Dental Research

Moisture susceptibility of resin-modified glass ionomer materiaisElizabeth Cho*/Hugh Kopel**/Shane N. Wliite***

The purposes of this experiment were to determine if resin modified glass-ionotnercements are less sensitive to moisture than are conventional glass-ionomer cetnents, toinvestigate the effects of barrier coatings, and to study the effects of different settingenvironments. Discoid specimens of a variety of resin-modified glass-ionomer materialsand a conventional glass-ionomer cement control were stored in different environmentsand were protected with different barrier coatings. The diametral tensile strengths ofthespecimens were determined and analyzed with three-way analysis of variance. Resin-modifiedglass-ionomer cements are less sensitive to moisture than is the conventional gla.ts-iononier cement control. Drier environments produced stronger resin-tnodified glass-ionomerspeciinens. Use of a fissure sealant as a barrier coating increased overall specimenstrength, and the individtial materials differed in strength.{Quintessence Int ¡995:26:351-358:)

Introduction

Conventional glass-ionomer materials arc sensitive tomoisture contamination during setting.' These mate-rials set by a complex series of acid-base reactionsbetween aqueous polyaciylic acid and aluminosilicateglass particles,^ Initial setting occurs in a few minutes,but precipitation, gelation, and hydration occur for atleast 24 hours, and setting continues slowly for muchlonger periods,''' A moist environment is necessary forall these reaction steps, but the presence of too muchwater may dissolve the reactants and prevent agglom-

* Clinical Assistant Professor, Depanment of Pédiatrie; Dentistry;Llniversity of Southern California, School of Deniislry, Los Angeles,Califomiai Head. Division ofPediatric Dentistry', Children's Hospi-tal Los Angeles. California,

*'• Professor Emeritus, Deparlment ofPediatric Dentistry. University ofSoulhern California, Sehool of Dentistry', Los Angeles, California;Division ofPediatric DentisLry. Children's Hospital, Los Angeles.California.

" • Assistant Professor and Director of Clinical Research, Depanmentof Restorative DenLislry/Biomaterials, Pincus Biomatcrials Re-search Laboratory', University of Southern California. School ofDentistry, Los Angeles, California,

Repnnt requests: Dr S, N, White, Pincus Biomaterjals ResearchLaboratory, University of Southern California, School of Dentistry No.4il2, Los Angeles, California 900S9-064L

eration ofthe setting matrix. Therefore, sensitivity ofconventional giass-ionomer materials to salivary con-tamination or to desiccation is considered to be animportant clinical problem,'

Recently, resin-modified glass-ionomer cementshave been introduced.*"' These materials containmodified polyacrylic acid polymers that may copoly-merize with other resins as well as participate in theconventional setting reaction. Thus, linked glass-ionomeric and resinous phases are formed. Setting ofthe resinous phase can be achieved by light-initiatedpolymerization, Resin-modified glass-ionomer ce-ments are known to absorb water,"*" but theirsensitivity to moisture has not yet been compared tothat of conventional glass-ionomer cements. Earlypolymerization of a resinous matrix might make thesematerials less moisture sensitive than conventionalglass-ionomer materials. Substantial clinical advan-tages would be provided by use of less moisture-sensitive materials and barrier techniques.'^"''' There-fore, it is important that the moisture sensitivity ofresin-modified glass-ionomer cements be investigatedand compared to that of conventional glass-ionomercements.

The purposes of this study were to compare themoisture susceptibility of resin-modified and conven-

2R NuNumber 5/1995 351

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tional glass-ionomer materials, to investigate theeffects of barrier coatings, and to study the effects ofdifferent setling environments.

Method and materials

The foiiowing resin-modified glass-ionomer materialswere investigated: Fuji II LC (FUJ) (GO; Geristore(GER) (Den-Mat); Variglass (VAR) (Caulk/Dentsply);and Vitrebond (VIT) (3M Dental). One conventionalglass-ionomer cement control. Ketac-Bond ApHcap(KET) (ESPE), was also included.

Discoid specimens, 6 mm in diameter and 3 mmthick, were made in Teflon molds in an atmosphere of6Û/0 humidity at 24°C. The molds were filled with asingle mix of material, covered with Mylar films, closedbetween glass plates, which were firmly clampedagainst the molds, and hght cured (Optilux 401,Demetron) according to manufacturers" instructions.Nine minutes after the start ofthe mix, the Mylar waspeeled off and the specimens were extruded from themoids and treated with a protective coaling,- Thus, thespecimens were isoialed from the start ofthe mix untilthe 9th minute,- and coatings were placed belween the9th and 10th minutes, after which the specimens wereplaced in their storage environments.

