Dental Research

Properties of resin-modified glass-ionomer restorative materialsand two polyacid-modifíed resin composite materials

Thomas Attin*/Michael Vataschki*/Eltnar Hellwig**

Abstract The objective of the study was to evaluate the physical properties of fourresin-modified glass-ionomer cements (Fuji ¡I LC, ¡onosit Fil, Vitremer,Photac-Fil) and nvo polyacid-modified resin composite materials (Dyract andVariglass VLC)). They were compared with a hybrid resin composite (blend-a-liix)and a chemically cured glass-ionomer cement (ChemFil Superior). The eom-pressive strength, flexura! strength, modulus of elasticity, and surface micro-hardness of the resin-modified giass-ionoiner materials and the poiyac id-modifiedresin-composite materials were inferior to those of the hybrid resin compositeand similar to those of the conventional glass-ionomer cement. The hybridresin composite exhibited the lowest resistance to wear caused by brushing. Some ofthe materials showed a marked decrease in hardness at depths exceeding 2.0 mm.Generally, the strength properties of the tested resin-modified glass-ionomermaterials and the polyacid-modified resin composite materials were inferior tothose of the hybrid resin composite. (Quintessence Int 1996:27:203-209.)

Clinical relevance

To achieve a sufficient depth of polymerization, thetested resin-modified glass-ionomer materials andthe polyacid-modified resin composite materialshould not he applied in increments exceeding 2 mm.In occlusal loaded areas, the use of the materialsshotild be restricted to small cavities without pre-paration of bevel-shaped cavosurface margins.

Introduction

Glass-ionomer cements are becoming increasinglypopular in restorative dentistry. They are used espe-cially in pédiatrie dentistry and for the restoration oferoded tooth areas,''^ However, some shortcomings in

* Assistant Professor, University Clinic of Dtntistry, Albert LudwigsUniversity, Freiburg, Germany.

*" Professor and Chaiiman, University Clinie of Deniistiy, AlbertLudwigs University, Freiburg, Germany,

Reprint requests: Dr Thomas Attin, Assistant Professor, UniversityClinic of Dentistry, Albert Ludwigs University, Hugstetter Strasse 55,D-79106, Germany,

physical properties and sensitive handling characteris-tics limit their routine use as a restorative material inpermanent teeth.-' A new generation of resin-modified(visible light-cured) glass-ionomer materials was in-troduced as liners and base materials to overcome theshortcomings of conventional glass-ionomer cements,''

Recently, new types of resin-modified glass-ionomer materials and a polyacid-modified resincomposite material (PMRC) have been introduced.These new materials were developed as restorativematerials for Class I, Class III, or Class V cavities. Theresin-modified glass-ionomer cements have to bemixed before apphcation.

The disadvantages of conventional glass-ionomercements include the brittleness and poor fracturetoughness of the materials,' Moreover, the hardenedsurface of glass-ionomer cement has low wear resis-tance against occlusal forces,̂ Glass-ionomer cements,which are frequently used as a restorative material forClass V cavities, are additionally subject to abrasionfrom toothbrushing.

Little is known about the fracture toughness,brittleness, and brushing abrasion of the newly devel-oped resin-modified restorative materials. Compres-

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Attin et al

Table ¡ Restorative materials tested

Material

Dyract

blend-a-luxPhotac-RI

ChemFil Superior

lonosit FilVi tremerVariGlass VLC

Fuji 11 LC

Manufacturer

DeTrey DentsplyBlendax

ESPEDeTrey Dentsply

DMG3M DentalDeTrey Dentsply

GC

Type

PMRCL hybridRM GIC

GICRM GIC

RM GICPMRC

RM GIC

Batch No.

KL 15-72-293230002007X 031

921214307067

199304169202265 (powder)930617 (liquid)070523 (powder)281021 (liquid)

Color

A3U/A3

A3

LYA3A3U

A3

Mixing

Capsule (no mixing)

Capsule (no mixing)Predosed capsule

Predosed capsulePredosed capsule

Manual

Manual

Manual

PMRC - pol>acid-inodified resin composite materialRM GIC = resin-modified glass-ionomer cement;L hybrid - light-eured hybrid resin composite^GIC - glass-Lonomer cement.

sive strength, fiexural strength, and modulus of elasti-city are physical parameters for fracture tougliness andbrittleness of a material.̂ -̂ Moreover, light-curingmaterials must be inserted in cavities in increments toprevent the occurrence of unpolymerized material indeeper layers.' Deeper layers may not be polymerizedeven if polymerization of the superficial layer iscomplete; deep unpoiymerized material may weakenthe restoration against occlusal loads. Therefore thedepth of cure of visible light-cured materials is ofinterest for the practical handling ofthe materials.

