Fluoride Release from Glass Ionomer with Nano Filled Coat and Varnish.

OBJECTIVE: This in vitro study compares the fluoride release from microlaminated glass ionomer based on glass hybrid technology coated with two different surface coating agents.
MATERIALS AND METHODS: A total of 18 samples were divided into three groups of six samples each: (1) glass ionomer Equia Forte Fil coated with Equia Forte Coat (Equia+EC), (2) glass ionomer Equia Forte Fil coated with GC Fuji Varnish (Equia+VC) and (3) uncoated glass ionomer Equia Forte (EQUIA cont). Fluoride release was measured using an ion-selective electrode (ORION EA 940) after 24 hours, 4 days, 30 days and 64 days. Repeated measures ANOVA, multiple comparisons, Tukey's test and paired t-test were used to test the differences between the groups.
RESULTS: The differences between the groups and four time points were statistically significant (ANOVA, p<0.0001). Cumulative fluoride ion release after 64 days was 66.01 mg/l, 123.54mg/l and 203.22 mg/l for EQUIA+EC, EQUIA+VC and EQUIA cont, respectively. All the differences were statistically significant except the difference between EQUIA+VC and EQUIA cont after 24 hours.
CONCLUSIONS: The amount of released fluoride was significantly lower in the samples coated with nanofilled surface coating agent compared to the samples coated with varnish and uncoated samples.

Introduction

Fluoride is an important therapeutic and preventive agent in dental caries prevention and remineralization of partly demineralized dental tissues, when topically administered in the oral cavity (). There are several mechanisms of anticariogenic fluoride action including inhibition of bacterial growth and metabolism, hindering demineralization and promoting remineralization (). Fluoride release is, therefore, considered to be a valuable property of restorative dental materials, and was shown to be influenced by several factors. The material composition, storage conditions and curing method influence the degreee of fluoride release. ().

Glass ionomer cements are widely used in contemporary dentistry (). Their advantages over other restorative materials such as chemical adhesion, biocompatibility, protective and remineralizing action on dental tissues are well documented (, ). Traditional GICs have unfavorable physical properties and they have been categorized as temporary materials not suitable for permanent restorations (). Better physical properties were achieved by optimizing acid-fluoroaluminosilicate glass ratio and particle size and distribution (). In 2007 a new restorative concept based on GIC technology consisting of Fuji IX GP Extra and nanofilled coat was developed, and it was renamed Equia Fil in 2011. In 2015, Equia Forte (GC, Tokyo, Japan) was launched as a new material based on glass hybrid technology, consisting of a highly viscous conventional GIC combined with a nanofilled coating material (Equia Forte Coat, GC, Tokyo, Japan) (). Equia's powder consists of 95% strontium fluoroaluminosilicate glass, including the newly added highly reactive small particles, and 5% polyacrylic acid. The liquid component consists of 40% aqueous polyacrylic acid. Strontium is responsible for increased radiopacity and it does not have any undesired effects on the appearance of the cement (). This substitution of calcium with strontium has enhanced fluoride release (). For the fluoride to be released, the salt needs to dissociate and diffuse through the bulk cement. Since calcium is more electropositive than strontium, CaF2 is less soluble than SrF2 (). Equia Coat consists of 50% methyl methacrylate and 0.09% camphorquinone. This hydrophilic low viscosity nanofilled surface coating seals the GIC surface, reduces abrasive wear and the fracture strength of the restoration during the first months until complete maturation is achieved. Besides it improves esthetics by glaze effect (, , ). Furthermore, it was shown that the clinical performance of the newly developed restorative system is quite satisfying (, ). One of the most important properties of GIC-based materials is their anticariogenic potential. Delayed demineralization of adjacent sound tissues and remineralization of demineralized underlying dentin are largely the result of fluoride release from the restorative material (, ).

The aim of this in vitro study was to evaluate and compare the fluoride release from Equia Forte Fil (GC, Tokyo, Japan), coated with two different surface coating agents Equia Forte Coat (GC, Tokyo, Japan) and Fuji Varnish protective coating (GC, Tokyo, Japan).

Materials and methods

Cylindrical aluminum molds (8 mm diameter and 2 mm depth) were used to prepare the samples. The diameter and depth were measured using an electronic digital caliper. Equia Forte Fil was prepared according to the manufacturer’s instructions and packed into the molds. The top surface of each specimen was covered with a celluloid strip and a glass slide at room temperature and the specimen was allowed to set at room temperature for 10 min. Equia Forte Coat was applied on six samples and light-cured for 20 s, six samples were coated with Fuji Varnish which was left to self-cure and six were left uncoated. Both agents were free of fluoride. Equia forte coat content includes low viscosity monomer methyl methacrylate, phosphoric acid ester monomer and photoinitiator, whereas Fuji Varnish contains isopropyl acetate, acetone, cornmint oil and cinnamaldehyde. The specimens were subsequently removed from the molds by applying pressure at one side and stored in a moist environment at 37°C for 24 h. Each specimen was immersed in 5 ml of deionized water in polyethylene vials and incubated at 37°C for 24 hours. After 24 h, the samples were removed from the vials and the concentration of fluoride ions in the distilled water was measured using a fluoride ion-selective electrode type 96-09 (Boston, Mass, USA) and a microprocessor analyzer ORION EA 940 (Orion Res Inc., USA). Prior to the measurements of the fluoride concentration, the accuracy of the measuring instrument was checked as well as the electrode inclination according to the manufacturer’s instructions, and 0.5 ml of TISAB III (Total Ionic Strength Adjustment Buffer; Merck KGaA, Darmstadt, Germany) was added to each sample to achieve constant ionic strength and ph.

