This article will review the physical properties of glass ionomer restorative materials, their gingival reaction, and their mechanisms of therapeutic action. A description will be given of how glass ionomer materials affect bacteria, and its effectiveness at preventing secondary, or marginal, caries. A summary of in vivo performance, together with descriptions of therapeutic clinical applications and recharging with fluoride to improve clinical performance of glass ionomer restorations will be presented.
It has been thought for many years that the use of a glass ionomer restorative containing fluoride provides some therapeutic advantage over other restorative materials.
This review of the literature will make recommendations as to specific therapeutic approaches with the use of these materials and their likely longevity.
There are three formulations of glass ionomer restorative materials: the regular self-cured glass ionomer (GI), the resin-modified glass ionomer (RMGI), and the polyacid-modified resin, also called a compomer. By adding resin to glass ionomer (RMGI) or glass ionomer to resin (compomer), the physical properties are significantly changed. Solubility in lactic acid is greatest for GI, ranging from 40 micron to 80 micron with 24-hour exposure to lactic acid, pH 2.74. The RMGI and compomer show little erosion.1 Compressive strength favors the compomers as significantly stronger than either the GI or RMGI, which is logical because they have a higher percentage of resin. The flexural strength (resistance to bending loads) of compomers is significantly greater than RMGI, which is significantly stronger than GI. The flexural modulus (stiffness — i.e., deformation under occlusal loads) is greatest for GI, followed by
some compomers, then RMGI5 (Figures 1-4).
Glass ionomers, as a class of material, bond more weakly to enamel and dentin than do resin-based composite restoratives. The micro-tensile bond strength to permanent dentin is usually stronger than that to caries-affected dentin. The micro-tensile bond strength is stronger for a compomer than for a self-cured glass ionomer. A resin-modified glass ionomer has the greatest bond strength2,3,4 (Table II). The strength of caries-affected dentin’s bond to glass ionomer will depend upon product formulation, amount of decalcification present in the affected dentin, and mixing method. Microleakage of glass ionomer restorations occurs less at enamel margins and is greater at dentinal margins.5
The surface texture of polished, resin-modified glass ionomers is improved slightly from chemically polymerized (self-cure) glass ionomer, though both have a rougher surface texture compared to resin-based composite materials. Glass ionomer, unpolished or polished, will collect more plaque because of its roughness and more positively charged inorganic ions on the surface (i.e., a higher surface energy), which collects more proteins and promotes better bacterial adherence.6 However, the gingival reaction to these materials is similar to amalgam restorations.7 Composite and resin-modified glass ionomer restorations can cause an increase in chronic inflammatory infiltrate and loss of attachment fibers in gingival tissues.8
Mechanisms of Action
Glass ionomer materials all contain fluoride due to the glasses from which they are formulated. “The composition and structure of fluoro-alumino-silicate glasses used to form glass (ionomer) polyalkenoate cements is critical to the setting reaction. Glass polyalkenoate cements are formed from reacting aqueous poly(acrylic acid) with a finely powdered fluoro-alumino-silicate glass. The glass is attacked by the acid, and there is Si-O-Al bond hydrolysis leading to the release of metal cations, which are then chelated by the carboxylate groups and serve to ionically crosslink the polycarboxylate chains. The number and type of cations and anions released from the glass determine the extent of ionic cross-linking of the polysalt matrix and the properties of the cement.”9
All glass ionomer materials release fluoride, moreso in acidic environments.10,11 Increasing the fluoride levels in glass ionomer cements increases the release of fluoride and provides greater protection against demineralization adjacent to the margin of the restoration.12 Glass ionomer, when placed adjacent to carious dentin, causes a decrease in the viable bacterial counts of Lactobacillus spp, Streptococcus mutans, and Actinomyces spp at 30 and 60 days. The dentin appears to be better organized compared to carious dentin, with more compact collagen fibrils and narrower dentinal tubules. This in vivo study suggests that the glass ionomer is able to halt carious progression and begin remineralization of previously demineralized tissues.13 Carious dentinal lesions remineralized adjacent to RMGI maintain the mineralized zone when subjected to a second round of demineralization. However, a second challenge of demineralization will not prevent advance of the carious lesion while maintaining the remineralization zone.14
Constituents within the glass ionomer are found in adjoining dentinal structures, indicating that different ions will migrate from the glass ionomer into dentin. This includes ions such as fluoride, strontium, aluminum, and silicon.15,16 Migration of these ions does not affect the radiopacity of the adjoining dentin.17 However, migration of these ions into adjacent dentinal surfaces can provide protection against a bacterial challenge of a biofilm made of Strep. mutans, Strep. sobrinus, Lactobaccillus rhamnosus, and Actinomyces naeslundii.18 Application of glass ionomer to incomplete removal of carious dentin, as in Atraumatic Restorative Treatment (ART) restorations, shows penetration of strontium and fluoride into underlying dentin, and continued adherence of the glass ionomer to the partially carious dentin.19 ART techniques will be discussed later.
