AbstractHybrid restorative materials comprising resins andcomponents of conventional glass ionomers havebeen widely introduced and accepted by the dentalprofession in recent years. These include the resin-modified glass ionomer cements and the polyacid-modified resin composites or compomers. They aredeveloped in an attempt to overcome the problemsof traditional restoratives, such as moisture sensi-tivity and reduced early strength, while at the sametime maintaining their clinical advantages ofcommand setting, adhesion to tooth structures,adequate strength to occlusal load, fluoride releaseand aesthetics. This paper reviews the development,composition and properties of these new materials.Their clinical performance appears to be promisingand they should be considered as good alternativesto amalgam and other conventional restorativematerials in the future.

Key words: Resin-modified glass ionomer cements,polyacid-modified resin composites, compomers, primaryteeth.

(Received for publication December 1996. Revised May1998. Accepted May 1998.)

IntroductionDental amalgam has been the restorative material

of choice for many decades. However, in recentyears, there has been increasing awareness about thesafety of dental amalgam, mainly in respect topossible mercury toxicity that may affect humanhealth and the environment. These concerns havehelped the dental profession to focus on the need todevelop alternative restorative materials. Among the

Resin-ionomer restorative materials for children: A review

K. M. Y. Hse, BDS, MDS*S. K. Leung, BDS†S. H. Y. Wei, BDS(Hons), DDS, MS, MDS, FRACDS, FDSRCS(Eng), FADI, FHKAM(Dental Surgery ) ‡

p rime candidates, resin composite and glass ionomerrestoratives appear to be viable alternatives.1,2

Stainless steel crowns remain as the most durablerestorations in primary molars, often surviving untilthe tooth exofoliates.3,4 The order of survival rates ofrestorations in paediatric dentistry from highest tolowest is: stainless steel crowns, amalgam, resincomposite and glass ionomer cements.4 R e s i ncomposite appears to be as durable as amalgam inthe short term, particularly with respect to the main-tenance of good anatomic form, but it has a highlong-term failure rate,3 mainly due to discolouration,loss of retention and secondary caries because thereis no cariostatic action.5 Moreover, placement of ahigh quality composite restoration requires excellentmoisture control which is more difficult to achievewith children.1

Glass ionomer cements were first introduced tothe dental profession by Wilson and Kent in 1972.Their main characteristics are an ability to chemi-cally bond to enamel and dentine with insignificantheat formation or shrinkage; biocompatibility withthe pulp and periodontal tissues; fluoride releaseproducing a cariostatic and antimicrobial action; lessvolumetric setting contraction; and a similar coeffi-cient of thermal expansion to tooth structure. Theseadvantages have made them successful as lutingcements and lining mat e ri a l s. Howe ver, as arestorative material, their sensitivity to moisture andlow mechanical strength and wear resistance makethem the least durable. This may be adequate forprimary teeth because they will exofoliate in anumber of years.6,7

Recently, there have been rapid developments inthe field of hy b rid resin-ionomer restorat i ve mat e ri a l s.These include the light-cured glass ionomer cementsand the compomers. The light-cured glass ionomerswere developed chiefly to overcome the problems ofmoisture sensitivity and low early mechanical

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*Acting Senior Dental Officer, Pamela Youde School Dental Clinic ,Department of Health, Hong Kong Government, Hong Kong.†Part-time Lecturer in Paediatric Dentistry, Faculty of Dentistry,The University of Hong Kong.‡Chair Professor of Paediatric Dentistry, Faculty of Dentistry, TheUniversity of Hong Kong.

R E V I E WAustralian Dental Journal 1999;44:(1):1-11

umbrella effect and protect the ongoing acid-basereaction within the cement.

Chemistry of setting reaction

When the powder and liquid in conventional glassionomers are mixed together, an acid-base reactionoccurs between the polyalkenoate acid and ion-leachable glass, resulting in a plastic paste whichthen hardens to a solid mass. The final set structureis a complex composite of the ori ginal glass part i c l e s ,s h e athed by a siliceous hydrogel and bondedtogether by a matrix phase of hydrated fluoridatedcalcium and aluminium polyacrylates.15,16

In the resin-modified glass ionomer cement, thesetting reaction is said to be a dual mechanism.13

The usual glass ionomer acid-base reaction beginson mixing the material, followed by a free radicalpolymerization reaction which may be generated byeither photoinitiators or by chemical initiators orboth. If chemical initiators are included, then thepolymerization reaction will begin on mixing as well.The acid-base reaction in this modified cementsystem is known to slow down as some of the waterhas been replaced by HEMA.13 Finally, two matricesare formed: a metal polyacrylate salt hydrogel and apolymer. The initial set of the resin-modified glassionomer cement is the result of the formation ofpolymer matrix and the acid-base reaction serves toharden and strengthen the formed matrix.13

