Dentinal bonding agents versus glass-ionomer cements · 2019. 9. 12. · Dentinal bonding agents versus glass-ionomer cements John W. McLean* Aiistract The long-term bonding of dental

Restorative Dentistry

Dentinal bonding agents versus glass-ionomer cements

John W. McLean*

Aiistract The long-term bonding of dental materiais to dentin remains an area of greatcontroversy and the results of in vitro testing do not always reflect those found invivo. T¡ie clinician is faced with a large number of dentinal bonding agents thathave had limited testing in vivo and are frequently replaced before any long-termclinical testing has been completed. Glass-ionomer cements, although having alonger history of good adhesion to dentin, are not suitable for use in high-stress-bearing areas. The selection of materials for specific clinical situations has becomemore and more difficult. This paper gives a personal view ofthe history andevolution of both resin bonding agents and glass-ionomer cements and theirpotential in clinical use. (Quintessence Int ¡996:27:659-667,)

Clinical relevance

Glass-ionomer cements should be useti to utilizetheir main advantages of fluoride release andIong-ierm adhesion to tooth structure. Dentinalbonding agents provide excellent serace when usedin small cavities. Because they exhibit lower me-chanical strength than do the currently availabledual-cured resin cements, polyacid-modifted resincomposite cetncnts and resin-mo dined glass-ionomer cements should only be used for conven-tional cementation.

History of dentinai bonding

The first attempt to develop an adhesive system forbonding to dentin was made by Hagger,' a Swisschemist working for the Amalgamated Dental Com-pany in London and Zürich in 1951, The firstcommercial product (Sevriton Cavity Seal), based on

* Honnrao' Senior Research Fdlow, Eastman Dental Institute, London,England,

Reprint requests: Dr John W, McLean, tO Witdwood Road, LondonNWIl 6TB, England,

Based on a paper presented to the Ameriean Academy of ResioiativeDentistry, Chicago, 19 Feb 1994,

glycerophosphoric acid dimethacrylate, was used tobond an autocuring acrylic resin, Sevriton, to dentin.Work at the Eastman Dental Hospital, London,showed that glycerophosphoric acid dimethacrylateincreased adhesion to dentin by penetrating the surfaceand forming an intermediate layer, now called thehybrid zone. This was detected because the dentinexhibited an intense affinity for hematoxylin staitiitig,and the zone of altered dentin had affinities similar tothose of calcified dentin in an exaggerated form,̂ It isinteresting to note that, despite the use of a methylmethacrylate resin as the restorative material, thepulpal reaction to Sevriton plus cavity seal was similarin intensity to that recorded for other autocuredresins.^ Although Sevriton was capable of causing anacute inflammatory reaction when placed in moder-ately deep, unlined cavities, the usual outcome of thisreaction was resolution,

Buonocore and Quigley'' used giycerophosphoricacid dimethacrylate in their early experiments andconciuded that the phosphate groups in SevHton cavityseal seemed ideally suited for chemical combinationwith constituents ofthe dentin, and they confirmed theearlier findings of Kramer and McLean,- Althoughthis work appeared to usher in a new era in dentinalbonding, the idea lay dormant for a long periodbecause the restorative materials used in the 1950swere all based on methyl methacrylates of relativelyhigh viscosity that contained free monomers and

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exhibited high shrinkage during polymerization, prop-erties that were less than ideal for securing long-termbonding to denlin. Subsequent clinical trials showedthat the phosphate bonds, supposedly formed withcalcium ions of the dentinal surface, were not of alasting nature and were hydrolytically unstable, aproblem that can still arise with the current resinbonding agents.

It was not until 1958 that the first low-shrinkageresin was produced by Schmidt and Purrmann inGermany; the material was based on their noveldevelopment of the low-viscosity epimine resins andmarketed under the trade name, P-Cadurit.^ In 1959,Bowen'' filed his first patent in the United States on hisnow-famous bis-GMA resin and produced a dentalrestorative material containing vinyl s il ane-treatedfused silica. More recently, the urethane dimethacry-lates (ICI Dental) were developed by Knight et al' forindustrial use. Subsequently, Forster and Walker,working at the Amalgamated Dental Company, made aurethane dimethacrylate resin for use in resin compos-ite dental materials. '̂̂ These new resins have theadvantage of higher molecular weight, lower viscosity,and a certain degree of toughness in the urethanemoiety, together with less staining than bis-GMA.Further work by ICI resulted in the introduction of thefyst visible light-curing systems, which started a newera in restorative dentistry and made possible themodem methods of dentinal bonding with low-viscosity photocured resins,'"

