Advances in glass-ionomer cements

This article describes the properties, advances and shortcomings of glass-ionomer cement as a restorative material. The adhesion of glass-ionomer to tooth structure is less technique sensitive than composite resins and its quality increases with time. Therefore glass-ionomer might turn out to be the more reliable restorative material in minimally invasive dentistry based on adhesive techniques. First published in J Appl Oral Sci 2006; 14:3-9.


Introduction
There is a continuous urge for novelties in dentistry originating from changing professional perceptions, changing demands from the patient and progress in industrial possibilities.The altering professional perceptions come along with raising consciousness that caries treatment is not merely technique, but requires a bio-medical approach, that less-invasive techniques are possible, that bio-compatibility requires increased interest, that there are challenging new possibilities and that there are new markets 1 .The patient has changed, as they demand more aesthetics, established biocompatibility and lower costs.Today's dentistry can be characterized by a move away from metal towards non-metal restorations.Motivation is mainly based on concern for aesthetics and biocompatibility.In direct restorative dentistry this means a shift from amalgam to composites.For direct restorations, three essentially different materials are at our disposal: amalgam, resin-based composites and glass-ionomer cements.
and edge strength and corrosion.The latter was main cause of undesired release of metal ions in the human body and poor aesthetics.Whether the use of amalgam in dentistry should be limited or avoided for biological reasons is still open for discussion.In the Netherlands, the official standpoint is that there is no scientific evidence that amalgam is a serious hazard for the patient, whilst the dental team can be at risk of mercury poisoning, if no adequate hygienic measures are taken 2 .The corrosion also leads to increased porosity, which on its turn contributes to higher brittleness.Because of its intrinsic brittleness, the restoration should be as bulky as possible, and by absence of adhesion, cavity preparation is based on macro-mechanical retention.Both measures imply that placement of amalgam is usually associated with excessive sacrifice of sound tooth structure ("extension for prevention").Today, the opinion holds that, if prevention has failed, the dentist should only minimally sacrifice sound tooth structure when restoring the tooth.Within this concept, adhesion is essential.There are at the present two classes of materials, which allow direct restorations with adhesive techniques.These are resin-based composites and glass-ionomer cements.

Resin-based composites
It goes beyond the scope of this paper to cover structure and properties of resin-based composites, but some essentials have to be addressed.In mechanical sense, the heavily filled resin-based composites with smaller filler particles can compete more or less with dental amalgam in mechanical perspective (Table 1).With the latest generations of adhesives and restorative materials, the lifetime of a composite restoration almost equals that of an amalgam.Greatest features of resin-based composites are their aesthetics and presumed ease of application.Indeed, the placement procedure seems easy and straight forward: minimal cavity preparation without special attention for macro-mechanical retention, recommended bonding procedure and placement is mandatory, where set-on-command is apparently guaranteed by sophisticated lightcuring.Successful bonding to dentin is only possible if a certain substrate condition is guaranteed.Proper bonding requires deep knowledge and great skill; more than before, the quality of the restoration is determined by the dentist factor.The latest bonding generations became more operator-friendly, but their clinical durability is reduced 4 .

Of Minimum Intervention In Dentistry
On top of this premature in situ degradation of bonding and composite limit the lifetime of these kinds of restorations 5 .Therefore, it has to be emphasized that use of rubber-dam is obligatory.Notwithstanding this high-technology approach of resin-based composite application, it has to be understood that it takes approximately 2-4 times more time to make a composite than an amalgam restoration.Therefore, resin-based composite restorations cost much more chair time and for that reason are relatively expensive.As a matter of fact, it takes a good dentist to make a good composite and a bad one to make a bad amalgam.If a plenty skilled dentists are available, resin-based composites may contribute to very satisfying dentistry, but problems arise if there exists a shortage.Table 2 shows the dentist density in a series of European countries.The variety in number of dentists per 1000 inhabitants is striking and might have repercussion on either the number of patients receiving dental care or on the quality of the dentistry in that particular area.This problem might be solved if the dental treatment was not becoming more and more demanding.The conclusion so far might be that consciously placing, the technique sensitive resin-based composite restorations offer highly aesthetic alternatives for amalgam.They can be used with minimal invasive treatment.Bonding to enamel is reliable, but that particularly the quality of the dentin bonding is questionable and that the bonding procedure is demanding and thus costly for wide-scale dentistry.Restorative systems, which demand highly skilled dentists for the creation of reliable and durable restorations are less desirable in the perspective of reducing the continuing, increase of costs of health services.

Journal Of Minimum Intervention In Dentistry
There is a general demand for delegation of simple treatments to health team members with a lower degree of education.In dentistry the delegation of the restoration of small cavities to dental hygienists or dental nurses is widely explored.For such an approach more simple restorative systems are required.A possible solution in this area might be found in application of the direct bonding glass ionomer cements as a less demanding alternative to resin-based composites.

