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Milestones in Adhesion: Glass-ionomer Cements

Martin J. Tyasa

a Professor, School of Dental Science, University of Melbourne,Australia.

Summary: The glass-ionomer cements (GIC), when introduced in the 1970s, were initially welcomed asthe first chemically-adhesive esthetic restorative materials. However, the clinical results were disappoint-ing, as these early products were brittle and were particularly susceptible to early water loss and uptake,and to long-term water loss. Continued development, notably the addition of resin polymerization com-ponents, has made the GIC an increasingly important part of clinical practice.

J Adhes Dent 2003; 5: 259–266. Submitted for publication: 10.12.02; accepted for publication: 16.04.03.

Reprint requests: Prof. M. Tyas, School of Dental Science, Universityof Melbourne, 711 Elizabeth Street, Melbourne 3000, Australia.Tel: + 61-3-9341-0231. Fax: + 61-3-9341-0437. e-mail: m.tyas@unimelb.edu.au

uch of the foundation of operative (“conserva-tive”) dentistry was established in the late

1800s and early 1900s by G.V. Black. The two prin-cipal direct restorative materials at that time wereamalgam and silicate cement, and there was a lim-ited understanding of the caries process. As a re-sult, Black’s cavity designs were rigidly mechanisticand resulted in serious weakening of the remainingtooth structure.52

The introduction of enamel etching in 19557

marked the beginning of “adhesive” dentistry. How-ever, it was nearly 15 years before resin compositematerials became available and allowed enameletching to become a clinical reality.8 Other adhe-sive systems have become available since 1955,mainly based on resins, and the bonding substratemay be enamel, dentin, metal, or ceramic. In thecontext of the replacement of enamel and dentin bydirect restorative materials, adhesive techniquesallow minimal cavity preparations and result in theconsequential benefits of maintenance of toothstrength and a reduction in pulp damage and sec-ondary caries.52

M Unique among direct restorative materials (ie,excluding liner/base materials) are the glass-iono-mer cements, as they are able to bond chemicallyto enamel and dentin, in contrast to resin compos-ites which bond micromechanically. Glass-ionomercements (GIC) are fundamentally water-based ma-terials, although many more recent products (fromabout 1990 onwards) contain a water-soluble res-in.

TERMINOLOGY

The term “glass ionomer” as applied to the originalcements is not strictly correct,27 which has led toconfusion among manufacturers and users. Thecorrect term for the original glass ionomers is“glass polyalkenoate cements”; however, this hasnot received general acceptance and is incorrectfor the later cements based on poly(vinyl phospho-nic) acid.27 For the purposes of this review, theglass ionomers which set only by an acid-base re-action (see below) will be referred to as “conven-tional” GICs. In some products, metal particleshave been added in an attempt to enhance thephysical properties. Silver or gold can be fused withthe glass powder, cooled, and ground to an appro-priate particle size to produce a “cermet” (ceram-ic-metal) material,26 eg, Ketac Silver (3M/Espe,Seefeld, Germany). Alternatively, amalgam alloycan be mixed with the GIC powder to produce an

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“admix”, eg, Miracle Mix (GC International, Tokyo,Japan).

The addition of a water-soluble monomer – com-monly 2-hydroxyethylmethacrylate (HEMA) – to theGIC liquid results in a “resin-modified glass iono-mer” (RM-GIC).27 Terms such as “light-cured glassionomer”, “hybrid ionomer”, and “resin ionomer”are often used instead of “resin-modified glass ion-omer”. However, the last is preferred as it is a moreaccurate description.27

COMPOSITION AND SETTING REACTION

Conventional Glass Ionomers

The first GIC became commercially available in themid-1970s (ASPA, DeTrey, Konstanz, Germany).The name “ASPA” was an acronym for aluminosili-cate polyacrylate, and reflects the origin of GIC asa natural development of silicate, zinc phosphate,and zinc polycarboxylate cements. Silicate cement,which became available in around 1880, wasformed essentially by mixing silica powder withphosphoric acid liquid; phosphate cement wasformed essentially by mixing zinc oxide powder withphosphoric acid liquid. In the mid-1960s, Smith48

developed polycarboxylate cement by combiningpolycarboxylic acid and zinc oxide. This was the firstdental material to bond ionically to tooth structure,and this was achieved by the attraction of the car-boxyl group (COO-) in the polycarboxylic acid to thecalcium in enamel and dentin.