A true resin-modified glass ionomer cement mustbe capable of setting without being photocured, thatis, it will set under conditions where no polymeriza-tion reaction occurs, with the acid-base reaction stillbeing active.8,14,17 In summary, a true resin-modifiedglass ionomer cement material is a two-part systemwhich is characterized by an acid-base reaction thatis critical to its cure, a diffusion-based adhesivebetween the tooth surface and the cement andcontinuing fluoride release.18 Typical examples oft rue resin-modified glass ionomer cements areshown in Table 1.19

Clinical properties Adhesion

Glass ionomer cements bond chemically toenamel and dentine with insignificant heat form at i o nor shrinkage of mat e rial during the hardeningreaction.20 An ion-enriched layer is developed at thei n t e r face between the cement and the tooth stru c t u r eso that the cement can firmly adhere to both enameland dentine without signs of marginal leakage.21

Resin-modified glass ionomer cements adhere todentine in the same way as a conventional cement.Secondary ion mass spectrometry depth profileshave the ion-exchanged process between the light-cured cement and the dentine surface.22 Laboratory

2 Australian Dental Journal 1999;44:1.

strength, at the same time maintaining their clinicala d va n t a g e s. The fundamental acid-base curi n greaction is supplemented by a second light-curingpolymerization reaction which increases the strengthand solubility resistance. They are commonlyrefered to as ‘resin-modified glass ionomercements’.2,8 Their command set facility and fluoridereleasing properties have made them very popular asliners and bases, especially for use under compositerestorations.

The compomers, as the name implies, combinethe characteristics of both composites and glassionomers into a single component. However, thisgroup of restorative materials probably should notbe classified in the same cat e g o ry as the resin-modified glass ionomers and a new term known as‘polyacid-modified resin composites’ appears moreappropriate.8 The rapid acceptance of these newresin-ionomer hybrid restorative materials by thedental profession was largely due to their ease of use.This paper aims to provide information about thecurrent status of these resin-ionomer restorativematerials, their properties and clinical performance.

Resin-modified glass ionomer cements

Composition

A conventional glass ionomer cement comprises apowder and a liquid component. The original ion-leachable glasses, which form the basis of thep owder, were based on an SiO2- A l2O3- C a F2-AlPO4-Na3AlF6 composition with a fluoride contentof up to 23 per cent.9 The liquid component of theglass ionomer cement is usually an acrylic acid-itaconic acid copolymer which has a lower viscositythan the originally used 50 per cent polyacrylicacids, thus improving the ease of mixing, providinglonger working time and an increased post-sethardening rate.10 About 10 per cent tartaric acid isalso incorporated into the liquid so as to prolong theworking time and increase the setting rate.11

The simplest forms of resin-modified glassionomer cements contain the addition of a smallquantity of resin component such as hydroxyethylmethacrylate (HEMA) or Bis-GMA in the liquid ofthe conventional glass ionomers.12 Some of the watercomponent of the conventional glass ionomercement is replaced by a water/HEMA mixture.13

More complex materials have been developed bymodifications of the polyacid with side chains thatcan be polymerized by a light-curing mechanism.Up to 18-20 per cent of additional resins are addedto the liquid and, depending on the powder/waterratio of the mixture, about 4-5 per cent of the finalcement mass can be regarded as extra resins.14 It isthen possible to light-cure, resulting in an immediatesetting reaction in the resins which will provide an

shear bond strength of the resin-modified cement todentine is significantly higher than that of conve n t i o n a lglass ionomer cement and the bond is a stable one.Adhesion studies on human dentine have also shownhigh adhesion values.23,24 It may be because of theslowness of the acid-base reaction in the modifiedcement that the polyacid is available for a longerp e riod, resulting in the form ation of a strongeradhesive bond.13

Resin-modified glass ionomers have the advantageof being able to directly bond to resin composite.They produce a catalyst rich air-inhibited layerwhich can polymerize with the composite, makingthem useful in glass ionomer/composite laminater e s t o r at i o n s.1 3 , 2 4 - 2 6 Studies have shown that theadhesion can be increased by acid etching theenamel27 or by application of dentine bonding agentsprior to the placement of the restorative.28,29

Biocompatibility

Polyacrylic acids are weak acids but their highmolecular mass and chain entanglement make itdifficult for them to penetrate through the dentinaltubules.30 Moreover, dentine is an excellent bufferand the polyacrylic acids are readily precipitated bythe calcium ions in the tubules.31 Being similar toc o nventional glass ionomer cement, the resin-modified glass ionomers are also highly biocompat i b l eto the pulp. The improved adhesion to dentinesignificantly reduces marginal leakage at thetooth/restoration interface, displaying substantiallybetter adaptation and seal to the cavity preparationthan conventional glass ionomer materials.24,32-34

It is now apparent that calcium hydroxide liningdoes not have a therapeutic effect on pulp tissue.35-37

It only serves to isolate the pulp from bacterial insultand is conducive to healing. Glass ionomer cement

can achieve the same purpose by its ability to closelyadhere to tooth structure without microleakage andthus essentially isolate the lesion. The presence of asub-lining would in fact decrease the surface area foradhesion with the inherent risk of encouragingleakage.38