The purpose ofthe present study was to evaluate thedepth of cure, surface microhardness, compressivestrength, fiexural strength, modulus of elasticity, andbrushing abrasion of four resin-modified glass-ionomer restorative materials and two PMRCs. Theproperties ofthe materials were compared to those of ahybrid resin composite and a conventional glass-ionomer cement.

Method and materials

The restorative materials tested in the study arepresented in Table 1. All materials were preparedaccording to the manufacturer's instructions. Thelight-cured materials were photocured (Optilux 400,Demetron Research) as described in the followingtestitig sections and removed from the molds. Thespecimens prepared for determination of compressive

strength and fiexural strength were additionally trans-ferred to a light-curing oven (PLC Schütz Dental) toachieve complete polymerization of the specimens.This procedure was necessary because the size of thesespecimens did not allow homogeneous polymerizationwith the light-curing unit (Optilux 400). After theywere light-cured, the specimens were stored in distilledwater for 24 hours at 37°C. The chemically curedglass-ionomer cement specimens were varnished andstored in 100% humidity for 24 hours prior to beingtested.

Hardness testing

Five cylindrical specimens (2.0 mm in height and 10.0mm in diameter) of each material were fabricated in asteel mold. The light-cured materials were photocuredfor 60 seconds from both sides and removed from themolds. The top surfaces ofthe cylinders were groundfiat, polished (1,200 grit), and exposed to the light-curing device again for 60 seconds. The specimenswere stored in distilled water for 24 hours at 37°C andsubsequently fixed on glass plates with a thermoplasticmaterial, aligned so that the pohshed surfaces ofthespecimens were parallel to the plates.

Microhardness measurements were obtained with aVickers Hardness Tester (Type 3212, Zwick) at 100 gof force for 15 seconds. The use of Vickers hardnessmeasurements to determine microhardness of restora-

204 Quintessence International Volume 27, Number 3/1996

Atlin et al

tive materials is in accordance with recent studies.*''According to Willems et al.̂ Vickers hardness measu-rements covered the requirements of the standard testmethod of materials as defined by the AmericanSociety for Testing and Materials.

A pilot study had proved that measurement at 100 gof force for 15 seconds initiated no cracks on thesurface of the materials, thereby providing a size ofindentation that allowed measurement of the surfacehardness of all tested materials. The indentations weremeasured immediately after removal of the indenter.Five microhardness indentations were made on thepolished surface of each specimen. From these, anaverage Vickers Hardness Number (VHN) was deter-mined.

Compressive strength testing

The compressive strength of each material was meas-ured in each of five cylindrical specimens. 6.0 mm inheight and 4.0 mm in diameter. The cylinders werephotocured for 60 seconds, transferred to the light-curing oven, and subsequently stored in distilled waterfor 24 hours at 37°C,

Compressive strength was determined in a universaltesting machine (UPM 81565. Frank) used at acrosshead speed of 0.7 mm/min. The forces requiredto produce fracture of the specimens were recorded.

flextiral strength and modulus of elasticity testing

Rexural strength of the materials was measured bythree-point bending of rectangular beams, 2.0 x 2.0mm in cross section and 25.0 mm long. Five beams ofeach material were fabricated. The light-cured mate-rials were photocured for 60 seconds from all sides,transferred to a light-curing oven and subsequentlystored in distilled water for 24 hours at 37°C.

Afterward the specimens were placed in a special jigwith a distance of 20.0 mm between the lower sup-porting points. The jig was mounted in a universaltesting machine and the beam was centrally loadedwith a crosshead speed of 1.0 mm/min. The forcemeasured at fracture of the specimen and the deforma-tion of the beam at fracture were recorded. Flexuralstrength {a^) and modulus of elasticity (£) werecalculated with the following formulas:

3x fx /

2 X

f X P

4 X Vf X h ' X

where F - force required to produce fracture; / -distance between the supporting points; ii' = width ofthe beam; /?= height of Ihe beam; and d= deformationof the beam at fracture.