In the 1970s, Wilson and Kent57 modified thesilica powder to produce a calcium fluoroalumino-silicate glass, which, when mixed with a modifiedpolyacrylic acid, resulted in the set glass-ionomercement. Since that date, significant improvementshave taken place. Other polyalkenoic acids (eg, ma-leic acid, itaconic acid, butene dicarboxylic acid,acrylic acid-itaconic acid copolymer, acrylic ac-id-maleic acid copolymer) have been used in vari-ous proprietary formulations.41 Tartaric acid is alsocommonly added in order to extend the workingtime and “sharpen” the set.32 Polyacrylic acid ofhigh molecular weight is viscous, and can increasein viscosity during storage, which compromises ac-curate dispensing; some formulations thereforehave the polyacrylic acid freeze-dried into the pow-der, and the liquid consists of tartaric acid or wa-ter.21 Some powder formulations consist of a stron-tium glass rather than a calcium glass.41

The setting reaction of the conventional GICs isa very complex acid-base reaction, and involves ashort initial setting reaction followed by a prolongedmaturation reaction. For the purposes of this re-view, a simplified description of the setting reactionis adequate.32,41 When the powder and liquid aremixed, the polyalkenoic acid attacks the fluoroalu-minosilicate glass, resulting in surface degradationof the glass and release of metal (eg, strontium,calcium, and aluminum), fluoride ions, silicic acid.These metal ions then combine with the carboxylicacid groups of the polyacid to form a polyacid salt,which becomes the matrix of the cement, and theglass surface is changed to a silica hydrogel. Theunreacted glass core remains as a filler. However,it should be noted that the presence of a filler doesnot imply that the set GIC is a composite; in GICsthere is a concentration gradient of ions from fillercore to matrix, whereas in a composite there is adistinct filler-matrix interface.41

Although conventional GIC is clinically set within3 to 5 min, “maturation”, that is, complete reactionof cations and anions, may take months or evenyears. In particular, the aluminum ions are veryslow to react.32 The prolonged maturation phase ofthe conventional GICs is a significant clinical disad-vantage, as the cement has to be protected fromsalivary (water) contamination for at least 24 h andfrom water loss for several months.32 The formerresults in dissolution of the metal ions before theycan react, and the cement surface becomesopaque and weak. Water loss results in shrinkage,cracking and staining, and possibly debonding ofthe restoration.32

Resin-modified Glass Ionomers

The resin-modified glass ionomers (RM-GIC) wereintroduced in order to lessen the sensitivity of con-ventional GICs to water balance, and enhance theirphysical properties of flexural and tensile strengthsand fracture toughness.41 The incorporation of res-in into the liquid can be achieved in several ways,for instance, by the addition of a water-soluble po-lymerizable methacrylate (eg, HEMA), or by the syn-thesis of polyalkenoic acids with pendant methacry-late side chains. The powder component is usuallyessentially the same as for the self-curing GICs.41

The first RM-GIC was a lining material (Vitrabond,now Vitrebond; 3M/Espe, St Paul, MN, USA). It con-sisted of a polycarboxylic liquid, with pendant meth-

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acryloxy groups. On mixing, the normal glass iono-mer acid-base reaction occurred, but on exposureto light, the methacryloxy groups cross linked andprovided immediate set. In practice, the photopoly-merization reaction occurred before the completionof the acid-base reaction, the latter then continuingafter clinical set.28

The setting reaction of RM-GIC is therefore two-fold; an acid-base reaction and a resin-polymeriza-tion reaction. In turn, the latter can be initiated bya photopolymerization reaction or by an oxidation-re-duction (“self-curing”) reaction. Following clinicalset by resin polymerization, the acid-base reactioncontinues, but much more slowly. This is becauseof the reduced amount of water in an RM-GIC com-pared to a conventional GIC,41 and because chem-ical reactions take place more slowly in a solidphase compared to a liquid phase. It has been sug-gested that the self-cure polymerization reactioncomprises only about 15% of the entire process.41

