**3.1 Composition and setting**

As has already been stated, compomers resemble traditional composite resins in that their setting reaction is an addition polymerization. It is usually light-initiated, and the initiator is camphorquinone with amine accelerator, and as such is sensitive to blue light at 470 nm [40]. There is, however, at least one brand, designed for use as luting cement, Dyract Cem, that is a two-paste system. Cure is brought about as a result of mixing the two pastes, each of which contains a component of the free radical initiator system. The set material, though, does not differ in any fundamental way from those compomers that cure photochemically.

A key feature of compomers is that they contain no water and the majority of components are the same as for composite resins. Typically these are bulky macro-monomers, such as bisglycidyl ether dimethacrylate (bisGMA) or its derivatives and/or urethane dimethacrylate, which are blended with viscosity-reducing diluents, such as triethylene

Filling Materials for the Caries 343

former band arises from the presence of carboxylic acid groups within the material, and gradually reduces in intensity on exposure to water. By contrast the latter band arises from the presence of carboxylate salts, and shows a corresponding increase in intensity with time. Neutralization has been shown to be controlled by rate of water diffusion and is therefore fairly slow. Although compomers are designed to take up water in order to promote a later neutralization, these processes have been shown to have an adverse effect on many of their

Compomers are designed to release fluoride in clinically beneficial amounts. Fluoride is present in the reactive glass filler, and becomes available for release following reaction of this glass with the acid functional groups, triggered by moisture uptake. In addition, commercial compomers contain fluoride compounds such as strontium fluoride or ytterbium fluoride, which are capable of releasing free fluoride ion under clinical conditions, and augment the relatively low level of release that occurs from the polysalt species that develops. Fluoride release occurs to enhanced extents in acidic conditions, and in lactate

The conventional way of determining fluoride release is to employ an ion-selective electrode, and to treat the sample solution with an equal volume of the decomplexing TISAB (total ionic strength adjustment buffer). This liberates fluoride from any potential

The authors speculated that the complexation was caused by the elevated levels of aluminium released under acidic conditions. As an example, for Compoglass F, aluminium concentration rose from 4.68 ppm in water to 104ppm in lactic acid solution. Aluminium is known to form complexes of the type AlF2+ and AlF2+ [2] and these have been widely assumed to occur, for example in glass-ionomer cements. However, an alternative suggestion has been made by Billington et al. [8], who have suggested that complexation as monofluorophosphate is also a possibility, and they note that phosphorus levels released by glass-ionomer cements are also typically elevated under acidic conditions. This is also possible for compomers, as their glass filler components are similar to those used in glassionomers, and they, too, show elevated phosphorus release under acidic conditions [25, 50].

Right from the time they were first launched, compomers have shown acceptable clinical performance in a variety of clinical applications. However, wear characteristics of early materials were poor and there were concerns about their durability. Despite this, the early results were promising, and more recently, results with newer formulations have also been

Compomers are designed for the same sort of clinical applications as conventional composites. These include Class II and Class V cavities, as fissure sealants, and as bonding agents for the retention of orthodontic bands. Their fluoride release, however, is seen as a useful feature for use in paedodontics, and certain brands have been produced that are

complexes, and enables to full amount of fluoride to be determined [50].

mechanical properties [50].

**3.4 Clinical performance** 

specifically aimed at children [25, 50].

good.

buffer has been shown to be diffusion-based [64].

**3.3 Fluoride release** 

glycol dimethacrylate (TEGDMA). These polymer systems are filled with non-reactive inorganic powders, such as quartz or a silicate glass, for example SrAlFSiO4. These powders are coated with a silane to promote bonding between the filler and the matrix in the set material. In addition, compomers contain additional monomers that differ from those in conventional composites, which contain acidic functional groups. The most widely used monomer of this type is so-called TCB, which is a di-ester of 2-hydroxyethyl methacrylate with butane tetracarboxylic acid [23]. This acid-functional monomer is very much a minor component and compomers also contain some reactive glass powder of the type used in glass-ionomer cements [25, 50].