Modern dentinal bonding agents

The principal barrier to effective adhesion to denta]tissue is water. Water will compete with a potentialadhesive for the surlace of a substrate and can alsohydrolyze adhesive bonds. Modern dentinal bondingagents have evolved from the original concept ofincreasing dentinal permeability and wettability andpromoting bonding to the smear layer, as in the case ofthe early Sevriton ca\nty seal, to the partial removal ofthe smear layer, and fmally to the use of strongeretchants to modify or remove the smear layer andobtain some form of micromechanical retention (Fig1). Nakabayashi et al" have described monomers,based on 4-methacryioxyethy] trimelhtate anhydride,that contain both hydiophilic and hydrophobic chemi-cal groups that can penetrate etched dentin andpolymerize in situ. This resin impregnation creates atransitional layer that is neither resin nor tooth, but ahybrid of the two (Fig 2). Sevriton cavity seal was thefirst commercial material to use this approach. Chemi-

cal bonding could further enhance the bond, but. at thepresent time, there is evidence that even^^wlienestablished, the bond is of a transitory nature.

A number of excellent reviews trace the history ofdentinal bonding through the various generations ofresin bonding agents.'^"'^ A comprehensive artidecomparing glass-ionomer cement and resin compositesystems has recently helped to clarify this controversialarea.''^ It is generally agreed that the major differencebetween the success of enamel bonding and that ofdentinal bonding lies in the substrate surface. Enamelis composed largely of hydroxyapatite and has a verylow water content." By contrast, dentin varies con-siderably and may be very dense, with only 1% of thesurface at the dentinoenamel junction consisting oftubules, or very porous at the pulpal floor, where asmuch as 22% of the total surface area may consist ofdentinal tubules. Dentin is permeated by fluids trans-ported from the pulp, and there is both loosely andtightly bound water even in enamel.

However, because enamel is an ion exchanger anddentin is a living material subject to change, one istrying to bond to sfufting sand rather than to solidrock. Under such conditions, the adhesive bond musthave a dynamic character too. It will be broken as thesubstrate changes and must be capable of beingre-formed. Once broken, covalent chemical bondscannot be re-formed. Dentinal adhesives may fail forthis very reason. In contrast, the ionic and polar bondsthat attach the glass-ionomer cement to the substratecan be reestablished; this, together with the multi-plicity of the adhesive bonds and setting stresscharacteristics of the cement,'^•^' may account for thismaterial's unique property of long-term adhesionunder oral conditions.

Although measurements of hond strengths revealmuch lower values for glass-ionomer cements than fordentinal bonding agents used with resin composites,when these two materials were first used for therestoration of erosion/abrasion lesions without cavitypreparation, the glass-ionomer materials had betterlong-term retention rates.'^"-- It is thought thatglass-ionomer materials are not affected by scleroticdentin as much as are some dentinal bonding agents.Also, the setting stress characteristics favor the glass-ionomer materials. '^•-' Davidson^' considered that theability of glass-ionomer cements to withstand stresscan be explained by an internal fracture mechanism bywhich material can easily reshape to enforced newforms.-'

It seems logical that the ideal method of clitiicallytesting the retention of bonded resin composite and

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Fig 1 Evolution of dentinal bonding, from fhe earlyattempts fo bond to the smear iayer to fotal removal by acidetching and bonding to infertubular dentin.

Fig 2 Formation of fhe hybrid layer in intertubular dentin.

Fig 3 Failure of wetfing by resin primers in ffie denfinaltubules.

Fig 4 Penetration of denfinal bonding agents to form tagsin fhe dentinal tubules. Ttie primer should wet the surface ofthe tubule prior fo the application of fhe bonding agenf.

glass-ionomer restorations should be in Class Verosion lesions without cavity preparation over aperiod of at least 3 years,-^ Unfortunately, clinicaltrials are difficult and take years to complete, so thateach generation of dentinal bonding agents tends to besuperseded before long-term clinical trials can bettndertaken and produce meaningful results. Thereforethe profession has had to rely mainly on short-term invitro testing of bond strengths as a measure of amaterial's success.