The addition of resin is highly significant in the con-text of water balance. Following clinical set by pho-topolymerization, the restoration can be trimmedand finished provided that is kept wet at all times.Dehydration still remains a significant problem, andsome manufacturers recommend the applicationand photocuring of a layer of unfilled resin prior todismissing the patient.41 Like all GICs, there will bea slight decrease in translucency over the first 24 hafter placement, followed by an increase over thenext 7 days, resulting in an enhancement of esthet-ics.32 However, the shade may darken slightly.41

Resin-modified glass-ionomer restorative and lut-ing cements are now also available. Examples ofthe former include Vitremer (3M/Espe), Photac-Fil(3M/Espe) and Fuji II LC (GC International, Tokyo,Japan), and examples of luting cements includeRely X (3M/Espe) and Fuji Plus (GC International).

CLASSIFICATION

The GICs can be classified in a variety of ways, how-ever, the most practical is based on the clinical us-age of the materials.32,58 Type I GICs are luting ce-ments, and are characterized by rapid set and lowfilm thickness. They are available as both self-cur-ing and resin-modified products, but as describedabove, the latter do not have a photopolymerizationsetting reaction.

Type II GICs are the “restorative” GICs, ie, theyare used as a final restorative material in prepared

cavities. Two subtypes are recognized; Type II.1comprises the esthetic GICs with appropriate colorand translucency, and Type II.2 comprises the “re-inforced” GICs. Type II.1 are available as both con-ventional and resin-modified versions, and theformer are characterized by the prolonged matura-tion reaction described above. Type II.2 GICs arethe metal-containing products described above, yetdespite the term “reinforced”, they are not signifi-cantly stronger than their Type II.1 counterparts.38

They do have better wear resistance than Type II.1conventional GICs,26 probably because the metalreduces the coefficient of surface friction and thusthe potential for “plucking out” of glass particles,and they are fast setting and resistant to early wa-ter uptake.

In the last few years, high powder:liquid ratioconventional GICs have been introduced, which aresometimes called “packable” or “high viscosity”GICs.41 These products (eg, Fuji IX, GC; Ketac Mo-lar, 3M/Espe; Chemflex, Dentsply) are self-curingand relatively fast setting, and are promoted mainlyfor small cavities in deciduous teeth,40 temporaryrestorations in permanent teeth, base and liningapplications, and for the Atraumatic RestorativeTreatment (ART) technique.12

BONDING MECHANISM

As described earlier, zinc polycarboxylate cementwas the first dental material to exhibit a chemicalbond to the calcified tissues. On ionization, each ofthe various GIC acids yields a carboxyl (COO-)group, two of which can theoretically bond ionicallyto a calcium ion (Ca++) in enamel or dentin. How-ever, this is a simplistic explanation of the bondingof conventional GIC, and additional bonding mech-anisms may occur for RM-GIC.32,55

Conventional Glass Ionomers

Theoretical mechanisms of adhesion include ad-sorption, involving chemical bonding (primary bonds:ionic, covalent; secondary bonds: hydrogen, Van derWaals’) and diffusion (bonding between mobile mol-ecules across the interface).55

A fundamental requirement of effective adhesionin any adhesive technology is a clean substrate sur-face.55 In addition, the surface energy of the sub-strate (enamel or dentin) should exceed the sur-

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face tension of the adhesive (the GIC). In the con-text of GIC, the tooth surface must therefore befree from biofilm, blood, saliva, and other contami-nants. Cleaning of unprepared cavities (eg, noncar-ious cervical lesions) is best achieved by a brief ex-posure to a slurry of pumice and water on a slowlyrotating brush or rubber cup. Commercial polishingpastes are not recommended as they may create asmear layer32 or leave an oily residue. All cavitiesshould then be “conditioned” with an appropriateconditioner. Several organic acid conditioners havebeen suggested over the last 25 years, such as cit-ric acid. However, the current consensus is thatpolyacrylic acid is the most appropriate, at a con-centration of ca 10%. Polyacrylic acid conditionerwill remove surface smear layer, leaving the smearplugs in the dentin tubules essentially intact.55 Ad-ditional benefits of conditioning with polyacrylicacid are that it increases the energy of the toothsurface and therefore enhances wettability by thefreshly mixed GIC.