Glass ionomer cements
The early conventional glass-ionomer materials were technique-sensitive, slow setting, opaque when set and sensitive to both desiccation and hydration during setting.This led to premature surface deterioration.Most of these problems have (more or less) been solved in newer generations of glass-ionomer cement.
Setting has been accelerated and hydration problems have been reduced.However, unlike composites, their use in stressed situations is still questionable.The most common indication of the newer, heavily filled, reduced particle size glass-ionomer cements is in non-stress bearing build-ups, root caries, tunnel restorations and long term provisional restorations in primary and adult dentitions.
The cement is formed as a result of poly-acidic attack of the outer shell of fluoride containing soluble aluminum-glasses.Dissimilar to resin-based composites that have no chemical reactivity after setting, glass ionomer cements remain reactive for a prolonged time.Also quite the opposite to resin-based composites, bond formation of glass ionomer cements to mineralized tissue is no problem.Although the bond strength reaches only 25% of that can be obtained with resinbased bonding systems, the bond is reliable and far more degeneration resistant than the resin systems, where the hybrid layer can break up with time 6 .Glass-ionomer cements do not require extra provisions for consistent retention or adhesion, as they adhere directly to, even humid, dental hard tissues 7,8 (Figure 1).The resin-modification of glass-ionomer cements, introduced to obtain command set glassionomer cements, did not contribute to higher wear resistance 9 .Resin modified glass-ionomer cements are materials in which a hydrophilic polymerizing resin is added to the glass-ionomer matrix.The admixed resin improves initial aesthetics and Within the framework of mixing resins with inorganic materials, it has to be realized that conventional glass-ionomer is a pure inorganic material and thus is predisposed to acid erosion.Figure 2 shows how decreasing pH affects wear significantly.This acid susceptibility is less present for the resin-modified glass-ionomers.Note that wear as such for the resinmodified types is considerably faster than for conventional ones.Figure 3 shows how conventional glassionomers seriously can erode when used interdentally in risk patients.Also excessive consumption of soft drinks might put conventional glassionomer restorations at risk (Table 3).
For direct restorative dentistry, the slow setting of conventional glassionomer cements is felt as an inconvenience.Apart from the nuisance of waiting for finishing the restoration, a drawback of the slow setting is that the water content of the freshly placed cement can easily be altered either by dehydration or water uptake from the saliva.
The loosely bound water may have a negative effect on initial solidity of glass-ionomer, but is at the same time responsible for positive characteristics

Images courtesy Dr. Raimond van Duinen
Journal Of Minimum Intervention In Dentistry such as curing shrinkage relieve and continuing chemistry throughout the bulk material, which reinforces the material and facilitates fluoride release.A, yet not fully customary technique, by which the cements' hardening is substantially accelerated with ultrasonic or heat treatment may solve many problems related to the slow setting of glassionomers 10 .Figure 4 shows that accelerated hardening prevents the glass-ionomer for dye penetration.
Mechanical properties are also significantly enhanced by heat or ultra-sound treatment.For this reason no specific values for glassionomer are given in Table 1.

Sealants
Thanks to their applicability under humid conditions and direct bonding to tooth enamel 11 , the inorganic glass-ionomer cements are also practicable alternatives for resin fissure sealing.Notwithstanding the affirmed low wear resistance of glass-ionomers, which causes the sealant to erode already after some months, its preventive effect was reported still effective after 5 years 12,13 . Arends et al., (1989), Campos Serra andCury (1992) and Glasspoole (2001) explained this result by effective fluoride released from the glass-ionomer, which forms in relatively short time a reservoir in the adjacent enamel in a fluoridated hydroxyl apatite structure [14][15][16] .Even the temporary presence of this material would already be responsible for the prolonged prevention efficacy.SEM images, obtained by replica techniques from the fissures showed clinically imperceptible, retained material (Figure 5).
The presence of this material may be responsible for the prolonged prevention efficacy 14,15 .Literature is not conclusive on the reason why this retained material is more resistant to erosion 17,18 .Shimokobe (1993) suggested that under oral conditions, glass-ionomer sealants might gradually change into a new, more durable structure with high retention 19 .He expected that with help of the mineralizing potential of saliva, glass-ionomers might transform into an enamel-like structure called "pseudo enamel".In addition to the satisfaction with glass-ionomer as an effective way of preventing fissure caries, Van Duinen  et al. (2004) observed clinically visible changes in the glass-ionomer as shown in Figure 5 and 6 20 .These changes referred to translucency, smoothness and hardness.In analogy to the (re-) mineralizing power on tooth structures 21 , the potential of saliva as a reinforcing agent for restorative materials was suggested.The ideal pit and fissure sealant should be a full proof obstruction for the damaging effects of dental plaque at sites of the tooth that hardly can be cleaned with domestic measures.A tightadhering, erosion-resistant, impermeable layer covering the tooth fulfils that goal.If the retention and its erosion resistance are guaranteed for a substantial number of years, there is nothing against the use of the, basically inactive, resinbased materials for this purpose.However, application of resins requires extensive tooth surface conditioning, whilst the hydrophobic material is essentially unwelcome in the humid oral environment.
In contrast to this, the hydrophilic glass-ionomer requires only minor substrate conditioning and shows a tight adhesion to enamel but unfortunately will erode easily.The findings of Mejàre and Mjör (1990) that teeth sealed with resins more frequently develop caries than teeth sealed with glass-ionomer, in spite of the fact that the bulk glass-ionomer sealant had visually vanished within a few months, were explained by SEM-imaging, which revealed still retained "glass-ionomer" in the depth of the fissure at sites, where clinically no remnants of the cement were detectable 12 .