Mechanical strength

Conventional glass ionomer cement is a very weakmaterial and it lacks early mechanical strength. Thefinal set structure shows a dramatic increase inc o m p r e s s i ve strength but is rather brittle andcomparatively low in tensile strength39 and has lowabrasion resistance40 making it unsuitable for highs t r e s s - b e a ring areas such as posterior teeth.Moreover, it is adversely affected by both moisturecontamination and dehydration.41 Water absorptioncauses the cement to lose its translucency and resultsin erosion of the weakened surface while dehy d r at i o nat this stage results in surface crazing.42 Therefore,protection of the cement during setting and finishingprocedures is required to ensure maximumtranslucency and mechanical strength.

Inclusion of the resin component into the conven-tional glass ionomers allows rapid development ofstrength and more resistance to early moisturec o n t a m i n ation. The set cement has improve ddiametral tensile strength, compressive strength andelastic modulus, when compared with its conven-tional counterp a rt s.1 2 , 1 3 , 2 4 , 4 3 - 4 5 The resinous componentrenders it tougher and less brittle.

Fluoride release

The glass powder particles contain up to 23 percent fluoride, some of which will be released fromthe glass, mainly in the form of sodium fluoride.Many investigators have demonstrated the ability of

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Table 1. Typical commercial examples of resin-modified glass ionomer cements and polyacid-modified composite resinsCategory Clinical characteristics Material Manufacturer Application

Resin-modified .Usually two-paste system Vitrebond 3M, St Paul, USA liner/baseglass ionomer Vitremer 3M, St Paul, USA restorativecements .Can harden without XR-ionomer Kerr, Romumus, USA liner/base

light-curing Zionomer Denmat, Santa Maria, USA liner/baseFuji Lining LC GC Corporation, Tokyo Japan liner/base.Possess properties of Fuji II LC GC America, Chicago, USA restorative

true glass ionomers Photac-Bond ESPE, Seefeld, Germany liner/basePhotac-Fil ESPE, Seefeld, Germany restorative

Polyacid-modified .Often one component Dyract Dentsply De Trey, Kanstanz, Germany restorativeresin composites with an adhesive system Dyract AP Dentsply De Trey, Kanstanz, Germany restorative(compomers) Compoglass Vivadent, Schaan, Liechtenstein restorative.Can only be hardened Compoglass F Vivadent, Schaan, Liechtenstein restorative

through light-curing Compoglass Flow Vivadent, Schaan, Liechtenstein restorativeIonoseal Voco, Cuxhaven, Germany fissure sealant.May lack the typical features Ionosit Fil DMG, Hamburg, Germany restorative

of true glass ionomer s Ionosit Baseliner DMG, Hamburg, Germany liner/baseIonosit Seal DMG, Hamburg, Germany fissure sealantVariGlass VLC LD Caulk, Milford, USA restorativeHytac Aplitip ESPE, Seefeld, Germany restorativeF2000 3M, St Paul, USA restorative

glass ionomer to increase the fluoride content inenamel and dentine adjacent to restorations.46-50 Theuptake of fluoride would increase its resistance toacid demineralization and prevent caries formationaround restorations.48,51-53 Fluoride release from glassionomers also has an antimicrobial action againstStreptococcus mutans in plaque.54-56 Several laboratoryand clinical studies have clearly demonstrated theability of the resin-modified glass ionomers torelease fluoride.24,57,58 The fluoride release from anduptake by the resin-modified products was higherthan or the same as that of conventional glassionomers59-61 and has no adverse effect on the bondstrength.50

Clinical use

Dentine pretreatment

It has been proposed that a brief 10 secondapplication of a 10 per cent polyacrylic acid on theprepared dentine surface will remove the grossdebris without opening up the dentinal tubules ordemineralizing the tooth surface.21 However, there isstill no clinical evidence that such conditioning willenhance the adhesion of glass ionomer. Previousstudies have shown an increase in bond strength ofthe conventional glass ionomer cements with dentinep r e t r e at m e n t ,6 2 , 6 3 while more recent laborat o rystudies have found no such effect.28,64,65

For the resin-modified glass ionomer cement, ithas been suggested that the cement is adhesive innature and does not require surface conditioning oftooth structure, probably due to the HEMA compo-nent.42 Recent studies have found no significantdifference between various pretreatment methods onm a r ginal gap form ation at the tooth/restorat i o ninterface of a resin-modified glass ionomer.66,67

Handling and manipulation

The resin-modified glass ionomer cement hasimproved setting characteristics. There is a longerworking time because the resin slows down the acid-base reaction. It sets sharply once the polymeri z at i o nreaction is initiated by light. This minimizes disru p t i o nof the ionomeric component and reduces the effectsof moisture contamination in the early stage ofsetting. It is also easier to apply since the consistencyremains constant until light-cured, and exactpositioning of the base may be undertaken withouthaste.