Three types of barrier protection were investigated:no protection (NO); a coating of petroleum jelly (VAS)(Vaseline, Chesebrough Ponds); and a coating of anunfilled light-curing fissure sealant (FS) (Delton,Johnson & Johnson), Barrier coatings or varnishesknown to contain organic solvents were not used inthis study because their potential interference withresinous setting reactions couid have had confoundingeffects. To determine if the fissure sealant had amechanical strengthening effect, five diametral testspecimens of solid fissure sealant were made asdescribed previously.

Ten minutes after the start of the mix, the barrier-coated specimens were transferred to their storageenvironments for 24 hours at 37°C, Three storageenvironments were studied; distiiled water (WAT):100% humidity (HUM); and desiccation, iess than0.01% humidity (DES), The desiccating environmentwas included so that the natures of the dominantsetting reactions, whether aqueous acid-base means orresinous polymerization, could be determined.

For each material, the following environment-protection combinations were investigated: WAT-NO;WAT-VAS: WAT-FS: HUM-NO; and DES-NO, Otherpossible enviromnent-protection combinations were

nol tested because they lacked clinical relevance anddid not assist the aims of this study. Ten specimenswere included in each material-environment-protec-tion group.

After storage, the specimens were mounted diamet-rically between disks of blotting paper on the hardenedsteel platens ofa universal testing machine (Instron),The blotting paper was used to prevent undue con-centration'^"-" of stress. The test specimens wereloaded at a constant strain rate of 0.05 cm/min, andstress was plotted against time with a chari recorderused at a sweep rate of 10 s/inch so that deformationcould be noted. Diametral tensile strengths, a, werecaicuiated for each specimen according to the formula

a - 2P/7idtwhere P is the load at failure, d is the diameter, and t isthe thickness,'^"-" Mean diametral strength values andtheir standard deviations were calculated for eachmaterial-environment-protection group. Diametraltensile strength was chosen to be the physical propertyinvestigated in this study, because specimens for thistest have a high surface area-volume ratio, whichensures that surface changes will profoundly infiuencespecimen properties, Furihermore, glass-ionomer-based materials are known to be susceptible to tensile

The effects of the three main factors of material,storage environment, and protection, as well asmaterial-envi ron ment and material-protection interac-tions, were determined by a three-way analysis ofvariance (ANOVA) (P< ,05), Investigation of possi-ble environment-protection interactions was pre-cluded, because not all possible combinations ofenvironment and protection types were studied. Over-all differences among materials, environments, andprotection types were determined by multiple-com-parisons testing with Tukey's honest least significantdifference method (P< ,05),

Differences caused by different environments andprotection types for each material were investigated byuse of a series of one-way ANOVA and Tukey tests(P< .05), Differences caused by different materials foreach environment-protection group were investigatedwhh a series of one-way ANOVA and Tukey tests(P<.OS).

Results

The five solid fissure sealant specimens all underwentconsiderable plastic deformation at minimal loads ontesting. Therefore, differences associated with fissure

352 Quintessence International Volume 26, Number

Dental Research

Table 1 Mean (SD) diametral strenths (MPa)

Environment

WAT WAT WATProtection^

Material* NO VAS FS

HUM

NO

DES

NO

KET

VIT

GER

FUJ

YAR

20,0(3.9)

11.1(1.4)

16.3(3.6)

15.8(0.9)

23.5(4.2) 21.1(5.7)

25.2(4.8)

22.4(1,7)

28.2(3,3)

20.0(2.1)

20.9(1,8)

24.7(3.0)

27.7(3.0)

26.1(1.4) 37.4(2.4)

24.8(3.3)

18.4(1.5)

25.2(3.0)

27.5(1.8)

20.7(2.4)

8.0(2.9)

29.5(1.7)

29.5(2.1)

35.8(5,1)

34.5(3.3)

Similar environment-protection groups are linked by horizontal lines, and similar material groups are linked by verticallines.* WAT = distilled water; HUM = 100% humidity; DES - desiccation.t NO = no protection; VAS = petroleum je Lly; FS - fissure sealant.± KET = Ketac-Bond; VIT - Vitrebond; GER - Geristore; FUJ = Fuji II LC; VAR - Variglass.

