Advances in glass-ionomer cements - Semantic Scholar...shortcomings of glass-ionomer cement as a...

JournalOfMinimum Intervention In Dentistry

J Minim Interv Dent 2009; 2 (1) - 3 -

Abstract

This article describes theproperties, advances andshortcomings of glass-ionomercement as a restorative material.The adhesion of glass-ionomer totooth structure is less techniquesensitive than composite resinsand its quality increases withtime. Therefore glass-ionomermight turn out to be the morereliable restorative material inminimally invasive dentistrybased on adhesive techniques.First published in J Appl Oral Sci 2006;14:3-9.

Address of author:Carel L. Davidson, PhD.Emeritus Professor in Dental Materials Scienceat the University of Amsterdam, TheNetherlands.

Introduction

There is a continuous urge fornovelties in dentistry originatingfrom changing professionalperceptions, changing demands fromthe patient and progress in industrialpossibilities. The altering professionalperceptions come along with raisingconsciousness that cariestreatment is not merely technique,but requires a bio-medical approach,that less-invasive techniques arepossible, that bio-compatibilityrequires increased interest, thatthere are challenging newpossibilities and that there are newmarkets1. The patient has changed,as they demand more aesthetics,established biocompatibility andlower costs. Today’s dentistry can becharacterized by a move away frommetal towards non-metalrestorations. Motivation is mainlybased on concern for aesthetics andbiocompatibility. In direct restorativedentistry this means a shift fromamalgam to composites. For directrestorations, three essentiallydifferent materials are at ourdisposal: amalgam, resin-basedcomposites and glass-ionomercements.

Amalgam

Of all direct dental restorations in theNetherlands, in 1993, 30% wereperformed in tooth colouredalternatives for amalgam; in 1997this number was 50% and in 2002this number was 70%! Restoring intooth coloured materials is nowadaysthe first option in the teachingprograms at the dental schools in theNetherlands. The multi-phase silveramalgam excellently served dentistryfor almost 200 years! Reasons forwanting alternative direct restorativematerials were the limited flexural

Advances in glass-ionomer cements

Carel L. Davidson



and edge strength and corrosion. Thelatter was main cause of undesiredrelease of metal ions in the humanbody and poor aesthetics. Whetherthe use of amalgam in dentistryshould be limited or avoided forbiological reasons is still open fordiscussion. In the Netherlands, theofficial standpoint is that there is noscientific evidence that amalgam is aserious hazard for the patient, whilstthe dental team can be at risk ofmercury poisoning, if no adequatehygienic measures are taken2. Thecorrosion also leads to increasedporosity, which on its turncontributes to higher brittleness.Because of its intrinsic brittleness,the restoration should be as bulky aspossible, and by absence ofadhesion, cavity preparation is basedon macro-mechanical retention. Bothmeasures imply that placement ofamalgam is usually associated withexcessive sacrifice of sound toothstructure (“extension forprevention”). Today, the opinionholds that, if prevention has failed,the dentist should only minimallysacrifice sound tooth structure whenrestoring the tooth. Within thisconcept, adhesion is essential. Thereare at the present two classes ofmaterials, which allow directrestorations with adhesivetechniques. These are resin-basedcomposites and glass-ionomercements.

Resin-based composites

It goes beyond the scope of thispaper to cover structure andproperties of resin-basedcomposites, but some essentialshave to be addressed. In mechanicalsense, the heavily filled resin-basedcomposites with smaller fillerparticles can compete more or lesswith dental amalgam in mechanicalperspective (Table 1). With the latestgenerations of adhesives andrestorative materials, the lifetime ofa composite restoration almostequals that of an amalgam. Greatestfeatures of resin-based compositesare their aesthetics and presumedease of application. Indeed, theplacement procedure seems easyand straight forward: minimal cavitypreparation without special attentionfor macro-mechanical retention,recommended bonding procedureand placement is mandatory, whereset-on-command is apparentlyguaranteed by sophisticated light-curing. Successful bonding to dentinis only possible if a certain substratecondition is guaranteed. Properbonding requires deep knowledgeand great skill; more than before,the quality of the restoration isdetermined by the dentist factor. Thelatest bonding generations becamemore operator-friendly, but theirclinical durability is reduced4.

