Fluoride Release by Restorative Materials after the Application of Surface Coating Agents: A Systematic Review

: Background: Fluoride is vital in dentistry for caries prevention, enhancing remineralization, and inhibiting bacteria. Incorporating fluoride into restorative materials like glass-ionomer cements, compomers, and giomers has significantly increased fluoride availability in the oral cavity. This review assesses how surface coatings influence fluoride release from various dental restorative materials. Methods: In December 2023, we conducted electronic searches in PubMed, Scopus, and Web of Science (WoS) databases. In the Scopus database, the results were refined to titles, abstracts, and keywords, while in PubMed, they were narrowed down to titles and abstracts. In WoS, the results were refined only to abstracts. The search criteria were based on the terms fluoride AND release AND (coating OR glaze OR layer OR film OR varnish) AND (composite OR glass OR compomer), following PRISMA guidelines and the PICO framework. Twenty-three studies were rigorously selected and analyzed for fluoride release from coated versus uncoated materials. Results: Surface coatings typically reduce the rate of fluoride release. Glass-ionomer cements had the highest release, followed by giomers and compomers. The initial release was greater in uncoated materials but stabilized over time, influenced by variables like artificial saliva and deionized water. Conclusions: Surface coatings generally decrease fluoride release rates from dental materials. Although initial rates are high, contributing to caries prevention, more standardized research is needed to better understand the impact of coatings and optimize materials for maximum preventive benefits.


Introduction
Fluorine is a light-yellow gas that, due to its high chemical activity, is not freely available in nature.The main sources of fluoride in the daily diet are water, tea, cereal products, leafy vegetables, nuts, fish, and potatoes [1].The incorporation of fluoride in public health measures, such as water fluoridation and the use of fluoride toothpaste, has significantly reduced the prevalence of dental caries in many parts of the world.Fluoride works by promoting the remineralization of tooth enamel and inhibiting the demineralization process.It also has antibacterial properties that help reduce the levels of harmful oral bacteria [2].
Appl.Sci.2024, 14, 4956 2 of 15 However, excessive fluoride intake during tooth development can lead to dental fluorosis, a condition characterized by discoloration and the mottling of the enamel.Dental fluorosis ranges from mild forms, which present as white spots on the teeth, to severe forms that result in brown stains and surface pitting.While mild fluorosis does not typically affect oral health-related quality of life, severe cases can lead to aesthetic concerns and social discomfort [3].Fluoride is considered the key component of caries prevention.It limits the impact of cariogenic bacteria by extenuating the production of acids and the deposition of bacterial plaque on tooth surfaces and holding back the metabolic transformation of carbohydrates in bacterial cells.It inhibits demineralization by the constant presence of low concentrations of fluoride ions, enabling the repositioning of mineral compounds lost during repeated acid attacks with the formation of fluorohydroxyapatite crystals that are less susceptible to dissolution.It supports and accelerates remineralization by providing higher concentrations of fluoride ions, which ensure the formation of calcium fluoride (CaF2), which is a reservoir of fluoride ions released during the action of acids on the enamel [4].
By leveraging the therapeutic role of fluoride, manufacturers of materials used in restorative dentistry have introduced a wide range of products that influence the increase in fluoride concentration in the oral cavity.Starting with conventional glass-ionomer cements (CGICs), progressing through compomers and giomers, and ending with composites coated with a fluoride-containing binding component.Glass-ionomer cements used in dentistry for over 30 years have undergone numerous changes and enhancements.CGICs consist of a liquid and a powder containing a fluoro-alumino-silicate glass filler, which serves as a base containing fluoride.The liquid contains acrylic acid or its copolymers with a possible content of aliphatic polycarboxylic acids.There are also water-activated glass-ionomer cements (WAGICs) that have the addition of an acidic polymer in powder.Upon mixing with water, an acid-base reaction occurs, influencing the material's hardening [5][6][7].These cements have the ability to replenish the pool of fluorides from the environment, acting as a fluoride reservoir and ensuring long-term fluoride release [8].
Compomers are another type of restorative material that combines the advantages of composites and glass ionomers [9].In the latest versions of this product, silicon-calciumaluminum-fluoride glass particles (42-67% by volume) are embedded in a conventional methacrylate matrix.This material hardens through two types of reactions: polymerization and acid-base reaction [10,11].These materials exhibit high aesthetics and fluoridereleasing capabilities, albeit to a lesser extent than CGICs [12].In turn, hybrid materials derived from glass ionomers and composites are also known as giomers.They contain fluoro-alumino-silicate filler particles that have undergone a prior (partial or complete) reaction with polyacrylic acid, known as pre-reacted glass-ionomer filler (PRG) technology [8,13].Giomers demonstrate superiority over compomers in terms of releasing a greater amount of fluoride and absorbing more from the oral cavity environment [14,15].
Coatings that prevent the development of caries and strengthen filling materials used in modern dentistry include varnishes; resins; glaze; and, for example, vaseline, depending on the method used.When using fluoride varnishes, fluorine ions diffuse from the tooth coating to its surface, improving the acid resistance of the tooth surface.Applying a coating to some surfaces of filling materials allows clinicians to modify the color of the tooth/filling and its susceptibility to staining.The most important and basic purpose of using coatings is to change the surface properties of the original substrate.The main problem that arises when using various coatings, and at the same time the greatest challenge, is the issue of interaction and cooperation between the applied coating, the substrate, and the surrounding environment [16].Despite the many advantages of using coating materials, it is also necessary to mention the downside aspect of their use, which is the negative impact on some properties of filling materials.Research shows that despite the improvement in the properties of glass-ionomer materials, e.g., their hardness, there is also a risk of a negative impact on the release and absorption of fluorine ions from the environment [17] (Figure 1). is also a risk of a negative impact on the release and absorption of fluorine ions from the environment [17] (Figure 1).The aim of this systematic review was to investigate the fluoride release from restorative materials subsequent to the application of surface coating agents.To date, no comparable study has been conducted.It is imperative to underscore the significance of this research area, as its thorough exploration has the potential to yield manifold benefits.Specifically, elucidating this aspect may enhance the efficacy of dental procedures, mitigate caries susceptibility, and thereby positively impact patient oral health outcomes.