Abrasion resistance testing

Six cylinders (2.2 mm in height and 10.0 mm indiameter) of each material were prepared and photo-cured from both sides for 60 seconds. Following waterstorage for 6 weeks at 37°C, the cylinders werepohshed (600 grit) until they were reduced 0.2 mm inheight. Tlie height and the diameter of the specimenswere controlled with a micrometer to an accuracy of5.0 \im and recorded. For determination of density ofthe materials, the specimens were weighed ( 1574 MP8,Sariorius), and the densities were calculated as amathematical function of volume and weight. Thedensities of the six specimens were averaged for eachmaterial.

The specimens were mounted in a toothbrushingmachine by means of impression material (Extrudemedium consistency, Kerr/Sybron). A pilot study hadproved thai no remnants of impression material couldbe detected on the surface of the specimens under alight microscope (x 40 magnification). The tooth-brushing machine consisted of a motor, which im-parted a reciprocating motion of one toothbrush head(blend-a-med medical, Blendax). The specimens werealigned so that the brushing head moved parallel to ihesurface of the specimens. For each specimen, a newbrushing head was used. A frequency of 40,000brushing strokes (120 strokes/min) was carried out ata load of 2.2 N in an abrasive slurry (33.4% dicaiciumphosphate in distilled water, glycerine, and formalin).After they were brushed, the specimens were weighedagain. Substance loss (in microns) of the specimenswas calculated as a mathematical flinciion of volume-tric loss and density.

Depth of cure testing

Three specimens each of the iight-curcd materials wereprepared for determining initial depth of cure. Thematerials were inserted in cylindrical steel molds(6.0 mm in height and 4.0 mm in diameter). Glassplates were placed forcefully on top of the molds tocreate a planar surface. The restorative materials werepolymerized by application of the visible light source,at a distance of 1.0 cm, perpendicular to the top of thecylinder. The light was activated for 40 seconds. Afterthe light activation was completed, the specimens were

'-rnnP |p*°'-natif-in-il WnliTnnW^.Nnmhpr 3/1996 205

Attin et al

Table 2 Mean (SD) surface microhardness(Vickers hardness number)

Material Microhardness

blend-a-luxChemFil SuperiorDyracIIonosit FilVitremerVariGlass VLCPhotac-FilFuji II LC

97.8 (2.9)59.8 (2.1)54.9 (0.6)41.7(1.7)41,4(3.0)38.3(1.7)37.4(1.7)36.2 (0.9)

Values connected by a vertical line are not signiticaijtly different (level ofsignificance» i ' < ,05 ) .

Table 3 Mean (SD) compressive strength (N/mm^)

Material Compressive strength

blend-a-luxDyractVitremerVariGlass VLCIonosit FilChemFii SuperiorFuji II LCPhotac-Fil

381.3 (29.1)255.6(18,8)176.1 { 8,4)167.0 ( 8.3)161.8(11,7)161.1 (12.5)159.7 ( 7.6)128.2(15.3)

Vatues connected by a vertical line aie not significanlly difièrent (level afsignificance - P < . 0 5 ) .

Table 4 Mean (SD) flexural strength and tnodulusof elasticity (N/mm^)

Material Flexural strength Modulus of elasticity

blend-a-luxDyractVitremerFuji II LCIonosit FilVariGlass VLCPhotac-FüChemFil Superior

130,6(3,8)123.9(2,4)55,9(2.5)54,6 (2,7)3S.8 (3,7)36,2(1,1)32.5(1.5)20,5(1,9)

10339(475)1

8395(615)

7596(792)

6249 (526)

6751 (581)

4749(315)

4326 (938)

11850(448)1

Values connected by asignificance =

/enicai line are not significantly different (level of

taken out of the molds, and the soft, uncured matertaiat the bottom of the cylinders was removed with aplastic sealer. The cylinders were bisected along theirlength whh a diamond saw, and the sectioned surfaceswere polished (1,200 grit).

The two halves were fixed on glass plates asdescribed for hardness testing, and, 10 minutes afterpolymerization, the Vickers hardness was measuredperpendicular to the long axis of illumination at bothhalves. The microhardness ( 100-g load for 15 seconds)was determined at steps of 0,5 mm each from thesurface of the cylinders that had been photocured. Thesites were located exactly in the middle axis of thesectioned surfaces.

Statistical analysis

Statistical analysis of the mean values among thematerials was achieved by analysis of variance andScheífé's test. Differences between hardness at 0,5-and 2,0-mm depths for each material were subjected toan unpaired t test followed by Bonferroni Correction,The level of significance was set at P< .05 in all tests.