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

Enamel Dentin Amalgam Microfill Hybrid

Hardness (KHN) 360 60 100 30 90

Compressive strength(MPa)

250 280 360 260 300

Tensile strength (MPa) 35 260 60 40 50

Elastic modulus (GPa) 50 12 3030

6 14



On top of this premature in situdegradation of bonding andcomposite limit the lifetime of thesekinds of restorations5. Therefore, ithas to be emphasized that use ofrubber-dam is obligatory.Notwithstanding this high-technologyapproach of resin- based compositeapplication, it has to be understoodthat it takes approximately 2-4 timesmore time to make a composite thanan amalgam restoration. Therefore,resin-based composite restorationscost much more chair time and forthat reason are relatively expensive.As a matter of fact, it takes a gooddentist to make a good compositeand a bad one to make a badamalgam. If a plenty skilled dentistsare available, resin-basedcomposites may contribute to verysatisfying dentistry, but problemsarise if there exists a shortage. Table2 shows the dentist density in aseries of European countries. Thevariety in number of dentists per1000 inhabitants is striking andmight have repercussion

on either the number of patientsreceiving dental care or on thequality of the dentistry in thatparticular area. This problem mightbe solved if the dental treatment wasnot becoming more and moredemanding. The conclusion so farmight be that consciously placing,the technique sensitive resin-basedcomposite restorations offer highlyaesthetic alternatives for amalgam.They can be used with minimalinvasive treatment. Bonding toenamel is reliable, but thatparticularly the quality of the dentinbonding is questionable and that thebonding procedure is demanding andthus costly for wide-scale dentistry.Restorative systems, which demandhighly skilled dentists for the creationof reliable and durable restorationsare less desirable in the perspectiveof reducing the continuing, increaseof costs of health services.

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

Country Active dentists Inhabitants Inhabitants/dentist

Austria 3 789 8 100 000 2 138

Belgium 7 600 10 020 000 1 342

Denmark 5 039 5 300 000 1 052

Finland 4 968 5 100 000 1 027

France 40 229 58 700 000 1 459

Germany 61 900 82 000 000 1 325

Greece 11 728 10 500 000 895

Iceland 322 275 000 854

Ireland 1 531 3 600 000 2 351

Italy 48 100 57 000 000 1 185

Luxemburg 269 418 000 1 554Netherlands 7 162 15 700 000 2 192

Norway 4 153 4 400 000 1 059

Portugal 4 200 10 000 000 2 381

Serbia and Montenegro 4 381 7 479 437 1 707

Spain 15 723 39 500 000 2 512

Sweden 8 650 8 850 000 1 023

Switzerland 4 650 7 000 000 1 505

Turkey 20 000 65 000 000 3 250

UK 25 170 58 000 000 2 304

Total 2 279 564 457 123 000 2 005



There is a general demand fordelegation of simple treatments tohealth team members with a lowerdegree of education. In dentistry thedelegation of the restoration of smallcavities to dental hygienists or dentalnurses is widely explored. For suchan approach more simple restorativesystems are required. A possiblesolution in this area might be foundin application of the direct bondingglass ionomer cements as a lessdemanding alternative to resin-basedcomposites.

Glass ionomer cements

The early conventional glass-ionomermaterials were technique-sensitive,slow setting, opaque when set andsensitive to both desiccation andhydration during setting. This led topremature surface deterioration.Most of these problems have (moreor less) been solved in newergenerations of glass-ionomercement. Setting has beenaccelerated and hydration problemshave been reduced. However, unlikecomposites, their use in stressedsituations is still questionable. Themost common indication of thenewer, heavily filled, reduced particlesize glass-ionomer cements is innon-stress bearing build-ups, rootcaries, tunnel restorations and longterm provisional restorations inprimary and adult dentitions.

