ReviewReview on fluoride-releasing restorative materials—Fluoride release and uptake characteristics, antibacterial activity and influence on caries formation
Introduction
Since the observation that secondary caries formation was rarely associated with fluoride-containing silicate cement restorations, increasing attention has been focused on the development of various fluoride-releasing products, to be used as restorative materials, lining cements, sealants and orthodontic cements.
Fluoride is well documented as an anticariogenic agent. A variety of mechanisms are involved in the anticariogenic effects of fluoride, including the reduction of demineralization, the enhancement of remineralization, the interference of pellicle and plaque formation and the inhibition of microbial growth and metabolism [1], [2], [3], [4]. Fluoride released from dental restorative materials is assumed to affect caries formation through all these mechanisms and may therefore reduce or prevent demineralization and promote remineralization of dental hard tissues.
Today, there are several fluoride-containing dental restoratives available in the market including glass-ionomers, resin-modified glass-ionomer cements, polyacid-modified composites (compomers), composites and amalgams. Due to their different matrices and setting mechanisms the products vary in their ability to release fluoride. However, it is assumed that the antibacterial and cariostatic properties of restoratives are often associated with the amount of fluoride released.
This paper aimed to summarize the current status of fluoride-releasing dental restoratives, their ability to release and recharge fluoride, their antibacterial activity and cariostatic properties. Therefore, original scientific papers and reviews listed in PubMed or available from peer-reviewed German journals (published between 1980 and 2004) were included in the review. Clinical studies concerning the development of secondary caries at the margin of restorations were taken into consideration when performed in split-mouth design with an observation period of at least three years. Papers dealing with orthodontic or endodontic topics were not included in the review.
Section snippets
Glass-ionomer cements
Glass-ionomer cements are composed of fluoride-containing silicate glass and polyalkenoic acids which are set by an acid–base reaction between the components. During the setting reaction a variety of ionic constituents is released from the glass, including fluoride. Two mechanisms have been proposed by which fluoride may be released from glass-ionomers into an aqueous environment. One mechanism is a short-term reaction, which involves rapid dissolution from outer surface into solution (process
Factors influencing the release of fluoride from restoratives
The elution of fluoride is a complex process. It can be affected by several intrinsic variables, such as formulation and fillers [27], [28], [31], [42], [86], [87]. It is also influenced by experimental factors, i.e. storage media, frequency of change of the storage solution, composition and pH-value of saliva, plaque and pellicle formation [9], [12], [21], [32], [35], [52], [88], [89], [90], [91], [92], [93], [94]. It was shown that the powder–liquid ratio of two-phase-systems, mixing
Fluoride recharge of restorative materials
The cariostatic action associated with fluoride-releasing materials is usually attributed to a sustained release of fluoride. Due to the fact that fluoride levels leached from fluoride-containing filling materials decreased over time (which is significant for glass-ionomers and resin modified glass-ionomers), the “recharging” of restoratives with fluoride has been suggested to maintain a continuously increased level of fluoride release. The ability of a restorative to act as a fluoride
Clinical relevance of fluoride released from restoratives in human saliva and plaque
In a number of studies reduced caries experience has been contributed to elevated salivary fluoride levels [138], [139], [140]. It has been stated that a constant supply of low levels of intraoral fluoride is of most benefit in preventing caries. An elevation of the fluoride level in saliva from 0.001 to 0.005–0.010 mmol/l, e.g. 5–10 times, for prolonged periods may be efficient for caries control [141].
After a single application of fluoride-containing dentifrices (1250 ppm) salivary fluoride
Antimicrobial activity of fluoride-releasing materials
The effects of fluoride on oral bacteria and plaque are well documented by a considerably amount of literature (for review see [153], [154], [155]). The mechanisms by which fluoride may interfere with bacterial metabolism and dental plaque acidogenicity include the inhibition of the glycolytic enzyme enolase and the proton-extruding ATPase as well as the bacterial colonization and competition. Furthermore, intracellular or plaque-associated enzymes such as acid phosphatase, pyrophosphatase,
Fluoride uptake of adjacent tooth structure
For many years the cariostatic effect of fluoride was attributed to the incorporation of structurally bound fluoride in the hydroxyapatite crystal lattice and the reduced solubility of the so-formed fluoridated hydroxyapatite. However, recent observations have found that fluoride in the aqueous phase surrounding the carbonated apatite crystals is much more effective in inhibiting demineralization than fluoride incorporated into the crystals [4]. Fluoride may precipitate onto tooth surfaces as
In vitro studies
In vitro, several fluoride-releasing restoratives have shown to inhibit enamel and dentin demineralization produced by acidic gels or demineralizing buffer solutions. Thereby, inhibition of enamel demineralization is located up to a distance of 7 mm from the edge of the material (‘remote effect’). When compared with non-fluoride-releasing restorations, the placement of glass-ionomers reduces mineral loss by almost 80% at 0.22 mm distance from the restoration and by 37% 7 mm distant from the
In vitro and in situ studies
It is also discussed whether fluoride-releasing restoratives might be effective in inhibition of incipient carious lesions of neighboring teeth located in interproximal contact to the restoration.
In vitro and in situ investigations found that glass-ionomer and resin-modified glass-ionomer restorations in direct contact to adjacent teeth prevent demineralization compared to non-fluoridated amalgam and composite [209], [242]. However, additional application of topical fluoride (fluoridated
Conclusion
There is a continuum of restorative materials that range from initially high fluoride release (conventional glass-ionomer and resin-modified glass-ionomer) to intermediate fluoride release (compomer) to low fluoride release (fluoride-releasing composite and fluoride-releasing amalgam) to no fluoride release (non-fluoridated composite resins and amalgam) [10], [11], [15], [55], [59], [252], [253]. However, the potential to release fluoride not only varies between different restorative materials,
Acknowledgement
The study was supported by a grant of the DENTSPLY DeTrey GmbH.
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