Polyacid-modified composite resins fluoride release

R.M.H. Verbeeck, E.A.P. De Maeyer, L.A.M. Marks, R.G.J. De Moor, A.M.C.J. De Witte, L.M. Trimpeneers, Fluoride release process of (resin-modified) glass-ionomer cements versus (polyacid-modified) composite resins. Biomaterials 19 (1998) 509-519. 

G. Eliades, A. Kakaboura, G. Palaghias, Acid base reaction and fluoride release profiles in visible light-cured polyacid modified composite resin restorations. Dent. Mater. 14 (1998) 57-63. 

Polyacid-modified composite resins were developed in an attempt to make a composite resin with the sort of ion-release capability of glass-ionomer cements, especially of fluoride [38]. They are similar to conventional composites in that they are mainly based on the hydrophobic monomers bis-GMA or urethane dimethaaylate, and their setting is typically initiated by light. In addition, they contain inert fillers of appropriate particle size. 

Polyacid-modified composite resins have undergone considerable development since they first appeared. The very limited nature of the acid-base reaction means that they have had to have the fluoride-releasing capability augmented, for example, through the inclusion of extra ytterbium fluoride in the formulation [38]. There has also been concern that the abihty to draw in water from the environment might also lead to staining and softening, and reformulation has partly been driven by the need to minimize any such moisture uptake, so as to preserve the physical properties of the composite. 

As mentioned, these materials seem to have found particular application in children s dentistry. The successive reformulations mean that they may have lost their original distinctive characteristic of having a small amount of acid-base reaction following post-cure moisture uptake. There is evidence that modem polyacid-modified composite resins primarily release fluoride as a result of the additional fluoride compound, as with fluoridated conventional composites, and that any acid-base reaction is so slight that it has little, if any, effect on the properties of the material. Overall, these materials do not duphcate the properties of either of the parent materials particularly well, and their current use in clinical dentistry is fairly limited [1]. 

The amount of fluoride released by composites tends to be much lower than that released by either conventional or resin-modified glass-ionomer. It is also lower than the level released by polyacid-modified composite resins. The reason for this is not... 

One of the properties of glass-ionomer cements that polyacid-modified composite resins are designed to possess is the ability to release fluoride. The reactive glass filler is an ionomer-type glass, and as such contains fluoride. This becomes available for release following its incorporation into the polysalt phase as a result of the moisture driven acid-base reaction with the acid-functional monomer component [1]. 

The early brands of polyacid-modified composite resin showed very low values of fluoride release [23,35], so to augment this, additional fluoride species have been incorporated into the re-formnlated materials. These include strontium fluoride and ytterbium fluoride [1,25]. 

As with glass-ionomer cements, flnoride release from polyacid-modified composite resins is snstained for long periods of time [23] and is enhanced by placing the polyacid-modified composite resin in acidic storage media [25,36]. This property has been snggested to be beneficial in the case of resin-modified glass-ionomers [37], since it wonld lead to enhanced release of protective fluoride ion under the very conditions that promote dental caries. A similar argnment can be advanced for polyacid-modified composite resins, and it may be that this ability to release extra fluoride under conditions of low pH is beneficial clinically. 

Like glass-ionomers, polyacid-modified composites have been shown to be capable of taking up fluoride from the surrounding medium where additional fluoride is included [25]. Several brands were studied, and recharge capacity was found to vary between the brands. Polyacid-modified composite resins that showed a high inherent fluoride release were found to have a greater recharge capacity than those with only a low inherent release [25]. 

There is debate about whether the relatively low level of fluoride is effective in preventing caries. It has been shown to be effective in vitro [41], but in the most detailed study, the conclusion was that it was not, and that there was no advantage in using polyacid-modified composite resins over amalgam in terms of any observable anti-caries effect of the fluoride release [42],... 

The release of ions other than fluoride and the ability to buffer organic acids by shifting pH towards neutral is a property of glass-ionomer cements that has been known for some years [43]. Similar properties have been observed in polyacid-modified composite resins. 

Storage under both neutral and acidic conditions causes ions such as Ca +, Al +, Na+ and to be released into solution [36,40]. Like fluoride, greater amounts of all of these species are released under acidic conditions [40]. For polyacid-modified composite resins, too, storage in acidic solutions, such as lactic acid, is associated with substantial shifts in pH towards neutral [43], showing that the capability of buffering storage media is conferred by the acid-base component of the polyacid-modified composite resin. 

Polyacid-modified composite resins are favoured by many clinicians over conventional composites to repair primary teeth [50]. This is on account of their fluoride release [1,50]. To emphasize their application in primary teeth, certain brands have been specifically produced for this purpose and are highly coloured. For example, in America, there is a dual-cure compomer called MagicFil (Zenith Dental, Englewood, New Jersey) which is produced in four colours (pink, green, blue and yellow) with gutter inclusions, and a similar material, Twinky Star (Voco, Germany) is available in Europe [51]. 

This means that polyacid-modified composites are essentially composite resins. As such, they must be bonded to the tooth with appropriate bonding agents, applied in increments, and show no ion-exchange properties, though they will release fluoride [38]. Similarly, resin-modified glass-ionomers are very similar to conventional glass-ionomers. They show inherent adhesion to the tooth [30], long-term fluoride release [31] and ion-release under neutral and acidic conditions [59]. 

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