Results

Micrahardness

Table 2 presents the mean values obtained from thedetermination of surface microhardness. All glass-ionomer materials had statisticaUy significantlylower microhardness than did the hybrid resin com-posite. The conventional glass-ionomer cement andthe PMRC, Dyract, demonstrated statistically highersurface hardness than did the remaining glass-ionomercements,

Compressive strength

The average compressive strengths of the materials areshown in Table 3. The hybrid resin composite yieldedthe highest compressive strength. No statisticallysignificant difference was observed between the hybridresin composhe and Dyract. Except for Fuji II LC andPhotac-Fil, all resin-modified glass-ionomer materialsexhibited higher compressive strengths than did theconventional glass-ionomer material and lower strengthsthan did the hybrid resin composite,

Flexural strength and modulus of elasticity

Table 4 presents the average flexural strengths andmodulus of elasticity of the tested materials. The hybrid

206 Quintessence Internationai Voiume 27. Number 3/1996

resin composite revealed the highest flexural strength,and the conventional glass-ionomer cement showedIhe lowest strength. The moduli of elasticity of thehybrid resin composite and of the chemically curedglass-ionomer cement were higher than those of theresin-modified glass-ionomer materials and thePMRCs.

Abrasion resistance

The average abrasion caused by brushing of thematerials is shown in Table 5. The hybrid resincomposite and one PMRC exhibited statisticallysignificantly higher abrasion than did the glass-ionomer matcriais. Except for Photac-Fil. all rcsin-modified glass-ionomer materials showed higher abra-sion resistance than did the conventional cement. Thedifferences between the conventional cement and theresin-modified glass-ionomer materials were not statis-tically significant.

Table 5 Mean (SD) abrasion of the tested materialscaused by brushing

Material

blend-a-luxDyractPhotac-FilChemFil SuperiorIonosit FilFuji II LCVi tremerVariGlass VLC

Abrasion

65.2 {3,7)53.6 (9.8)35,0(3,3)34.4(3.6)30.4(1.5)30.2(2.3)28.8 (2.6)21.4(1.2)

Values connected by a vertical line are not significantly different (level a(significance »

Depth of cure

To simplify the illustration, the means of the micro-hardness values at various depths of the materials arepresented in two separate figures (Figs la and Ib).Uncured material at the bottom of the cylindricalspecimens did not allow for determining microhard-ness of Ionosit Fil, VariGlass VLC, and Photac-Fil atdepths exceeding 3.0, 3.0, and 4.0 mm, respectively.VariGlass VLC, Dyract, Ionosit Fil, and blend-a-luxexhibited a continuous decrease in microhardnessfrom 0,5 mm depth to the bottom of the cylinders. Themicrohardness profiles of Fuji II LC and Vitremerstayed nearly constant. Photac-Fil showed a loss inmicrohardness at a depth exceeding 3.5 mm.

In Table 6, the difference in hardness between 0.5and 2.0 mm in depth is expressed in relation to thehardness at 0.5 mm in depth. The relative decrease inhardness from 0,5 to 2.0 mm in depth was highest forDyract and the hybrid resin composite (blend-a-lux).Dyract and Photac-Fil revealed statistically significantdifferences in hardness between 0,5 and 2.0 mm indepth. The resin-modiñed glass-ionomer materials,Ionosit Fil, Vitremer, and Fuji II LC, showed merely aslight loss in hardness at 2,0 mm in depth.

Discussion

The results of the study demonstrated marked differ-ences in physical properties among the tested mate-

rials. The rankings of the glass-ionomer restorativematerials varied among the different tests, Fxcept forthe depth of cure, all glass-ionomer materials exhibitedphysical properiies that were different from those ofthe hybrid resin composite, Dyract revealed propertiessimilar to those of the hybrid resin composite in alltests. Similar results had been described in previousinvestigations, which have described its properties asresembling those of a resin composite rather thanthose of a glass-ionomer material.'"''^

The average surface microhardnesses of the glass-ionomer restorative materials and the PMRCs werelower than those of the control materials. The wearresulting from abrasion, unexpectedly, did not cor-relate with the Vickers hardness value of the materials.It was expected that the coefficient of wear woulddecrease as the hardness of the material increased. '̂This was not true for the present results. In the presentstudy, the hybrid resin composite yielded the lowestwear resistance and the highest surface hardness.

