article

Laboratory Strength of Glass Ionomerand Zinc Phosphate Cements

Andree Piwowarczyk, Dr med dent,1 Peter Ottl, Dr med dent,1 andHans-Christoph Lauer, Dr med dent2

Purpose: The present in vitro study examined 3 mechanical properties, namely compressive,

flexural, and diametral tensile strength, of various commercially available cements and core

materials as a function of time after mixing.

Materials and Methods: The examined materials were 2 cermet cements (Ketac Silver [ESPE,

Seefeld, Germany] and Chelon Silver [ESPE]), 1 metal-reinforced glass ionomer cement (Miracle

Mix [GC Dental Industrial Corp, Tokyo, Japan]), 2 conventional glass ionomer cements (Ketac Bond

[ESPE] and Ketac Cem [ESPE]), 1 standard cure zinc phosphate cement (Harvard Cement [Richter

and Hoffmann, Berlin, Germany]), and 1 zinc phosphate cement with the addition of 30% silver

amalgam alloy powder (Harvard Cement 70% with Dispersalloy 30% [Richter and Hoffmann/

Johnson and Johnson, East Windsor, NJ]). Properties were measured using a universal testing

machine at 15 minutes, 1 hour, and 24 hours after first mixing.

Results: Compressive strengths varied widely between the 3 times of measurement from 5.8 66.6 MPa for Ketac Cem to 144.3 6 10.2 MPa for Ketac Silver. Twenty-four hours after mixing, the

Bonferroni test showed significant (p < .01) differences between Ketac Silver and all other

materials tested. Diametral tensile strengths ranged widely from 4.4 6 0.9 MPa for Ketac Cem to

11.5 6 2.2 MPa for Chelon Silver. At 15 minutes, 1 hour, and 24 hours after first mixing, the analysis

of variance did not show any significant differences between Ketac Silver, Chelon Silver, and

Miracle Mix. The 3-point flexural strength of Ketac Silver showed, at 15 minutes with 13.5 6 3.9

MPa and at 24 hours with 27.2 6 7.4 MPa, the highest values.

Conclusions: Setting time influences the mechanical properties of the materials tested in this

study. Ketac Silver, a glass ionomer cement reinforced with sintered glass-silver particles, showed

the highest mechanical properties of the examined materials.

J Prosthodont 2001;10:140-147. Copyright © 2001 by The American College of Prosthodontists.

INDEX WORDS: core restoration, mechanical properties, compressive strength, flexural

strength, diametral tensile strength

FIXED PROSTHODONTIC restorations oftenrequire the placement of foundation restora-

tions to establish retentive and resistance forms tothe teeth used as prosthesis retainers. These resto-rations will ideally possess sufficient mechanicalstrength, retention to the hard dental tissues, and ahigh degree of biocompatibility.1

Glass ionomer cements2,3 have been used as corebuild-up materials. They provide mechanical and

chemical adhesion to the hard dental tissues that ison the order of one fifth to one third of that ob-tained with composite resins using the acid-etchtechnique.2 The reported results differ on the basisof brand of material4 and conditioning techniques.5

Metal-reinforced glass ionomer cements, known ascermet cements, are a mixture of glass and metalpowder6 sintered to high density. This material setsto form a cement via an acid-base reaction.7 Bysintering metal particles into the glass phase, it ispossible to improve the material’s abrasion,8,9 andto yield a modest increase in diametral tensilestrength.10

The objective of this study was to determine andcompare the mechanical properties of various ce-ments used as core build-up materials relative totime after first mixing. The parameters examinedwere compressive strength, flexural strength, anddiametral tensile strength at times of 15 minutes, 1hour, and 24 hours.

From Department of Prosthetic Dentistry, School of Dentistry, JohannWolfgang Goethe University, Frankfurt, Germany.

