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Year : 2021  |  Volume : 13  |  Issue : 2  |  Page : 90-95

Effect of Time, Temperature, and Storage on Fluoride Release and Recharge of Esthetic Restorative Materials

1 Undergraduate Student at Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka, India and Presently Doctor of Dental Medicine (D.M.D) Candidate, University of Pennsylvania, Philadelphia, United States
2 Associate Professor & Incharge Head, Department of Dental Materials, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka, India
3 Research Scholar, Department of Dental Materials, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka, India
4 Professor & Head, Department of Oral Pathology and Microbiology, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka, India
5 Professor, Department of Dental Materials, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka, India

Date of Submission30-Nov-2019
Date of Decision15-Dec-2020
Date of Acceptance01-Mar-2021
Date of Web Publication14-Jan-2022

Correspondence Address:
Dr. Prashanthi S Madhyastha
Department of Dental Materials, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education, Manipal, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jofs.jofs_236_20

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Introduction: The fluoride-containing restorative materials act as fluoride reservoir and release fluoride into the oral fluids gradually, thereby inhibiting secondary caries and restoration failure. The study can be utilized to develop improved regimes for topical fluoride delivery. The study evaluated and compared the influence of temperature, time, and storage conditions on the fluoride release and recharge of esthetic restorative materials. Materials and Methods: Silorane-based composite, methacrylate-based composite, compomer, and glass ionomer cement (GIC) were investigated for fluoride release and recharge with the fluoride selective ion electrode after immersion in distilled water and artificial saliva at 4°C, 37°C, and 55°C. Comparison between immersion media was performed with student t test, and comparison between materials and time interval (weeks and days) was performed with one-way ANOVA and Tukey post hoc test. The assessment of variation over time was assessed with repeated measures ANOVA. P-value was established to be significant at P < 0.05. Results: The highest rerelease was seen with GIC, followed by Dyract AP, Filtek P90, and Z100. Fluoride rerelease was greatest in GIC followed by Dyract AP in artificial saliva when compared to distilled water. Fluoride release was more significant at a higher temperature of 55°C. Also, fluoride recharge was highest at week 3 where the greatest rerelease was seen on day one. Conclusion: Fluoride release and recharge increase with temperature, time interval, and in artificial saliva.

Keywords: Esthetic dental material, fluoride recharge, fluoride release, fluoride selective electrode

How to cite this article:
Sen M, Madhyastha PS, Bangera MK, Natarajan S, Kotian R. Effect of Time, Temperature, and Storage on Fluoride Release and Recharge of Esthetic Restorative Materials. J Orofac Sci 2021;13:90-5

How to cite this URL:
Sen M, Madhyastha PS, Bangera MK, Natarajan S, Kotian R. Effect of Time, Temperature, and Storage on Fluoride Release and Recharge of Esthetic Restorative Materials. J Orofac Sci [serial online] 2021 [cited 2023 Jun 8];13:90-5. Available from:

  Introduction Top

The role of fluoride in oral health promotion is crucial with the development, progress and prevention of dental caries’.[1],[2] Restorative materials act as fluoride reservoirs and aid in gradual fluoride release into the oral fluids, thereby averting caries development and progression.[3],[4] Release of fluoride ions from the restorative materials can be highly beneficial as fluoride ions augment the neighbouring enamel or dentine preventing caries.

Conventional glass ionomer cements (GICs) are the top contenders possessing active fluoride release abilities to resist the development and progression of secondary caries.[5],[6],[7] However, in comparison to composite resins, GICs possess low mechanical properties immediately after set, and low translucency. Hybrid materials like compomers were developed to counteract the associated disadvantages of GICs while maintaining their anticaries properties.[4] Though these materials release fluoride, the fluoride-releasing property is influenced by varying temperature, time, and storage conditions.[8],[9],[10],[11]

A sharp decrease in fluoride release ability of restoratives is observed after 3 days. Topical fluorides or fluoridated dentifrice can be partly used to recharge the depleting fluoride content. But the rechargeability differs broadly between different groups of fluoride-releasing materials.[12] The study was performed to estimate the fluoride release and recharge from restorative materials under varying conditions of temperature, time, and storage that simulate the oral cavity conditions.

  Materials and Methods Top

Ethical approval for this study (Protocol Ref. No.: 13052) was provided by the Institutional Ethical Committee of Manipal College of Dental Sciences, Mangalore, on December 23, 2013. The study consisted of four groups:
  • Group I - Resin-modified GIC (GC Gold Label Light Cured Universal Restorative – GC America Inc, USA),
  • Group II- Microhybrid composite (Filtek P90–3M ESPE, USA),
  • Group III- Compomer (Dyract AP–Dentsply, India),
  • Group IV- Hybrid composite (Z100–3M ESPE, USA).

