the RMGICs exhibited pattern A of fluoride release (Figure 2A). Pattern B (Figure 2B) was exhibited by seven GICs (Bioglass R, EQUIA Forte Fil, Fuji 9, Ionofil Plus, Ketac Molar Easymix, Riva Self Cure, and Vitro Molar) and four RMGICs (Fuji 2 LC, Resiglass R, Riva Light Cure, and Vitro Fil LC). Pattern B was characterized by an initial higher release of fluoride in the De- solution compared to the Re- solution. However, this difference was not maintained over time. Pattern C (Figure 2C) was exhibited by one RMGIC (Vitremer), which showed a very similar pattern of fluoride release in the De- and Re- solutions over the 12-day period.
Citation: Operative Dentistry 46, 4; 10.2341/19-296-L
Quantitative Evaluation of Fluoride Release
Daily Release in De- + Re- Solutions — The amount of fluoride released daily in the De- and Re- solutions was summed. All ionomeric materials exhibited a surge in fluoride release during the first five days, followed by a decline and stabilization (Figure 3). ANOVA showed statistically significant differences for materials, time, and interaction (p<0.0001). The materials differed regarding the time period between the surge of fluoride release and stabilization. All GICs and RMGICs differed from the control group (see Supplementary Material, available online).
Citation: Operative Dentistry 46, 4; 10.2341/19-296-L
Certain GIC materials (Ionglass R, Ionofil Plus, Vitro Fil R, and Vitro Molar) exhibited a very fast surge in fluoride release that stabilized after day 2 of pH-cycling. Other GIC materials (Bioglass R, Fuji 2 LC, Ketac Molar Easymix, and Maxxion R) and two RMGIC materials (Riva Light Cure and Vitro Fil LC) stabilized after day 3. Two GIC materials (EQUIA Forte Fil and Fuji 9) and two RMGIC materials (Resiglass R and Vitremer) stabilized after day 4. In comparison, one GIC (Riva Self Cure) stabilized after day 5.
When the materials were compared on each day, one GIC (Maxxion R) exhibited the greatest fluoride release over all periods, while one RMGIC (Resiglass R) exhibited the lowest fluoride release. The statistical difference among the materials on each day is shown in the Supplementary Material (available online).
By day 6 of evaluation, the burst effect of fluoride release ceased for all materials, after which all materials released a constant amount of fluoride daily. Figure 4 shows the statistical difference among the materials on day 7 of evaluation. All fluoride materials differed from the resin composite negative control (p<0.05) on fluoride release. The GIC Maxxion R released the highest amount of fluoride (p<0.05), and the RMGIC material, Resiglass, released the lowest amount. Between the highest fluoride release by Maxxion R and the lowest by Resiglass R, groups of materials, irrespective of being GIC or RMGIC, were found by decreasing order of fluoride release; the statistical significance of fluoride release among these materials is depicted in Figure 4.
Citation: Operative Dentistry 46, 4; 10.2341/19-296-L
Cumulative Fluoride Release Over the 12 Days — Figure 5 shows the cumulative amount (μg F/cm2) of fluoride released in the De- + Re- solutions over the 12 days. ANOVA showed a statistically significant difference among all materials. All ionomeric materials significantly differed (p<0.0001) from the resin composite negative control with respect to fluoride release. Out of all of the GIC and RMGIC materials, one GIC material (Maxxion R) released the most fluoride, while one RMGIC material (Resiglass R) released the least fluoride. The other GICs and RMGICs released intermediate amounts of fluoride between these two materials.
Citation: Operative Dentistry 46, 4; 10.2341/19-296-L
Summary of the Results — The main results described above are summarized in Table 2, facilitating comparisons among all materials and among the GIC and RMGIC materials.