1Assistant Professor.2Professor and Director.Accepted June 25, 2001.Correspondence to: Dr. A. Piwowarczyk, Department of Pros-

thetic Dentistry, School of Dentistry, Johann Wolfgang Goethe Uni-versity, Theodor-Stern-Kai 7, D-60590 Frankfurt, Germany. E-mail:[email protected]

Copyright © 2001 by The American College of Prosthodontists1059-941X/01/1003-0003$35.00/0doi:10.1053/jpro.2001.27338

140 Journal of Prosthodontics, Vol 10, No 3 (September), 2001: pp 140-147

Materials and MethodsThis study evaluated 2 cermet cements (Ketac Silver[ESPE, Seefeld, Germany] and Chelon Silver [ESPE]), 1metal-reinforced glass ionomer cement (Miracle Mix[GC Dental Industrial Corp, Tokyo, Japan]), 2 conven-tional glass ionomer cements (Ketac Bond [ESPE] andKetac Cem [ESPE]), 1 standard cure zinc phosphatecement (Harvard Cement [Richter and Hoffmann, Ber-lin, Germany]), and 1 zinc phosphate cement with theaddition of 30% silver amalgam alloy powder (HarvardCement 70% with Dispersalloy 30% [Richter and Hoff-mann/Johnson and Johnson, East Windsor, NJ]) undercompressive, tensile, and flexural forces (Table 1).

Powder was weighed using an analytical balance (61mg) following the manufacturer’s recommendation,while the liquid volume suggested by the manufacturerwas established using a measuring pipette with an accu-racy of 60.03 mL. One material, Ketac Silver, was sup-plied in premeasured capsules. For this material, themanufacturer’s recommendation of a trituration time of

10 seconds was used for the Capmix (ESPE) amalgam-ator. To measure the compressive and tensile strengths,specimens were prepared in cylindrical Teflon molds(Figs 1 and 2) to the dimensions of 6 and 8 mm diameterand 12 and 4 mm height, respectively, as dictated by theBritish Standards Institute Specification 6039.11 Thespecification BS 6039 refers to the manufacturer’s rec-ommendations concerning the mixing process along withthe need to fill the molds within 2 minutes of the initia-tion of mixing.

Specimens used to determine flexural strength wereproduced in brass molds 25 3 2 3 2 mm in size that hadbeen insulated with polytetrafluoroethylene spray (DIN13922/EN 24049).12 The molds were placed in an incu-bator (Heraeus Kulzer, Hanau, Germany) at 37.0 61.0°C and at 95% to 100% relative humidity 3 minutesafter mixing. Specimens were removed from the moldwithin 60 minutes of testing. Specimens for the 24-hourmeasurements were stored in the incubator submergedin distilled water until examined. The dimensions of thespecimens were checked with a micrometer (Mauser,

Figure 1. Precision mold and specimen for determiningthe compressive strength (6 3 12 mm).

Table 1. Core Materials and Manufacturers

Material Classification ManufacturerMixingProcess

Powder/LiquidRatio

Ketac Silver Cermet cement Espe, Seefeld, Germany MechanicalChelon Silver Cermet cement Espe, Seefeld, Germany Manual 3.8:1Miracle Mix Metal-reinforced glass

ionomer cementGC Dental Industrial

Corporation, Tokyo, JapanManual 5.0:1

Ketac Bond Conventional glassionomer cement

Espe, Seefeld, Germany Manual 3.4:1

Ketac Cem Conventional glassionomer cement

Espe, Seefeld, Germany Manual 3.4:1

Harvard Cement Zinc phosphate cement Richter & Hoffmann, Berlin,Germany

Manual 2.5:1

Harvard Cement 70%/Dispersalloy 30%

Zinc phosphate cementwith admixtureamalgam alloypowder

Richter & Hoffmann, Berlin,Germany/Johnson &Johnson, East Windsor, NJ

Manual 2.5:1

Figure 2. Precision mold and specimen for determiningthe diametral tensile strength (4 3 8 mm).