A single operator prepared specimens to reduce variability using Teflon molds of 10 mm diameter and 2 mm thickness. The uncured material was intentionally overfilled into the mold, sandwiched between transparent matrix strip and glass slide with slight pressure to expel the excess. Light curing was achieved through the top and bottom of the mold for the manufacturer’s recommended time. The intensity of the curing lamp was calibrated before each run via a Hilux Curing Light Meter (Dental Hilux Curing Light Meter, Dental Benlioglu, Inc., Ankara, Turkey). After curing, the retrieved cylindrical specimens were conditioned at 100% relative humidity at 37°C for 24 hours.

Based on the Key Article by Xu and Burgess,[12] (as mentioned in Table 2) comparing compressive strength, fluoride release, and recharge of fluoride-releasing materials, the following sample sizes can be derived. With a 5% alpha error, 80% power of the study, and a clinically significant difference of one unit, the required sample (n) in each group is five. A total of 120 pieces (30 samples of each restorative material) were fabricated. Fifteen samples were immersed in either 20 mL of distilled water or artificial saliva in a graduated 30 mL test tube. Samples were further subdivided into three groups (n = 5) to be stored at 4°C (in the refrigerator), 37°C (in an incubator), and 55°C (in a water bath) for 1 day.

Fluoride release

The samples were stored in sterile plastic containers.[10] At the end of 24 hours, the samples were removed, washed with distilled water, and dried with blotting paper. The samples were then transferred and immersed in a fresh jar containing 20 mL of distilled water or artificial saliva till day 7. The procedure was repeated and continued similarly till day 14 and day 28. The solutions (artificial saliva/distilled water) thus collected at the end of various time intervals, namely, 1 day, 7 days, 14 days, and 28 days were subjected to fluoride ion release measurement. This was done using a fluoride sensitive electrode (pH/Ion 510, Oakton (CE), Eutech Instruments Pvt Ltd, USA) by adding a 1:1 quantity of total ionic strength adjustment buffer solution. Each analysis was performed in triplicates, and mean data were obtained.

Fluoride recharge

A total of 40 samples (n = 10) were stored at 37°C for analysis of fluoride recharge. Fluoride recharge was done by immersing all the samples in 5000 ppm neutral sodium fluoride (NaF) solution for 5 minutes on the first day of weeks 1, 2, and 3. Following each recharge episode, each group’s samples were randomly divided into two categories of 5 and stored either in distilled water or artificial saliva. The fluoride re release was determined after 1, 3, and 7 days of each recharge episode.

Comparison between immersion media was performed with student t test, and the correlation between materials and time interval (weeks and days) was performed with one-way analysis of variance and Tukey post hoc test. The assessment of variation over time was assessed with repeated measures analysis of variance. Differences were considered to be significant at P<0.05.

  Results Top

Fluoride release was highest in group I, followed by group III, group II, and least in group IV in both the immersion media [Figure 1]. Higher fluoride release was seen in artificial saliva when compared to distilled water. Fluoride release increased with time from day 1 to day 28 in all the groups except in group I, which showed decreased fluoride release when measured on day 14. Also, all the groups showed an increase in fluoride release with temperature in both immersion media.
Figure 1 Fluoride release in (a) distilled water and (b) artificial saliva

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Post fluoride recharge, the highest rerelease was seen in group I, followed by groups III, II, and IV [Figure 2]. Fluoride rerelease was higher in group I than group III in artificial saliva when compared to distilled water. However, the observed difference in fluoride rerelease between groups II, III, and IV was not statistically significant. Rerelease increased from week 1 to week 3 in all the groups. Group IV showed higher rerelease in week 3 compared to group II. All four groups showed decreases fluoride rerelease from day 1 to day 3 and then increased fluoride rerelease from day 3 to day 7.
Figure 2 Fluoride rerelease in (a) distilled water and (b) artificial saliva

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  Discussion Top

Fluoride release is one of the most expected requisites from all restorative materials because of its beneficial effects, such as caries inhibition and remineralization. Esthetic restorative materials like composites, compomers, and GIC have exhibited properties of fluoride release and recharge.[4],[13] Topical fluoride treatments help in the formation of potential fluoride reserves within restored teeth.[13] This capability raises the possibility of fluoride-containing restorative materials exhibiting a lower incidence of recurrent caries than nonfluoride-containing materials.[13] Thus, the concept of fluoride recharge of restorations using topical fluorides becomes important to enable them to release fluoride ions for prolonged periods in an oral environment.[14]

The present in vitro study evaluated the fluoride release and recharge potential of four esthetic restorative materials (Filtek P90, Z100, Dyract AP, and GIC) over an extended period. Although taken together, the glass ionomer materials had more significant potential than other materials followed by compomers. It has been found that the extent of the glass ionomer matrix of the glass filler is an essential factor for the fluoride release and recharge capacity of resin-based materials.[15] The fluoride release and recharge of the composites were low, and the values probably represented fluoride remaining on the surface after the recharge and wash cycle.