The cement is formed as aresult of poly-acidic attack of theouter shell of fluoride containingsoluble aluminum-glasses. Dissimilarto resin- based composites that haveno chemical reactivity after setting,glass ionomer cements remainreactive for a prolonged time. Alsoquite the opposite to resin-basedcomposites, bond formation of glassionomer cements to mineralizedtissue is no problem. Although thebond strength reaches only 25% ofthat can be obtained with resin-based bonding systems, the bond isreliable and far more degeneration

resistant than the resin systems,where the hybrid layer can break upwith time6. Glass-ionomer cementsdo not require extra provisions forconsistent retention or adhesion, asthey adhere directly to, even humid,dental hard tissues7,8 (Figure 1).

Figure 1. GIC bond strength [MPa] tocontaminated substrates.

As filling material, glass-ionomercement mimics tooth colour not asgood as composites do and showfaster surface loss by wear, but sinceit is less technique demanding it mayserve in many ways moresuccessfully than resin-basedcomposites. Dependant on tradition,in some countries (e.g. Australia,UK) full glass-ionomer Class IIIrestorations are generally accepted,while the material is merely used asonly dentine replacement insandwich restorations. For the timebeing resin-based compositespossesses superior surfacecharacteristics.

The resin-modification ofglass-ionomer cements, introducedto obtain command set glass-ionomer cements, did not contributeto higher wear resistance9. Resinmodified glass-ionomer cements arematerials in which a hydrophilicpolymerizing resin is added to theglass-ionomer matrix. The admixedresin improves initial aesthetics and

8

7

6

5

4

3

2

1

0clean saliva blood clean

--------------dentin---------------

enamel

Fig. 1. GIC bond strength [MPa] to contaminated substrates



tensile strength and fracturetoughness. Also desiccation andhydration problems are reduced.Resin-modified glass-ionomercements set partly through an acid-base reaction and a polymerizationof the resin component of the matrix.The resin component can be light-cured. Another portion of the settingprocess involves the typical acid-base process between the filler andthe poly-acid matrix. The latterreaction does not progress ascomplete as is the case withtraditional glass-ionomers. Thehydrophilic character of the resincomponent also contributes toosmotic swelling.

Another step in merging thecharacteristics of resin-basedcomposites with those of traditionalglass-ionomer cements was theintroduction of the poly-acrylic acidmodified composite resins, alsocalled compomers. Compomers wereintended as to optimally combine theproperties of glass-ionomers andresin-based composites. If regardedas a more or less temporaryrestorative, compomers can replaceresin-based composite in anteriorproximal restorations and havebecome in many countries thematerial of first choice in paediatricdentistry. In almost all otherapplications, traditional compositesand glass-ionomer cements arepreferred because of greaterstrength and wear resistance andbetter dimensional stability. As amatter of fact, a disadvantage ofcompomers is that ease of handlingwas obtained at the cost theestablished specific properties ofhybrid resin-based composites and aproper glass-ionomer reaction.

Within the framework ofmixing resins with inorganicmaterials, it has to be realized thatconventional glass-ionomer is a pureinorganic material and thus ispredisposed to acid erosion. Figure 2shows how decreasing pH affectswear significantly. This acid

susceptibility is less present for theresin-modified glass-ionomers. Notethat wear as such for the resin-modified types is considerably fasterthan for conventional ones. Figure 3shows how conventional glass-ionomers seriously can erode whenused interdentally in risk patients.

Figure 2. pH-dependent erosive wear ofconventional and light-curing glass-ionomers.

Also excessive consumption of softdrinks might put conventional glass-ionomer restorations at risk (Table3).

For direct restorative dentistry, theslow setting of conventional glass-ionomer cements is felt as aninconvenience. Apart from thenuisance of waiting for finishing therestoration, a drawback of the slowsetting is that the water content ofthe freshly placed cement can easilybe altered either by dehydration orwater uptake from the saliva.

The loosely bound water mayhave a negative effect on initialsolidity of glass-ionomer, but is atthe same time responsible forpositive characteristics

Table 3. pH values of some soft drinks

Water 7.0 Mineral water 4.1Coffee 3.8 Orange juice 3.2Beer 4.3 Seven-up 3.2Yoghurt 3.8 Apple juice 2.8Wine 3.4 Coca cola 2.7

pH=7pH=6pH=5

Ketac Fil Fuji II Chemfil Photac Fuji II VitramerSuperior Fil LC

200

150

100

50

0

Fig 2. pH-dependent erosive wear of conventional and light-curingglass-ionomers.