Modem dentinal banding agents used with acid-etching procedures produce high bond strength yal-ues, but these figtires should not be confijsed withlong-term resistance to microleakage, which has beenshown to occur even in the absence of gaps under aresin composite placed in Class V cavities with an

adhesive resin system,̂ ^ The dentinal surface and thesmear layer after tooth preparation show many varia-tions, with the result that short-term in vitro studies ofthe strength of dentinal bonds cannot always give theclinician an accurate picture of the future clinicalsituation, Paul and Schärer-'' believed that there hasbeen a discrepancy between the results of laboratorytesting and the in vivo performance of the dentinalbonding systems; they attributed the difference todentinal fluid, that under pressure, leaked out ofnumerous cut tubules and changed the conditions ofthe chemical reaction of the adhesive resin to thedentin (Fig 3), The interlocking ofthe bonding agentwith the collagen network of the intertubular dentin(see Fig 2), and not the tag formation into the tubuli(Fig 4), was considered to be the main substrate that

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Fig 5 Glass-ionomer cement used as a dentin subsf itute ina Class lil preparation.

yields high bond stretigths to dentin. It is nowgenerally agreed that where the area of iiitertubulardentin is maximal, as in outer dentin, better bondstrength figures can be obtained.

Glass-ionomer cements, being water-based materials,are not as affected by dentinal fluids, and, althoughthey exhibit lower tensiie bond strengths than do theresin bonding agents, the bond area generally shows acohesive fracture that reflects the low tensile strengthofthe cement and not the actual strength ofthe bond.Until a much stronger cement is produced, it will notbe possible to measure the true strength of this bond.For this reason, current use of glass-ionomer cementsas restorative materials should be confined to low-stress-bearing areas; their greatest value lies in bondingto dentin. In cavities where peripheral enamel bondingcan be achieved, the resin composite restoration usedwith a resin bonding agent is superior in strength,surface integrity, and esthetics.

Clinical use

There are two schools of thought with regard to theclinical use of dentinal bonding agents and glass-ionomer cement linings and bases. One group advo-cates the exclusive use of photocured resin bondingagents to form a hybrid layer in the dentinal surface,-'while the proponents of glass-ionomer materials rec-ommend, for the deeper cavity, placement of glass-ionomer bases as a dentinal substitute to which com-posite or other restorative materials can be attached-^(Fig 5).

The use of a single bonding agent and restorativematerial that can be attached to the tooth in oneprocedure appeals to the hard-pressed clinician but, aswith most dental materials and techniques, a price hasto be paid. Enamel and dentin are very differentsubstrates; to achieve long-term adhesion, the clinicianmust recognize this fact and not lightly dismisstechniques that may take more time but take account ofthese differences. The clinician is faced with innumer-able bonding systems, and making a correct choice foreach clinical case has become more and more difficultas the manufacturers move from one generation ofbonding agents to the next.

Dentinal bonding agents

Considerable support is emerging, both in the clinicaland research fields, for the use of mild acid etchants,such as maleic acid, to demineralize the intertubulardentin, allowing hydrophilic primers to infiltrate thecollagen network and form a thin hybrid layer or zoneof resin-impregnated dentin,""'-^ The only questionthat still remains unanswered is how long this seal willlast when placed under stress and subjected tolong-term exposure to oral fluids. As previouslydescribed. Sano et al^' have performed experimentsusing a cryo-scanning electron microscope and a silverion penetration method, Microleakage was observed,even in the absence of gaps, under a composite placedin Class V cavities with an adhesive resin system. Theysuggested that the bonding monomer was not fully ableto penetrate the demineralized dentin after phosphoricacid etching, thus leaving a porous zone as a pathwayfor microleakage beneath the resin-impregnatedlayer,̂ ^ The resistance ofthe restoration to long-termmicroleakage should be the main consideration whentechniques and materials are selected for adhesivebonding.

There is general agreement that the larger the bulk ofthe resin composite restoration, the greater the effectof stresses produced by polymerization shrinkage.'''^'In the large posterior restoration, bonding to cervicaldentin involves greater risks of microleakage becauseinner dentin has less intertubular dentin and becausethere is an increase in dynamic occlusal stresses.Whenever possible, the clinician should retain anenamel margin, particularly at the cervical area. Atpresent, the use of composites placed on dentinalmargins is better confined to small cavities andlow-stress-bearitig areas.

However, the use of resin bonding agents to seal

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dentin under porcelain, gold, or amalgam restorationsis enjoying greater success, although as yet much of theevidence for this is anecdotal. Pashley et al-' con-sidered the method worthy of further investigation andexamined the ability of six différent dentinal bondingagents to seal the dentin of crown preparations ofhuman teeth in vitro. They concluded that, althoughthe bonding agents tend to accumulate on chamfers,thereby increasing their thickness to 200 to 300 um,the method cotUd be a simple way to protect the pulpfrom the consequences of microleakage.

