Fig. 25.5 Formation of a resin-modified polyacid chain by esterification of some acid groups with HEMA.

(4)Water, which is an essential component required to allow ionization of the acid component so that the acid–base reaction can occur.

Other minor components include polymerisation activators and stabilizers. Some products are simply formulated by blending together polyacids and dimethacrylates in aqueous solution. Most products, however, contain specially formulated resins in which both methacrylate and acid groups are present as active groups on a single polymer chain. This can be achieved, for example, as illustrated in Fig. 25.5, where some of the acid groups on the polyacrylic acid molecule are replaced with methacrylate groups by carrying out an esterification with HEMA.

When the powder and liquid components of the cement are mixed together the acid–base reaction can begin immediately as the acid groups are able to react with the glass in the presence of water. Polymerisation takes over as the primary setting reaction as soon as sufficient free radicals become available to initiate the reaction. For lightactivated products this corresponds to the time at which light irradiation occurs. The light source used to activate polymerisation is of the same type as that used for light activated composites (see Chapter 23). Polymerisation involves the pendant methacrylate groups of the modified resin (as shown in Fig. 25.5) along with any free HEMA

present in the liquid components. Most resinmodified glass ionomers contain activators which enable polymerisation to occur both in the presence or absence of the activating radiation. Where the chemically activated polymerisation plays an important part, the presence of three setting mechanisms may be claimed by the manufacturer:

(1)Acid-base setting;

(2)Light-activated polymerisation;

(3)Chemically-activated polymerisation.

The latter is sometimes described as a dark cure, but in essence it is similar to the chemically activated setting achieved with conventional twocomponent composites. In one product, mixing releases a previously microencapsulated persulphate/ascorbic acid redox catalyst system which activates polymerisation in the absence of light.

The glass components of both modified composites and resin-modified glass ionomers have been modified in most materials in order to incorporate a heavy metal (e.g. strontium) which imparts radiopacity.

25.3 Setting characteristics

Resin-modified glass-ionomers

The setting characteristics of these materials are controlled by the two reactions which occur, often simultaneously. The acid-base reaction occurs comparatively slowly whilst the free-radical polymerisation reaction occurs very rapidly and becomes the predominant mode of setting, particularly for light activated products.

Although the resin-modified glass ionomers used for restorative applications are all light activated many do not possess the long working times associated with light-activated composites. There are three possible explanations for this behaviour. First, the acid–base reaction proceeds immediately after mixing and before exposure to activating light. Second, many materials contain chemical activators and initiators which enable a chemicallyactivated polymerisation to proceed before exposure to activating light. Third, some materials are very sensitive to ambient visible light. Sensitivity to ambient light is tested as part of the ISO test for these materials (ISO 9917-2). The sensitive nature of the materials is identified by the fact that the material is required to remain homogenous for

only 30 seconds when exposed to ambient light equivalent to that which would be found in the dental surgery.

In order to achieve acceptable results with these products it is best to carry out placement and shaping as soon as possible after mixing. Many encapsulated materials are provided with extrusion nozzles which allow the mixed material to be extruded directly from the capsule into the prepared cavity. The modified composite products have setting characteristics similar to those for light-activated composites. In Chapter 22 the ‘limited depth of cure’ for light-activated composites was listed as one of the potential disadvantages of those materials. The resin-modified glass ionomers exhibit a similar behaviour except that unexposed material may set through an acid–base reaction or through a chemically-activated polymerisation. Nevertheless, only the material properly activated by light will be optimally cured. The concept of ‘depth of cure’ can therefore be applied to these materials. The modified composites exhibit a similar behaviour to other composites. The setting reaction in all cases is highly exothermic and a temperature rise of 10ºC above ambient can be readily produced in a medium sized cavity. An important feature of conventional glass ionomer products is their sensitivity to moisture contamination during and shortly after setting. Although the resin-modified glass ionomers contain water and set partly through an acid–base reaction, the presence of resin appears to protect the cement from the effects of contamination by moisture. Hence, protection with varnish or a resin is not advocated for most materials within this category. Another significant factor is that whereas finishing of conventional glass ionomers is best delayed for 24 hours to allow maturation of the cement, the light-activated resin-modified materials can be finished immediately after curing.