Journal Of Minimum Intervention In Dentistry
These remnants may be the same as the "intermediate" layer as postulated by Wilson et al. (1983)  22 , being the product of an exchange reaction between the poly-acid and the hydroxyl-apatite.That deeply hidden, difficultly accessible inorganic layer should possess a high acid-resistance, as it constantly will be covered by dental plaque.It is reasonable to attribute this quality for a great deal to fluoride from the glass-ionomer.Van Duinen et al.  (2004) demonstrated that glassionomer adjacent to tooth structure and in contact with the oral fluids, frequently altered into a material with unexpected cutting resistance and displaying raised Calcium-and Phosphate content 23 .It was remarkable that such an altered layer was only detectable after a couple of years' performance, whilst its thickness increased with time.This indicates that, with time, the exchange process continues and consequently the glass-ionomer restoration gains in quality, starting from the outer surface and the junction with tooth structure.It appears that, glass-ionomer performs clinically better than from laboratory research may be expected 18,24 .As saliva and its minerals play a crucial role in mineralization processes 25 , it can be understood that only under in vivo circumstances the glass-ionomer surface changed into the new structure.Okada et al. (2001)  showed that glass-ionomer stored in saliva has an improved surface hardness compared to samples stored into water 18 .Also in deeper areas exchange processes has been reported.Geiger and Weiner (1993)  demonstrated between dentin and glass-ionomer an intermediate exchange layer containing fluoridated carbonate-apatite 26 .
Yet literature is not conclusive on the clinical efficacy of glassionomer cements fluoride as measure to prevent demineralization or promote remineralization of adjacent tooth structure 22,27,28 .It has to be stressed that in these review articles on clinical trials on secondary caries prevention by glass-ionomer vs. amalgam or composite restorations, the reason why glassionomer was used was not given.It might very well be so that glassionomer was merely selected in caries prone patients.
It appears that, glass-ionomer performs clinically better than from laboratory research may be expected 18,24 .As saliva and its minerals play a crucial role in mineralization processes 25 , it can be understood that only under in vivo circumstances the glass-ionomer surface changed into the new structure.Okada et al. (2001)  showed that glass-ionomer stored in saliva has an improved surface hardness compared to samples stored into water 18 .Also in deeper areas exchange processes has been reported.Geiger and Weiner (1993)  demonstrated between dentin and glass-ionomer an intermediate exchange layer containing fluoridated carbonate-apatite 26 .
The fluoride content in glass-ionomers is much higher than in the tooth.With ion exchange over time, fluoride ions might diffuse from the cement to the tooth.In the process, some of the hydroxy-apatite in the tooth would be permanently transformed into fluoro-hydroxy-apatite 29 .The lower is the pH, the greater gets the fluoride release, a feature that justifies glass ionomer cements to be called intelligent materials 17 .

Conclusions
In contrast to resin bonding, the adhesion of glass-ionomer to tooth structure is not technique sensitive and its quality increases with time.

Figure 3 .
Figure 3. Poor hygiene can cause severe and progressive erosion in conventional glass-ionomer restorations.

Figure 6 .
Figure 6.Various SEM magnification aspects of the altered glass-ionomer sealing of Figure 5.

Figure 5 .
Figure 5. Clinical and SEM aspect of a two-year old glass-ionomer fissure sealing in a 47.The transformation is visible at the borderline of the fissure.

Table 1 .
Some mechanical properties of a lathe-cut amalgam compared with tooth structure resin-based composites 3

Table 2 .
Dentist density in various West-European countries (EU Manual of Dental Practice 2000)

Table 3 .
pH values of some soft drinks