More recently, photocured resin bonding agentsbased on glass-ionomer technology have been devel-oped (Scotchbond Multipurpose, 3M Dental; PertacUniversal Bond. ESPE); carboxylic acid groups thatbecome available for attachment to dentin are incor-porated. In addition, these materials are useful forattaching resin composites to glass-ionomer cementsurfaces. This line of chemistry may have a promisingfuture, because the introduction of carboxylic acidgroups could faciiitate some chemical bonding. Thesenew bonding agents could be classified as glass-¡onomer bonding agents, because they have a dual rolein bonding to both dentin and glass-ionomer cement.

Swift and Triólo,'^ in an in vitro study on Scotch-bond Multipurpose Universal Dental Adhesive, foundthat dentin and even enamel bond strengths areimproved when the adhesive is applied to slightlymoist surfaces, a finding that mirrors the ideal surfacefor glass-ionomer bonding. They found that improvedbond strengths are obtained on enamel when thestronger 35% to 40% phosphoric acid is used insteadof the 10% maleic acid advocated by the manufacturer.The manufacturer has started to recommend thisprocedure, which, as previously discussed, iiiustratesthe difficulty of using one material or etchant tocondition enamel and dentin. In the pursuit ofuniversality, maximal physical properties are oftensacrificed on the altar of speed. The clinician is dealingwith two very different surfaces, and dentin is a vitaltissue that is better treated with mild etchants orsur&ce conditioners.^"

Glass-ionotner cetnents

The use of glass-ionomer cement as a base or dentinsubstitute for attaching composite restorations to toothstructure was ftrst described by McLean and Wilson-'in 1977. The main question today is whether cementbases are obsolete and have any value in operativedentistry. To answer this question it is necessary toagain consider the basic requirement of ail restor-

ations, resistance to microleakage. The efficacy ofcavity sealing by glass-ionomer bases has had a longhistory of success and has been confirmed in numerousclinical studies.'""-'-'" In addition, because the glass-ionomer cements liberate fluoride, they possess somecariostatic properties.'*'^" Provided that these cementsare placed on a clean dentinal surface, their long-termresistance to microleakage has been proven overperiods of more than 15 years. ̂ °

A fiirther advantage is that, when these cements areused as dentin substitutes in the so-called sandwichtechnique (see Fig 5 ), they reduce the bulk of overlyingcomposite and subsequent polymerization shrinkage.In the case of the Class II! restoration, a glass-ionomerbase can ofteti improve esthetics because the cement'stransmission of iight is ciose to that of dentin,preventing the halo effect sometimes observed with themore translucent microfilled composites.

Criticism is still leveled, however, at the laminate, orsandwich, technique and some in vitro studies appearto show leakage around glass-ionomer bases where anacid-etched composite has been inserted.^'•^- It ispostulated that the polymerization shrinkage of thecomposite restoration breaks the seal of the glass-ionomer cement to dentin. However, a recent studyfound no loss of seal in vital teeth, and microleakagewas more related to the atmosphere in which therestoration was placed.^^ In this in vivo test, thepartially humid environment favored the applicationprocess and, as previously discussed, the stress reliefexhibited by glass-ionomer cements may contribute tothis result.'^-' The use of glass-ionomer bases hasenjoyed considerable clinical success over the iast 20years in countries such as the United Kingdom andAustralia, and this anecdotal evidence should not belightly distnissed.

In the shallow cavity, loss of seal in the sandwichtechnique is often related to the strength and thicknessof the base used, and thin linings of less than 1.0 mmare not aiways satisfactory. Essentially, glass-ionomercement should be used as a dentinal substitute toprotect the dentin from any acid penetration duringinsertion of the composite. The cement itself should beprotected with a glass-ionomer bonding agent, aspreviously described, prior to acid etching (Fig 6). Inthe shallow cavity, the clinician should continue to usedirect dentinal bonding.

