Fluoride Release and Recharge in Conventional Varnishes, Compared to a Giomer and a Resin Modified Glass Ionomer

Introduction

Fluoride varnishes are used worldwide for caries prevention and treatment of dentin hypersensitivity. These materials were introduced as an alternative method of topical application (1).  Their effectiveness, safety, and easy application have made them a perfect candidate for dental prevention protocols (2). Studies have mentioned that the use of a topical fluoride could be the most effective protocol in the prevention and control of early childhood caries (3). The risk of developing cervical dentin hypersensitivity increases in treatments of caries and periodontal disease, and fluoride varnishes are a useful option for the treatment of this pathology (4). Dentin hypersensitivity is one of the most common symptomatic conditions affecting oral comfort and can be defined as a short sharp pain arising from exposed dentin in response to stimuli that cannot be ascribed to any other dental pathology (5).

This pathology, which may affect 8%-57% of the adult population, is associated with exposed dentin, gingival recession, abrasion, attrition, erosion, abfraction, and improper brushing habits (6). The prescription of fluorine-containing products has become very popular, since the application of fluoride salts causes calcium fluoride precipitation inside the tubules through the interaction between sodium fluoride and calcium contained in dentin hydroxyapatite crystals (7). The highest and constant fluoride release in dental materials is indispensable to prevent caries and treat hypersensitivity.

New fluoride varnishes are designed to release fluoride slowly over a prolonged period and are supposed to have the ability of recharging. Therefore, it is necessary to identify the varnish which has the highest and more stable amount of fluoride and is capable of recharging. The present study aimed to compare the amount of fluoride released and the recharge capacity from three

conventional sodium fluoride varnishes with a Coat Giomer and Resin modified glass ionomer (RMGI).

Materials and Methods

Sample preparation

The materials used in this study were three fluoride-releasing varnishes (Duraphat “DP”, Clinpro White Varnish “CWV”, Fluor protector “FP”), a Giomer (PRG Barrier Coat “BC”) and one Resin modified glass ionomer (Clinpro XT Varnish “CXT”) divided in 5 groups: with eight samples of each group. The materials used were manipulated in accordance with the instructions of the respective manufacturers. (Table I).

Table I. Materials used in this study, instructions and detail compositions

Forty rectangular blocks were obtained from human premolars and molars extracted for orthodontic reasons. The teeth received prophylaxis with low speed handpiece, later were cleaned with deionized water to remove any surface debris. The samples were sectioned using a carbide disc under irrigation, with dimensions (8mm x 4mm x 2mm). The teeth blocks were washed with deionized water and dried for 20 seconds. All light-curing materials were polymerized using a led device (Elipar Light Curing Unit 3M ESPE) verifying with a radiometer that the intensity of light emitted by the led lamp had a minimum value of 400 mW/cm². The samples were placed at room temperature for 10 minutes, after were stored in plastic bottles with 5 ml of deionized water, the samples were conserved at 37º C for 30 days, and measured in days 1, 2, 5, 15 and 30.

To determinate the amount of fluoride in varnishes, an ion selective electrode for sodium fluoride (model 1011, Hanna Instruments, USA) and a potentiometer (model HI 3222, Hanna Instruments) were necessary. The total ionic strength adjustment buffer (TISAB) solution was used to keep the pH stable and to prevent that fluoride ion produce complexes with different cations (8-10).

The fluoride solutions used in this study were prepared in concentrations of (1ppm, 2ppm, 10ppm, 100ppm, and 1000ppm). TISAB was used to get calibration slope with fluoride solutions; in equal volumes (25ml of each fluoride solution and 25ml of TISAB) were placed and mixed in a plastic bottle of 100ml; the device was calibrated until the readings were reached.

Fluoride determination

At the end of each period, the samples were removed from their respective plastic bottle and each sample was washed with 1ml of deionized water inside the bottle that originally contained it, in order to preserve the total amount of fluoride released by each sample. After we proceed to introduce the sample in a new 5ml plastic bottle with deionized water. Five ml of solution used to store the sample, and 1ml used to wash the sample gave a total of 6ml that were mixed with 6 ml of TISAB, because this solution works in a proportion 1:1.

The readings were performed with the electrode immersed at least 3 centimeters in the solution where was previously the sample, under magnetic stirring with a time interval of 3 minutes for each measurement. The values ​​of the readings were expressed in parts per million.

The data were analyzed with analysis of variance (ANOVA) one way and for repeated measures using the statistical program SPSS Statistics (IBM New York, USA).

Fluoride recharge

After thirty days of study, the samples were immersed in 1 ml of 2% (20,000ppm) sodium fluoride gel (Ionite Borgatta, Mexico) for 4 minutes and subsequently rinsed with a sterile solution. The fluoride released after the recharge was determined at 24, 48 and 72 hours through the procedure previously described.

Results

The highest fluoride release of each varnish occurred on the first day. From the second day, a slow steady decline began in fluoride release and continued until day 30, except for Fluor protector (FP), demonstrating a marked decline since the first day. Figure 1 clearly displays the pattern of fluoride released for each varnish during study time. The amount of fluoride release according to the time intervals is represented in TableII, and Duraphat (DP) and ClinPro White Varnish (CWV) showed a greater concentration of fluoride during the first day

Table II. Fluoride release and recharge in varnishes under study

Fluoride release from each varnish under study represented in ppm (parts per million), analyzed in different periods and three days after a recharge with a sodium fluoride gel.  (AR: After Recharge)

Figure 1. Fluoride released from varnishes during 30 days.

The analysis of variance (Table III) illustrated significant differences during the first day when DP, CXT, and CWV were compared with FP and BC since these varnishes released a higher concentration of fluoride. On the second day, only CXT displayed significant differences from BC and FP since this varnish showed a lower decrease. From the fifth day, CXT showed a more constant fluoride release since significant differences were observed compared to other varnishes. The behavior of materials in the established periods of the study was determined using the repeated measures ANOVA after Mauchly's test of sphericity (Table IV).Differences were observed in terms of time, and FP was the only material that did not display any significant differences when the first day was compared with other periods of study since it showed the lowest values from the first day. The DP displayed significant differences between days one and two, signifying a marked decrease of fluoride in this varnish after the second day.

The CWV showed significant differences between day one and the other established periods. This varnish did not demonstrate any difference between days 5 and 15, as well as days 15 and 30, since fluoride levels were minimal. In the BC group, marked differences were observed only between the first day and the rest of the periods, and this material presented the most significant decrease on the second day. The CXT presented a more constant fluoride release, and this could be the reason why differences were observed between all of  time intervals. The first day showed significant differences with the other periods, and day two demonstrated a difference when compared with day 30. Day 5 was different from days 15 and 30, and significant differences were observed between day 30 and the rest of the time periods.

The concentration of fluoride released once the samples were recharged with the fluoride gel is presented in Table 1. The highest concentration of fluoride mainly occurred in groups BC and CXT 24 h after the application of the gel. For group CWV, the amount of fluoride was minimum, whereas DP and FP did not show any release. After 48 h of recharge, the amount of fluoride decreased to minimum levels in BC and CXT groups. After 72 h, the fluoride released in all groups was practically null.

Table III. One-way ANOVA test. Significative differences between the groups at different periods.

Table IV. ANOVA test for repeated measures