Fig.4. Dye infiltration in normally (a) and ultra-sonically (b) set GICafter 3 days water storage.

a b

Courtesy Dr. Raimond van Duinen

Fig. 3. Poor hygiene can cause severe and progressive erosion inconventional glass-ionomer restorations

Courtesy Dr. Raimond van Duinen

Figure 4. Dye infiltration in normally (a) and ultra-sonically (b) set GIC after 3 days water storage.

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

Images courtesy Dr. Raimond van Duinen



such as curing shrinkage relieve andcontinuing chemistry throughout thebulk material, which reinforces thematerial and facilitates fluoriderelease. A, yet not fully customarytechnique, by which the cements’hardening is substantiallyaccelerated with ultrasonic or heattreatment may solve many problemsrelated to the slow setting of glass-ionomers10. Figure 4 shows thataccelerated hardening prevents theglass-ionomer for dye penetration.

Mechanical properties are alsosignificantly enhanced by heat orultra-sound treatment. For thisreason no specific values for glass-ionomer are given in Table 1.

Sealants

Thanks to their applicability underhumid conditions and direct bondingto tooth enamel11, the inorganicglass-ionomer cements are alsopracticable alternatives for resinfissure sealing. Notwithstanding theaffirmed low wear resistance ofglass-ionomers, which causes thesealant to erode already after somemonths, its preventive effect wasreported still effective after 5years12,13. Arends et al., (1989),Campos Serra and Cury (1992) andGlasspoole (2001) explained thisresult by effective fluoride releasedfrom the glass-ionomer, which formsin relatively short time a reservoir inthe adjacent enamel in a fluoridatedhydroxyl apatite structure14-16. Eventhe temporary presence of thismaterial would already beresponsible for the prolongedprevention efficacy. SEM images,obtained by replica techniques fromthe fissures showed clinicallyimperceptible, retained material(Figure 5). The presence of thismaterial may be responsible for theprolonged prevention efficacy14,15.Literature is not conclusive on thereason why this retained material ismore resistant to erosion17,18.Shimokobe (1993) suggested thatunder oral conditions, glass-ionomer

sealants might gradually change intoa new, more durable structure withhigh retention19. He expected thatwith help of the mineralizingpotential of saliva, glass-ionomersmight transform into an enamel-likestructure called “pseudo enamel”. Inaddition to the satisfaction withglass-ionomer as an effective way ofpreventing fissure caries, Van Duinenet al. (2004) observed clinicallyvisible changes in the glass-ionomeras shown in Figure 5 and 620. Thesechanges referred to translucency,smoothness and hardness. Inanalogy to the (re-) mineralizingpower on tooth structures21, thepotential of saliva as a reinforcingagent for restorative materials wassuggested. The ideal pit and fissuresealant should be a full proofobstruction for the damaging effectsof dental plaque at sites of the tooththat hardly can be cleaned withdomestic measures. A tight-adhering, erosion-resistant,impermeable layer covering thetooth fulfils that goal. If the retentionand its erosion resistance areguaranteed for a substantial numberof years, there is nothing against theuse of the, basically inactive, resin-based materials for this purpose.However, application of resinsrequires extensive tooth surfaceconditioning, whilst the hydrophobicmaterial is essentially unwelcome inthe humid oral environment. Incontrast to this, the hydrophilicglass-ionomer requires only minorsubstrate conditioning and shows atight adhesion to enamel butunfortunately will erode easily. Thefindings of Mejàre and Mjör (1990)that teeth sealed with resins morefrequently develop caries than teethsealed with glass-ionomer, in spite ofthe fact that the bulk glass-ionomersealant had visually vanished withina few months, were explained bySEM-imaging, which revealed stillretained “glass-ionomer” in thedepth of the fissure at sites, whereclinically no remnants of the cementwere detectable12.



