Fluoride release

Resin-modified glass—ionomer lining and restorative materials add a multifunctional acidic monomer to the poly(acryhc acid) [9003-01 Hquid component of the system. Once the glass powder and Hquid are mixed, setting can proceed by the acid—glass—ionomer reaction or the added monomer can be polymerized by a free-radical mechanism to rapidly fix the material in place (74,75). The cured material stiH retains the fluoride releasing capabiHties of a glass—ionomer. 

Fluorides. Most woddwide reductions in dental decay can be ascribed to fluoride incorporation into drinking water, dentifrices, and mouth rinses. Numerous mechanisms have been described by which fluoride exerts a beneficial effect. Fluoride either reacts with tooth enamel to reduce its susceptibihty to dissolution in bacterial acids or interferes with the production of acid by bacterial within dental plaque. The multiple modes of action with fluoride may account for its remarkable effectiveness at concentrations far below those necessary with most therapeutic materials. Fluoride release from restorative dental materials foUow the same basic pattern. Fluoride is released in an initial short burst after placement of the material, and decreases rapidly to a low level of constant release. The constant low level release has been postulated to provide tooth protection by incorporation into tooth mineral. 

Gl ss-Ionomers. Glass-ionomers show fluoride release at levels that are usually higher than those found in composite materials. The fluoride is found within the aluminosihcate glass, which is melted with fluoride fluxes and ground to form powder filler. The fluoride is added as calcium fluoride [7789-75-5] aluminum fluoride [15098-87-0] and sodium fluoride [7681-49-4] in a combined proportion of approximately 20% by weight in the final powder (284,285). 

The movement of fluoride through the atmosphere and into a food chain illustrates an air-water interaction at the local scale (<100 km) (3). Industrial sources of fluoride include phosphate fertilizer, aluminum, and glass manufacturing plants. Domestic livestock in the vicinity of substantial fluoride sources are exposed to fluoride by ingestion of forage crops. Fluoride released into the air by industry is deposited and accumulated in vegetation. Its concentration is sufficient to cause damage to the teeth and bone structure of the animals that consume the crops. 

Chamberlain Powers, 1976 Jendresen Trowbridge, 1972). The addition of stannous fluoride to the cement increases dissolution, but this is an advantage rather than a disadvantage, for the fluoride released is taken up by neighbouring enamel (Bitner Weir, 1973). 

The most important modification of these materials was the discovery of the effect of adding stannous fluoride (Foster Dovey, 1974, 1976). Originally added to provide fluoride release, it was found to improve the mixing qualities of the cement and to increase strength by about 50 %. This is reflected also in improved adhesion to enamel and dentine (Section 5.7.4). 

Forsten, L. (1977). Fluoride release from a glass ionomer cement. Scandinavian Journal of Dental Research, 85, 503—4. 

Kasten, F. H., Pineda, L. F., Schneider, P. E., Rawls, H. R. Foster, T. A. (1989). Biocompatibility testing of an experimental fluoride releasing resin using human gingival epithelial cells in vitro. In Vitro Cellular Development Biology, 25, 57-62. 

Kuhn, A. T. Jones, M. P. (1982). A model for the dissolution and fluoride release from dental cements. Biomaterials, Medical Devices and Artificial Organs, 10, 281-93. 

Meryon, S. D. Smith, A. J. (1984). A comparison of fluoride release from three glass ionomer cements and a polycarboxylate cement. Interruitional... 

Thornton, J. B., Retief, D. H. Bradley, E. L. (1986). Fluoride release from and tensile bond strength of Ketac-Fil and Ketac-Silver to enamel and dentine. Dental Materials, 2, 241-5. 

Although fluoride is added as the tin salt, fluoride release is accompanied by the release of aluminium and not tin (de Freitas, 1973). There is little leaching out of tin apart from an initial wash-out. Of course, aluminium is not released from the normal cement (Wilson, 1976 Wilson, Abel Lewis,... 