A recent study has found that most of the currentresin-modified glass ionomers have greater curingshrinkage than the conventional chemically-curedcements.68 Incremental placement techniques shouldalways be used to ensure complete curing at depthand to minimize polymerization shrinkage.18 Mostmanufacturers state that immediate polishing can be

carried out after light-curing. However, the settingreaction will continue slowly for at least 24 hoursand the best result can be obtained if finishing isdelayed. When immediate polishing is required, caremust be taken not to overheat the restoration as thismay cause excessive drying and cracking and mayprevent setting of the ionomeric component.69 It isrecommended to place a layer of resin seal or afissure sealant coating over the polished surface tocover up the exposed porosities and to reinforce thesurface in the short term.14,43,69,70

Problems of resin-modified glass ionomercements

In summary, the resin-modified glass ionomercements are superior to the conventional glassionomer cements by providing a longer working timewith a command set upon light-curing, easier clin-ical procedures and manipulation, improve dmechanical strength and aesthetics towards that ofresin composite. This makes them a highly desirablealternative to amalgam for restoring primary teeth.However, compared with resin composites, they aregenerally more difficult to handle because theyrequire skillful mixing techniques in order to give thecorrect consistency, otherwise the paste may be toosticky during placement or harden too quicklybefore contouring can be finished. Moreover, theiroverall strength and aesthetic properties are stillinferior to that of resin composites. When comparedwith the conventional glass ionomers, there may bedifficulties for intraoral placement of the curing tipto different parts of the mouth especially in smallchildren. Finally, most of these products lack thelong-term research needed to evaluate their clinicalp e r f o rmance and wear resistance and their lifeexpectancy still remains unknown.

Polyacid-modified resin composites(compomers)

Recently, other resin-ionomer hybrid restorativeshave been marketed as multipurpose materials or areresins that may release fluoride but have only limitedglass ionomer properties. One such new material isthe ‘compomer’ which contains the major ingr e d i e n t sof both composites (resin component) and glassionomer cements (polya l k e n o ate acid and glassfillers component) except for water.17 However, incontrast with the resin-modified glass ionomers,they have a limited dual setting mechanism. Thedominant setting reaction is the resinous photopoly-merization and no acid-base reaction can occur untillater when the material absorbs water.

The name ‘compomer’ means that the materialpossesses a combination of the characteristics ofboth composites and glass ionomers,71 but actually it

4 Australian Dental Journal 1999;44:1.

shows minimal glass ionomer reactions. In fact, amore preferred nomenclature of ‘polyacid-modifiedresin composites’ has been suggested but is lesswidely accepted.8,17,72-75 Typical examples of thesenew products are shown in Table 1.17,75 All of themhave in common the following characteristics: singlepaste light-curing materials with glass particles asfillers and at least two different resins for the matrix,including a light-curable monomer like urethanedimethacrylate (UDMA) or Bis-GMA.75

The clinical performance of the polyacid-modifiedresin composites or compomers has not been fullyevaluated since they have only been marketed for afew years. Among them, Dyract (Table 1) was one ofthe earliest compomer restoratives available and ithas been used in many clinical trials for comparisonwith the resin-modified glass ionomer cements.M a ny new compomer restorat i ve mat e rials areavailable which are claimed to have better physicalproperties, lower wear, more fluoride, higher qualityof marginal seal and smoother surface than the firstand second generations of compomers. Theseinclude Dyract AP, Compoglass F, CompoglassFlow, F2000 and Hytac Aplitip (Table 1). Most ofthe information regarding the composition, physicalp r o p e rties and performance of the polya c i d - m o d i f i e dresin composites is based on short-term clinicalreports, abstracts, laboratory studies or from theproduct information leaflets supplied by the manu-facturers.71 Results of long-term clinical trials are notyet available.

Composition

Taking Dyract as an example, such a system isc h a r a c t e rized by a unique single-componentcompomer restorat i ve and a newly deve l o p e dprimer/adhesive liquid for enhanced adhesion totooth tissues and improved seal of the cavity. Thefilling material consists of two resins forming them at rix of the final paste. The urethane dimethacry l at e(UDMA) monomer is well-established for its tissuecompatibility in light-cured lining materials. Thei n gredient that significantly contri butes to theinnovative character of Dyract is TCB resin. Itconsists of a new monomer of dual functionality,made up of a butane tetracarboxylic acid backbonewith a polymerizable hy d r ox ye t hy l m e t h a c ry l at e(HEMA) side chain. The resultant new monomercontains two methacrylate groups as well as twocarboxyl groups. The former can cross-link withother methacrylate terminated resins when initiatedthrough radical polymeri z ation while the lat e rgroups can undergo acid-base reaction to form a saltwith metal ions and water.