Resin-modißed glass-ionotner cements

The introduction of resin-mcdiñed glass-ionomercements that can be photocured has created great

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Fig 6 Glass-ionomer cement protected with a glass-ionomer bonding agent prior to the insertion of the resincomposite. The bonding agent wiil act as a separator andprevent slicking ol the appiicator during insertion of thecement.

interest. Their advantages are ease of placement,setting on command, and early resistance to moisturecontamination. It is not possible to photocure a regularglass-ionomer cement, and it is necessary to modify thepolyacid by grafting methacryiate groups onto thepoly (acrylic acid) chain. Because the modifiedpoly (acrylic acid) is less soluble in water than itsparent, hydroxyethyl methacrylate (HEMA) is addedas a cosoivent. When this hydrophilie species isincluded, the set cement will act. to some extent, like ahydrogel, swelling in water and becoming weaker.'''"^^In general, the greater the amount of HEMA in-corporated, the greater the swelling and reduction instrength,

A proposed classification for these new cements hasattempted io differentiate between the true glass-ionomer cement and the newer hybrid varieties":

1, The unqualified term glass-ionomer cement shouldbe reserved exciusively for a material consisting ofan acid-decomposable glass and a water-solubleacid that sets by a neutralization reaction,

2, Materials that retain a significant acid-base reac-tion as part of their overali curing process, ie, theywill cure in the dark, are classified as resin-modifiedglass-ionomer materials.

3, Materials that contain either or both ofthe essentialcomponents of a glass-ionomer cement but at levelsinsufficient to promote the acid-base curing reac-tion in the dark should be referred to as polyacid-modified resin composites.

Clinical use of resin-modijied and regular glass-ionomer cements

With the introduction of the photocured resin-modi-fied glass-ionomer cements, it might be thought thatthe regular glass-ionomer cements that set by anacid-base reaction could become obsolete. However,certain properties of the photocured materials needclose examination. The clinical implications for theswelling in water ofthe resin-modified materials haveyet to be established,^^ and their use as a base or corefor inlays or crowns could resuit in a misftt if thecement absorbs water and swells after the impression istaken. In the case of bases under resin composite oramalgam alloy restorations, the amount of swelling isunlikely to affect the stability of the restoration,provided that the resin-modified glass-ionomer ce-ment that is selected has a significant glass-ionomeracid-base reaction and will still cure rapitíly in the

has described a method of overcoming thestresses placed on glass-ionomer bases by the polymer-ization shrinkage of composites: the resin-modifiedglass-ionomer base and resin composite are curedsimultaneously. He postulated that the resin compositecures before the resin-modified glass-ionomer materialand that shrinkage stresses could be absorbed by themore plastic glass-ionomer base. This is an interestingapproach and deserves fUnher study, particularly whertconventional glass-ionomer bases are used.

If the resin-modified materials are used as a totalrestorative in Class III and Class V cavities, theimplications of the water uptake must be taken intoaccount, with regard to not only marginal adaptationbut also color stability. The formation of hydrogels inthe resin-modified glass-ionomer cements and sub-sequent swelling in water may result iti discolorationover time, and the results of long-term clinical trials arestili awaited. It is for this reason that research aimed atproducing faster-setting and stronger regular glass-ionomer cements that set by an acid-base reactionshould be continued, together with attempts to intro-duce alternatives to HEMA in the resin-modifiedmaterials.

The polyacid-modlfied composite materials alsorequire longer-tenn clinical trials, because althoughthey are stronger than either glass-ionomer or resin-modified materials, they are still significantly weakerthan regular hybrid or microfilled composites. Theyalso do not cure in the dark, which indicates theabsence of any significant degree of acid-base reaction.A clinical question still remains as to their perform-

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anee compared with that of the hybrid or small-particlecomposites, particularly in posterior restorations. Inaddition, there seems to be little evidence that thesematerials can adhere to dentin by chemical bonding, asoccurs vflth the glass-ionomer acid-base reactioncements, and they still require an acid-etching pro-cedure, as used for conventional dentinal bondingagents, to obtain high bonding strengths.

Properties of polyacid-modified resin composite andresin-modified iiiting cements

The introduction of these new cements has arousedconsiderable interest, because both materials arestronger and are claimed to have better resistance toearly solution than the glass-iononner luting cements,although no results have yet been published on theiractual loss of cement-forming ions or organic con-stituents when exposed to moisture. The physicalproperties of both types of material appear to indicatethat they could make a useful contribntion to im-proving the retention of crowns and inlays whenconventional cementation tecliniques are used, andthey are worthy of further clinical study

However, a warning should be given where directbondirtg of porcelain inlays or crowns is contemplated.Even in the case of the polyacid-modified compositecements, their mechanical properties are itiferior tothose of the current dual-cured or autocured resincements. The prevention of crack propagation ordebonding of porcelain restorations is dependent onthe strength of the bond between the porcelain surfaceand the tooth. Therefore, the strength of the bondingresin plays a vita] part in securing long-term resistanceto fracture or debondirtg of the restoration. At thisstage, more clinical evidence is needed before eitherresin-modified glass-ionomer or polyacid-modiliedreshi composite cements are used for purposes otherthan for conventional cementation.