Kuhn Jones (1982) examined various models for fluoride release and showed that release did not fit the membrane and homogenous monolith model. Instead, they concluded that the cement behaved as a porous granular monolith, as described by Kydonieus (1980). The release of fluoride appears to be an ion exchange phenomenon, as dental silicate cement takes up rather than releases fluoride from solution if it is present in sufficient concentration (Kuhn, Lesan Setchell, 1983). 

Brauer, Stansbury Flowers (1986) modified these cements in several ways. The addition of various adds - acetic, propionic, benzoic etc. -accelerated the set. The use of zinc oxide powders coated with propionic add improved mixing, accelerated set, reduced brittleness and increased compressive strength from 63 to a maximum of 72 MPa. The addition of plasticizing agents such as zinc undecenylate yielded flexible materials. Incorporation of metal powders had a deleterious effect and greatly increased the brittleness of these cements. The addition of fluorides was not very successful, for fluoride release was not sustained. 

L. Forsten, Fluoride release and uptake by glass ionomerand related materials and its clinical effect. Biomaterials 19 (1998) 503-508. 

A.M.J.C. de Witte, E.A.P. De Maeyer, R.M.H. Verbeeck, L.C. Martens, Fluoride release profiles of mature restorative glass ionomer cements after fluoride application, Biomaterials 21 (2000) 475-482. 

Not surprisingly, the use of acidified water increased the level of fluoride release from the glass, and this effectively models what happens in a setting cement. The acid-base reaction between the glass and the water-soluble polymeric acid liberates fluoride from the glass, causing it to move to the matrix, from where it is gradually leached as the cement releases fluoride [227,228]. 

As mentioned, fluoride release from set cements follows a distinctive pattern. First, there is a rapid burst of ion release that is non-linear with respect to time. This has sometimes been referred to as early wash-out , but actually is not confined to particularly early periods in a cement s life, as it has been shown to be discernible after 28 days [224]. 

Later, there is a period of sustained release, typically showing release proportional to indicating that the process of fluoride release is diffusion controlled [228]. Alternatively, in some studies, release has been shown to be proportional to t, which means that for those cements, release is dissolution controlled [232]. This sustained release phase is the one that prevails over long periods of time. For example, Forsten [225] carried out a study in which cement specimens were exposed to running tap water for 5 years, with short periods in static water to allow samples to be collected for analysis, after which they were returned to the running tap water. The data he obtained have been analysed by Billington et al. [233], who showed that the pattern of fluoride release right up to the end of the experiment (i.e., 5 years) continued to show a linear relationship with Fluoride release is thus diffusion controlled for a considerable period of time. 

Glass-ionomer cements have been shown to be dynamic materials and, following the suggestion of Walls [238], to be capable of taking up fluoride from surrounding media. There have been several reports that fluoride release is enhanced following prior exposure to fluoride solution [239-243]. This effect has been demonstrated both in vitro and in vivo, and has been claimed to be evidence of fluoride uptake and re-release. 

Fluoride release is most frequently determined using an ion-selective electrode. Because such electrodes are incapable of detecting complexed fluoride, a decomplexing agent is generally added to the mixture prior to analysis. This frees up fluoride from most complexes as the F ion, and the total quantity of fluoride released can then be determined by the ion-selective electrode. The usual complexing agent is TISAB (total ionic solubility acid buffer) [250]. 

This approach may not account for all the fluoride released by glass-ionomers [252], A recent study has used two methods of decomplexation of fluoride, using the same solutions for both methods, by dividing a given storage volume into two and treating each aliquot differently. One aliquot was diluted with an equal volume of TISAB, as is usual in the determination of fluoride by ion-selective electrode. The other solution was treated with a small volume of 4 M hydrochloric acid, allowed to stand for 3 h, then neutralised with an equal amount of 4 M sodium hydroxide. A volume of TISAB equal to the initial volume of the aliquot was added. This technique is known to liberate fluoride from monofluorophosphate as well as from aluminofluoride complexes [253],... 

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