DISCUSSION
To have some validity, models should simulate real life. Thus, the mechanisms of fluoride release by GICs and RMGICs, and their anticaries potential, must be evaluated under conditions that simulate the physicochemical process of caries development.7 Through mimicking the caries process of demineralization and remineralization, to which teeth restored with these materials are subjected daily, we evaluated the ability of ten conventional GICs and five RMGICs to release fluoride.
Our findings clearly showed qualitative (Figures 1–2) and quantitative (Figures 3–5) differences in fluoride release among the evaluated materials. However, these differences might not be due to any common or specific components reported by the manufacturers described in Table 1. For example, when the 10 GICs were compared with the five RMGICs based on the qualitative results of fluoride release, nothing differentiated the type of material (Table 2). The same was true when comparing GIC and RMGIC materials from the same manufacturer; thus, resin and aluminum fluorosilicate doped with other ions (Table 1) do not confer specific patterns in terms of qualitative fluoride release from RMGIC materials or GIC materials, respectively.
When evaluating the quantitative data of fluoride release and composition of the materials (Table 1), we did not identify any distinct or common element. No differentiation was detected between conventional GICs (doped or not with ions) and RMGICs during the early surge of fluoride release27 nor for the time required to stabilize and sustain release. The early surge of fluoride release among all tested materials ranged from 48 to 120 hours (Figure 3; Supplementary Material, available online), irrespective of the composition of the material (Table 1). The early surge is induced by a superficial rinse that causes an initial higher dissolution of the complexes of aluminum or phosphate with fluoride, which form during the acid-base reaction.16 This process is mainly regulated by glass particles inside the cement that have not reacted.27 After the higher release of fluoride on the first day, sustained release during the remaining period might be the result of diffusion through pores, fissures, and bulk diffusion.28,29
This study also ranked fluoride concentration on day 7 of the experiment and over the cumulative 12-day period (Table 2). However, no association between the composition of the materials and their ability to release more or less fluoride was detected. Conventional aluminum fluorosilicate GICs tended to release more fluoride compared to RMGICs (Figures 4 and 5). Consequently, the GIC Maxxion R and the RMGIC Resiglass R released the greatest and lowest amounts of fluoride, respectively. However, one RMGIC (Vitremer) was still among the four materials that released the most fluoride, and there was also one conventional GIC (Ketac Molar Easymix) (Figures 4 and 5).
Although we could not explain the qualitative and quantitative patterns of fluoride release based on the composition of the evaluated materials, the results could be used to indicate their anticaries potential. Fluoride interferes with the physicochemical process of caries development, reducing the demineralization of enamel and dentin, and enhancing remineralization during restoration.7 Therefore, maintaining constant low fluoride concentrations in the oral cavity for prolonged periods are more important than maintaining high concentrations for short periods of time. Thus, the burst effect of fluoride release found for all materials might be less relevant in terms of efficacy on caries control compared to the sustained release shown by all materials after day 6 (Figure 3; Supplementary Materials, available online). However, the early burst of fluoride release might have some clinical relevance for repairing caries lesions on the surface of teeth located close to areas of GIC or RMGIC restorations. This possibility is remote, even for the GIC material Maxxion R, which had the highest ability to release fluoride during the first three days of the burst effect. During the first three days, this material released a mean of 55 μg F/cm2. Thus, restoration of a 1 x 1 cm surface area would release 55 μg of fluoride in the mouth, which could be relevant in terms of remineralization, if it was not diluted very fast by salivary clearance.30 Indeed, the biofilm formed around the restoration site is the only area where fluoride that is released in the oral cavity by GIC or RMGIC is retained and shows an anticaries effect.6 Thus, the capacity of this particular GIC’s fluoride release might be balanced with its mechanical properties.
