Comparative Evaluation of the Fluoride Recharge Ability of Two Glass Ionomers Obtained from Fluoridated Dentifrice and CPP-ACFP Paste: An In-vitro Study

Introduction

Dental caries is a prevalent chronic disease, which particularly affects young children. Dental professionals have been concerned with the restoration of carious teeth with minimal aggression to the tooth structure using cariostatic materials (1, 2). Glass ionomer cements (GICs), also known as resin infiltration, are used as a minimally invasive approach involving the removal of tissue decay using manual instruments alone and resin sealants (3). In this technique, restoration of carious teeth with fluoride-releasing restorative materials has been proposed as a possible mechanism to reduce the occurrence of secondary caries. In addition, these materials diminish the counts of residual bacteria under the restoration (4, 5).

GICs are essential materials for such purposes due to their releasing of fluoride and chemical adhesion to the tooth structure (6). Glass ionomers are able to take up and rerelease fluoride ions from exogenous sources (7, 8). Fluoridated dentifrices are considered to be the most common sources of fluoride with the capability of daily use. In this regard, Freedman et al. (9) had denoted that home care fluoride exposure provides sufficient measurable fluoride uptake and rerelease. Furthermore, Rao has stated that fluoride is diffused into the GIC matrix material and increases its fluoride reservoir, from which it is gradually released (10). However, several factors may affect the process of fluoride release from GICs, such as formulation, solubility, and porosity of the material (11).

Recently, a new restorative material has been introduced (EQUIA Forte, GC, Tokyo, Japan), which contains a high-viscosity, conventional GIC (EQUIA Fil, formerly known as Fuji IX GP extra), as well as a novel nanofilled coating material (EQUIA Coat, formerly known as G-coat plus). According to the indications of the manufacturer, EQUIA Forte is optimal for class I, II, and V restorations. The self-adhesive, nanofilled resin (G-coat) of EQUIA Forte infiltrates the surface of GICs, thereby providing long-lasting protection and marginal integrity and increasing the strength and wear resistance of the GIC surface (12, 13).

According to the literature, casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) has anticariogenic properties (14, 15). The combination of CPP-ACP and fluoride (CPP-ACFP) has been reported to enhance the incorporation of fluoride into the plaque and subsurface enamel, which could substantially improve the remineralization of the subsurface lesions in the enamel (16).

To the best of our knowledge, no prior studies have evaluated the use of CPP-ACFP paste to recharge GICs. The tested null hypotheses of the current research were as follows:

1)    There is no difference between the fluoride-releasing properties of the two selected glass ionomers;

2)    There is a difference between fluoride rerelease with exposure to various fluoride sources.

The present study aimed to investigate the fluoride release and uptake capacity of two glass ionomers in the presence of various fluoride sources.

Materials and Methods

This in-vitro study was conducted using light-cured, resin-reinforced GIC (group I, Fuji II, GC Corporation, Japan) and bulk fill hybrid GIC (group II, EQUIA Forte System, GC, Japan) (Table I). The capsules of the materials were only activated before mixing, set into the amalgamator (ultramat2, SDI, Australia), and triturated for 10 seconds. In total, 60 specimens were prepared in each group and placed in customized Teflon molds (diameters: 5×2 mm). During the fabrication of the specimens, the top and bottom surfaces of the molds were covered by polyester strip, supported by glass slabs on either side, and clamped in order to provide a smooth surface. The excess extruded material was removed by gentle pressure.

In accordance with the instructions of the manufacturer, the setting time of EQUIA Forte GIC is 2.5 minutes since the beginning of mixing. The EQUIA coat was applied and photocured for 20 seconds using dental curing light (Coltolux 75, Coltene Whaledent, Switzerland). The light-cured, resin-reinforced GICs from each surface were polymerized for 20 seconds.

GIC disks were removed from the molds and stored in a humid environment at the temperature of 37°C for 24 hours. Afterwards, all the specimens were suspended independently in plastic bottles containing five milliliters of deionized water and placed in an incubator at the constant temperature of 37°C. After 24 hours since the preparation of the suspension, the first fluoride concentration was measured. The deionized water in the plastic container was buffered with total ionic strength adjustment buffers (TISAB II) in order for stable pH, as well as to prevent the generation of fluoride ion complexes with various cations. In addition, five millilitersof the storage media was mixed with five millilitersof TISAB II, and the fluoride level was assessed using a digital ion analyzer and fluoride electrode (Mettler Toledo, United States). The instrument was calibrated with a series of standard fluoride solutions at the concentrations of 0.50, 1.00, 2.00, 10.00, 20.00, and 100 ppm through diluting the fluoride standard of 1,000 mg/l. The specimens were transferred to a new bottle, and the solution was refreshed every 24 hours for the first week, followed by weekly refreshment for 21 days (17, 18).

After measuring the fluoride release for 21 days, the samples were divided into three subgroups. Each subgroup of specimens was subjected to one treatment, including no fluoride treatment (subgroup A; control), application of fluoridated dentifrice (subgroup B; Oral B 1450 ppm; once a day for one minute), and application of MI Paste Plus (subgroup C; CPP-ACFP 900 ppm; once a day for one minute) (Table II).

After the treatment, each disk was wiped clean with a tissue and placed in five milliliters of deionized water for 24 hours. The treatments were repeated during the first week, and fluoride measurement was performed every 24 hours. In the second week, no treatment was carried out, and the fluoride release of the samples was measured every 24 hours on days 7-14 and 21 (8, 19).

Data analysis was performed in SPSS version 17 using Shapiro-Wilk test to assess the normal distribution of the data. In addition, repeated measures analysis of variance (ANOVA) and Tukey’s HSD post-hoc test were applied (α=0.05), and the P-value of less than 0.05 was considered statistically significant.

Table I. Description of Materials Used in Study

Table II. Examined Pastes in Study 

Results

Mean values of fluoride release (ppm) in the samples of the glass ionomers are presented in Table III. Accordingly, the mean values showed a significant reduction in the fluoride release from day one until day seven in both groups (P<0.001). On days 14 and 21, cumulative fluoride release was observed. According to our findings, there were statistically significant differences between various days of the experiment in this regard (P<0.001). Moreover, the results of one-way ANOVA indicated that fluoride release was higher in Fuji II compared to EQUIA Forte (P<0.001). Until day seven, the mean values of fluoride release (daily recharge in subgroup B) were significantly higher compared to subgroups A and C (P<0.001). However, no significant difference was denoted between subgroups A and C in this regard (P=0.135) (tables IV&V).

After seven days of daily recharge, no significant difference was observed in terms of fluoride release between subgroups A, B, and C of group two on days 1-7, 14, and 21 of specimen incubation (P=0.416). Furthermore, the fluoride release following the recharge between subgroups A, B, and C of group one was showed a significant difference from day one until day 21 (P<0.001). Significant differences were also denoted in the mean values between groups one and two (P<0.001) (Tables VI &VII).

Table ІІІ. Mean value of fluoride release for two types of GIC

Table IV. Mean value of fluoride uptake of light-cured resin-reinforced GIC (Fiji II) after seven days of daily recharge

Table V. Mean fluoride uptake of Equia Forte after seven days of daily recharge

Table VI. Mean value fluoride release of light-cured resin-reinforced GIC(Fuji II) following recharge

Table VII. Mean value fluoride release of Equia Forte following recharge