Placement of the freshly mixed GIC onto condi-tioned calcified tissue results in an initial polar at-traction, with weak hydrogen bonds predominat-ing.55 The free acid in the cement dissolves any re-sidual smear layer, yet there is minimal demineral-ization because of the buffering action of the phos-phate ions from hydroxyapatite and because poly-acrylic acid is quite weak.55 As phosphate ions(negative) are displaced from the hydroxyapatite,calcium ions (positive) are also displaced in orderto maintain electrolytic balance, and both calciumand phosphate ions are adsorbed into the cement.This results in an “ion-exchange”59 layer, which isstrongly bound to the hard tissue and to the ce-ment.32,55 However, one of the problems of investi-gating the ion-exchange layer is that conventionalscanning electron microscopy requires dehydrationof the specimen, resulting in shrinkage and crack-ing.36 New “environmental” imaging techniquesare now increasing our understanding of GIC adhe-sive mechanisms.36

The nature of the ion-exchange layer59 has re-ceived attention from various authors, and severalnames have been ascribed to it. This has led tosome confusion in the literature with mechanismsof resin composite bonding, particularly to dentin.

Using a scanning electron microscope equippedwith a high-resolution field emission gun and a cry-ostage, in order to stablise water in the GIC, Ngo etal36 described a “zone of interaction” between GICand enamel/dentin. This layer was 1 to 2 µm thick,

resistant to acid etching, but of unknown nature. Itseems probable that it was equivalent to the “ionexchange layer” of Wilson et at.59 Subsequently,Ferrari and Davidson10 noted a similar acid-resis-tant layer between GIC and enamel/dentin, approx-imately 6 µm thick, which they termed the “inter-dif-fusion zone”. The nature of this zone was not dis-cussed.

Hosoya and Garcia-Godoy17 employed SEM tostudy the interface between two conventional GICsand enamel/dentin, following pretreatment with20% polyacyrlic acid/3% aluminium chloride solu-tion. They were unable to demonstrate the “interdif-fusion zone” of Ferrari and Davidson,10 but indicat-ed that the “interdiffusion zone” could also betermed the “hybrid layer”, analogous to that formedbetween resin-modified GIC and dentin37 (see be-low). This has given rise to some confusion regard-ing bonding systems (GIC and resin composite),and it is recommended that the term “hybrid layer”should not be used in relation to conventional GIC;“ion-exchange layer” is preferred.

The nature of the ion-exchange layer was investi-gated by Sennou43 using x-ray photoelectron spec-trometry (XPS), and the name “interphase” given.The interphase was found to consist of “reciprocaldiffusion” of ions from GIC and dentin. The dentinyielded calcium and phosphate ions, and the glassionomer yielded aluminium, silica and fluoride ions,together with calcium and/or strontium, dependingon the glass composition.

A further XPS study61 clearly demonstrated achemical bond between the carboxyl groups of poly-alkenoic acid and the calcium component of hy-droxyapatite (HA). These authors used HA insteadof enamel/dentin, in order to validate the instru-mentation and thus the findings. The carboxylgroups were found to replace the phosphate ions ofthe HA in creating these ionic bonds, and about68% of the carboxyl groups became bonded.

Tay’s group49 further investigated the ion ex-change layer, and proposed that various “zones”and “layers” described in the literature10,36,43 wereequivalent. Geiger,15 using Fourier-transformed in-frared spectroscopy, identified this layer as “fluori-dated carbonatoapatite”. Tay et al49 confirmed thepresence of the “interphase” or “intermediate lay-er”, as containing “elements of bidirectional diffu-sion of ions between GIC and dentin”. A furtherstudy60 using SEM and transmission electron mi-croscopy measured the “intermediate layer” at 0.5to 1.5 µm thick (in agreement with Ngo et al36 but

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not with Ferrari and Davidson10). In addition, na-nometre-sized plate-like particles were identifiedwithin the intermediate layer, which they speculat-ed may be the reprecipitation of calcium and phos-phate.