Focused Question
This systematic review followed the PICO framework as follows [18]: PICO question: In the case of restorative materials (population), will the addition of surface coating (investigated condition) cause a change in fluoride release (outcome) compared to restorative materials without surface coating (Comparison Conditions)?

Protocol
The article selection process for this systematic review was meticulously outlined according to the PRISMA flow diagram (see Figure 2).The aim of this systematic review was to investigate the fluoride release from restorative materials subsequent to the application of surface coating agents.To date, no comparable study has been conducted.It is imperative to underscore the significance of this research area, as its thorough exploration has the potential to yield manifold benefits.Specifically, elucidating this aspect may enhance the efficacy of dental procedures, mitigate caries susceptibility, and thereby positively impact patient oral health outcomes.

Focused Question
This systematic review followed the PICO framework as follows [18]: PICO question: In the case of restorative materials (population), will the addition of surface coating (investigated condition) cause a change in fluoride release (outcome) compared to restorative materials without surface coating (Comparison Conditions)?

Protocol
The article selection process for this systematic review was meticulously outlined according to the PRISMA flow diagram (see Figure 2).

Eligibility Criteria
All studies included in this systematic review had to meet the following criteria: They had to investigate fluoride release by dental materials, explore fluoride release with different coating materials, and be published in English with no restrictions regarding the date of publication.The reviewers established the following exclusion criteria: studies in non-English languages, clinical reports, opinions, editorial papers, review articles, and studies without a full-text version available [20][21][22][23]

Information Sources, Search Strategy, and Study Selection
In December 2023, we conducted electronic searches in PubMed, Scopus, and Web of Science (WoS) databases.In the Scopus database, the results were refined to titles, abstracts, and keywords, while in PubMed, they were narrowed down to titles and abstracts.In WoS, the results were refined only to abstracts.The search criteria were based on the terms fluoride AND release AND (coating OR glaze OR layer OR film OR varnish) AND (composite OR glass OR compomer), following PRISMA guidelines and the PICO framework.The search parameters were constrained to studies meeting eligibility criteria.Additionally, a supplementary literature search was conducted to identify any articles not captured during the initial database search.Only articles with full-text versions were included in the analysis.

Eligibility Criteria
All studies included in this systematic review had to meet the following criteria: They had to investigate fluoride release by dental materials, explore fluoride release with different coating materials, and be published in English with no restrictions regarding the date of publication.The reviewers established the following exclusion criteria: studies in non-English languages, clinical reports, opinions, editorial papers, review articles, and studies without a full-text version available [20][21][22][23].