Class V restorations and erosion/abrasion lesions

The retention rate of glass-ionomer cements in theerosion/abrasion lesion solely involving dentin isgenerally better than that of dentin-bonded compositerestorations. "•̂ "•̂ ^ For this reason, other questions ofchoice vWU depend on esthetic demands and themaintetiance of polished surfaces. Modem compositerestorations have great esthetic appeal, but both thestandard and resin-modified glass-ionomer materialshave been considerably improved with regard totranslucency and color. Although the composite res-

toration may exhibit superior color in the early years,the standard glass-ionomer cements can maintaingreater color stability because of their chemical stabil-ity.20,29..10 However, poor finishing techniques canproduce rough surfaces that stain, and preservation ofthe original gel surface should be the clinician's primeobjective, as described by McLean and Wilson^^ in1977. Provided that this goal is achieved, glass-ionomer cements are very durable in cervical restora-tions and compete with the composites, particularlywhere bonding to cervical dentin is required. Scleroseddentin remains the greatest obstacle to obtaining goodbonding with dentinal bonding agents, and failure atthe cervical margin as a result of microleakage is notalways easily detected.

Class V cavities may also be restored with glass-ionomer cements, but esthetic considerations takepriority when more extensive facial areas are involved,and the srttall-particle or microfilled composites aresuperior. The sandwich technique is only practical inthe deeper cavity and should be the first choice wherecaries control is a priority for the older patient.Shallow cavities are better restored with direct dentin-bonded composites, but again the area of cervicaldentin involved will influence the lotigevity of therestoration. An alternative is to place a conventionalglass-ionomer restoration, which can later be cut backand overlaid with an acid-etched composite.

Class II restorations

Direct bonding of composites to dentin in the pos-terior restoration is generally regarded as the mostchallenging operation by the clinician. He or she has tocontend not only with moisture control but also withdentinal surfaces that are not as receptive to hybridiza-tion. In addition, as described previously, the bulk ofthe restoration will determine the amount of polymeri-zation shrinkage and subsequent distribution of stress.

Unless the entire restoration can be bonded toperipheral enamel, a strong case can be made for usinga glass-ionomer base (conventional, cermet-based,^^or resin-modified) to act as a biologic seal and offerprotection to the surrounding tooth struettire throughthe release of fluoride. The glass-ionomer base alsoavoids the risk of damage by acid etching in the deepercavity, postoperative sensitivity is reduced, and hulkshritikage of the composite restoration is lessened.

The sandwich technique has been misapphed to thistype of restoration. Attempts to use glass-ionomercement as a base, so that the cement is extended to thesurface at the cervical margin, may result in dissolution

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ofthe cement, Glass-ionomer cements should only beused as internal bases. However, in a recent 3-yearclinical study comparing direct composite inlays withconventional restorations, Wassell et al'^ observed thatthere was no evidence of dissolution of a cermet base,even when it extended to the cervical margin, in eithertype of restoration, and no secondary caries wasdiagnosed. It is possible that the fluoride release andbetter abrasion resistance of this material may havecontributed to this result.

The imperfections of the large mesio-occlusodistalcomposite restoration are well known, and, despitecurrent attempts to ban amalgam alloy restorations, thepublic must be made aware of these deficiencies.Composites can provide good service in small Class Iand Class II preparations with minimal direct stress,because occiusal contact is generally confined to thetooth enamel""' and the restoration is to some extentprotected. However, despite improvements in thechetnistry and particle-size distribution ofthe fillers,the hydrolytic stability of the filler-matrix interface stillremains questionable and may result in loss of materialin ñinctional occlusion, where occlusal stability is sovital. Wear in the contact area also remains a prob-lem,"" Government health departments should bewarned that compulsory replacement of amalgam alloyrestorations with tooth-colored materials could provean expensive exercise in public health care.