141September 2001, Volume 10, Number 3

Nurnberg, Germany; precision: 0.01 mm). Testing wasperformed at 23.0 6 1.0°C and 50% 6 5% relative hu-midity. A Zwick 1435 Universal testing machine (Zwick,Ulm, Germany) was used at a crosshead speed of 1mm/min to test compressive and diametral tensilestrengths (Fig 3). Specimens were loaded until fractureat a constant crosshead speed of 1 mm/min (BS 6039).11

Measurements for the 3-point flexural strength test (Fig4) were performed at a constant crosshead speed of 1mm/min (DIN 13922/EN 24049). Flexural, compressive,and diametral tensile strengths were assessed at 15 min-utes, 1 hour, and 24 hours after first mixing. Ten mea-surements each were made for each material and foreach of the 3 parameters examined.

Statistical data analysis was performed by one-wayanalysis of variance with regard to the time factor and by2-sided t tests with regard to the compressive, flexural,and diametral tensile strength variables. The level ofsignificance was fixed at p # .05 for significant differ-ences. The Bonferroni test was used for post-hoc analysisof variance comparing multiple means.

ResultsCompressive Strength

At the earliest testing period of 15 minutes aftermixing, mean compressive strength values rangedfrom 35.8 6 6.6 MPa for Ketac Cem to 74.9 6 7.2MPa for Harvard zinc phosphate cement with ad-mixture of silver amalgam alloy powder (Fig 5).Compressive tests at 1 hour showed increased com-pressive strength for all materials. Metal-reinforcedglass ionomer cements Ketac Silver and Chelon

Silver increased by 40.1% and 47.8%, respectively.The lowest value was found for the glass ionomercement Ketac Cem with 64.2 6 7.3 MPa after 1hour. At 24 hours, the compressive strength valuesshowed further increases to 144.3 6 10.2 MPa byKetac Silver, followed by Chelon Silver and MiracleMix with values of 119.2 6 17.8 MPa and 111.4 613.2 MPa, Harvard cement with 95.7 6 19.1 MPa,and the admixture of Harvard cement with amal-gam alloy powder with 91.5 6 15.0 MPa. TheBonferonni test identified statistically significantdifferences among groups concerning the timesof measurement (Table 2).

Diametral Tensile Strength

Diametral tensile strength means and standarddeviations are displayed in Fig 6. At 15 minutes, thehighest value of 7.8 6 1.0 MPa was obtained withthe combination of Harvard cement and amalgamalloy powder; conventional glass ionomer cements(Ketac Bond, Ketac Cem) were measured at 5.3 60.7 MPa and 4.4 6 0.9 MPa. At 1 hour, results forthe tested materials ranged from 6.3 6 1.0 MPa(Ketac Cem) to 8.5 6 1.9 MPa (Miracle Mix). At 24hours, Chelon Silver at 11.5 6 2.2 MPa, KetacSilver at 11.2 6 1.6 MPa, and Miracle Mix at 10.7 61.5 MPa showed the highest values. Between 1 and24 hours after first mixing, Chelon Silver, KetacSilver, Miracle Mix, and Ketac Cem showed a sig-nificant ( p # .01) increase in diametral tensilestrength. In addition to the 3 times of measure-

Figure 3. Testing procedure for determining the diame-tral tensile strength.

Figure 4. Testing procedure for determining the flex-ural strength.

142 Laboratory Strength of Cements ● Piwowarczyk, Ottl, and Lauer

ment, the analysis of variance again identified sig-nificant differences among the materials (Table 3).

Three-point Flexural Strength

Figure 7 illustrates the 3-point flexural strengthmeans and standard deviations. At 15 minutes afterfirst mixing, Harvard cement with amalgam alloypowder at 16.2 6 1.4 MPa, Harvard cement at14.5 6 1.7 MPa, and Ketac Silver at 13.5 6 3.9 MPayielded the highest values. At 15 minutes, KetacCem showed the value of 4.7 6 1.7 MPa. Duringthe first hour, the flexural strength value for Mir-acle Mix increased by 87.3% to 17.7 6 4.3 MPa. Thecorresponding values for Chelon Silver and KetacSilver were 15.9 6 5.0 MPa and 15.0 6 4.6 MPa.Twenty-four hours after first mixing, Ketac Silver,at 27.2 6 7.4 MPa, yielded the highest values of allthe cements examined, while also showing the