Information Sources, Search Strategy, and Study Selection
In December 2023, we conducted electronic searches in PubMed, Scopus, and Web of Science (WoS) databases.In the Scopus database, the results were refined to titles, abstracts, and keywords, while in PubMed, they were narrowed down to titles and abstracts.In WoS, the results were refined only to abstracts.The search criteria were based on the terms fluoride AND release AND (coating OR glaze OR layer OR film OR varnish) AND (composite OR glass OR compomer), following PRISMA guidelines and the PICO framework.The search parameters were constrained to studies meeting eligibility criteria.Additionally, a supplementary literature search was conducted to identify any articles not captured during the initial database search.Only articles with full-text versions were included in the analysis.

Data Collection and Data Items
Seven reviewers (J.K., J.K., M.S., O.T., D.T., T.D., and W.D.) diligently selected the articles that fulfilled the previously established criteria.The essential data were then compiled into a standardized Excel file.

Assessing Risk of Bias in Individual Studies
During the initial stage of study selection, the authors independently reviewed the titles and abstracts of each study to minimize potential reviewer bias.The level of agreement among reviewers was assessed using Cohen's κ test [24].Any discrepancies regarding the inclusion or exclusion of a study were resolved through discussions between the authors.

Quality Assessment
Two independent assessors (J.M. and M.D.) conducted an evaluation of the procedural quality for each study included in the article.The criteria used to evaluate study design, implementation, and analysis included research conducted on molds, differentiation of materials used, use of artificial saliva, usage of fluoride ion electrodes, adherence to manufacturer's instructions for material use, and frequency of checkups (hours/days cycle).Studies were scored on a scale of 0 to 6 points, where a higher score indicated better study quality.The risk of bias was categorized as follows: 0-2 points denoted a high risk, 3-4 points denoted a moderate risk, and 5-6 points indicated a low risk.Any discrepancies in scoring were resolved through discussion until a consensus was reached [25][26][27][28][29][30][31].

Study Selection
The initial search of the electronic databases generated 645 records.Subsequently, 261 duplicates were identified and removed, leaving 384 unique records for abstract screening.Following the process, 69 articles were excluded based on abstracts, which resulted in 27 articles remaining for full-text evaluation.Three articles were excluded due to the unavailability of full-text versions, and one was removed because of failure to meet predefined inclusion criteria.This culminated in a final selection of 23 articles for both qualitative and quantitative analyses.
The results revealed a broad spectrum of fluoride release behaviors influenced by material composition, surface coating, and environmental conditions.Overall, uncoated samples exhibited higher fluoride release compared to coated samples across all cases [17,.
Glass-ionomer cements typically exhibited higher fluoride release compared to other materials [33,34,38,40,41,46,49].Surface coatings, such as varnishes and resins, generally reduced fluoride release, although the extent of reduction varied depending on the material and type of coating.The initial fluoride release was typically higher but decreased over time.Additionally, studies explored fluoride recharging abilities and the impact of external fluoride sources, such as toothpaste and varnish, revealing their potential to enhance fluoride release from dental materials [33,34] (see Supplementary Table S1).

Main Study Outcomes
The objective of this study was to assess the fluoride release of different materials, after applying a coating agent.The findings across studies revealed diverse outcomes.Various materials and coatings were used to measure the amount of the cumulative fluoride ions released.In most papers, the most frequently examined material is glass ionomer [17,, while the next common material is composite [34,38,40,41,45,47].In four studies, giomers were mentioned [33,36,40,45], two studies analyzed compomers [38,48], and only one study was conducted on bioactive glass [46] In terms of a system to check the amount of released fluoride, SEM analysis was used, in addition to a fluoride ion-selective electrode type 96-09 (Boston, MA, USA) and a microprocessor analyzer ORION EA 940 (Orion Res Inc., Salt Lake City, UT, USA).
In one article, the authors stated that freshly set glass ionomer released the highest number of fluoride ions, which decreased every 24 h till day 21st [43].In another study, we found data indicating that sealant emission led to the highest fluoride ion release during the first day, which then steadily decreased until the fifteenth day [33] (see Table 1).