Modified composites and giomers

The setting characteristics of these materials are almost identical to those of composites (see Section 22.4). Although both products contain glass polyalkenoates in varying proportion, the acid-base reaction involved in its formation does not occur during the initial intra-oral setting process. For acid-modified composites the acid-base reaction is limited in extent and is delayed until after the initial setting and until the set material begins to

absorb water. For giomers, the acid-base reaction occurs prior to blending the glass-polyalkenoate complex with resin.

25.4 Dimensional change and dimensional stability

All categories of materials discussed in this chapter undergo a significant shrinkage on setting. The major component of the shrinkage is due to the polymerisation of methacrylate groups in the resin component. Acid–base setting makes a minor contribution to the observed shrinkage. The magnitude of the shrinkage is similar to that observed for resin–matrix composites (Chapter 22) and the same arguments about shrinkage stress, cavity size and configuration factor apply.

The resin-modified glass ionomers undergo a rapid and marked expansion when placed in water. This more than compensates for the shrinkage and can result in the material protruding from the cavity and exerting a positive pressure against the cavity walls. This observation can be explained by a value of water absorption after 7 days of 100–250 μg mm−3 compared with a maximum of 40 μg mm−3 allowed for resin-based materials in ISO 4049. The large value of water absorption for the resin-modified materials is due to the use of the hydrophilic monomer, HEMA, as a primary constituent in these products. The clinical significance of the observed swelling is yet to be fully explained. Swelling does not occur to the same extent with modified composites as these products generally do not contain HEMA. The full reaction type giomers contain HEMA and they have a very high value of water absorption.

The effect of the radial pressure generated by water absorption of these products is likely to depend upon the stiffness of the set material and the cavity configuration (see Section 22.5). It is conceivable that the expansion and radial pressure generated within a low-stiffness material may have a beneficial effect in helping to seal the cavity margins. In high stiffness materials water absorption is likely to produce high radial pressures which could lead to excessive pressures on cavity walls. The effect will be magnified by a high cavity configuration factor (C-factor) and would be particularly concerning for luting applications where cements are constrained as thin films with very high configuration factors (see Chapter 30).

25.5 Mechanical properties

The flexural strength values for a range of resinmodified glass ionomers and modified composites are given in Table 25.3 along with the minimum values required in the ISO Standards. It is clear that whereas some of the weaker resin-modified glass ionomers are only marginally stronger than the conventional glass ionomer products (Chapter 24), some of the single component acid-modified composites have a strength value which clearly satisfies the requirement of ISO 4949 for resin-based materials. The latter products compare favourably with composites (Table 22.3). The values given in Table 25.3 support the hypothesis that the materials have a structure/property characteristic which would place them somewhere on a continuum which connects the purely acid–base reaction materials at one extreme and the purely resin–matrix products at the other. In general terms, incorporating increasing amounts of resin in a glass-ionomer system does seem to have the effect of strengthening and toughening these otherwise brittle materials. There is some evidence that the resin-modified glass ionomers become weaker after a period of water storage. This may be expected in light of the large water absorption values for these materials. The modified composites appear to be relatively unaffected by water storage.

25.6 Adhesive characteristics

All materials within the group of resin-modified glass-ionomers and related products contain polyacids of various types and at varying concentrations. All materials therefore have a potential to interact with the tooth surface to produce bonding

as for glass ionomer cements (see Section 24.4). However, the magnitude and efficacy of the bond is often limited by one of the following:

(1)lack of sufficient concentration of free acid groups to form an effective bond

(2)lack of a sufficiently ionic character to enable bonding to occur

(3)lack of mobility of the active acidic species to enable interaction with the tooth surface.

