Resin-modified glass-ionomer cements fluoride release

M.F. Costa Cabral, R.L. de Menezes Martinho, M.V. Guedes-Neto, M.A.B. Rebelo, D.G. Pontes, F. Cohen-Cameiro, Do conventional glass ionomer cements release more fluoride than resin-modified glass ionomer cements Restor. Dent. Endod. 40 (2015) 209-215. 

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. 

J.A. Williams, R.W. Billington, G.J. Pearson, A long term study of fluoride release from metal containing conventional and resin modified glass ionomer cements, J. Oral Rehabil. 28 (2001) 41 7. 

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]. 

K.C. Kan, L.B. Messer, H.H. Messer, Variability in cytotoxicity and fluoride release of resin-modified glass-ionomer cements, J. Dent. Res. 76 (1997) 1502-1507. 

Resin-modified glass-ionomers, like their conventional counterparts, are capable of releasing fluoride [224,264,265], and in greater amounts under acid conditions than neutral ones [265], Release rates and release profiles have been shown to be comparable with those from conventional glass-ionomer cements [264,265], Other ions have also been shown to be released by these materials and, as for fluoride, these ions show a greater release under low pH conditions [265], However, the level of phosphorus released has been shown to be much lower from resin-modified glass-ionomers than from conventional ones [263], This suggests that there is little or no possibility of association of fluoride as monofluorophosphate, but rather that almost all of the fluoride is released either as the free fluoride ion or as alumino-fluoride complex ions. 

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. 

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. 

In this section, three aspects of the interaction of resin-modified glass-ionomers with water are dealt with, namely water uptake, fluoride release and other ion release and its associated buffering effect. Like conventional glass-ionomer cements, resin-modified glass-ionomers are deployed in an essentially wet environment and their behaviour in terms of each of these features differs more or less from that of conventional glass-ionomers, due to the presence of the polyHEMA phase. These are considered in turn. 

Ihe quest for interactive or bioactive dental restorative materials is not a totally new endeavor in dental materials. For example, as a general concept, glass ionomer cements (GICs) have been endorsed as a bioactive material because of their dynamic release of fluoride, as well as their unique mineral-based poly-salt matrix composition that is claimed to also contribute to the ability to remineralize calcium-depleted tooth structure. The continuous release of fluoride by GICs and resin-modified glass ionomers (RMGIs) has also been positioned as a potential mechanism to delay or inhibit secondary caries at teeth restored with these materials at the margins of the restorations [47,48]. 

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 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. 

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