Discussion
In the present systematic review, our objective was to investigate the fluoride release by restorative materials following the application of surface coating agents.The findings of the studies indicate that the utilization of coating agents impacts the release of fluoride ions from filling materials, with coating materials demonstrating a significant reduction in fluoride ion release [17,36,37,[39][40][41]44,45,49].Moreover, the quantity of released fluoride ions also varied depending on the material utilized in the study.In the analyzed studies, researchers also examined how the release of fluoride ions from filling materials changed over time.The studies have shown that the release of fluoride ions is highest in both coated and uncoated samples in the first few days of observation and then decreases, maintaining a steady level [32,33,37,39,44,47,52].It was also found that the use of coating agents can improve the ability of filling materials to absorb fluoride ions from the

Discussion
In the present systematic review, our objective was to investigate the fluoride release by restorative materials following the application of surface coating agents.The findings of the studies indicate that the utilization of coating agents impacts the release of fluoride ions from filling materials, with coating materials demonstrating a significant reduction in fluoride ion release [17,36,37,[39][40][41]44,45,49].Moreover, the quantity of released fluoride ions also varied depending on the material utilized in the study.In the analyzed studies, researchers also examined how the release of fluoride ions from filling materials changed over time.The studies have shown that the release of fluoride ions is highest in both coated and uncoated samples in the first few days of observation and then decreases, maintaining a steady level [32,33,37,39,44,47,52].It was also found that the use of coating agents can improve the ability of filling materials to absorb fluoride ions from the surroundings [48].The release of fluoride ions also varied depending on the conditions in which research was carried out.The influence of deionized water, artificial saliva, or specific demineralization conditions caused the release of fluorine ions from the tested materials to be different, and the highest number of ions were released in deionized water [39].Deionized water was the most commonly used solution in studies utilizing the fluoride ion electrode at the same time [17,32,34,35,[37][38][39][40][41][44][45][46]48,49,[51][52][53], allowing for standardization and an accurate comparison of results across these studies.
Various adhesive systems, particularly when used in conjunction with glass-ionomer cements, exert a significant influence on the fluoride release process.This suggests that certain adhesive systems may function as mechanical barriers, thereby resulting in a reduction in the quantity of fluoride released [47].Variations in the efficacy of fluoride release are contingent upon the formulation of a particular adhesive system, encompassing factors such as its viscosity, thickness, pH, and uniformity of application.The emphasis placed on these parameters implies that, beyond the quantity of fluoride released, other inherent properties of adhesive systems are pivotal for their effectiveness within a clinical milieu.The functionality of these adhesive systems as potential barriers prompts discourse regarding their role in safeguarding tooth structure and modulating remineralization processes.
An essential facet explored in contemporary research is the influence of fluoride release on bacteria, specifically those implicated in the caries process.Numerous investigations have elucidated the capacity of fluoride emanating from diverse dental materials to impede the metabolic processes of cariogenic bacteria, thereby mitigating acidogenesis [47,50].For example, findings from the study conducted by Seppä et al. [47] revealed substantial quantities of fluorine ions released by freshly mixed glass-ionomer cements, resulting in a marked suppression of pH decline within the liquid phase.Conversely, aged samples exhibited significantly diminished fluoride release and failed to exert any discernible impact on attenuating acid production by Streptococcus mutans.Furthermore, the application of fluoride gel induced a noteworthy augmentation in fluoride release and associated inhibitory efficacy across all the tested glass-ionomer cements.Notably, variability in the degree of inhibition was observed among different formulations of glass-ionomer cements.Nevertheless, the scientific literature consistently underscores the potential of sustained fluoride release, particularly facilitated by specialized fluoride preparations, to sustain a bacteriostatic effect [47,50].Consequently, these findings show the potential merits of employing dental materials that not only facilitate remineralization but also actively deter bacterial proliferation within the oral milieu.
The primary limitation of this systematic review stems from the notable lack of standardized methodologies across the dental research conducted.Discrepancies in experimental conditions, including variations in the type of water utilized, the composition of artificial saliva, and specific demineralization parameters, can introduce considerable variability in outcomes.This lack of uniformity complicates the comparison and generalization of findings.Additionally, disparities in methodologies for assessing fluoride release further compound the complexity of interpreting results across studies.Hence, it is imperative to conduct further research aimed at investigating the effect of coatings on fluoride release with stringent experiment standardization.This will be instrumental in substantiating the findings obtained in various studies.

Conclusions
In conclusion, this systematic review focused on the fluoride release from different restorative materials both with and without coatings.The results show that coating a filling material with coatings such as bonds or varnishes tends to reduce the fluoride release, which leads to the conclusion that fluoride release is dependent only on the filling material type.However, the fluoride released from various dental materials has demonstrated antibacterial properties, inhibiting the metabolic activities of cariogenic bacteria and reducing acid production, which is crucial for caries prevention.However, due to the lack of homogeneity of research, there is a need to conduct more accurate and heterogeneous research in order to draw more specific and thorough conclusions.