Fig 1 Group means by material. Each material is identiiiedby simiiariy coiored bars oriented from ieft to right. Eachenvironment-protecticn lype is oriented from front to back.The tauest columns are the strongest. Desiccation (DES);no protection (NO); 100% humidity (HUM); dfStilled water(WAT); fissure sealant (FS); petroleum ¡elly (VA); Ketac-Bond(KET); Vitrebond (VIT); Geristore (GER); Varigiass (VAR); FujiII LC (FUJ).

Fig 2 Group means by en^/ironment-protection group.Each environment-protection type is identified by similarlycolored bars oriented from lett to right. Each maleriai isoriented trom tront to back. The tauest columns are thestrongest. Fuji 11 LC ÍFUJ); Variglass (VAR); Geristore (CER);Vitrebond (VIT); Ketac-Bond (KET); distilled water (WAT); noprotection (NO); petroleum jeliy (VA); fissure sealant (FS);100% humidity (HUM); desiccation (DES)

sealant coatings were not caused by mechanicalstrengthening but were attributed to a barrier-sealingeffect.

Five of 10 \*lt-Wat-No specimens exhibited notice-able deformation during tesüng, precluding calculationof their diametral tensile strengths; these specimens

were eliminated from the analysis. All other specimensproduced diametral fractures and exhibited no notice-able deformation prior to fracture, thus allowingcalculation of their diametral strengths.

Mean diametral strengths are displayed in Table 1and Figs 1 and 2. Means ranged from only 8.0 MPa

Q u i ntuuuuiiuc II i :6, Number 5/1995 353

Dental Researoh

30

20

10

WAT

^

HUM

\

\

FUJ

GER

DES

Fig 3 Interaction ot material and environmenl. The resin-modiiied glass-ionomer cements all behaved in a fairlysimilar manner because their interaction line plols are allapproximately parallel. However, the conventional glass-ionomer cement behaved in a strikingly different manner,indicated by its divergent plot. Variglass (VAR): Fuji II LC(FUJ); Genstore (GËR}; Vitrebond (VIT); Ketao-Bond (KET);distilled water (WAT); 100% humidity (HUM); desiccation(DES).

Fig 4 Interaction of material and protection. The materialsgenerally behaved in different ways in response lo differenttypes of protection because few of their plots are parallel.This is suggestive of differences in formulation and settingmechanism. Variglass (VAR); Fuji II LC (FUJ); Vifrebond (VIT);Gerisfore (GER); Ketac-Bond (KET); no protection (MO);petroleum jelly (VAS); fissure sealant (FS).

Table 2

squaresMain-effect three-way ANOVA, sums of Table 3 Least square means (MPa) and multiple

comparsions tests

Source of

variationSum ofsquares

Level Mean StE

df F ratio

Material (M)Environment (E>Protection (P)

2880

1153

0865

Interaction M * E 3724Interaction M * P 0945

Residual 5250 220

302418

2005

<.OO1< .001<,001

< .001

< .001

Total (corrected) 16255 255

(KET-DES-NO) to as great as 37,4 MPa (VAR-WAT-

FS). Such large differences are expected to have

clinical relevance.

The main efFects of material, environment, and

protection, as well as material-environment and

material-protection interactions, all had highly signif-

icant effects on the diametral tensile strength of

specimens (Table 2). An interaction plot of material

type and environment type (Fig 3) showed that the

conventional glass-ionomer cement control (KFT)

behaved in a different manner than did resin-modified

Grand mean

Material"

VAR

FUJGERVIT

KET

Environment

DES

HUMWAT

Protect i o ir

245

5050504550

5050

145

26.0

32.131.325.624.516.4

29,7

25.6

22.7

0.5

1.21,2i.21.31,2

0.9

0.9

0.4

FSVAS

NO

50

50145

29.4

24,823.7

0.9

0.90.4

Similar overall groups are connected by vertical line

" VAR = Variglass; FUJ = Fuji II LC; GER = Geristore-VIT = Vitrebond; KET = Ketac-Bond.

t DES = desiccation; HUM - \m% humidity;WAT = distilled water.

X fissure seaiand; VAS = petroieum jelly;NO = no protection.

