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Combining the technology and chemistry of GICs and composite resins resulted in development of a glass-ionomer/composite resin hybrid materiah^^'^^). There are a wide range of formulations possible for this hybrid which can produce materials encompassing those similar to composites with some incorporated ionomer-glass

particles to those similar to conventional GICs with some incorporated monomers. This has led to a diverse group of materials with widely different properties appearing on the market. These new materials have caused considerable confusion to dentists. Several terms have been used by the manufacturers to describe their products, for example, light-cured or light-curable, dual-cure, tri-cure or triple-cure, resin- reinforced, resin-ionomer, and resin-modified. Some of these terms {e.g. dual-cure, resin-ionomer) may give an impression to the dentists that the materials are in the same class as the 'true' glass-ionomer cement and the acid-base reaction contribute to the setting processes. Others {e.g. light-cured or light-curable) improperly implies that the acid-base reaction can be photo-initiated.

According to McLean et typical resin-modified glass-ionomer cements are glass- ionomer materials where the acid-base reaction occurring during the setting reaction is complemented by a polymerisation reaction, which may be chemically or light induced. These materials are capable of setting without polymerisation reaction, but the process is slower than the conventional GIC and an inferior material will be formed. Throughout this thesis, the term 'resin-modified glass-ionomer cement', abbreviated as

RMGIC, will be used.

2 .3 .2 C om p osition

RMGICs contain the basic ingredients derived from both conventional GICs and composite resins. These are an ion-leachable glass, poly(alkenoic acid)s, water, polymerisable monomers, fillers, and an initiator/activator system(^^). These materials were initially developed as photo-polymerisable lining cements but currently they are also available as photo-polymerisable restorative materials and as self-cured luting cements. From a chemistry point of view, there are two different routes towards modifying the conventional GICs into RMGICs. The aims of both methods are to adapt

the resin matrix to the GIC matrix so that the two different setting mechanisms lead to an interpenetrating network.

The first method was introduced by Mathis and Ferracane^^^^ with the ideas of reducing the brittleness of GICs by adding organic additives and a free radical initiator system into the GIC liquid. They combined the liquid component for the commercial dental composites with that used in a commercial glass-ionomer cement (13/87 wt% ). The mixture was added to the glass to produce a hybrid glass-ionomer/composite^^^. Although physical and mechanical properties of the cement were improved, this simple combination of the two systems was unlikely to function as the primary monomeric components used in composite resins are hydrophobic in nature, thus being incompatible with the aqueous environment present in the conventional GICs(^^\

The system using water-soluble or water-compatible vinyl monomers, such as 2- hydroxyethyl methacrylate (HEMA) was introduced by McKinney and Antonucci^^^^) and Antonucci and Stansbury<^^°). The formulation was found to have improved wear resistance, higher diametral tensile and compressive strength, reduced moisture sensitivity during setting and degradation after exposure to aqueous acids, and showed higher measured bond strength to dentine and composites than conventional

GICsC^'^90),

As with the conventional GICs, the powder and liquid components of the material are separated to avoid the reaction between the polyacids and the glass. Mixing of the two components initiates the acid-base neutralisation reaction since there is still some water in the system. However, the rate of this reaction is slower than that in the conventional GICs due to a reduction in the level of water in the composition and the presence of organic species^^^^'^^^). The initial set of the cement is a result of the polymerisation reaction, usually photo-polymerisation, which forms a polymer matrix.

The polyacrylate matrix is formed at a later stage as a result of the acid-base reaction and may serve to harden and strengthen the already set cement(^^\ Thus, the set cement consists of glass particles and two matrices which are different in nature, i.e. an ionomer salt hydrogel which is hydrophilic and polyHEMA which is hydrophobic. For thermodynamic reasons, the two matrices may not interpenetrate, resulting in phase- separation as suggested by Wilson^^^) and Nicholson and Anstice^^^^\