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

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



These remnants may be the same asthe “intermediate” layer aspostulated by Wilson et al. (1983) 22,being the product of an exchangereaction between the poly-acid andthe hydroxyl-apatite. That deeplyhidden, difficultly accessibleinorganic layer should possess a highacid- resistance, as it constantly willbe covered by dental plaque. It isreasonable to attribute this qualityfor a great deal to fluoride from theglass-ionomer. Van Duinen et al.(2004) demonstrated that glass-ionomer adjacent to tooth structureand in contact with the oral fluids,frequently altered into a materialwith unexpected cutting resistanceand displaying raised Calcium- andPhosphate content23. It wasremarkable that such an alteredlayer was only detectable after acouple of years’ performance, whilstits thickness increased with time.This indicates that, with time, theexchange process continues andconsequently the glass-ionomerrestoration gains in quality, startingfrom the outer surface and thejunction with tooth structure. Itappears that, glass-ionomerperforms clinically better than fromlaboratory research may beexpected18,24. As saliva and itsminerals play a crucial role inmineralization processes25, it can beunderstood that only under in vivocircumstances the glass-ionomersurface changed into the newstructure. Okada et al. (2001)showed that glass-ionomer stored insaliva has an improved surfacehardness compared to samplesstored into water18. Also in deeperareas exchange processes has beenreported. Geiger and Weiner (1993)demonstrated between dentin andglass-ionomer an intermediateexchange layer containing fluoridatedcarbonate-apatite26.

Yet literature is not conclusiveon the clinical efficacy of glass-ionomer cements fluoride asmeasure to prevent demineralizationor promote remineralization of

adjacent tooth structure22,27,28. It hasto be stressed that in these reviewarticles on clinical trials on secondarycaries prevention by glass-ionomervs. amalgam or compositerestorations, the reason why glass-ionomer was used was not given. Itmight very well be so that glass-ionomer was merely selected incaries prone patients.

It appears that, glass-ionomerperforms clinically better than fromlaboratory research may beexpected18,24. As saliva and itsminerals play a crucial role inmineralization processes25, it can beunderstood that only under in vivocircumstances the glass-ionomersurface changed into the newstructure. Okada et al. (2001)showed that glass-ionomer stored insaliva has an improved surfacehardness compared to samplesstored into water18. Also in deeperareas exchange processes has beenreported. Geiger and Weiner (1993)demonstrated between dentin andglass-ionomer an intermediateexchange layer containing fluoridatedcarbonate-apatite26. The fluoridecontent in glass-ionomers is muchhigher than in the tooth. With ionexchange over time, fluoride ionsmight diffuse from the cement to thetooth. In the process, some of thehydroxy-apatite in the tooth wouldbe permanently transformed intofluoro-hydroxy-apatite29. The lower isthe pH, the greater gets the fluoriderelease, a feature that justifies glassionomer cements to be calledintelligent materials17.

Conclusions

In contrast to resin bonding, theadhesion of glass-ionomer to toothstructure is not technique sensitiveand its quality increases with time.Therefore glass-ionomer might turnout to the more reliable restorativematerial in minimal invasivedentistry based on adhesivetechniques. Glass-ionomer is not



only bioactive, but has even featuresof an intelligent material.

1. For socio-economical reasons,direct restorative techniquesare preferred over indirectones.

2. Interest in amalgam is fading.3. Resin based composites are

unforgiving, and still are farfrom perfect.

4. Shortcomings of compositesinvariably have to be tackledby sophistication of placementtechniques.

5. Glass-ionomers and theirapplication technique are stillopen for improvements.

6. Glass-ionomers are forgiving,bioactive and intelligentmaterials.

7. Glass-ionomers holds a greatpotential to become the firstchoice direct restorativematerial.

J Appl Oral Sci 2006; 14: 3-9

Resumen

El presente artículo describe laspropiedades, avances ydeficiencias de los cementosionómero vítreos como materialrestaurador. La adhesión delionómero vítreo a la estructuradental es menos susceptible a lastécnicas que los compuestos deresinas, y su calidad aumentacon el tiempo. Por ello, losionómero vítreos pueden resultarser un material restaurador másseguro en la odontología deinvasión mínima basada en

técnicas adhesivas. Publicadoprimero en J Appl Oral Sci 2006; 14:3-9.

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