greatest increase in strength (81.0%) over the1-hour value. The other materials showed resultsranging from 9.6 6 4.6 MPa for Ketac Bond to24.8 6 8.0 MPa for Chelon Silver (Fig 7). Theanalysis of variance showed that Ketac Silver andChelon Silver differed significantly from MiracleMix (p # .01), Ketac Cem (p # .01), Ketac Bond(p # .01), Harvard cement (p # .01), and Harvardcement with amalgam alloy powder (p # .05). As aresult of the Bonferonni test at p # .05, the verticallines in Table 4 bracket statistically equivalentgroups.

DiscussionIn designing this study, the decision was made tomake evaluations on standardized molds. This al-lowed the testing to conform to the British Stan-

Figure 5. Compressivestrength (BS 6039) of variouscore restoration materials(0.25, 1, and 24 hours afterfirst mixing; n 5 10 measure-ments each per material andtime). KS, Ketac Silver; CS,Chelon Silver; MM, MiracleMix; KB, Ketac Bond; KC,Ketac Cem; HZ, Harvard ce-ment; HA, Harvard cementwith amalgam alloy powder.

Table 2. Compressive Strength (BS 6039) of Various Core Restoration Materials (0.25, 1, and 24 Hours After FirstMixing; n 5 10 Measurements Each per Material and Time)

Material

0.25 h 1 h 24 h

x (MPa) x (MPa) x (MPa)

Ketac Silver 66.0 6 3.5 92.5 6 9.5 144.3 6 10.2Chelon Silver 55.1 6 2.6 81.5 6 5.9 119.2 6 17.8Miracle Mix 40.7 6 4.1 77.6 6 6.1 111.4 6 13.2Ketac Bond 37.7 6 4.0 66.2 6 6.7 84.1 6 18.3Ketac Cem 35.8 6 6.6 64.2 6 7.3 80.1 6 201Harvard cement 72.9 6 5.9 84.0 6 13.3 95.7 6 19.1Harvard cement 70%/

amalgam alloy powder(Dispersalloy) 30% 74.9 6 7.2 86.9 6 11.4 91.5 6 15.0

Note. The vertical bars in respective lines to the right of the points of measurement indicate groups that do not significantly differ asmeasured by the Bonferroni test (p # .05).Abbreviations: x, arithmetic mean; s, standard deviation.


dards Institute Specification 6039.11 The alternativeuse of extracted teeth brought with it the variabilityassociated with differences in dimensions, degree ofcalcification, water content, and the potential forpre-existing hard tissue defects.13-15 Although thelatter approach may allow simpler interpretation ofclinical expectations, it eliminates standardizationand complicates meaningful comparisons.

Considerable differences in compressive, flex-ural, and diametral tensile strength were seenamong the tested materials. All 3 of the recognizedtests showed a similar ranking order for the mate-rials (Figs 5, 6, and 7). Under the conditions of thisstudy, Ketac Silver presented the highest values ofcompressive and flexural strengths at 24 hours ofall the cements examined. This material showed

statistically significant differences (p # .05) in test-ing compressive strength at all times of measure-ment.

Between 1 and 24 hours, Ketac Silver showed asignificant ( p # .01) increase in strength within allthe 3 testing parameters. Previous reports showedthat the setting reaction of glass ionomer cementsis complex, requiring 24 hours for maturation.7,16,17

This is in contrast to the zinc phosphate materials,Harvard cement, and Harvard cement with addedamalgam alloy, which showed less increase instrength over time.

The data in this report failed to show a consis-tent relationship between the method of mixing(manual or mechanical) and strength properties.Strict adherence to the prescribed mixing ratio is

Figure 6. Diametral tensilestrength (BS 6039) of variouscore restoration materials(0.25, 1, and 24 hours afterfirst mixing; n 5 10 measure-ments each per material andtime). KS, Ketac Silver; CS,Chelon Silver; MM, MiracleMix; KB, Ketac Bond; KC,Ketac Cem; HZ, Harvard ce-ment; HA, Harvard cementwith amalgam alloy powder.