Summary

Dentinal bonding agents and glass-ionomer cementshave a usefijl role in adhesive dentistry, and neithersystem has a monopoly on clinical success. It isimportant to recognize the optimal properties of eachmaterial when it is being selected for clinical use,

Glass-ionomer cements, because of their ability torenew broken bonds, have better cavity-sealing prop-erties and resistance to microleakage over longperiods. In addition, because of their ability to leachfluoride, they possess some cariostatic properties.They are particularly suited to the restoration oferosion/abrasion lesions and as dentinal substituteswhen resin composite or other restorations are placedand iong-term resistance to microleakage is a priority,Theh- excellent clinical performance has considerableclinical and scientific backing if confined to low-stress-bearing areas, and postoperative sensitivity isreduced particularly where stronger acid etching isemployed or faults in the dentinal bonding techniqueoccur.

In high-stress-bearing areas or when thin sectionsof material are required, glass-ionomer cements lackstrength and are easily damaged during function or byearly finishing procedures and contamination withmoisture. The modern hybrid and microfiHed compos-ites are superior in esthetics, strength, and retention ofsurface polish where large areas of facial enamel areinvolved.

Dentinal bonding agents have been firmly estab-lished for bonding anterior resin composites, porcelaitiveneers, inlays, and some metal restorations to toothstructure. They possess higher bond strengths than doglass-ionomer cements but require greater attention tothe preparation of surfaces for bonding. Modemtechniques are employing weaker acids, such as maleicadd, to prevent damage to the pulp in the deepercavity, and these etchants facilitate the fonnation of ahybrid layer. The success of dentinal bonding is stilldependent on the morphology ofthe dentin, and inareas lacking a high percentage of intertubular dentin,problems can arise.

Judgments on the merits of using glass-ionomercement bases or resin dentinal bonding agents that arebased on short-term in vitro testing on extracted teethare contradictory. Long-term clinical trials are stillneeded to establish the veracity of this testing.

Future research on dentina! bonding agents em-ploying glass-ionomer technology could provide somechemical bonding as well as materials with increasedfracture toughness, negligible setting shrinkage, andthermal expansion similar to that of tooth structure.Long-term stability at the enamel-dentin interface canonly be achieved with restorative materials that havethese properties.

Acknowledgment[ am indebted to my daughter Dee McLean for the ¡lltjsuation.

References1, HageerO, Swss patent 278946; Briüsh patent 687299, 1951.2, Kratner IRH, McLean JW, Alterations ¡n the staining reaction of

dentine resulting ftom a constituent of a new self-polymerising resin.BtDentJ 1952i93:l50-15.T,

3, McLean JW, Kramer IRH, A clinical and pathological evaluation ofa sulphinic acid activated resin for use in restorative dentistr>', BrDent J 1 9 5 : Í 9 3 Í 3 5 5 - 2 6 9 , 291-293

4, BuonocoreMG, Quigley M, Bonding ofasjnlhetic resin materialtohuman dentin: Preliminary study ofthe bond area J Am Dent Assoc1958;;7,-807-8]2,

5, McLean JW, Some physical properties ofa new cross-linked plasticTilling material. Br Dem J 1961^10:375-378,

6, Bowcn RL, Denial filling materials comprising vinyl.silane treatediliscd siiica and a binder consisting of the reaction product ofbispheno! and glycidyl methacrylate. US patent 3,066,112, 1962

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7, Knight MH, Milcliel B, Ratcliffe MI. Vmyl urethane resins, Britishpatent 135:O6J, 1971,

S, Forster I, Walker RJ. Dental filling materials. British patent1,401.505, 1975.

9, Forster J, Dental filling materials, British patent 1,430,303, t976,10, Dan EC, NemcQck J, Ptiotopolymerisable composiliün having as

catalyst alpha-dikelone and a reducing agenl, e.g. DMAEM, Britishpatent 408,265, 1971,

t l , Nakabayashi N, Nakamura M, Yasuda N. Hybrid layer as adentin-bond ing mechanism, J Esthet Dent 1991^3:13.1-138,

t2, Asmussen E, Miinksgaard EC, Adtiesion of restorative resins todentinat tissues, tn: Vanherle G, Smith ElC (eds). Proceedings of anIntemalional Symposium on Poslerior Composile Resin Denta!Restorative Materials. Si Paul, MN: 3M, 1985:217-238.

13, Walts A, Paterson RC, A review of current proprietary bondingsystems. Rest Dent 1991;7:56-61,

14, Eiek JD, Cobb CM. Ctiappell RP, Spencer P, Robinson SJ, Thedentinal surface: Its influence on denlinal adhesion. Pan I.Quintessence Inl t99t;22:967-977,

15, Walson TF, Bartlett DW. Adhesive systems: Composites, dentinebonding agents and glass ionomers. Br DentJ t994it75:227-231.