Furthermore, the specimens were rinsed, dried, weighted and then reimmersed into a new vial containing 5 ml of deionized water. The changing of distilled water and the fluoride content measurements were performed on days 4, 30 and 64 in triplicate for each sample and expressed in mg/L (ppm F-).

Data were statistically analysed using SAS statistical package. ANOVA was used for the comparison of means, Tukey's test for multiple comparisons, and paired t-test with Bonferroni correction for the comparison of means at different time points. The significance level for all tests was p<0.05.

Other specimes than those used for the fluoride release measurements were analysed using SEM. The specimens were placed into an electrically conductive polymer mass, grinded at 300 rpm under water cooling using sand paper (P320, P500, P1000, P2400, P4000), polished at 150 rpm with 30 N force applied using diamant pastes (3µm and 1 µm) and lubricant.

Results

The results for fluoride ion release are given in Table 1. The fluoride release significantly differed between groups (p<0.0001) and at different time points (ANOVA, p<0.0001). The least fluoride release was noted in EQUIA+EC group, followed by EQUIA+VC group. The greatest fluoride release was in the group with uncoated samples. The Tukey’s test showed that after 24 hours the release of fluoride forms EQUIA+VC and EQUIA cont. samples were similar. After 64 days a significant difference in fluoride release was noted between EQUIA+EC and EQUIA cont. The results of cumulative fluoride ion release are given in Table 2 and shown in Fig. 1.

Table 1

Fluoride release in mg/l (Mean and standard deviation, st.d.)

	EQUIA+EC	EQUIA+EC	EQUIA+VC	EQUIA+VC	EQUIA cont	EQUIA cont
	mean	st.d.	mean	st.d	mean	st.d
24h	41.57	(21.3)	70.97	(11.2)	75.95	(17.9)
4 days	12.76	(9.2)	27.61	(14.1)	68.97	(12.6)
30 days	8.60	(4.2)	16.63	(10.5)	39.16	(11.5)
64 days	3.08	(3.4)	8.33	(6.6)	19.15	(14.6)

Table 2

Cumulative fluoride ion release in mg/l (Mean and standard deviation, st.d.)

	EQUIA+EC	EQUIA+EC	EQUIA+VC	EQUIA+VC	EQUIA cont	EQUIA cont
	mean	st.d.	mean	st.d	mean	st.d
24h	41.57	(21.3)	70.97	(11.2)	75.95	(17.9)
4 days	54.33	(30.2)	98.58	(24.7)	144.92	(21.4)
30 days	62.93	(33.4)	115.21	(33.7)	184.07	(27.5)
64 days	66.01	(33.6)	123.54	(36.4)	203.22	(34.6)

Figure 1

Fluoride release over time for the three groups of samples: EQUIA+EC, EQUIA+VC and EQUIA cont.

Regression analysis revealed the following relation between cumulative fluoride release (y) and time (t):

EQUIA+EC y=13.0∙ln t+43.5

EQUIA+VC y=27.6∙ln t+75.3

EQUIA cont y=66.6∙ln t+87.3

Descriptive statistics for sample weights are given in Table 3.

Table 3

Sample weights (Mean and standard deviation, st.d.)

	EQUIA+EC	EQUIA+EC	EQUIA+VC	EQUIA+VC	EQUIA cont	EQUIA cont
	mean	st.d.	mean	st.d	mean	st.d
24h	0.304	(0.04)	0.296	(0.02)	0.269	(0.02)
4 days	0.304	(0.04)	0.302	(0.02)	0.280	(0.02)
30 days	0.298	(0.04)	0.299	(0.02)	0.273	(0.02)
64 days	0.317	(0.04)	0.307	(0.02)	0.276	(0.02)

The differences between the groups were not significant (ANOVA test, p=0.15), but weight significantly changed over time (ANOVA test, p=0.0001). Post hoc comparison showed that changes occurred in all samples.

The SEM analysis showed that Equia Forte Coat adhered better to Equia Forte Fil glass ionomer than Fuji Varnish (Figure 2).

