Chapter 10 The tooth-coloured restorative materials IV
Resin-modified glass ionomer cements
Introduction and History of Development
Resin-modified glass ionomer cements (RMGICs) were developed in an attempt to address the perceived limitations of the conventional glass ionomer cements and to utilize the advantages of resin composite materials (Table 10.1). In other words, RMGICs aim to maintain the benefits of fluoride ion release and adhesion of glass ionomer cements while overcoming their disadvantages with the more favourable attributes of resin-based composites.
|Advantages of resin-based composites||Disadvantages of glass ionomer cements|
|Command set by visible light||Low early strength|
|Better wear resistance||Susceptibility to erosion|
|Lower solubility||Slow setting phase|
|Early high compressive and flexural strength||Risk of desiccation|
|Ability to retain its polish|
Components of A Resin-Modified Glass Ionomer
Essentially RMGICs are conventional glass ionomer cements containing glass, polyacid, tartaric acid and water, with the addition of a water-soluble resin and modified poly(acrylic acids) (Figure 10.1). These modified polyacrylic acids have pendant methacrylate groups or methacryloxy groups grafted onto the polyacid chain and are called copolymers. Chemicals that allow light activation are also incorporated. These materials may therefore be regarded as hybrid glass ionomers. They behave primarily as glass ionomer cements as only a small amount of resin has been added, which is usually hydroxyethylmethacrylate (HEMA).
Fig. 10.1 Generic composition of RMGICs. The upper (mauve) constituents are found in a conventional glass ionomer cement while the lower (blue) components are from conventional resin composites. The middle constituent (yellow) is the resin that will bind all these together.
What is HEMA?
HEMA is the monomer that is used to make the polymer polyhydroxyethylmethacrylate (polyHEMA). In the monomeric form HEMA is a small, highly reactive molecule. The polymer is hydrophilic and so once it is exposed to water it will swell. Depending on the physical and chemical structure of the polymer, it will absorb between 10% and 600% of its own dry weight of water. This property has been utilized successfully in the manufacture of flexible contact lenses. HEMA’s hydrophilic properties mean that it has become a common constituent in a range of dental materials including RMGICs and adhesive resin systems. It is one of the few resins which is miscible with water.
Biocompatibility of HEMA
The HEMA monomer is cytotoxic and so care should be taken in the application of the unset material to any soft tissue. The cytotoxicity is minimal after polymerization, however, if the level of conversion is low then unconverted monomer can leach out as polyHEMA, which will absorb water very quickly. HEMA is known to cause chemical dermatitis (Figure 10.2) if it comes into contact with skin or mucous membrane.
Any dental materials containing HEMA should be handled with extreme care because of its potentially adverse dermatological effects. Unfortunately, surgical gloves are porous and HEMA is known to penetrate through the glove to reach the skin. For this reason, the dental team should avoid touching any HEMA-containing products when mixing and placing these materials.
Furthermore if the polymer degrades, the resulting release of freed monomer into the surrounding dental hard and surrounding tissues may potentially have toxicological effects on the dental pulp and osteoblasts (see Chapter 3).
Even in its polymerized form bound in the RMGIC, if HEMA is placed directly onto vital dental pulpal tissue it may cause the death of the pulpal tissue. For this reason the use of RMGICs in direct contact with the pulp is contraindicated. In a very deep cavity, micro-exposures may be present and not obvious to the dentist. If there is any doubt, consider the use of another lining material such as a setting calcium hydroxide cement placed on the pulpal floor as a sublining. A layer of RMGIC may then be placed over the set calcium hydroxide cement and the surrounding dentine. This will seal the dentinal tubules opened during cavity preparation and decrease the risk of microleakage (Figure 10.3).
Fig. 10.3 (A) Sublining of setting calcium hydroxide cement (Life, Kerr Hawe) placed over the pulpal floor of a deep cavity in tooth 24. (B) A RMGIC (Vitrebond Plus, 3M ESPE) has then been applied to cover the sublining and the rest of the pulpal floor to seal the dentinal tubules on the cavity floor.