In vitro testing of the various GI materials has shown that they do have some antibacterial properties. Freshly prepared GI and ZOE samples show no growth when inoculated with Strep. mutans and Actinomyces viscosus.20 In vivo research where plaque is allowed to accumulate for five days on various surfaces reported that glass ionomer and compomer restorations harbor an almost entirely dead biofilm (vital micro flora <8%).21 Different glass ionomer products seem to exhibit greater zones of inhibition of bacteria in vitro and are rated as Vitrebond> 0.2% chlorhexidine> Ketac Molar> Fuji IX.22 Other in vitro tests have shown self-cure GI to have greater inhibitory activity against bacteria than RMGI.23 Such in vitro tests often use a chemical demineralization model in which samples are cycled in acids or a model in which bacteria and sucrose are used on the samples. In such a study, GI and RMGI are superior to resin-based composites and amalgam at preventing demineralization and caries formation adjacent to the restorative surfaces.24,25,26,27
In vitro tests show GI as being inhibitory to bacterial growth. However, there is bacterial adhesion on the surface of GI, RMGI, and compomers due to their surface roughness and ionic charge.28,29 In vivo Strep. mutans counts are lowered with the use of fluoride-containing restorations, including GI.30 Two in situ studies wherein test materials were placed in a removable appliance and worn in the mouth for a period of time with specific treatment protocols were inconclusive in showing prevention of caries formation in dentin blocks.31,32
In vivo testing confirms that glass ionomer with or without chlorhexidine significantly reduces aerobic and anaerobic bacterial counts after seven days in carious lesions that had GI placed.33 Laboratory studies utilizing bacterial challenge show there is an inhibitory effect of bacterial growth adjacent to GI restorations in enamel and dentin which results in fewer wall lesions, less demineralization, and greater inhibition of caries formation compared to fluoride-containing composites or non-fluoridated restorative materials.34,35,36 However, in an in vivo study that reviewed proximal lesions in deciduous teeth restored with GI two years later, it was found that those restorations did not affect the rate of caries progression at the adjacent proximal deciduous enamel surfaces.37
In Vivo Research
American Dental Association testing protocol looks at the in vivo retention rate of direct restoratives and tooth bonding using a protocol that calls for the restoration of non-carious cervical lesions with no preparation other than cleansing of the surface prior to bonding. In this most demanding protocol, RMGI are retained after two years 96% of the time, compared to 81% for resin-based composites38 (Figure 5).
A common finding about restoration longevity is that replacement of restorations is more common than new restorations; composite restorations are replaced more frequently than amalgam restorations; recurrent caries is the most common diagnostic reason for restoration replacement; secondary or marginal caries is the most common failure of glass ionomer restorations; material failures such as marginal degradation, discoloration, bulk fracture and loss of anatomic form, are more frequent in tooth-colored restoratives than with amalgam.39 A survey of Icelandic dentists showed that 47.2% of restorations are placed for failed restoration replacement, 45.3% for primary caries, and 7.5% for non-carious cervical lesions.
Secondary caries is the primary reason for replacement of all types of restorations.40 In a study of Finnish restorations, secondary or marginal caries is also the most common reason for replacement of restorations, occurring in 36% of composites, 52% of glass ionomers, and 41% of amalgam. The median age of restoration failure is seven years for a glass ionomer.41 Annual failure rates reported in a meta-analysis of in vivo studies of glass ionomers of posterior stress-bearing restorations is 1.1% for compomers, 7.2% for GI, and 6.0% for ART GI.42
Cariostatic effects of RMGI and compomers were measured in a seven-year Danish study of restorations placed in children 3 to 15 years old. Fifty percent survival rates were greater than five years for both types of restorations, with 4.5 years for adjacent unfilled surfaces. Eighteen percent of the restorations failed, and operative treatment was required on 24% of the adjacent surfaces.43 In the same Danish study, after eight years 37% of the RMGI and 44% of the GI had been repaired or replaced. Progression of caries on adjacent surfaces required intervention on 20% of surfaces next to RMGI and 14% of GI restorations.44
Longevity of a highly viscous glass ionomer, Fuji IX GP, was measured over six years in Class II load-bearing cavities. The study showed no losses in the first 18 months. Survival falls to 93% at 18 months and more dramatically at 42 months. By 72 months the survival rate is 60%. It is noted that there is a progressive loss of GI material in proximal areas, just below contact areas.45 Use of a RMGI also results in poorer performance in maintenance of surface quality compared to resin-based composites.46
In three-year studies of pediatric patients, RMGI performs better than GI in primary molars.47,48
As a caution, there are several authors who point out that the correlation between the in vivo and in vitro studies is poor. The anti-cariogenic effect of glass ionomer with in vivo research shows mixed results, with some protection possible, but also secondary caries quite possible.49,50
Use of Glass Ionomer as Pit and Fissure Sealants
Glass ionomer materials used as pit and fissure sealants can increase the hardness of the adjacent enamel surfaces,51 but they have significantly poorer retention than resin-based composite sealants.52,53,54 For that reason, glass ionomers are not suggested as pit and fissure sealants.
