Polyacid-modified composite resins conventional composites

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 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... 

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]. 

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. 

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. 

This ability to buffer external solutions in contact with the polyacid-modified composite resin may be a useful one under clinical conditions. The potential reduction in acidity of caries-generating acids, the main one being lactic acid, would be expected to reduce the rate of caries development in patients [44], and this would give them a useful clinical advantage over conventional composite resins. 

Polyacid-modified composite resins are aimed to be used in similar applications to conventional composites, such as Class II [44] and Class V cavities [45,46], and fissure sealants [47], They have also been used as bonding agents in orthodontics [48]. 

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]. 

N.S. Koupis, L.C. Martens, R.M.H. Verbeeck, Relative curing degree of polyacid-modified and conventional resin composites determined by surface Knoop hardness. Dent. Mater. 22 (2006) 1045-1050. 

A fundamental criticism of the resin-modified glass polyalkenoate cements is that, to some extent, they go against the philosophy of the glass polyalkenoate cement namely, that the freshly mixed material should contain no monomer. Monomers are toxic, and HEMA is no exception. This disadvantage of composite resins is avoided in the glass polyalkenoate cement as the polyacid is pre-polymerized during manufacture, but the same cannot be said of these new materials. For this reason they may lack the biocompatibility of conventional glass polyalkenoate cements. These materials also absorb excessive amounts of water because of the hydrophilic nature of polyHEMA (Nicholson, Anstice McLean, 1992). 

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... 

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. 

Table 4.1 Variation in equilibrium water content (%) of polyacid-modified and conventional composites resin stored at 37°C (specimens cured for 40 s)...

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. 

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