The experimental data showed that GICs and RMGICs interfere with caries development in the enamel or dentin adjacent to restoration sites by releasing fluoride. This phenomenon reduces demineralization and enhances remineralization.7 However, the key mechanisms of this process have not been identified. Our findings showed three patterns of fluoride release in De- and Re- solutions (Figures 1 and 2). If reducing demineralization is more relevant, the GICs and RMGICs that showed pattern A of fluoride release would be more effective at controlling caries. If fluoride is as effective at reducing demineralization as it is at enhancing remineralization, pattern C (as exhibited by Vitremer) would be highly effective at controlling caries. For the anticaries effect of toothpaste, the effect of fluoride on the remineralization of enamel and dentin might be more relevant than reducing demineralization.31 Thus, further studies are required to test these potential pathways in GIC materials.
Dental caries is a chronic disease and caries lesions progress each time sugar is ingested.7 Consequently, fluoride must be constantly available in biofilm to interfere with the caries process. Thus, the anticaries potential of the tested GIC and RMGIC materials should be reflected by their ability to sustain fluoride release, as shown in the quantitative results for daily (Figure 4) and cumulative (Figure 5) fluoride release. From day 6 of evaluation, all materials presented sustained fluoride release, differing statistically on day 7 (Figure 4). Maxxion R released the most fluoride (27.7 μg F/cm2), whereas Resiglass R released the least (0.41 μg F/cm2). Only a very low physicochemical fluoride concentration is needed to interfere with caries,7 and it could be achieved in biofilm fluid by all materials;6 however, the 10-fold difference in fluoride release between Maxxion R and Resiglass R might be relevant in mitigating caries, but this requires further evaluation. In comparison, the high release of fluoride by Maxxion R could provoke the erosive dissolution of the restoration site.32 Thus, the persistence of restoration might be balanced by the anticaries property of this material when considering the cost-effectiveness of this intervention.
The distinct qualitative and quantitative patterns of fluoride release by these materials might reflect their anticaries properties. Thus, future studies should evaluate how different doses of fluoride release influence enamel-dentin demineralization and/or remineralization in validated models.7
CONCLUSION
In conclusion, the GICs and RMGICs evaluated exhibited distinct qualitative and quantitative patterns of fluoride release under conditions simulating the caries process, which might reflect their anticaries potential.
Mean (n=6) daily fluoride concentration (μg F/ml) in demineralizing (A) and remineralizing (B) solutions by glass ionomer cements (continuous line) materials, resin-modified glass ionomer cements (dotted line), and the control (Resin Z250) over the 12-day period of the pH-cycling regime to which the specimens were subjected.
Representative patterns (A, B, and C) of fluoride release in De- and Re- solutions (μg F/ml) by glass ionomer cements and resin-modified glass ionomer cement materials over the 12 days of pH-cycling (resin-modified glass ionomer cements in italic).
Daily fluoride released (mean, n=6) (μg F/cm2) in demineralizing + remineralizing solutions by glass ionomer cement materials (continuous line), resin-modified glass ionomer cement materials (dotted line), and the control (Resin Z250) during the 12 days of pH-cycling.
Mean (standard deviation; n=6) of fluoride release (μg F/cm2) by the glass ionomeric cements (solid fill) and resin-modified glass ionomer cement materials (textured fill), and the control (Resin Z250) in De- + Re- solutions on day 7 of the pH-cycling regime. Capital letters show differences among the materials (p<0.05).
Mean (standard deviation; n=6) cumulative fluoride release (μg F/cm2) by the glass ionomer cements, the resin-modified glass ionomer cement materials (dotted lines), and the control (Resin Z250) over the 12 days of pH-cycling in De- + Resolutions. Capital letters show differences between the materials (p<0.05).
Contributor Notes
Alejandra Brenes-Alvarado, Piracicaba Dental School, UNICAMP, Piracicaba, São Paulo, Brazil; University of Costa Rica, School of Dentistry, San José, Costa Rica
* Jaime Aparecido Cury, professor, Piracicaba Dental School, UNICAMP, Piracicaba, São Paulo, Brazil
Clinical Relevance
Glass ionomer cement and resin-modified glass ionomer cement release different amounts of fluoride when subjected to conditions simulating the process of caries development; consequently, these materials have different potentials to control caries.