C o n s i d e ring the recently available compomer,Dyract AP, the ingredients of the restorative arebasically similar to the original Dyract. The organic

matrix of the restorative has been modified byadding a small amount of a highly cross-linkingmonomer which brings an enormous increase inhardness and strength of the matrix almost equal tothat of a hybrid resin composite, Spectrum TPH,§and considerably higher than the original Dyract.76

In F2000 compomer restorative, the resin matrix iscomprised of three monomers: the dimethacrylatefunctional oligomer (CDMA oligomer) derived fromc i t ric acid, the hy d r oxypropylene dimethacry l at ewhich is commonly known as glyceryl dimethacry l at e(GDMA) and a high molecular mass hydrophilicpolymer. The CDMA oligomer has a greater ratio ofm e t h a c ry l ate groups to carboxyl groups whichallows greater cross-linking of the resin matrix.GDMA is chemically and functionally similar toHEMA with a hydrophilic hydroxyl group whichacts as a diluent for the CDMA and copolymerizeswith the oligomer. The high molecular masshydrophilic polymer is an essential and uniqueingredient in the F2000 compomer formulation. Itrapidly takes up a controlled amount of fluid fromthe oral cavity which fa c i l i t ates the transport offluorides. Due to its large size and flexibility, it actsas a rheology modifier that contributes to the clinicalhandling characteristic of the compomer.77

Of equal importance for the final properties of therestorative system is the reactive silicate glass filler.Dyract, the first generation compomer, containssolely glass ionomer fillers. The finely milled glass,with a mean particle size of 2.5 µm, accounts for 72per cent (m/m) of the composition and also contains13 per cent (m/m) of fluoride. In the secondgeneration product Compoglass, the fillers are ac o m b i n ation of methacry l ate monomers andc o nventional glass ionomer fillers with a meanparticle size of 1.5 µm, which produce an additionalstability to the cross-linkage, with improvements inp hysical propert i e s.7 8 In the recently marketedcompomers, great variations exist among the formu-lation, particle size and loading of the glass filler. InF2000, the fluoro-aluminosilicate glass filler has anaverage particle size of about 3 µm and a maximumof about 10 µm. A small amount of colloidal silica isadded with the filler and contributes to a loading ofabout 84 per cent by mass. Ground Ca-Al-Zn-fluoroglass is the filler used in Hytac Aplitip. It has amean particle size of 5 µm and accounts for 66 percent of the composition by mass. Glass fillers withsmaller size, 1 µm or less, are found in other recentlymarketed compomers. Dyract AP, had its strontium-f l u o r o - s i l i c ate glass filler of mean particle sizereduced to 0.8 µm from 1.5 µm and a loading of

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§Dentsply De Trey, Kanstanz, Germany.

about 73 per cent by mass. The Ba-Al-fluorosilicate-glass filler with a particle size of 1 µm accounts for60 per cent by mass of Compoglass F compomerrestorative. The general reduction of filler particlesize in the newer compomers has resulted in ani m p r ovement in wear and strength and betterpolishing and fluoride release in this type of restorat i vematerial.

When Dyract was developed, the prime objectivewas to build adhesiveness into the system at a levelsufficiently high to make acid etching unnecessary.The pri m e r / a d h e s i ve consists of three resins:PENTA, a patented dipentaerythritolpentacrylatephosphoric acid, which contains an acidic monomermade up of phosphoric acid with a polymerizablemethacrylate group attached and is responsible forthe formation of ionic bonds to the inorganic part ofthe tooth; TGDMA and an elastomeric resin whichare specially synthesized to determine the level ofcross-linking among the different monomers and,more importantly, the elasticity of the cured primer/adhesive. Acetone acts as a solvent that carries theresins, helps to wet the tooth surface and assists thepenetration of the resin in the dentine surface.

Recently, a new bonding agent, Prime & Bond2.1,, has been developed and used with Dyract APcompomer restorative material. It retains the ease ofuse of original Dyract-PSA but has an increasedability to absorb the stresses caused by chewing andtemperature fluctuations. Major ingredients of thenew bonding agent are similar to the ori gi n a lDyract-PSA, with the exception of the presence ofcetylamine hydrofluoride to deliver an additionalamount of fluoride to teeth.76 Other bonding systemsalso have been developed to be used for individualcompomer restorative materials by different productm a n u fa c t u r e rs. For example, Syntac Single-component is used with Compoglass F, F2000compomer primer/adhesive is used with F2000,Hytac OSB is used with Hytac Aplitip. Differentfrom the previously developed pri m e r / a d h e s i vesystems, the F2000 primer/adhesive and SyntacSingle-component bonding adhesive are hydrophilicin nature. They contain resin monomer (HEMA),methacrylate modified polyacrylic acid and maleicacid in an aqueous solution of water. Thus, they areless volatile and highly suitable for use on moistdentine surfaces. Acetone is the solvent in the Prime& Bond 2.1 and Hytac OSB bonding adhesive. Allthe bonding systems in the recently marketedcompomer restorative materials require light-curing.With these adhesive systems, adequate bondstrength to tooth substance is obtained clinically

without the need for traditional acid etchingprocedures.