354 Quintessence |njernational Vr>li im» ?p 5/1995

Dental Research

Table 4 One-way ANOVA for materials

Material

KET

VITGERVARFUJ

Sum of squaresbetween groups

15591519374

2169656

df

44444

Sum of squareswithin groups

05090851070124500738

df

4540454545

F ratio

3418061010

P

< .0001< ,0001< ,0001< ,0001< ,0001

KET = Ketac-Bond; VIT = Vitrebond; GER = Geristore; VAR = Variglass; FUJ = Fuji II LC,

Table 5 One-way ANOVA for environment-protection groups

Group

WAT-NOWAT-VAS

WAT-FSHUM-NODES-NO

Sum of squaresbetween groups

7541263

1958538

5045

df

44

444

Sum of squareswithin groups

870791

1011962

6661

df

4045454545

F ratio

0918220635

P

< .0001< .0001< .0001

,0004< ,0001

WAT = distilled water; NO - no protection; VAS = petroleum jelly; FS - fissure sealant; HUM - 100% humidity;DES = desiccation.

glass-ionomer materials (VAR. FUJ. VIT, and GER)when setting took place in a desiccator. Unlike theresin-modified glass-ionomer cements, the convention-al glass-ionomer cement was adversely affected by adry environment, A statistical interaction plot ofmaterial and protection method revealed many dis-similarities in the behavoir of the diiferent materials(Fig 4); this is indicative of differences in formulationor setting mechanism among the materials tested.

Overall trends were discerned by muhiple-com-parisons testing (Table 3), Materials were ranked intothe following groups, from strongest to weakest: FUJ.VAR > VIT. GER > KET. Each environment type wasdifferent from the others, and the rank order from mostfavorable to least favorable was DES > HUM > WAT.Protection with a coating of fissure sealant producedsignificantly stronger specimens than did coating withVaseline, which was no better than immersion in waterwithout any protection: FS > NO, VAS,

The one-way ANOVAs showed highly significantdifi"erences among different environment-protectiongroups within all materials (Table 4) and highlysignificant difTerences among difFerent materialswithin each environment-protection group (Table 5).The multiple-comparsions tests disclosed whichgroups were similar to other groups (see Table 1 ).

Discussion

This study showed that resin-modified glass-ionomercements were less affected by moisture than was aconventional glass-ionomer control. Results also re-vealed that a fissure sealant barrier coating had abeneficial effect and that the driest environmentproduced the strongest specimens.

All materials exhibited some moisture sensitivitybecause differences were found among environment-protection groups for all materials. However, in

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Dental Research

contrast to the resin-modified glass-ionomer cements,the conventional glass-ionomer cement groups storedin water were all significantly weaker than those storedin 100% humidity. Additionally, when stored in adesiccated environment, the conventional glass-ionomer cement was significantly weaker than theresin-modified glass-ionomer materials. Therefore, thelight-curing glass-ionomer materials were less suscep-tible to moisture than was the conventional glass-ionomer control material, which may be a substantialclinical advantage.

Storage in water produced the weakest specimens,storage in 100% humidity produced intermediatestrengths, and storage in a desiccator produced thestrongest specimens overall. Unfortunately, the oralenvironment cannot be changed for long periods, butthe dentist can select the materials and protectionmethods that will perform best in the wet oralenvironment.

The conventional glass-ionomer cement was adverselyaffected by both water and desiccation, which isconsistent with its acid-base setting reaction mecha-nism'-' and with the results of prior investigations,"-'Therefore, as first recommended by Wilson and Kent,'conventional glass-ionomer restorations should not bedesiccated by excessive drying with an air syringe or beexposed to saliva until initial setting has occured.

In contrast, the resin-modified glass-ionomer mate-riais were less adversely affected by water and werestrengthened by desiccation. This is consistent with asetting mechanism dominated by resinous polymeriza-tion and is inconsistent with one dominated byacid-base reactions,' ' Furthermore, a plot of statisticalinteractions ofthe materials and environments showedthat the conventional material behaved differently thandid the resin-modified materials (see Fig 3), Thissuggests that the dominant setting mechanism oftheresin-modified materials is resinous polymerizationrather than ionomeric acid-base reactions. However,the results of this study do not exclude the possibilitythat an acid-base reaction occurs in any of thematerials tested. Different mechanisms may piay differ-ent roles during different stages ofthe setting reaction,and because the data in this study were recorded at 24hours, the conclusions are limited to the overall result.

