Preventive Measures and Minimally Invasive Restorative Procedures

Abstract

16 fluorides include sodium fluoride (NaF), sodium monofluo-rophosphate (NaMFP), and amine fluoride (AmF, such as dectaflur and olaflur or amine fluoride 297). While the fluorides AmF and NaF can be easily split in aqueous solutions into the ions, an intraoral enzymatic or acid-driven hydroly-sis process is necessary to split the fluoride ion from the NaMFP. Four reactions between the dental hard tissue and the fluoride can be distinguished. (1) The enamel is slightly dissolved at the surface and forms during reprecipitation CaF 2-like precipitates. (2) The enamel is slightly dissolved and forms during reprecipitation fluoridated hydroxyapatite or fluorapatite. (3) The fluoride diffuses into the enamel and specifically adsorbs to free binding places at the crystals such as OH − , Ca 2+ , or phosphate compounds. (4) The fluoride diffuses into the enamel and binds unspecifically in the aqueous coverage of the enamel crystals. In particular the first mentioned mode of action, the formation of calcium fluoride (CaF 2)-like precipitates on the tooth surfaces, is of major importance. The CaF 2-like precipitates are stabilized intraorally on the tooth surfaces by phosphate groups and saliva proteins. In case of a plaque accumulation and bacteria-induced pH decline in the bio-film, parts of the CaF 2-like precipitate are dissolved releasing fluoride ions. This fluoride can diffuse into the enamel, and the modes of action 2, 3, and 4 could take place. A sufficient plaque removal with fluoridated toothpaste recovers the CaF 2-like precipitate, which again forms a reservoir for fluoride ions. The CaF 2-like layer is insofar of major importance as it can protect the underlying enamel against acid impacts. If this layer is incomplete, the underlying enamel can be dissolved , even if fluoride has been incorporated into the crystals. However, the critical pH of fluoridated enamel is decreased, which can lead to a lower solubility of the enamel itself [66]. The amount of CaF 2 formed on the tooth surface depends on the concentration of fluoride in the preparation (the higher the more), the application duration (the longer the more), the pH of the preparation (the lower the more), and the fluoride compound [67]. It has been shown that AmF leads to a higher CaF 2 formation than NaF as well as the fluorides containing polyvalent metal cations [67, 68]. All fluoride preparations lead to an enrichment of fluoride in the upper structures of enamel [69]; the penetration depth is limited and depends also on the compound used. While readily dissociating fluorides easily diffuse into the upper enamel structures, this process takes much more time in case of NaMFP, since this compound has to be split enzymatically [70], with the consequence that after NaMFP application, the fluoride uptake is lower than after the other compounds [70, 71]. In case of healthy enamel, there is a chemical balance between the saliva and the dental hard tissue, leading also to a delivery of fluoride into the environment. In case of a carious demineraliza-tion and a fluoride application, however, the fluoride uptake is much higher, leading to a permanent enrichment with fluoride ions. > The formation of CaF 2-like layers on the tooth surface is of major importance for caries prevention as it constitutes a fluoride reservoir from which fluoride can be released during a cariogenic acid attack. The amount of CaF 2 formed on the tooth surface depends on the concentration of fluoride in the preparation, the application duration, the pH of the preparation, and the type of fluoride compound. Fluoride is also retained in the dental plaque in form of ionized , ionizable (weakly bound), and bound fluoride. The bound fluoride is connected to organic components in the plaque and to the bacteria. The weakly bound fluoride is the abovementioned phosphate-stabilized fluoride. At neutral pH, only low amounts are present as ionized fluoride; in case of pH decline, bound fluoride will be set free, both from plaque, CaF 2-like precipitates, and dental hard tissue, and the absolute amount of ionized fluoride increases [72]. The basic principles of anticariogenic effect of fluorides are (1) reduction of acid solubility of the dental hard tissue and (2) inhibition of demineralization as well as promotion of remineralization. The hydroxyapatite in the dental hard tissue is not the stoichiometric form of this compound but a deficient one. Several crystals show deficits and imperfections. In these areas, fluoride can be incorporated during maturation of the teeth, in both preeruptive and Post-eruptive enamel maturation. By the filling of the deficiencies with fluoride, the crystalline structure is stabilized which modifies or reduces the solubility of hydroxyapatite. Fluoride ions, however, do not only fill the deficiencies but also replaces the hydroxyl ions (conversion of hydroxyapatite into fluorapatite). This leads also to a reduced acid solubility. The replacement is limited: during preeruptive formation of hydroxyapatite, approximately 10% can be replaced. Posteruptively, the surface can also be enriched with fluoride ions by a topical application of fluoride preparations. Comparable, the interaction between the fluoride ion and the surface causes also a substitution of hydroxyl with fluoride ions. However, the depth penetration is limited. The caries inhibition can only be partly be explained by the mentioned effect on solubility. An equally important mode of action is the impact of fluorides on de-and remin-eralization. The enrichment of the tooth surface by the topical application of fluorides with weakly bound fluoride (CaF 2-like precipitates) allows that during a carious pH decline, fluoride can be released into the aqueous phase. These free fluoride ions can protect the crystals of the dental hard tissue by forming fluorapatite and by reducing their solubility. During the neutralization process, the fluoride ions precipitate together with calcium ions and dissolved hydroxyapatite, forming fluorapatite and fluoridated apatite. These fluoride-enriched minerals precipitate earlier, as the solubility product of hydroxyapatite and fluorapatite differs. Therefore it can be concluded that the demineralization is reduced by the inclusion of the fluoride but also the remin-eralization due to the earlier precipitation [72-74]. This chemical and dynamic balance prevents, if a good oral