Chemical constituents of RMGICs
|Powder||Liquid||Purpose for their inclusion|
|Barium, strontium or aluminosilicate glass||Improved strengthImparts radiopacity|
|Vacuum-dried polyacrylic acid||Polyacrylic acid||Reacts with the glass to form the poly salt matrix|
|Potassium persulphate||Redox catalyst system to provide the methacrylate (dark) cure|
|HEMA||Water miscible resin|
|Polyacrylic acid with pendant methacrylates (copolymer)||Ability to undergo both acid–base and polymerization reactions Helps form interpenetrating network|
|Tartaric acid||Sharpens the acid–base reaction set|
|Water||Permits reaction between the polyacid and the glass|
|Photo-initiators||Achieves light curing|
Effect of particle size
As with all cements, the particle size of the glass is related to the application. The finer glasses are used in the cements intended for luting. Particle size also affects the setting reaction. The smaller the particle, the faster the setting reaction as its surface area available for reaction is greater with respect to its volume. The size of the particles is therefore often a compromise to achieve set in a clinically acceptable time while leaving an adequate surface finish. Generally speaking, in RMGICs intended for use as restorative materials there is a progressive loss in translucency as the particle size is reduced.
The polyacrylic acid reacts with the glass to form the poly salt matrix as for the conventional glass ionomer cement (see Chapter 9). This extended setting reaction means that the salt matrix is liable to damage for some time after placement if exposed to water. The addition of a water-miscible monomer to the material permits a polymerization reaction to take place during the initial stages of the setting of the material, which provides it with early strength as the secondary acid–base reaction between the glass and the polyacrylic acid goes to completion. In other words, the polymerized resin phase is designed to form a scaffolding while the ionomer cement matrix is being formed. As the resin content is increased so the acid–base reaction is slowed. The effect of the resin addition is primarily seen during the initial stages of set when the glass ionomer cement is at its weakest. It is essential that the resin is soluble in water as the cement remains water based. In the absence of water, no reaction will occur between the polyacid and the glass. This restriction limits the number of resin systems which may be used in these materials as most are hydrophobic.
Thus two types of setting reaction take place within the material: an acid–base reaction and a polymerization reaction. The polymerization reaction is a free-radical methacrylate reaction effected by light activation. Unfortunately, light transmission through these materials is limited and in thicknesses greater than 0.5 mm the base of the material does not polymerize adequately. The methacrylates in the material essentially remain uncured in the absence of light. Therefore in materials designed for restoration or core build-up, it is essential to place the material in increments and light cure each layer in order to obtain a thoroughly cured material. With lining materials this is not a problem as only a thin layer of material is required.
To compensate for the limited light-cure polymerization reaction, a second dark-cure initiator system has been incorporated into some RMGIC products. This system aims to achieve adequate free-radical methacrylate polymerization within the deeper parts of the restoration where light cannot penetrate. This gives the dentist confidence that the material will reach a full cure. The disadvantage is that the end product is not as well cross-linked as in the areas where light activation is achieved and the mechanical properties of the material where dark curing has occurred are reduced by approximately 25–30%.
The dentist should always ensure that light curing is effective over as much of cement surface as possible because the chemical dark cure is less effective and results in a material which has usually about 70% strength of the light-cured component.
Reliance on the redox reaction for the cure leads to a reduction in the level of conversion of the material and reduced mechanical properties. This is illustrated in Figure 10.4, which shows the difference in compressive strength of a RMGIC when light cured and when the redox setting reaction is relied on alone. The magnitude of shrinkage is in excess of the resin-based composites so considerable care must be taken to control the effect of the stresses which are generated. In thin sections, particularly when used as a liner, the margins of the cement layer may curl up away from the surface to be protected. As with the conventional acid–base cements, the acid–base reaction is not involved with the shrinkage. It will only contribute if there is desiccation of the material.
Dual- and tri-cure
Stages of the RMGIC setting reaction
Fig. 10.5 The sequence of the two setting reactions in a dual-cured resin modified glass ionomer cement. The boxes coloured mauve indicate the glass ionomer cement reaction, while those in blue indicate the resin polymerization reaction initiated by light.
• Acid–base reaction: This commences at the start of mixing and often continues for a substantial time after all other setting reactions have been completed, which may be up to 6 hours from start of mixing. During this time, the matrix is susceptible to damage by extraneous water.
• Light activation: This takes place at the end of placement and is completed within 10 seconds of light activation. Little post curing occurs but the material in the path of the light will have formed a solid resin matrix at this point. Water uptake by the polymer will start from the saliva in the oral cavity at this time.
Fig. 10.6 The sequence of the three setting reactions in a tri-cure resin-modified glass ionomer cement. The boxes coloured mauve indicate the glass ionomer cement reaction, while those in blue indicate the resin polymerization reaction initiated by light. Boxes in grey indicate a redox polymerization reaction initiated chemically.
Fig. 10.7 A working time-line illustrating the relationship of the various setting reactions to the clinical handling of a tri-cure RMGIC. A dual-cured product is identical but with the omission of the dark-cure reaction.