Chemistry of setting reaction

Restorative materials such as Dyract AP can onlybe hardened through photopolymerization. Thereare two stages of setting reaction. The first stage isthe dominant free radical polymerization identicalwith that occuring in resin composite. Upon light-curing, the polymerizable molecules of UDMA andthe patented TCB resins are interconnected into athree dimensional network which is reinforced bymeans of the enclosed filler particles. At this stage,the existing carboxyl groups on the TCB moleculesstill remain inactive since Dyract AP is an anhydrousformulation and therefore no ion-exchange processtakes place. Exclusion of water is essential inpreventing premature setting of the material in thecontainer but also ensures that setting occurs onlyby photopolymerization.17

After the initial set, the polymerized bulk ofDyract AP begins to absorb water in the moistenvironment of the mouth. With the presence ofwater, Dyract AP now contains all the ingredientst h at are necessary to initiate an ionic acid-basereaction as in glass ionomers. The acidic conditionsin Dyract AP, by vitrue of the carboxyl group on theTCB molecules, cause metal cations to be liberatedfrom the reactive silicate glass, which eventuallyleads to the formation of hydrogels in the resinstructure of the compomer, although the rigidity ofthe set material at this stage means that the extent towhich such a reaction can occur is limited.17 Thisadditional acid-base reaction results in further cross-linkages of the entire matrix. Depending on the sizeof the restoration, the absorption of water willcontinue for several months until the filling materialhas reached its maximum level of water content,which is estimated to be approximately three percent of water (m/m) at the most in Dyract.71 Thevolumetric change following water absorption isconsidered to be low and insignificant.68 Settingreactions of all the recently marketed compomersare also based on the dominant light-initiated freeradical polymerization followed by a later acid-basereaction.

Clinical Properties

Adhesion

In the Dyract AP restorative system, two differentmechanisms are responsible for the formation ofadhesive bonds to the cavity wall. One of these is theself-adhesive property of the restorative itself. Fiftyper cent of the reactive units of the patented TCBmonomer in the restorative consists of hydrophiliccarboxyl (-COOH) groups. Such polyelectrolytes

6 Australian Dental Journal 1999;44:1.

,Dentsply De Trey, Kanstanz, Germany.

can bond to both enamel and dentine without acidetching. The functional carboxyl groups can formionic bonds with the calcium ions of the toothsurface. It is suggested that some secondary valencebonding like hydrogen bonding may occur as well.

The second mechanism is adhesion to the toothsurface through the primer/adhesive system. Thehydrophilic phosphate group of the PENTA resin inthe adhesive will form ionic bonds with the calciumions of the hydroxyapatite. In addition, when light-cured, the three methacrylate-based resins in theadhesive will undergo free radical addition polymer-ization. The cross-linked resins form a reinforcedzone, similar to the hybrid zone, of the surfacedentine, and make both the enamel and dentinecompatible for the actual restorative.71,76 The forma-tion of hybrid layers at the dentine interface isfurther supported by a scanning electron microscopestudy investigating the mechanism by which theprimer/adhesive of a recently marketed compomerbonds to tooth substance.77 In the study, the etchingon the conditioned enamel surface appeared moreshallow. Partial opening of the dentinal tubules wasfound on dentine but not to the extent typically seenwith phosphoric acid treatment. Additionalmechanical bonding with resin tags formation wasfound in the dentinal tubules which confirmed thepenetration of the compomer primer/adhesive.

After placement of the restorative over the curedadhesive, reactions between the methacrylate groupsof the coupling agent and the restorat i ve are assumedto take place. The manufacturer demonstrated thatif the Dyract primer/adhesive is not applied prior tothe placement of the restorative, the adhesion toenamel and dentine will be reduced by a factor 2 and4 respectively.71

Dyract has a significantly higher bond strength todentine than other resin-modified glass ionomercements and chemically cured glass ionomer.71,79-81

When comparing the dentine bond strength of thisfirst generation product, a higher value is obtained inrecently marketed compomers.7 7 , 8 2 The bondstrength to dentine of Hytac Aplitip was found to becomparable to a commercially available resincomposite, TPH.83 However, the bond strength ofthese compomers to enamel is definite and moremodest. Adhesion of Dyract to enamel is enhancedwith the use of acid-etching technique. A higherbond strength can be obtained when the enamel hasbeen etched by 10 per cent or 35 per cent phosphori cacid prior to the placement of the pri m e r /adhesive.27,84,85 Perhaps this procedure should be aroutine step to further increase the bond strength ofthe restorative system.