Despite the presence of these additional components, compomers are similar to composite resins in that they are fundamentally hydrophobic, though less so than conventional composite resins. They set by a polymerization reaction, and only once set do the minority hydrophilic constituents draw in a limited amount of water to promote a secondary neutralization reaction [23]. They lack the ability to bond to tooth tissues, so require bespoke bonding agents of the type used with conventional composite resins, and their fluoride release levels are significantly lower than those of glass-ionomer cements. Such low levels of fluoride release have been shown to compromise the degree of protection afforded by these materials in *in vitro* experiments using an artificial caries medium [41].

#### **3.2 Effect of water uptake**

A distinctive feature of compomers is that, following the initial polymerization reaction, they take up small amounts of moisture *in situ*, and this triggers an acid–base reaction between the reactive glass filler and the acid groups of the functional monomer [51, 60]. Among other features, this process causes fluoride to be released from the glass filler to the matrix, from where it can readily be released into the mouth, and act as an anticariogenic agent [41]. Polymerization is associated with contraction and the development of measurable stresses, and it may be that the sorption of water plays some part in reducing these stresses *in vivo* [25, 50].

The role of the reactive glass in the water uptake process has been considered in one report [1]. A conventional composite resin formulation was used as the matrix phase, with filler being either an unreactive glass, Raysorb T-4000, or the ionomer glass G338, whose composition and properties have been described extensively in the literature. In each case, the glass was used both with and without a coating of silane coupling agent (ϒmethacryloxy propyl trimethoxysilane). The results show that silanation reduced the water uptakefor both of the glasses and also improved the strength. However, incorporating G338 rather than Raysorb T-4000 gave an inferior material since it took up more water and was of lower strength. Previous studies of water sorption by composite resins have shown that the water accumulates around the filler particles [69], so that one conclusion of the study is that G338 is more hydrophilic than Raysorb T-4000. This suggests that it provides part of the driving force for water uptake by compomers, and also that it is responsible for a decline in their overall mechanical properties relative to conventional composite resins.

Compomers are designed to absorb water, and are able to up of the order of 2–3.5% by mass of water on soaking. This water uptake has been shown to be accompanied by neutralization of the carboxylic acid groups, as shown by changes in bands at 1705 and 1555 cm−1. The

glycol dimethacrylate (TEGDMA). These polymer systems are filled with non-reactive inorganic powders, such as quartz or a silicate glass, for example SrAlFSiO4. These powders are coated with a silane to promote bonding between the filler and the matrix in the set material. In addition, compomers contain additional monomers that differ from those in conventional composites, which contain acidic functional groups. The most widely used monomer of this type is so-called TCB, which is a di-ester of 2-hydroxyethyl methacrylate with butane tetracarboxylic acid [23]. This acid-functional monomer is very much a minor component and compomers also contain some reactive glass powder of the type used in

Despite the presence of these additional components, compomers are similar to composite resins in that they are fundamentally hydrophobic, though less so than conventional composite resins. They set by a polymerization reaction, and only once set do the minority hydrophilic constituents draw in a limited amount of water to promote a secondary neutralization reaction [23]. They lack the ability to bond to tooth tissues, so require bespoke bonding agents of the type used with conventional composite resins, and their fluoride release levels are significantly lower than those of glass-ionomer cements. Such low levels of fluoride release have been shown to compromise the degree of protection afforded by these

A distinctive feature of compomers is that, following the initial polymerization reaction, they take up small amounts of moisture *in situ*, and this triggers an acid–base reaction between the reactive glass filler and the acid groups of the functional monomer [51, 60]. Among other features, this process causes fluoride to be released from the glass filler to the matrix, from where it can readily be released into the mouth, and act as an anticariogenic agent [41]. Polymerization is associated with contraction and the development of measurable stresses, and it may be that the sorption of water plays some part in reducing