Another version of RMGIC has been developed by Mitra^^^) to prevent phase- separation in the set cement. In this system, the polyacid co-polymer is modified by substituting some carboxylic acid groups by methacrylate groups capable of free radical polymerisation. Other polymerisable monomers such as HEMA are also incorporated into the liquid component to serve as a co-monomeri^^'^^^). This system has been shown to have high early strength, improved water resistance, and higher bond strength to tooth structure compared to conventional GICs^^’^J. The amount of fluoride release is similar to the conventional GICs(^^^\

In this system, the powder and liquid are also kept separate. The setting reaction of this cement involves an acid-base reaction between glass and polyacids and two polymerisation reactions, i.e. homopolymérisation of HEMA into polyHEMA and co­ polymerisation of HEMA and methacrylate side groups of the modified polyacid. It is also possible that homopolymérisation of the carboxylic acid side chains into cross- linked polyacid occurs(^^\ In the set cement, the polyHEMA is thought to chemically link to the polyacrylate matrix which may reduce the occurrence of phase- separation(^^'^^^). The acid-base reaction is also slowed down in this system due to a decrease in the amount of water and the effect of the organic species.

2 .3 .3 S ettin g reaction

The setting of RMGICs involves two different reactions, i.e. an acid-base reaction of the conventional GIC and a free-radical polymerisation reaction typical of the composite material. The latter reaction is designed to supplement the reaction of the former and can be chemically and/or light initiated. This setting mechanism was experimentally confirmed by Mitra et using Fourier Transform Infrared Spectroscopy (FTIR) and Bourke et using Differential Thermal Analysis (DTA).

Recently, products have appeared which have been claimed to utilise a 'tri cure' or a triple-curing mechanism. The manufacturers incorporated an initiator for a self-cure, free radical polymerisation into the formulations in addition to that for light-initiation. However, this term is misleading since the materials only have a two-mechanism cure where one of the process, i.e. the polymerisation reaction, has two possible modes of initiation - chemical and light initiations^).

In practice, the acid-base reaction and the polymerisation cannot take place without reference to each other. Eliades and PalaghiasS^^^) and Kakaboura et

demonstrated experimentally that the presence of the hydrophobic species, as well as the reduced diffusion coefficient due to the reduction of the concentration of water, will lower the rate of the acid-base process. Anstice and NicholsonS^^) showed that when some of the water used to mix a water-hardening GIC was replaced by HEMA or methanol (50 wt%), the working time and setting time of the cement slowed down. The delay of the setting reaction was assumed to arise for two reasons. The organic species are poorer solvents for polyacids than pure water, hence affecting the configuration of the acids in the solution and resulting in less dissolution. In addition, the ionic setting reaction is not favoured in the presence of organic species in the mixture(^^^"^°°). This result agreed with the work of Crisp et although a lower level of methanol (5 wt%) was used in their study. When all the water was replaced, the setting was so slow

that the specimens had not set after 1 houH^^\ As expected from these studies, if there is too little water or no water in the RMGIC system, the acid-base reaction cannot occur and only the polymerisation reaction will take place when properly initiated.

A possible clinical implication of the reduced rate of the acid-base reaction might be a prolonged acidic behaviour of the cements to the surrounding tissues. This can be determined by measuring the pH of the setting cements. The initial pH of RMGIC liner/bases ranged from 2 to 6 during the first 90 minutes followed by a slow increase over 24 hours(^°^'^°^\ These values were comparable to those of conventional GIC

liners.(204.205).

The polymerisation reaction in some RMGICs where the modified polyacid is used, has been shown to be enhanced by the acid-base reaction through the steric orientation effecb^^^^. The suggested mechanism was based on the preferential orientation of the modified polyacid chains, the carboxylic acid groups being towards the glass particles. This brought the pendant methacrylate groups into a position favourable for a cross- linking reaction.

Another feature of the setting reaction of RMGICs is a tendency for the reaction mixture to phase separate as the reaction proceeds(^^^\ This may be explained as follows. When HEMA undergoes polymerisation, its solubility in water decreases. In addition, HEMA becomes less soluble when the pH of the mixture increases as a result of neutralisation of the acid-base reaction^^^^l This also leads to the product containing domains of different phases, i.e. the ionic discrete assemblies in the organic matrix.