Strength and wear performanceIn order to withstand high chewing forces in the

oral cavity, a filling material intended for long-term

use needs to have high compressive and flexuralstrength. The initial and long-term compressivestrength, diametral tensile strength and transversestrength of Dyract are found to exceed that of thec o nventional and resin-modified glass ionomers.7 1 , 8 1 , 8 6

A recent study on Dyract also showed that itscompressive strength, flexure strength and abrasionresistance were not significantly different from thatof a hybrid resin composite.87 Laboratory studies onDyract AP revealed that the value of compressivestrength and flexure strength were almost equal tothat of the hybrid resin composite, Spectrum TPH,and considerably higher than original Dyract.76

Substance loss or wear is one of the most criticalparameters that significantly affects the longevity ofa dental restorative. The manufacturer performed alaboratory three-body abrasion test on Dyract andthe results indicated that the total substance losscorresponding to five years was approximately 200-250 µm, about twice the amount recorded forPrisma APH,¶ a posterior resin composite.71 Thesame test was carried out to evaluate the wear resis-tance of the recently marketed compomers. Similarwear resistance was demonstrated among them anda higher wear resistance was found when comparedwith the first generation products.76,77,88-90 They weresignificantly more wear resistant than the resin-modified and chemically cured glass ionomercements.77,89 Laboratory studies on Dyract AP andHytac Aplitip even showed a wear value similar tothat of resin composites.76,89 The introduction ofsmaller, submicrometre filler in newer compomers isprobably the reason for the improvement in theirwear resistance.

Long-term clinical studies are more reliable tod e t e rmine the wear performance of restorat i vematerials. Recent clinical studies on Dyract placedin Class I and II cavities in primary molars havefound an overall wear of 100 µm in six months, and190 µm in 12 months.91,92 In the latter, the authorsalso reported that after two years of clinical service,visible wear without exposure of dentine had beendetected in most of the restorations. A much lowerwear value for Dyract was obtained in other studies.In one study, the wear value was only 20.8 µm in sixmonths, about one-fifth that of the first study.93 Inanother controlled clinical study, the wear value ofDyract was 43.3 µm in six months and 72.7 µm in 12months which was about three times the wear of ahybrid resin composite, Prisma TPH, being investi-gated in the same study.94 In a later study, the 24months wear values of Dyract and TPH were 113 µmand 63.9 µm respectively, which were considered tobe low and clinically acceptable.95 The reasons for

¶LD Caulk, Milford, USA.

Australian Dental Journal 1999;44:1. 7

such differences in the wear value are unknown butmay be related to different methods of measurementand criteria used. Little is known about the clinicalwear performance on the recently marketedcompomer restorative materials.

Fluoride release

The release of fluoride ions from Dyractcompomer is well documented.7 1 , 9 6 - 9 9 L a b o r at o ryinvestigations on the release of fluoride ions lastingfor more than 12 months have been carried out bythe manufacturer. Their results indicated that evenafter one year, Dyract not only continued to releasefluoride ions but, most importantly, maintained thesame rate of diffusion. Secondary ion mass spec-trometry in prepared and filled third molars hasshown fluoride uptake into the enamel which wasthe highest at the contact zone and in the adjacentlayer of some 20 µm in thickness. The concentrationdropped as the sites were moved further away fromthe cavity wall and eventually reached the level of thenatural fluoride concentration of sound enamel at adepth of 75 µm. The increase in concentration offluoride in the adjacent tooth structure was equal tothat of traditional glass ionomer with proven anti-cariogenic properties.71

Recently, studies have found that the release offluoride by Dyract was significantly less than resin-modified glass ionomer cement or other fluoridereleasing resin composite.59,96,100-102 Moreover, unlikeglass ionomer cement, it was not affected by fluoridereplenishment.59 Similar to glass ionomer cement, itacted as a fluoride reservoir. When it was exposed tofluoride ion sources, such as toothbrushing with afluoridated dentifrice, fluoride would be adsorbedand slowly released into the surroundings after theion source was removed.96-98 This may be an effectivecaries preventive measure for an adjacent tooth. It isfurther supported by a laboratory study investigatingthe influence of fluoride release on Dyract thats h owed an initial inhibitory effect on theStreptococcus mutans growth.99 However, the antibac-terial action decreased significantly over time. Thecumulative fluoride release of the recently marketedcompomers was found to be higher than the firstgeneration products.76,77,82 One newer compomer,Compoglass F, has as much as 50 per cent morefluoride release than its original, Compoglass.82 Theincrease in fluoride release is partially due to thefiner particle size of the fluoride glass contained inthe newer compomer restoratives and the incorpora-tion of additional fluoride in some of theprimer/adhesive systems.76,82 A laboratory study onDyract showed that it could release more fluoride inan acidic solution of pH 3.1 0 1 Another newe rcompomer could also demonstrate similar effectwhen immersed in a medium of pH 4.5 without

adversely affecting the strength of the material.77

This could be assessed as a beneficial anticariogeniceffect arising from the presence of a low pH softdrink causing a high fluoride outflow each time apotential carious site was challenged. However, theglass ionomer material was essentially unaffected bythe change of pH.77 In addition, the caries inhibitioneffect of compomer restorative material was higherthan the conventional type of resin composite.103,104 Itappears, therefore, that Dyract or other polyacid-modified resin composites are probably effective interms of cariostatic properties.

