Polyacid-modified resin composites

Compomers are properly called polyacid-modified composite resins and are a group of aesthetic materials chemically similar to the well-established composite resins [266], They were introduced to the dental profession in the early 1990s [267], and were intended to combine the benefits of traditional composite resins and glass-ionomer cements, and their trivial name reflects this, being derived from the names of these two parent materials, the comp coming from composite, and omer from ionomer [268], These materials are now considered a distinct class of dental restorative, with well established uses in clinical restoration, particularly in children s dentistry [269],... 

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.W. Nicholson, Polyacid modified composite resins ( compomers ) and their use in clinical dentistry. Dent. Mater. 23 (2006) 615-622. 

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. 

The essential features of the two basic types of restorative material are given in Table 2.1. From this, it can be seen that each type has its own advantages and disadvantages. In terms of overall properties, modem composite resins appear to be favoured, and there is evidence that these materials are the ones used in the majority of aesthetic repairs in dentistry, particularly in adults. However, as the development of the polyacid-modified composite resins (compomers) shows, these materials are far from perfect, and there is unquestionably scope to enhance their properties. Glass-ionomer cements have properties that would seem to indicate the direction in which improvements could be made, despite the technical difficulties in doing so. 

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... 

G.O. Adusei, S. Deb, J.W. Nicholson, The role of the ionomer glass component in polyacid-modified composite resin dental restorative materials, J. Mater. Sci. Mater. Med. 15 (2004) 751-754. 

Polyacid-modified composite resins are a class of composite material used in dental repair [1], Like conventional composite materials, they consist of two distinct phases that differ in form and chemical composition and are mutually insoluble in each other. They are combined to form a mixture that has superior mechanical properties to those of the individual phases. 

One of the key features of polyacid-modified composite resins is their lack of adhesion to tooth tissnes [5]. This is a feature that they share with conventional dental composite resins, and the contrasts with the behaviour of the glass-ionomer cement. It is further evidence that these materials are essentially composite resins, and have very little of the anticipated hybrid character of composites and glass-ionomers. Bonding therefore reqnires the type of bespoke bonding agents used for conventional composite resins, together with the appropriate preparation of the freshly cut tooth surface [6]. 

Like conventional composites, the polymerization reaction in polyacid-modified composite resins is associated with a contraction in the overall volume of the material and a corresponding contraction stress [13], Experiments showed that the values... 

The volume fraction of filler and its effect on properties has been studied in experimental polyacid-modified composite resin systems [14]. As expected, the viscosity of the uncured paste was increased by the inclusion of filler, as were both the compressive and diametral strengths. However, these changes reached their limit at filler volumes of 20-30%, and above this range there were no significant differences in any of the properties. These findings are similar to those of conventional composite resin systems, and danonstrate that polyacid-modified composites have entirely conventional behaviour in this regard. 

As part of their formulation, polyacid-modified composite resins also contain a small fraction of basic glass filler of the type used in glass-ionomer cements [1]. Such glasses are typically calcium (or strontium) alumino-fluorosiUcates, and react with acids in the presence of water to release ions. The ions released, particularly calcium (or strontium) and aluminium react with the acid to form salts, which in the case of glass-ionomers, are insoluble because they form ionic crosslinks with the polymeric acid. The ions effectively insolubilize the acid-functional polymer chains by this reaction, as well as stiffening the material due to coil expansion and ion-binding. This type of chemistry is available to polyacid-modified composite resins once moisture is present, and these materials are designed for this reaction to occur in the early part of their existence. 

Moisture uptake by polyacid-modified composite resins... 

As already mentioned, the distinctive property of polyacid-modified composite resins is that, once the polymerization reaction has occurred, the set material is able to take up traces of moisture. This activates the acidic character of the carboxylic functional monomer and triggers an acid-base reaction with the glass [1,2]. The water uptake behaviour of these materials has been studied in detail [15], along with the corresponding water desorption processes. Three commercial polyacid-modified composite resins were used in a study of water uptake and loss, and cured samples were prepared as small discs of size 6 mm diameter x 2 mm thickness. Water uptake was allowed to take place in a controlled humidity environment at 93% relative humidity. Following the initial water uptake, there was an intervening desorption cycle in which specimens were stored in a dry atmosphere over concentrated sulfuric acid. 

In fact, studies of water uptake with a direct comparison of water uptake in com-pomers and conventional composite resins do not show particularly large differences between the two different types of composite material. For example, when the polyacid-modified composite resin brands Dyract and Compoglass were compared with the conventional composite resin Pekafill , there were only minor differences in equilibrium water uptake in both pure water and in 0.9% saline solution (Table 4.1) [18]. Pekafill showed lowest equilibrium water uptakes in both storage media, but only by a very small amount, and one that was not statistically significant in the case of pure water. 

Selection of the glass component of the filler blend in polyacid-modified composite resins appears straightforward. It needs to have a basic character, as in conventional glass-ionomers, and to be capable of reacting with the acid-functional monomer once sufficient water has been drawn into the set composite structure [21], However, there are also complications that need to be considered, from which it can be seen that the nature and incorporation of the ionomer glass component into polyacid-modified composite is not as straightforward as it appears at first sight. 

Table 4.5 Effect of Water on the properties of polyacid-modified composite resins [30]...

The study showed that water affected the properties of the conventional composite resin to slight but statistically significant extents, a finding which is consistent with them taking up small amounts of water under these conditions [31], Despite the possibility of the water uptake in polyacid-modified composite resins triggering the secondary add-base reaction, they showed similar behaviour to the conventional composites, with a reduction in both properties on taking up water. This was a greater problem for these materials, as they had lower values to start with. 

Re-formulated polyacid-modified composite resins have also been found to have properties that decline following water uptake and secondary acid-base reaction (Table 4.6). In one study that demonstrated this, Dyract AP was shown to have lower compressive and biaxial flexure strength values following soaking in water for 4 weeks than they did at 24 h [32] (Table 4.6). By contrast, when stored in dry conditions, strengths were not significantly different between 24h and 4 weeks. 

Table 4.6 Properties of the polyacid-modified composite resin Dyract AP stored in wet and dry conditions [32]...

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]. 

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