Figure 2

Representative Equia Forte glass hybrid specimens with (A) Equia Forte Coat (EQUIA+EC); (B) covered with Fuji Varnish (EQUIA+VC); and (C) without coating agent (EQUIA cont). SEM analysis revealed that there was better adhesion of Equia Forte Coat than Fuji Varnish onto the underlying GIC material.

Discussion

Generally, there are two types of coatings used for the protection of GICs after placement and initial hardening to avoid contamination by moisture and loss of unbound water: simple solutions of polymer in solvent and light-curable low viscosity monomers. There are experiments revealing that light-curable coats protect GICs more effectively from drying out than simple varnish, and that they improve the physical properties of GICs (). Coating agents used in our study were Equia Forte Coat containing a low viscosity monomer methyl methacrylate, phosphoric acid ester monomer and photoinitiator, and Fuji Varnish containing isopropyl acetate, acetone, cornmint oil and cinnamaldehyde. Our results show that there was significantly more fluoride released from Equia Forte Fil specimens when they were coated with Fuji Varnish indicating that Equia Coat seals the GIC surface more effectively. The SEM analysis of the specimens showed that the surface was smoother when covered with both coating agents, which could imply a reduced tendency of bacteria to adhere to the surface. (). Furthermore, the SEM analysis also showed that Equia Forte Coat adhered better than Fuji Varnish to the underlying GIC. This is probably due to the nanofiller technology used in Equia Forte Coat enabling uniform dispersion of the filler particles (). Fuji Varnish on the other hand, contains polymer molecules dissolved in organic solvent. After the GIC filling is covered with varnish, the solvent evaporates, leaving the solute as a thin layer or film. The solute molecules are larger than the nano-particles of the Equia Forte Coat and this probably influences the film thickness of both coatings (Figure 2).

It was previously shown that curing method, either light or chemical curing, influences fluoride release from resin modified glass ionomers and dual-cured resin cements, and it was shown that the photoinitiated polymerization enhances cross-linking density resulting in the reduced resin matrix permeability for fluoride ions (, ). However, our results of increased fluoride release in the samples coated with Fuji Varnish that was not light cured can hardly be explained with the enhanced cross-linking upon light curing, since Equia Forte Fil is a material that sets by chemical curing alone.

In our study, fluoride was released in a logarithmic time dependence in all three groups (Figure 1), similarly as in previous studies (, ). As it was already mentioned, this initial burst effect is desirable in the context of anticariogenic action because it stimulates remineralization of enamel and dentin and has an antibacterial effect (, , , ). In the case of samples coated with Equia Forte Coat, the period of fairly constant fluoride release rate was reached after 4 days, similarly as in uncoated and varnished samples, but the initial fluoride release was significantly smaller in the group coated with Equia Forte Coat. This is in concordance with other studies where 60-76% of reduction in fluoride release from the coated GICs was reported (, ). This probably occurred because the superficial layer of immature GIC is more readily dissolved and eroded if it is not protected (). Generally, our results are in line with previous studies reporting that the highest values of released fluoride occur in the first 24 - 48 hours ranged from 5 to 155 ppm for different GICs (, ).

After the initial burst, constant fluoride release occurs because fluoride ions do not react chemically during the setting reaction, and since they remain unreacted they can diffuse down their concentration gradient and are released into the oral environment, or taken up by the glass ionomer if it is exposed to solutions with high fluoride concentration (, ). In our study, fluoride release after setting continued to follow the same pattern in all three groups because the release is determined by filler particles composition and the matrix of set material: fluoride ions diffuse through the pores of the GIC (). The same pattern of fluoride release was observed in previous studies, in conventional GICs and modified GICs (, -). The recordings and their time dependence enable predictions about the rate of fluoride release in the future, and a point in time when the release would cease. In the study of Arbabzadeh-Zavareh et al. () the amount of fluoride release measurement on day 60 was considered the base measurement of fluoride release after exhaustion of the materials. However, it was shown that glass ionomer materials are able to release fluoride at a sustained rate for long periods of time (at least 5 years) ().

It was noted that the Equia Forte samples released somewhat more F- ions than some other GICs (). The reason could be a replacement of Ca2+ with Sr2+ ions which slightly enhances fluoride release rate because SrF2 is more readily dissociated in less acidic environment than CaF2 resulting in a higher fluoride release ().

Our results show that weight increase was the highest in the case of specimens that were coated. This is probably due to the presence of hydrophilic monomer within the light-curable coat which surface remains partly unreacted due to the polymerization inhibition by oxygen (). This could lead to water sorption contributing to weight increase.

When interpreting the results of this study in clinical context, it must be considered that oral conditions with oscillating pH, temperature and occlusal loading were not simulated.

Within the limits of this study, it can be concluded that the nanofilled coat inhibits the release of fluoride from the GIC material compared to varnish and control samples, but the quantities still seem satisfactory for caries protective action, especially considering beneficial effects of coating on mechanical properties of GIC as a possible alternative material for long-term restorations.