16, Pashley DH, Pashley EL, Dentin permtabitity and restorativedentislri: A status reportfor the American Journal of Dentistry, AmJDenl t991;4:5-9.

17, Pashley DH, The effects of acid etching on the pulpodentin complex,Buonoeore Memonal Lecture. Oper Dent t992;17:2;9-24;,

18, Johnson GH, Powell LV Gordon GE, Dentin bonding systems. Areview of current products and techniques, J Am Dent Assoct99t;122:34-41.

19, Erickson R, Giasspoole EA. Bonding to tooth siructure; Acomparison of glass-ionomer and composite resin systems, J EsthelDent 1994;6:227-244.

20, Wilson AD, McLean JW, Glass-ionomer Cement, Chicago; Quint-essence, 1988:83-99, 107-115.

21 Davidson CL. Glass-ionomer bases under posterior composites, JEsthel Dent 1994:6:223-226,

22, Beech DR, Bonding of restorative resins to dentine. In: Vanherle G,Smith DC (eds). Proceedings of an Intemalional Symposium onPosterior Composite Resin Dental Restorative Matenals, Si Paul,MN: 3M, 1985:231-238,

23. Sano H, Shona T, Takatsu T. Hosoda H, Microporous dentin ionehenealh resin impregnated layer, Oper Dent 1994; 19:59-64.

24. Paut SJ, Schârer P. Factors in dentin bonding. Part I. A review ofthemorphology and physiology of human dentin, J Esthet Dent1993i5:5-8,

25, Cox CF, Suzuki S, Reevalualing ptjip protection: Catcium hydrox-ide liners vs, cohesive hybridization. J Am Dent Assoc 1994;125:823-831.

26. McLean JW, Prosser HJ, Wilson AD, The use of glass-ionomercements in bonding composite resins to dentine. Br Denl J198i;158:410-414.

27. Pasbtey EL, Comer RW, Simpson MD, Horner JA, Pashtey DH,Caiigbman WF, Dentin permeability: Seating the deniin in crownpreparations. Oper Dent 1992;17:t3-2O,

28. SwiflEI, Triólo PT, Bond strengths of Scotehbund Multi-Purpose tomoist dentin and enamel. Am J Dent I992;5:3IS-32O,

29. McLean JW, Wilson AD, The clinical development ofthe glass-ionomer cement, II, Some clinical applications, Aust Denl J1977;22;12O-I27,

30. Mount GJ. An Atlas of Glass-ionomer Cements, ed 2. London:Martin Dunitz, 1994:1-31,

31. Dietschi D, De Siebenthal G, Neveu-Rosen stand L, Hoiz J.Inflticncc of the reslorative technique and new adhesives on thedentin maiginal seal and adaptation of resin composite Class IIrestorations: An in vitro evatuation. Quintessence Int 1995i26:7t7-727,

32. Wieczkowski G, Joynt RB, Davis EL, Yu XY, Cleary K., Leakagepatterns associated with gtass-ionomer-based resin restorations,Oper Denl t992;17 21-25.

33. Mason PN, Ferrari M. In vivo evaluation of glass-ionomer cementadhesion to deniin. Quintessence Int I994;25:499-5O7.

34. Nicholson JW. Anstice HM. McLean JW, A preliminary report onthe effect of storage in water on the properties of commerciallight-cured glass-ionomer cements, BrDentJ 1992:173:98-101.

35. Anstice HM, Nicholson JW. Studies on the structure of light-curedglass-ionomer cements J Maler Science Mater Med I992;3:447-455.

36. Wilson AD, McLean JW, Heller]. J Am Dent Assoc 1994;125:1046-1048,

37. McLean JW, Nicholson JW, \Wlson AD. Proposed nomenclature forglass-ionomer denial cemenls and related materials, QuititessenceInl 1994:25:587-589,

38. Knight GM, The co-cured light-activated glass-ionomer cement-composite resin restoration. Quintessence Int 1994^25:97-100,

¡9. WassellRW.WallsAWG, McCabe JF, Direct composile inlays versusconventional composite restoraliuni: Three-year clinical results. BrDentJ 1995;l79:343-349,

40. Bayne SC, Heymann HQ, Swift EJ, Update on dental compositerestorations, J Am Dent Assoc 1994:125:687-701.

41, ZiemieckiTL.WendtSL,LeinfelderKF. Methodology for proximalwear evaluation in posterior resin composites. Am J Dent 1992;5:202-207, O

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