Hence many materials, despite having the potential for bonding require the use of intermediary agents or primers in order to achieve effective bonding.

Resin-modified glass ionomers

Some of the products within the resin-modified glass ionomer groups give an inherent adhesion to enamel and dentine by a mechanism which is similar to that for conventional glass ionomers (Chapter 24). Free polyacid groups are thought to interact with the mineral of the tooth surface. When no conditioning or etching is used shear bond strengths of 4–10 MPa to both enamel and dentine are claimed for some products. Improvements in bond strength are obtained when the enamel and/or dentine are etched or conditioned and most manufacturers advocate the use of a conditioner such as aqueous polyacrylic acid in order to remove the dentine smear layer. Other manufacturers advocate conditioning and priming of dentine and enamel prior to placement of the restorative material. The primer may be a solution of the methacrylated polyacid (one of the major ingredients of the liquid component of the cement),

which is able to partly demineralize the surface of the hard tissue and leave a surface which is more readily wetted by the restorative. The use of conditioning and priming can increase the shear bond strength into the range 8–24 MPa.

Modified composites and Giomers

It is clear from the previous discussion that some materials must be treated in a similar way to composites in order to achieve an effective bond, i.e. special treatments of the enamel and dentine are required in order to produce effective bonding. To achieve adhesion with the modified composite products they must essentially be treated as composites and bonded using adhesive bonding agents similar to those described in Section 23.8.

Likewise, with giomers there are insufficient quantities of accessible acid groups to enable effective bonding to occur. Dentine and enamel bonding systems similar to those used for composites must be used.

25.7 Fluoride release

Fluoride release has been considered a major advantage of the conventional glass ionomers. Various studies have demonstrated reduced levels of demineralization and caries around materials which are able to release fluoride ions in vitro. The pattern of fluoride release for conventional glass ionomers indicates a high initial release rate followed by a rapid reduction in the rate of release over time (Fig. 25.6). The relative importance of the ‘initial burst’ of greater levels of fluoride and the sustained release at a lower level over an extended period has not been fully explored. Furthermore the levels of fluoride release required to produce a therapeutic effect are not known and in the absence of such information it is often assumed that greater levels of release are better, whereas this may not be the case. One factor which may be significant is that fluoride release from conventional glass ionomers is reversible and when these materials are bathed in an environment containing a high fluoride concentration (e.g. a topically applied fluoride gel) they are capable of taking up fluoride. Following such treatments the fluoride release rate of these cements can be significantly increased as they are able to slowly re-release the freshly absorbed fluoride. It is, therefore, said that conventional glass iono-

Fig. 25.6 Daily (24 hour) fluoride release over time for typical glass ionomers (solid-line), resin-modified glass ionomers (dashed line) and modified composites (compomer, dotted line).

mers can act like a fluoride ‘sink’ which is capable of releasing fluoride but which can be ‘topped up’ at regular intervals.

Figure 25.6 shows daily (24 hours) fluoride release of small disc specimens of cements (10 mm diameter by 1.5 mm thick) into 5 ml of water as a function of time. The storage water is refreshed daily throughout the test. The results show that typically the resin-modified glass ionomers behave in a similar fashion to the conventional glass ionomers in terms of both the pattern of release and the daily amount of fluoride released. It is emphasized that results are for ‘typical’ materials and some considerable variation is seen between commercial materials. Key factors in the rate of fluoride release from resin-modified materials are the extent to which the acid–base reaction occurs during setting and the presence of HEMA, which results in the formation of a polymeric hydrogel through which water can diffuse quite rapidly. The fluoride release behaviour of the modified composite products is somewhat different to that of the other materials (Fig. 25.6). Hence, there is no initial burst, but there is a low and sustained level of fluoride release which typically, after about 40 days, becomes equivalent to that from a conventional glass ionomer. The lower level of fluoride release from these products is a function of the limited extent to which any acid–base setting occurs combined with a lower rate of water diffusion through resins which do not contain the hydrophilic monomer HEMA.