The role of the reactive glass in the water uptake process has been considered in one report [1]. A conventional composite resin formulation was used as the matrix phase, with filler being either an unreactive glass, Raysorb T-4000, or the ionomer glass G338, whose composition and properties have been described extensively in the literature. In each case, the glass was used both with and without a coating of silane coupling agent (ϒmethacryloxy propyl trimethoxysilane). The results show that silanation reduced the water uptakefor both of the glasses and also improved the strength. However, incorporating G338 rather than Raysorb T-4000 gave an inferior material since it took up more water and was of lower strength. Previous studies of water sorption by composite resins have shown that the water accumulates around the filler particles [69], so that one conclusion of the study is that G338 is more hydrophilic than Raysorb T-4000. This suggests that it provides part of the driving force for water uptake by compomers, and also that it is responsible for a decline in

Compomers are designed to absorb water, and are able to up of the order of 2–3.5% by mass of water on soaking. This water uptake has been shown to be accompanied by neutralization of the carboxylic acid groups, as shown by changes in bands at 1705 and 1555 cm−1. The

their overall mechanical properties relative to conventional composite resins.

materials in *in vitro* experiments using an artificial caries medium [41].

glass-ionomer cements [25, 50].

**3.2 Effect of water uptake** 

these stresses *in vivo* [25, 50].

former band arises from the presence of carboxylic acid groups within the material, and gradually reduces in intensity on exposure to water. By contrast the latter band arises from the presence of carboxylate salts, and shows a corresponding increase in intensity with time. Neutralization has been shown to be controlled by rate of water diffusion and is therefore fairly slow. Although compomers are designed to take up water in order to promote a later neutralization, these processes have been shown to have an adverse effect on many of their mechanical properties [50].

#### **3.3 Fluoride release**

Compomers are designed to release fluoride in clinically beneficial amounts. Fluoride is present in the reactive glass filler, and becomes available for release following reaction of this glass with the acid functional groups, triggered by moisture uptake. In addition, commercial compomers contain fluoride compounds such as strontium fluoride or ytterbium fluoride, which are capable of releasing free fluoride ion under clinical conditions, and augment the relatively low level of release that occurs from the polysalt species that develops. Fluoride release occurs to enhanced extents in acidic conditions, and in lactate buffer has been shown to be diffusion-based [64].

The conventional way of determining fluoride release is to employ an ion-selective electrode, and to treat the sample solution with an equal volume of the decomplexing TISAB (total ionic strength adjustment buffer). This liberates fluoride from any potential complexes, and enables to full amount of fluoride to be determined [50].

The authors speculated that the complexation was caused by the elevated levels of aluminium released under acidic conditions. As an example, for Compoglass F, aluminium concentration rose from 4.68 ppm in water to 104ppm in lactic acid solution. Aluminium is known to form complexes of the type AlF2+ and AlF2+ [2] and these have been widely assumed to occur, for example in glass-ionomer cements. However, an alternative suggestion has been made by Billington et al. [8], who have suggested that complexation as monofluorophosphate is also a possibility, and they note that phosphorus levels released by glass-ionomer cements are also typically elevated under acidic conditions. This is also possible for compomers, as their glass filler components are similar to those used in glassionomers, and they, too, show elevated phosphorus release under acidic conditions [25, 50].

#### **3.4 Clinical performance**

Right from the time they were first launched, compomers have shown acceptable clinical performance in a variety of clinical applications. However, wear characteristics of early materials were poor and there were concerns about their durability. Despite this, the early results were promising, and more recently, results with newer formulations have also been good.

Compomers are designed for the same sort of clinical applications as conventional composites. These include Class II and Class V cavities, as fissure sealants, and as bonding agents for the retention of orthodontic bands. Their fluoride release, however, is seen as a useful feature for use in paedodontics, and certain brands have been produced that are specifically aimed at children [25, 50].