Table 3. Diametral Tensile Strength (BS 6039) of Various Core Restoration Materials (0.25, 1, and 24 Hours AfterFirst Mixing; n 5 10 Measurements Each per Material and Time)

Material

0.25 h 1 h 24 h

x (MPa) s (MPa) x (MPa) s (MPa) x (MPa) s (MPa)

Ketac Silver 6.5 1.5 8.1 1.5 11.2 1.6Chelon Silver 7.2 1.6 8.2 1.4 11.5 2.2Miracle Mix 7.2 1.0 8.5 1.9 10.7 1.5Ketac Bond 5.3 0.7 6.9 1.2 7.4 1.2Ketac Cem 4.4 0.9 6.3 1.0 7.5 1.3Harvard cement 6.9 0.9 7.4 1.1 7.9 1.7Harvard cement 70%/

amalgam alloy powder(Dispersalloy) 30% 7.8 1.0 8.3 1.1 9.2 1.8



said to be especially important for the successfuluse of glass ionomer cements. Negative influenceson strength, solubility, and surface structure as aresult of having too little powder have been cited inthe literature. Conversely, with too high of a pow-der-to-liquid ratio, a reduction in setting time andbond strength has been described.18,19 The manu-ally mixed product Chelon Silver, chemically iden-tical to Ketac Silver, has a lower compressivestrength at 15 minutes ( p # .01), 1 hour ( p # .05),and 24 hours ( p # .01), but Chelon Silver andKetac Silver showed no significant differences inflexural and diametral tensile strengths over thesetime periods ( p # .05).

The present study showed significantly ( p # .05)lower values for the glass ionomer cements KetacCem and Ketac Bond concerning compressive, flex-ural, and diametral tensile strengths at all times of

measurement, compared with the cermet cements,Ketac Silver and Chelon Silver. The data reportedby Chung10 showed higher diametral tensilestrength values for metal-reinforced materials,Ketac Silver and Miracle Mix, compared with theglass ionomer cement, Fuji II (GC Dental Indus-trial Corp). Cermet cements were intended to pos-sess higher flexural strength than glass ionomercements.20 Walls and coworkers21 showed an in-crease in compressive strength and compressivefatigue limit for cermet-type materials comparedwith the strength of a conventional glass ionomercement from the same manufacturer. In contrast tothe above-mentioned studies, Cho et al22 docu-mented that silver-reinforced glass ionomer cementKetac Silver did not improve the compressive anddiametral tensile strength in comparison with theconventional glass ionomer cement, Ketac Fil

Figure 7. Three-point flex-ural strength (DIN 13922/EN24049) of various core resto-ration materials (0.25, 1, and24 hours after first mixing;n 5 10 measurements eachper material and time). KS,Ketac Silver; CS, Chelon Sil-ver; MM, Miracle Mix; KB,Ketac Bond; KC, KetacCem; HZ, Harvard cement;HA, Harvard cement withamalgam alloy powder.

Table 4. Three-point Flexural Strength (DIN 13922/EN 24049) of Various Core Restoration Materials (0.25, 1, and24 Hours After First Mixing; n 5 10 Measurements Each per Material and Time)

Material

0.25 h 1 h 24 h

x (MPa) s (MPa) x (MPa) s (MPa) x (MPa) s (MPa)

Ketac Silver 13.5 3.9 15.0 4.6 27.2 7.4Chelon Silver 11.8 4.1 15.9 5.0 24.8 8.0Miracle Mix 9.5 1.1 17.7 4.3 18.5 1.9Ketac Bond 8.4 3.2 7.3 4.4 9.6 4.6Ketac Cem 4.7 1.7 8.2 3.4 11.6 5.6Harvard cement 14.5 1.7 17.7 2.9 20.3 1.6Harvard cement 70%/

amalgam alloy powder(Dispersalloy) 30% 16.2 1.4 18.8 1.3 22.0 1.6



(ESPE). The work of Williams et al23 did not showany significant reduction in compressive and diame-tral tensile strengths for the glass ionomer ce-ments, Chelon Fil (ESPE) and Fuji II (GC DentalIndustrial Corp), compared with reinforced mate-rials, Chelon Silver and Miracle Mix. Peutzfeldt24

reported a similar flexural strength of Ketac Silverand Miracle Mix to 5 conventional glass ionomercements.