Resin-modified glass-ionomer cements' have beendescribed by several different names. They are morecommonly known as light-curing giass-ionomer ce-tnent.f or as glass-ionomer/resin-eomposite hybrids.''However, none of these names is ililly satisfactorybecause, as suggested by this study, some materials may

posses minimal or no convenrional acid-base glass-ionomer cement setting reactions. Perhaps, materialslacking any acid-base reaction should be calledfittoroaluminosilicate glass-modified resin composites.A consensus on nomenclature has not yet beenreached. However, a recent editorial by McLean et aP^suggested that gias.s-ionomer cement be defined as "Acement that consists of a basic glass and an acidicpowder which sets by an acid-base reaction betweenthese components," They suggested that "glass-ionomer hybrid materials," setring partly by an acid-base reaction and partly by polymerization, be calledresin-modified glass-ionotner.-^^ They suggested a thirdterm, polyacid-modified composite resin, to describematerials containing "either or both of the essentialcomponents of a glass-ionomer cement but at levelsinsufficient to promote the acid-base reaction.""However, the same authors have used different termi-nology in other recent publications,'•'°'^'' Additionalresearch should more fully characterize the compo-nents and setting reactions of current materials,allowing an accurate application of the proposednomenclature.

Although the results of this study showed thatdesiccation improved the tensile strength of all resin-modified glass-ionomer cements, a prior study onClass V lesions reported that dehydration duringsetting caused cracking of Vitrebond and adjacenttooth structure,^^ Those cracks may not have beencaused by desiccation per se, but may have beensecondary to excessive shrinkage produced by loss ofsorbed water and by polymerizarion contraction.'"Excessive contractile stresses within those restorationswere not relieved because the tooth-restoration hondremained intact," thus causing tensile failure oftherestorative material and surrounding tooth struc-ture,"'^^ Therefore, deliberate desiccation of settingresin-modified glass-ionomer cements is not recom-mended. However, carefiil isolation and early lightcuring are recommended because they increase bondstrength and protect the unset material from moisturecontamination,-'

The type of barrier coating protection used had asignificant effect on strength. Overall, a fissure sealantcoating was superior. Therefore, as suggested byGarcia-Godoy,'^ it is recommended that fissure sealantcoatings be routinely placed over both conventionaland resin-modified glass-ionomer restoraüoiis." Vas-eline had no overall benefical effect and had individualdeleterious effects on some materials (KET and GER)The deleterious effect of Vaseline on the conventional

356 Quintessence International Volume 26,

Dental Research

glass-ionomer cement may have been caused byblockirtg of metal salt bridging by Vaseline that wasattracted to the nonpolar parts of polyacrylic polymers.

The type of material used had a significant effect onstrength. The resin-modified materials VAR, FUJ, andGER were as strong as, or significantly stronger than,KFT, the conventional material, in all environment-protection groups. Therefore, based on diametraltensile strength and early water sensitivity, it isrecommended that the resin-modified materials VAR,FUJ. and GER be used as alternatives to the conven-tional material, KET, The diametral strengths reportedin this study are similar to those reported in priorstudies of conventional and resin-modified glass-ionomer materials,'"-^'^''---*-^

The results of this study are also consistent withthose of prior studies by Nicholsen et al'" and Ansticeand Niciiolson," They found that storage in waterresults in decreased compressive strength and in waterSorption for two of the first commercially availableresin-modified glass-ionomer cements. However, theydid not include a conventional glass-ionomer cementas a control. They noted that all specimens stored inwater underwent plastic deformation prior to fail-ure, '*'•" However, in the current study, plastic deforma-tion was only noted for five VIT specimens stored inwater without protection. The greater incidence ofplastic deformation reported in some prior sludiescould be accounted for by differing test geometry,specimen sizes, materials, storage conditions, andstrain rates."'•-'"^'

Because the clinical consequences of in vitro phe-nomena and physical properties are unknown, clinicaltriais should be instigated, Resin-modified glass-ionomer materials have many desirable properties,including adhesion, fiuoride release, strength, pleasingesthetics, the ability to set on command, and ease ofygg i,s,w,2i,22,27-2s,32 j^^j undesifabk properties, such ascontinued water sorption, loss of ionomeric matrix-forming metal ions, and plastic deformation, have alsobeen reported,"*'""

Summary

1. Resin-modified glass-ionomer cements were iessmoisture sensitive than was a conventional glass-ionomer cement control.

2. The dominant setting mechanism of resin-modifiedglass-ionomer materials was resinous polymeri-zation.

3, Drier storage environments produced strongerresin-modified glass-ionomer cement specimens,

4, A conventional glass-ionomer cement was ad-versely affected by water and by drying,

5, Use ofa fissure sealant barrier coating significantlyincreased overall specimen strength.

6, Various resin-modified glass-ionomer materialsexhibited significantly different strengths.

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358

An IntroductionPhilip Worttiington. Brien R, Lang, and William E. LaVelle

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