Filling Materials for the Caries 345

that were more water tolerant as well as methods of treating the dentine with oxalates to gain adhesion. The concern of many clinicians at that time was the potential damage phosphoric acid was going to cause the dental pulp if dentine was etched [79]. The first work to investigate the mechanism of bonding to the dentine was by Nakabayashi [47]. His paper of 1982 has now become one of the classic papers to first identify a layer between the resin and dentine substrate referred to as 'hybrid' dentine, in that it was the organic components of the dentine that had been permeated by resin (Fig. 2). The term 'hybrid layer' has now become synonymous with bonding of resins to etched dentine. There has been a tremendous amount of research done on the hybrid layer, its structure, formation and how it can be improved. Without a hybrid layer a bond will not be formed to the dentine. Therefore, it is essential for some modification to be made to the dentine surface so a mechanical interlocking of resin around dentinal collagen can occur. This layer has also been

Fig. 2. Bonded specimen in which the dentine (mineral and protein) has been removed. The infiltration of resin into the acid-etched dentine can be seen with an associated permeation

Dentine bonding agents have gone through many changes over the last 10 years. This has led some people to refer to the changes as 'generations' of material, implying that there has been some chronological development. This is a falsehood — for example, the first 'selfetching' type material was introduced by Coltène (Altstätatten, Switzerland) as 'ART Bond'. Therefore, it is more logical to classify materials by the number of steps needed to complete

This group represents those materials that have separate etching, priming and adhesive steps. It just so happens that this group of materials is also the oldest. However, they are still widely used and have been shown to provide reliable bonding. The greatest problem with this group would seem to be that three distinct steps are needed, which gives rise to possible

of resin throughout the dentine tubular network and its lateral branches.

**4.1 Classification** 

the bonding process.

**4.1.1 'Three-step' or 'Conventional' systems** 

referred to as the 'resin-dentine interdiffusion zone' [79].

Compomers have been widely used in Class V restorations. For example, the compomers Dyract AP, Compoglass F and F2000 were evaluated for use in this application over a 2-year period [37]. This study concluded that, after this time, all three materials showed an acceptable level of clinical performance.

Colour stability has been found to be somewhat of a problem with compomers in a few studies. This is not entirely surprising, given that they are designed to take up water, which is likely to alter appearance through a change in refractive index, and also to carry with it coloured chemical species (stains) from certain foodstuffs such as coffee and red wine. In a 3-year study of Class V restorations of Dyract, Demirci et al. [21] found that all Ryge criteria were good, except those relating to colour change, *i.e.* colour stability and marginal discoloration. In both of these there were significant changes [21].

Compomers have been used as fissure sealants [28], and a clinical study examined the teeth of children aged between 7 and 10 years sealed by the compomer Dyract Seal. Sealed teeth were examined post-operatively at 3, 6, 12 and 24 months, and were also evaluated by the Ryge criteria. In general Dyract Seal behaved as well as a conventional composite resin sealant, except on the criterion of marginal integrity, showing that this material was acceptable for its clinical application, at least of the 24 months period of the study [28].

Compomers have also been used for Class I [56] and Class II restorations. In the Class I study, they were used in composite laminate restorations, and were shown to perform as well as conventional composite resins [56]. In the Class II study, they were studied over 7 years in children aged between 3.6 and 14.9 years. Again performance was indistinguishable from that of conventional composite resins.

Lastly, compomers have been employed as cements for orthodontic bands and there have been a number of full studies of compomers in this application [34]. Results have been generally extremely good for compomers, except in the realms of taste, as determined by the patient, and in which compomers scored less well than glass-ionomers. Thus, compomers have been shown to have acceptable performance as materials for use in orthodontic band retention, though the final choice of cementing agent could be left to patients. If they found the taste of the compomer particularly objectionable, a resin-modified glass-ionomer could be used equally effectively instead [50].

Overall, the major conclusion from these clinical results is that compomers perform well, and are suited to their suggested uses in dental restoration. The reduction in strength due to water uptake does not seem to be important clinically and these materials are suited to use in vivo.