Our study showed an increase in fluoride release over time (from day 1 to day 28) which was in agreement with the findings of similar studies,[16],[17],[18] which explains that the restorative materials sustain fluoride release over time. This is in contrast to demonstrating a high “burst” of fluoride release immediately following placement. However, these findings appear to be in contrast with those of Preston et al.,[19] Attar et al.,[20] and Gao et al.[21] The authors noted the “initial burst” of fluoride release which showed decrease in fluoride release over time.

The elution of fluoride is complex, even though the exact mechanism of fluoride release and recharge is unknown. Certain factors that play a crucial role in the short-term and long-term fluoride release include the material matrices, setting mechanism, environmental conditions,[22],[23] permeability of the material, form, and concentration of fluoride used.[24]. Among these, the permeability determines the depth to which both the absorption and then fluoride rerelease can occur.[10] The high water content of the materials, coupled with an increase in permeability, allows for higher fluoride release from the specimens’ depth. This explains the highest fluoride release and rerelease observed in GIC.

In GIC, fluoride release occurs either by a short-term reaction which involves the rapid dissolution of fluoride from the outer surface into the solution or a more gradual release resulting in a sustained diffusion of fluoride through the bulk cement.[4],[25],[26] The compomer also contains reactive glass fillers containing SrF2 which are identical to the ion leachable glass fillers used in conventional GICs, but in smaller sizes than those used in most composite resins. The initial setting is due to photopolymerization, followed by an acid–base reaction that arises from the sorption of water.[27] The hydrogel cannot form until water has diffused into the polymer matrix, and therefore, it shows a steady increase in fluoride release potential when compared to GICs as explained in other studies.[19] In vitro studies have shown that compomers released considerably less fluoride than conventional GICs over time with no initial “burst” of fluoride.[28],[29] But, the levels of fluoride release remained low and relatively constant over time.[10] The results obtained with Dyract AP in our study are in agreement with these findings. On the other hand, composites diffuse water very slowly and have lower water permeability (sorption). In composites, fluoride diffuses past a polymer network which is integrally demanding than diffusion through a hydrogel. Thus, the low fluoride release present during the study may be due to fluoride absorbed onto the surface.[30],[31]

In our research, we have used different temperatures to check whether an increase in environmental temperature increases the materials’ fluoride release and recharge abilities. As broad temperature fluctuations occur in the oral environment, thermal circuits may frequently challenge the restorative materials placed in this environment.[8] Results of our study suggested that an increase in temperature increases fluoride release in all the materials tested. Also, fluoride treatment of restorative materials at a high temperature is clinically recommended to improve their recharging ability. This can be used to develop regimes for improving the delivery of topical fluoride.

Sterile plastic containers were used throughout the present study for storage as glass vessels absorb or leach fluoride.[10] Test medium also plays a significant role in fluoride release. Some researchers utilized distilled water as the test medium.[22] However, artificial saliva simulates conditions similar to the in vivo environment than distilled water. Our study witnessed higher fluoride release in artificial saliva than in distilled water. This phenomenon can be explained with respect to the pH of the dissolving medium. Studies have suggested that the restoratives tested at acidic pH were able to increase the fluoride release significantly.[32],[33] Our study used artificial saliva with a pH of 5.3 to 5.5, which explains the higher fluoride release in artificial saliva than distilled water.

The capability for fluoride “recharge” is essential than fluoride release alone. The daily exposure of the restorations in the oral cavity to topical fluorides from toothpaste or mouthwash acts as a potential recharge.[19] In the prevention of dental caries, besides using an antibacterial agent, remineralization should be promoted. In our study, NaF was used for fluoride recharge as it is a common ingredient in commercial toothpaste and rinses. While most toothpaste has fluoride at around 1000 parts/106 F level, we used lower concentrations (500 parts/106 F level) to determine rechargeability at a lower fluoride concentration. The concentration for fluoride replenishing was 0.2% NaF solution. An increase in fluoride release after exposure to 0.2% NaF solution could be credited to the retention of fluoride in the pores or on the surface. Our result agreed with the findings of a previous study by Itota et al.[34] Previous recharge studies have also implied that the GIC’s have fairly high recharge potentials as seen in the present study.[10] In our study, rerelease increased from week 1 to week 3 in all the groups suggesting that topical applications of fluoride would help in the prevention of caries and failure of restorations. GIC and compomer (Dyract AP) have better fluoride recharging capabilities than the composites (Filtek P90 and Z100) suggesting the more frequent application of fluoride is needed for composites.

  Conclusion Top

The study demonstrated that GIC has the highest fluoride release and recharge capacity, followed by compomer and composites. Fluoride release was more when stored in artificial saliva than in distilled water. Fluoride release increased with an increase in temperature and time. On recharge, the highest fluoride rerelease was shown by GIC and least by composites. The authors recommend GICs over composites in high carious conditions.

Financial support and sponsorship

This study was funded by Indian Council of Medical Research (ICMR) under the short-term Student Research Scholarships (STS-2012) bearing research proposal no. 2014-00811.

Conflicts of interest

There are no conflicts of interest.

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