Optical propertiesThe aesthetic qualities of a material are deter-

mined by its colour and opacity. The colour isobviously important for the overall match of a fillingto its surroundings if the material is to look at allnatural. Dyract AP is available in a range of twelvedifferent shades which follows the Vita shade guide76

and, in general, most of the recently marketedcompomers have a broader range of shades for selec-tion than the first generation compomers. However,only F2000 and Compoglass F compomers havespecialty shades for pri m a ry teeth.7 7 , 8 2 Less obviously,the opacity of a material must also be correct andDyract AP has an optimal range for opacity of about40-45 per cent, similar to that of the resincomposite materials.76 Furthermore, the determinedradiopacity of Dyract AP is 2.5 times that of dentinewhich has a radiopaque value of 2. Thus for DyractAP, it should be around 5 which is slightly higherthan that of enamel (3.5). This value is consideredto be desirable for radiographic detection of recurr e n tcaries105 and offers an easy method for documenta-tion of dental work.

Handling and manipulationEase of manipulation is another advantage of the

compomer restoratives. Similar to resin composites,they are supplied in compules which require nomixing. The gun provided allows easy dispensingdirectly to cavities and surfaces. The manufacturerof Hytac Aplitip compomer has even designed a fineaplitip system in conjunction with its small appli-cator that allows a better view of the working area.Since the adhesive can provide sufficient bondstrength for retention, no acid etching procedure isrequired prior to placement of the restorative. Theconsistency makes it easy to apply and contourwithout stickiness and, therefore, less time will berequired for final finishing. These properties areespecially beneficial in treating children becauserestorations usually can be completed much fasterand within the tolerance of the child patient.

As with other light-curing restoratives, polymer-ization shrinkage is a problem. A recent study has

8 Australian Dental Journal 1999;44:1.

shown t h at curing shrinkage is similar to that of theconventional hybrid resin composites.68 Therefore,placement in increments of 3 mm or less is recom-mended for Dyract AP, 2 mm or less for other newercompomers, and then each to be cured for at least40 seconds. Finishing can be undertaken immediat e l yafter curing using fluted tungsten carbide finishingburs or polishing discs.71,76,77,82,90

Clinical performanceReports of 6 month, 12 month and 24 month

evaluations of the clinical performance of Dyract asa restorative in primary molars have been publishedrecently. Direct evaluation results using the UnitedStates Public Health Service (USPHS) system andindirect replica techniques have shown promisingresults.91-94,106 In these studies, baseline assessmentshave shown restorations of excellent quality. Nopost-operative sensitivity was reported at any timethroughout the study period. In a controlled clinicalstudy comparing the performance of Dyract with aresin composite, Prisma TPH, on restoration ofprimary teeth, Dyract was comparable to TPH incolour match, marginal integrity, anatomical formchanges and formation of secondary caries after oneyear. The only significant difference was marginaldiscolouration and wear, in which TPH was betterthan Dyract. In this study, an overall failure rateafter one year was 1.7 per cent.94 The two yearresults of the same study demonstrated an additionalstatistically significant difference in the colour matchb e t ween the two mat e rials in which Dyract wa sinferior to TPH.95 The cumulative failure rate fort wo ye a rs was only 1.9 per cent which was comparablewith another study with a failure rate of 2.5 percent.92 However, the wear of these restorations in t woye a rs was unlikely to necessitate their replacement.The results of these short-term clinical reports haveshown that the polyacid-modified resin compositeor compomer is a reliable restorative material for usein primary teeth. Unfortunately, in some studies,carefully designed control groups were not includedand, therefore, their clinical performance could notbe directly compared with those of another materialused for similar purpose. The results of these studiestended to be over-optimistic, especially when theresearch was directly supported by manufacturers.

ConclusionVariations in composition and chemistry among

the commercial products marketed under the samegroup of hy b rids or resin-ionomer restorat i vematerials may directly affect their properties andclinical characteristics. They may or may not havethe typical features of true glass ionomers such aschemical adhesion to tooth structures and long-termfluoride release. Therefore, they should be used

carefully, closely following the instructions of themanufacturers because different handling methodsmay influence their clinical behaviour.

The development of both resin-modified glassionomers and polyacid-modified resin compositeshas greatly enhanced the efficacy and effectiveness ofrestoring carious teeth. The extent of success ofthese restorative materials is very promising as morelong-term controlled clinical trials evaluating theirclinical performance become available. In reviewingtheir advantages and clinical characteristics, theyappear to be extremely suitable alternatives to theconventional restorative materials. They also have aparticular role in the restoration of primary teeth.

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Address for correspondence/reprints:Professor S. H. Y. Wei,

Paediatric Dentistry,Faculty of Dentistry,

The University of Hong Kong,Prince Philip Dental Hospital,

34 Hospital Road,Hong Kong.

Australian Dental Journal 1999;44:1. 11