2 .3 .4 M on itorin g o f settin g reaction o f RMGICs

Infrared (IR) spectroscopy has been carried out to monitor the setting reaction of RMGICs. Fourier transform infrared spectroscopy (FTIR)(^('^'^99.206) revealed that the

photo-polymerisation reduced the acid-base reaction during the early setting stages. Kakaboura et also found that the degree of conversion in these materials, measured immediately after irradiation, ranged from 33-50%. Irradiation after 20 min of dark/dry storage significantly reduced the degree of conversion. Although IR spectroscopy can provide the evidence of whether a glass-ionomer reaction takes place in the cements, the method is not suitable for reliable quantification since the typical absorption pattern of the acid-base reaction are obscured by the presence of water and

HEMA(207).

Most dental materials undergo setting reactions which involve heat generation. The changes in tem perature can be monitored using thermocouples. Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA). These methods are useful when the materials under investigation have a marked exothermic setting reaction. The calorimetric studies showed that the temperature rises during setting of some RMGICs, particularly the liner/bases, were greater than those of the composites^^’^'^®^ The presence of the photo-polymerisation and the acid-base reactions were clearly shown by this method(^°^'^°^\

2 .3 .5 P roperties o f resin -m o d ified g la ss-io n o m er cem en ts

A d h esio n and b o n d stren gth

Little information is available regarding the mechanisms of adhesion of RMGICs to tooth structure. Since RMGICs contain both polyacrylic acid and HEMA in the formulations, the adhesion of these materials may combine the mechanisms found in both conventional GICs and composite resins. For these materials, pretreatments of the tooth surface with conditioning agents (polyacrylic acid or a combination of polyacrylic acid and HEMA) are recommended by the manufacturers. Bond strengths of RMGICs to conditioned enamel and dentine (9-15 MPa) were higher than those of conventional GICs (4-6 MPa) but lower than those of composite resins (20-28

'^^2iyi67;iii-2i5)^ Failures occurred cohesively within the materials which indicated that the interfacial bond strengths may be higher than the inherent strengths of the materials. Higher bond strengths of RMGICs compared to conventional materials may relate to their higher cohesive strengths. Generally, the bond strengths of RMGICs to those conditioned enamel were higher than to conditioned dentine(^"\ This is probably due to the micromechanical interlocking with the etched enamel.

RMGICs also showed significantly higher bond strengths to conditioned than unconditioned dentine^^^^'^^^). Pretreatment with polyacrylic acid improves adaptation(^°), removes the smear layer and partially déminéralisés the dentine which helps HEMA present in the materials penetrate the exposed collagen fibres^^^). HEMA has been reported to react with the collagen in dentine both mechanically by entangling the demineralised dentine matrix and chemically via hydrogen bonding^^»^\ Prado et alS?24) found no difference in shear bond strength of RMGICs to dentine when dentine was conditioned with either 10% polyacrylic acid or 10% phosphoric acid. Shear bond strengths of RMGICs to conditioned enamel and dentine were not adversely affected when the materials were stored in water for a period up to 6 months(^"\

Recently Tam et used fracture toughness tests to measure the fracture resistance of the glass-ionomer/dentine interface. Mini short-rod fracture-toughness specimens were stored for 24h in 37°C water. They found no differences in the fracture resistance between conventional GlC/dentine interface and RMGIC/dentine interface when the manufacturers' recommendations were followed. The failure occurred within the smear layer in both cases(^^^\ as also observed in other studies(^^^\ suggesting that the cements could not penetrate or reinforce the smear layer structure. A significant increase in the fracture resistance of RMGICs to dentine was achieved by pre-treatment of the smear layer and the use of dentine adhesives(^^^'^^\ the values of which were similar to those of composite resin/dentine interface(^^^\

One study^^^ has shown that the thickness of some lining materials has an effect on the bond strength of RMGICs to dentine; thin layers of materials resulted in higher bond strength. This may be due to a greater cure in the thin specimens compared to the thicker ones.

S tren gth p ro p erties

The initial compressive, tensile^^^'^*^^ and flexural strengths^^^»^^) of RMGICs have been shown to be greater than those of conventional GICs. The toughness and fracture toughness of RMGICs were also higheri^^-^), implying less brittle behaviour of these materials. The monomeric components present in the materials lower their elastic moduli and increase their resistance to crack propagation.