Atraumatic Restorative Treatment (ART)
Glass ionomers are used in the ART technique, which was developed to provide treatment to populations in the third world, where there is no electricity available, as an alternative to extraction due to caries. The technique calls for the removal of open carious lesions by the use of hand instruments rather than the use of rotary instruments. The technique may be used with or without anesthesia with partial or complete caries removal. Following tooth preparation, some form of glass ionomer is used, most often self-cured. This technique has been extended in many countries to include pediatric populations, and it now also utilizes RMGI and compomers. The first report of the use of this technique was by Frencken in a rural Thailand village. He reported that after one year, 79% of the one-surface ART restorations were intact and 59% of the > than one-surface restorations were intact on deciduous teeth. Restoration of permanent teeth (mainly one-surface) was 93%, and they had a retention rate of 78% for sealants.
The fact that caries is left behind makes this technique quite controversial. However, in vivo studies have sampled the dentin before and after placement of GI and found that while the original dentin was infected and intertubular dentin was loosely organized with well-defined collagen fibrils, after GI placement there is a drastic reduction in bacteria, denser intertubular dentin, and more compact collagen fibrils. There is an increase in the calcium concentration suggesting remineralization. However, no fluoride was found in the dentin.55
In vivo studies looking at longevity and survival in pediatric populations show that ART GI and RMGI restorations outlast amalgam restorations placed in deciduous teeth.56,57,58 A six-year study of a pediatric population comparing occlusal amalgam restorations with occlusal GI restorations in occlusal restorations of permanent teeth showed that the ART technique was as effective as the conventional rotary tooth preparation technique in terms of restoration longevity. The rate of marginal or secondary caries was 2% for GI and 10% for amalgam restorations.59
ART has also been suggested for use with elder populations, especially for root restorations. An in vivo study of institutionalized adults compares the restoration of root caries using a conventional rotary preparation restored with RMGI versus the use of ART and restoration with a high-strength GI. The 12-month survival rates were 91.7% conventional versus 87% ART restorations.60 This has been confirmed by a different author.61
The use of ART does have some distinct problems associated with it. The smaller the gap formation at the margins, the less likely it is recurrent caries or residual caries will develop. Conversely, the greater the marginal gap, the more likely it is the restoration will be an early failure.49,62,63
Recharging Fluoride in Glass Ionomers
After initial placement, GI, RMGI, and compomers all release fluoride into the oral environment. The greatest release is early on, and then it diminishes over time. GI releases more fluoride than RMGI, which releases more than compomers. The release of fluoride is greater in a lower pH environment such as a high-caries-risk patient. In vitro studies have shown that fluoride containing GI, RMGI, and compomers can be recharged with more fluoride after initial placement. The material that releases the greatest amount of fluoride will recharge with the greatest amount of fluoride. 0.2% NaFl was the most effective fluoride agent at recharging.64,65,66,67
As seen earlier, in vivo studies show mixed results in preventing recurrent caries with glass ionomer restorations. Recharging the lost fluoride may be a key toward improving the outcomes with these restorations in high-caries-risk individuals.
Treating High Caries Risk Patients
An interesting in vivo study compared GI, RMGI, and resin-based composite Class V restorations in xerostomic head and neck radiation patients. Restorations were placed and followed for two years. In addition to the restoration of root caries, the patients were asked to use daily neutral sodium fluoride tray applications. Patients were later classified as either compliant (used the trays >50% of the time) or non-compliant (used the tray <50% of the time). Compliant patients had no caries during the study period, whereas the non-compliant patients had a reduction of 80% of caries with GI and RMGI compared to composite placement.68
High viscosity GI such as Fuji IX has been tested in high-caries-risk patients in stress-bearing restorations and has been found to have high retention. As well, it appears to prevent marginal or secondary caries.69,70
There is clearly some therapeutic benefit in using glass ionomer materials, especially in high-caries-risk individuals, to prevent marginal caries, and together with other therapeutic approaches eliminate the infection of the teeth. However, the glass ionomer restorations may require recharging of its fluoride content at intervals consistent with oral hygiene practices, risk, and diet in the mouths of high-caries-risk individuals to maintain its therapeutic effect.
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*Dr. Larson is associate professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota 55455. E-mail is firstname.lastname@example.org.