Fluoride release composite resins

J. Arends, J. Ruben, A.G. Dijkman, The effect of fluoride release from a fluoride-containing composite resin on secondary caries An in vitro study, Quintessence Int. 21 (1990) 671-674. 

Composite Resins. Many composite restorative resins have incorporated fluoride into the filler particles. One commonly used material, yttrium trifluoride [13709-49-4] is incorporated as a radiopaque filler to aid in radiographic diagnosis, and is also responsible for slow release of fluoride from the composites (280). This same effect is achieved with a barium—alumina—fluoro-siUcate glass filler in composite filling and lining materials. Sodium fluoride [7681-49-4] has also been used in composites by incorporating it into the resin matrix material where it provides long-term low level release (281-283). 

Unlike glass-ionomers or compomers, composite resins are not inherently fluoride-releasing and they do not generally contain any fluoride compounds. However, they can be formulated with such compounds [281], for example NaF, YbFs or ion-leachable glass [201]. Organic fluorides can be used, too, such as methacryloyl fluoride-methyl methacrylate (MF-MMA) or tetrabutyl ammonium tetrafluoroborate. These latter substances impart the property of slow release of fluoride to the surrounding tissue without the creation of voids within the material. 

Fluoride release from composite resins differs from that in glass-ionomers (conventional or resin-modified) in two important respects. First, it tends to be a gradual, sustained process throughout the life time of the restoration... 

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. 

P. Karantakis, M. Helvatjoglou-Antoniades, S. Theodoridou-Pahini, Y. Papadogiannis, Fluoride release from three glass ionomers, a compomer, and a composite resin in water, artificial saliva, and lactic acid, Oper. Dent. 25 (2000) 20-25. 

P. Weidlich, L.M. Miranda, M. Maltz, S.M.W. Samuel, Fluoride release and uptake from glass ionomer cements and composite resins, Braz. Dent. J. 11 (2000) 89-96. 

Use of resin-modified glass-ionomers has grown considerably since their introduction in 1991, and versions are available that are suitable for use as full restorations [34]. However, because of limited penetration by light, deep cavities may need to be filled using the incremental build-up technique usually associated with composite resins. Resin-modified glass-ionomers show good adhesion to dentine [31] and also release useful amounts of fluoride [31,35]. 

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]. 

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]. 

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... 

Fluoride release from composite resins does not show the early burst that glass-ionomers display and also there appears to be no ability for fluoride to recharge in most systems. However, recharge has been demonstrated for an experimental composite system [148]. On the other hand, fluoride release has been shown to continue over long periods of time, at least a year having been reported in a number of studies [149,150],... 

The kinetics of fluoride release from composite resins vary widely, and are straightforward than those of glass-ionomers. Various types of behaviour have been observed, including fluoride release being directly proportional to time and fluoride release being approximately proportional to either the logarithm of time or to the inverse square root of time, [150,151]. The clinical significance of these differences is not clear. 

B.J. Cohen, A.S. Deutsch, B.L. Musiant, Fluoride release from four reinforced composite resins a one year study. Oral Health 85 (1995) 7-8. 

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]. 

Since this time, there have been numerous studies to compare the clinical effectiveness of glass-ionomer cements with that of composite resin sealants. These have typically determined the relative retention rates, and they have usually found that glass-ionomer sealants show inferior performance [149]. However, when caries rate in teeth are compared it turns out that glass-ionomers are at least as effective as composite resins [150,151]. It has been suggested that this is due to retention of glass-ionomer cement deep within the fissure and also because of the fluoride release into the enamel prior to the loss of the bulk cement [1]. 

Resin-modified glass-ionomer cements have been used for both Class II and Class III restorations in primary teeth [97,98]. Their lower brittleness compared with conventional glass-ionomers make them appropriate for these applications, though composite resin again appears preferred for these types of cavity in permanent teeth [94], Various clinical studies have shown resin-modified glass-ionomers to perform well in these restorations [99,100], and in addition they have been shown to have useful caries inhibition properties as a result of their fluoride release [101]. 

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