In an early study, when stored in air at ambient humidity, RMGICs showed an increase in compressive strength with time and the specimens fractured in a brittle manneri^’^, implying that the maturation process occurred in the cements. When stored in distilled water and physiological saline, however, the cements became much weaker; the strength decreased with time up to a period of 3 months. The mode of failure changed from brittle to tough, with considerable plastic deformation^^’j). Between the two storage media, the specimens stored in saline showed smaller reduction in the compressive strengths. By contrast, Mitra and Kedrowski^^^^) demonstrated a slow increase in compressive and diametral tensile strengths for RMGICs during long-term storage in water. Surface protection of RMGICs with light-cured resins enhanced the strength only during 24h storage in distilled wateri^^). After 24h, water balance may be achieved in these materials and thus this gives the strength independent of moisture contamination.

M icro h a rd n ess

Surface hardness has been used as a measure of the extent of the setting reaction of RMGICs. Bourke et found that for one RMGIC the ultimate hardness was significantly higher than that obtained at the termination of light activation, indicating th at the post-hardening reaction occurred in that material. In the other cement, no evidence of the post-hardening was observed. The hardness of RMGICs was significantly greater when light-cured than when allowed to set without irradiation^^). The hardness of some RMGICs was similar to that of composites(^). The surface hardness tests have also been used to evaluate the depth of cure of the materials^^»^^. Immediately after light activation, the upper surfaces of RMGICs stored in distilled water at 25°C were harder than the deeper layers, but the hardness in the deeper layers increased to that of the superficial layer within 7 days^^^). RMGICs generally attained maximum surface hardness Id after light irradiation^^^»^®). As in the case of composite resins, specimen thickness, exposure times, and distance from the light source affected the hardness of RMGICs^^^).

The effects of storage medium on the hardness of RMGICs have been investigated by Mante et RMGICs were stored in distilled water, ethanol, heptane, and O.IN NaOH solution. After 30d, the surface hardness of all RMGICs decreased.

The effects of delayed curing on the surface hardness of RMGICs have been studied by Puckett et In their experiment, the RMGICs were either cured immediately after mixing or delayed up to 180 s before curing. Delayed curing did not affect the surface hardness of Vitremer and Fuji II LC. These two materials showed a significant increase in hardness after 1 wk storage. However, the hardness of the other cement decreased upon delayed curing.

W ater so rp tio n

When they first appeared on the market, RMGICs were thought to be less water sensitive than conventional GICs due to the formation of the polymerised network and did not require surface p r o t e c t i o R e c e n t l y , RMGICs have been shown to absorb water and swell in an aqueous environmenb^^^'^'^^^. This finding can be explained in term s of the underlying chemistry of the materials. The matrix of the set cement contains a polymerised network of HEMA as well as a polyacrylate network. Poly(HEMA) with its high proportion of hydrophilic hydroxyl groups has a strong affinity for water. This results in the cement behaving as a hydrogel, i.e. a water- swollen polymer. Synthesised poly-HEMA hydrogel has been shown to absorb a large amount of water (typically 40 wt% for the pure poly(HEMA) and 12.5 wt% for the co­ polymer) and expand(^^\ Thus, the mechanical properties of RMGICs may depend on the amount of water taken up. Nicholson et reported a reduction in compressive strengths and plastic behaviour of RMGICs aged in water.

The water uptake and expansion of the hydrogel is dependent on the osmotic pressure surrounding it. Anstice and Nicholson(^^^) found that when the cements were stored in a saline solution, the increase in mass and volume is smaller compared to the cements stored in distilled water. The alteration of the osmotic pressure as a result of the dissolved solutes, such as sodium, in the saline reduces the degree of swelling of the cements. This result implies that when the cements are exposed to the oral environment, the swelling may be less than that observed in distilled water.

In contrast to the conventional GICs which show a continuous gain in weight during long-term storage in water, RMGIC showed more varying patterns. A continuous increase in weight was observed in some RMGICs with the amount being greater than