In addition to the invigorating bonding mecha-nisms described, chemical bonding of GIC to col-lagen has also been proposed.34 This possibilitydoes not appear to have been further investigated.

The cohesive strength of the ion-exchange layeris unknown,41,55 since in laboratory bond strengthtesting, cohesive failure of the cement is usually re-sponsible for bond failure.49 This finding greatlycomplicates the evaluation and the significance ofbond strength testing of GICs, since measuredbond strengths are usually in the range of 3 to10 MPa (ie, the cohesive strength of the GIC),whereas for resin-dentin bonds adhesive failure of> 30 MPa is frequently reported.4 Despite theselaboratory values, GIC has excellent clinical reten-tion in noncarious cervical lesions.

Resin-modified GICs

Since resin-modified glass-ionomer cements (RM-GIC)contain a polymerizable resin, commonly 2-hydroxy-ethyl methacrylate (HEMA), the theoretical opportuni-ty exists for bonding to occur through micromechani-cal retention in the same way as resin composite.

Lim et al34 used a range of imaging techniques(SEM, confocal microscopy, XPS and secondary ionmass spectrometry) to investigate the interface be-tween an RM-GIC and dentin. They reported thepresence of resin tags in the dentinal tubules, andalso that an ion-exchange process had takenplace, but no resin-dentin hybrid layer was found.Friedl et al14 were also unable to demonstrate hy-brid layer formation, and confirmed the presenceof resin tags about 3 µm long in the dentin tu-bules.

Titley et al50 investigated separately the liquidonly and the mixed cement, and their effect on den-tin. The liquid initiated a mild peritubular etch andthe formation of tubular “plugs”. Continued expo-sure to the liquid (> 30 s) resulted in progressivedisintegration of the plugs, possibly due to an “ef-fervescent reaction”. The authors concluded thatbonding is primarily chemical, but that the surfacedemineralization caused by the liquid componentprior to setting may allow improved penetration ofHEMA and “later mechanical interlocking”. Howev-

er, it is not clear whether the authors meant “inter-locking” into the dentinal tubules or into the col-lagen of the tubules.

Pereira et al37 used SEM to examine theRM-GIC/enamel and dentin interfaces. Followingtreatment of the substrate with 20% polyacrylic ac-id/3% aluminium chloride, resin tags were found inthe dentinal tubules. In addition, a “resin-rich layer”and an “indistinct zone” were observed betweenthe RM-GIC and the dentin. The authors implied,but did not specifically state, that the resin-rich lay-er was the penetration of resin into the collagen lay-er, ie, the classical “hybrid layer” in resin compos-ite bonding. Regarding enamel, no results of theSEM examination were given. However, the authorsimplied that they identified micromechanical inter-locking as being responsible for bonding.

Ramos and Perdigão39 used field-emission SEMin a study of the bonding of an RM-GIC to dentin.They reported no penetration into dentin, and noresin-dentin interdiffusion zone (hybrid layer). Col-lagen fibres exposed by the 10% citric acid condi-tioner remained exposed, and the authors suggest-ed that the measured bond strength (2.9 MPa) wasdue to ionic bonding.

Sidhu and Watson46 and Sidhu et al44 examinedthe interface between RM-GIC and tooth tissue us-ing confocal microscopy and 2-photon imaging. Theauthors described a “structureless” or “absorp-tion” layer in the cement adjacent to cut dentin tu-bules, up to 25 µm thick, although one of the fourRM-GICs tested did not show this feature. It wasalso absent in association with enamel. There wasno suggestion by the authors that this “absorptionlayer” was relevant to the bonding mechanism. Noevidence for resin tag formation into enamel or den-tin was found.