This finding can partly be explained with anobservarion made by Lugassy and Greener.''' Theyshowed that the abrasion of an unfilled resin is lowerthan that of a filled resin and that the pattern of weardiffers. It is assumed that glass-ionomer cements reactlike unfilled materials. With unfilled materials, the wearis more uniform than is wear exhibited by conventionalresin-based, filled resin composites, in which fiUer"pluck-out" predoniinates.'* A slurry containing 33.4%dicalcium phosphate with low abrasivity was used,'*

,.Number 3/1996 207

Attin et al

100

80

60

40

20

[VHN]

b1end-a-lux «Dyract âlonositFJI

40

30

20

10

[VHN]

-»Fuji II LC -*-Vitretner -^Photac-Fil

2 3

Depth [tnm]

Figs la and Ib Mean Vickers micro-hardtiess values (VHN) at variousdepths.

Fig la

Fig Ib

Table 6 Average loss of microhardness from 0,5 to2,0 mm in depth {%)

Material Loss of hardness

Dyract*blend-a-luxPhotac-Fil*"VariGlass VLClonosit FilVitremerFuji II LC

31.6(3.2)22.6 (7.0)13.4(1.4)12.2(5.4)6.8 (2.9)4.5 (1.6)

- 0.6(2.6)

* Statistically significant loss of hardness from 0,5 to 2,0 ram in depth.Values connected by a vertical line are not significantly different (level ofsiEnifieance - /'<,05¡,

The detached particles of the hybrid resin compositeact as an additional abrasive agent themselves andenlarge the abrasion property of the slurry.

The finding observed in this study may be extra-polated to the in vivo situation, because it correspondswith the findings of Krejci et al.'^ They also found thatthe in vivo wear of a hybrid resin composite is higherthan the wear of the PMRC Dyract. However, clinicalinvestigations are necessary to determine the long-term wear performance of these new materials,

Mexural strengths of the glass-ionomer materialsand the PMRC:s were lower than that of the hybridresin composite. Except for the conventional glass-ionomer cement, the flexural strength of all materials

208 Quintessence International Volume 27, Nutnber 3/1996

Attin et al

correlated with their modulus of elasticity; ie, thehigher the recorded fiexural strength, the higher themodulus of elasticity. The conventional glass-ionomercement revealed a high modulus of elasticity and alow fiexural strength. This finding underlines thesusceptibility to brittle fracture of conventional glass-ionomer cements.^ Except for one PMRC. the resin-modified glass-ionomer materials showed significantlylower fiexural strengths than did the hybrid resincomposite. It is conceivable that the low fiexuralstrengths of these materials exerted an infiuence on thefracture toughness of these materials.

The compressive strengths of the resin-modifiedglass-ionomer materiais were significantly lower thanthat of the hybrid resin composite but comparable tothe compressive strength ofthe conventional cement.The compressive strength of Dyract was not signif-icantly different from that ofthe hybrid resin compos-ite. The compressive strength of a restoration contrib-utes to its fracture resistance against loading.** Thereduced compressive strengths and the concomitantlow fiexural strength of the resin-modified giass-ionomer restorative materials may limit the use of thesematerials to smali cavities with little occlusal stress.

Depth of cure was determined in a way similar tothat described by Baharav et al." Most ofthe testedmaterials exhibhed a decrease in hardness from thesurface to the depth ofthe specimens. For statisticalanalysis, the loss of hardness between 0.5 and 2.0 tnmin depth was calculated in relation to the hardness in0.5 mm in depth. The hardness at 2.0 mm in depth wasselected as a reference because the manufacturersrecommend use of layers of approximately 2.0 mmwhen the materials are applied in an incrementaltechnique. In this study, the initial depth of cure wasdetermined 10 minutes after polymerization ofthelight-cured materiais. Resin-modified glass-ionomermaterials undergo ¡iirther chemical reactions afterphotocuring, thereby increasing depth of cure.'** How-ever, the present results indicated that, for most ofthetested materials, the thickness of increments in a cavityshould not exceed 2.0 mm to achieve maximal initialhardness ofthe restoration.

Summary

The tested resin-tnodified glass-ionomer materials andthe PMRCs showed inferior strength properties but

higher resistance to wear caused by brushing than did ahybrid resin composite. To achieve higher initialhardness, the thickness of increments of the testedmaterials in a cavity should not exceed 2.0 mm.

J Am Dent Assoc

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