In the present study, Miracle Mix, a metal-reinforced, nonsintered glass ionomer cement,showed a marked increase in compressive, tensile,and flexural strengths during the first hour afterfirst mixing. At 24 hours, Miracle Mix was inferiorto the cermet cements Ketac Silver ( p # .01) andChelon Silver ( p # .05) in flexural strength. After 7days, Peutzfeldt24 found, in the 3-point flexuralstrength tests with specimens of 10 3 2 3 2 mm, aflexural strength of 13 6 2 MPa for Miracle Mixand 29 6 13 MPa for Ketac Silver. Nakajima et al25

showed no significant differences in 3-point flexuralstrength tests on Miracle Mix glass ionomer ce-ments (23.0 6 3.1 MPa) and Ketac Silver (22.6 610.4 MPa) when tested at 24 hours after storagein 100% humidity at 37°C. When measuring com-pressive strengths (American Dental Association[ADA] specification #66), values of 132.1 6 13.7MPa and 121.9 6 12.7 MPa for Miracle Mix andKetac Silver, respectively, were shown.

Cohen et al26 examined the diametral tensilestrength and compressive strength (ADA specifica-tion #27) of titanium-reinforced composite resins,Ti-Core (Essential Dental Systems, Hackensack,NJ) and Flexi-Flow Cem (Essential Dental Sys-tems), compared with commercially available corebuild-up materials and cements. All specimens hadbeen stored at 100% humidity for 24 hours. Theglass ionomer cements Ketac Silver, Miracle Mix,and Ketac Cem yielded 60.1% to 80.8% lower valuesthan titanium-reinforced composite resins. Of the3 glass ionomer cements tested, Ketac Silverachieved the highest values with regard to diame-tral tensile strength and compressive strength at12.5 6 2.2 MPa and 115.1 6 16.0 MPa, respectively.

The present study showed that after 24 hours,zinc phosphate cement, with and without the ad-mixture of silver amalgam alloy powder, is strongerthan conventional glass ionomer cements andweaker than cermet cements under compressive,tensile, and flexural forces. The cement strength isalmost linearly dependent on the powder:liquid ra-tio.27 For zinc phosphate cement, Abraham28 deter-

mined an increasing compressive strength and de-clining solubility by increasing the powder-to-liquidratio. On a graphic curve, this results in a rise to amaximum value and subsequently a decline in re-lation to the compressive strength. Smith29 con-cluded that the compressive and tensile strengthsof properly mixed zinc phosphate cement are ade-quate to resist masticatory stress.

The reported results concerning the mechanicalproperties of cements are broad and variable. Inparticular, measurements of strength are depen-dent on the method of specimen preparation, theoperator, and labor variability.30 In addition to me-chanical properties, other factors such as pulp com-patibility,31-33 allergic potential,34-36 and expansioncaused by water absorption37-39 should be consid-ered when choosing a material for a foundationrestoration. To confirm the results of laboratorystudies, longitudinal clinical trials should be con-ducted to establish the most important predictorsof clinical success for core materials.

Conclusions1. Compressive strength, flexural strength, and

diametral tensile strength varied among thetested materials.

2. Under the conditions of this study, 24 hours afterfirst mixing, cermet cements were stronger thanmetal-reinforced glass ionomer cement, conven-tional glass ionomer cements, and zinc phos-phate cement without and with the addition ofsilver amalgam alloy powder.

3. The compressive strength of Chelon Silver wassignificantly lower than the chemically identicalcapsule product Ketac Silver at the 3 times ofmeasurement.

AcknowledgmentThe authors would like to thank the Institute for MedicalInformation Processing, Biometry, and Epidemiology (di-rector: Professor K. Uberla) of the University of Munichfor its support in performing the statistical analysis of thedata.

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