Currently, therefore, there is no overwhelming ev-idence that RM-GICs bond by the formation of a hy-brid layer (ie, the infiltration of the resin componentinto collagen exposed by acid pretreatment). Saitoet al41 have published scanning electron micro-graphs claiming to show hybrid layer formation forRM-GICs but not for conventional GICs, but the ex-perimental details were not given.

BIOLOGICAL PROPERTIES

The biocompatibility of GICs has been extensivelyreviewed by Sidhu and Schmalz,45 and the readeris referred to this excellent paper for more detail.

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CLINICAL PERFORMANCE

In terms of adhesion in non-undercut noncariouscervical lesions, the GICs have shown excellentperformance.25,29,51 The technique is easy andquick, in contrast to the more demanding resin-den-tin bonding systems.19 However, the conventionalGICs do suffer from a number of disadvantages asdiscussed above; low fracture toughness, subopti-mum polish and sensitivity to water balance for sev-eral months. Despite the benefits gained by incor-porating resin to produce the RM-GIC, the problemsof strength and polish remain.32 Consequently, theideal situations for GICs are in non-stress-bearingareas, ie, anterior approximal cavities and cervicalcavities. Although they can be used in occlusal andposterior approximal cavities, this should only bedone after a careful assessment of the anticipatedload. The clinical procedure for the placement ofconventional and resin-modified GICs in all theabove cavity types has been described in detailelsewhere,19,32 and successful clinical perfor-mance for 10 years25 and 15 years31 for conven-tional GICs has been reported. However, clinicalstudies on RM-GICs are fewer and short term, be-cause of the more recent introduction of theseproducts.1-3,5,6,9,11,18,23,33,53 To date, there is noreal consistency among the results, both for differ-ent brands of RM-GIC and for comparative studiesof RM-GIC and polyacid-modified resin composites.One criticism which has been made against theRM-GICs is the potential for water uptake and swell-ing due to the HEMA content.32 However, the clini-cal significance, if any, is unclear.

The recent trend towards minimum-interventiondentistry52 has focused increasing interest on theGICs.32 One of the main tenets of minimum-inter-vention dentistry is the concept of removing only“infected” dentin when restoring a carious lesion,and leaving the “affected” dentin.24 Affected dentinis demineralized, but the collagen remains intact.Such dentin is clinically stained, but firm and dry,and has the potential to remineralize.13 Recent pre-liminary studies suggest that the fluoride releasedfrom GIC, in conjunction with the metal ion originat-ing from the glass (eg, calcium or strontium), maypromote such remineralisation.35 This has particu-lar relevance in the Atraumatic Restorative Treat-ment (ART) Technique,13 which is being stronglypromoted in environments where conventional den-tal facilities are not available. Such environmentscan include developing countries, remote rural

communities, nursing homes, and for the home-bound, where, for example, rotary instruments andmixing machines are not available. The ART tech-nique also plays an important role in the treatmentof young children, dental “phobics” and the medi-cally compromized, and has additional potential inconjunction with chemomechanical caries removalsystems.22,42 The fundamentals of the ART tech-nique are removal of caries with hand instruments(thereby leaving the affected dentin), and restora-tion of the resulting cavity with GIC.

TYPE I AND III GLASS-IONOMER CEMENTS

This review has concentrated mainly on Type II (re-storative) GICs. Space constraints do not allow a de-tailed evaluation of Type I (luting) or Type III (liner,base, and fissure sealant) GICs. Nevertheless, GICsare important in these applications. The “cervicallining” technique was recommended in 1984,26 andclinical trials are demonstrating its usefulness inmaintaining the integrity of the cervical margin of ap-proximal posterior cavities, provided that an appro-priate material is used.20,30,54,56 Glass-ionomer ce-ments are also useful as fissure sealants where thestrict moisture control for resin-based sealants can-not be obtained, eg, for partly erupted teeth.47 Al-though the clinical retention rate of GICs is less thanthat of resin sealants, the prevention of fissure car-ies appears comparable.47 The use of GICs in thedeciduous dentition has been discussed extensive-ly elsewhere.16

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