Factors affecting the accuracy of elastometricimpression materials

S.Y. Chena,*, W.M. Liangb, F.N. Chenc

aSchool of Dentistry, China Medical University, 91 Hsueh-Shih Road, Taichung, Taiwan, ROCbSchool of Public Health, China Medical University, Taichung, Taiwan, ROCcDepartment of Social Medicine, China Medical University, Taichung, Taiwan, ROC

Received 16 December 2003; received in revised form 12 April 2004; accepted 16 April 2004

KEYWORDSImpression material;

Accuracy; Storage time;

Filler

Summary Objectives. The purpose of this study was to evaluate the effects of (1)various impression materials, (2) different storage times and (3) the proportionof inorganic filler on the accuracy and stability of elastometric impressionmaterials.

Methods. The impression materials studied included three alginate impressionmaterials (Algiace Z, CAVEX and Jeltrate), five commercial silicone impressionmaterials (Aquasil, Exaflex regular type, Express, Coltex fine and Rapid liner) andtwo experimental silicone impression materials designed for this study (KE106Aand KE106B). Impressions were made of 10 metal dies that mimicked preparedcrowns. After an impression was taken, dental stone was immediately pouredinto the alginate impressions, while the silicone impressions was poured 30 minlater and waited for 1 h for setting. The second and third stone dies were made1 and 24 h later, respectively. The diameters of the occlusal surfaces of themetal dies and stone casts were determined using photographs of the surfacestaken with a Kodak DC 290 digital camera. The pictures were then measuredusing a photomicrograph digitized integration system to calculate any discre-pancy. Because each impression was used to make three rounds of stone dies,two-factor mixed factorial ANOVA was used to evaluate the effect of materialsand storage time on the accuracy of the stone casts. The simple effects analysis,combined with multiple comparisons considering the per family type I error rate,was performed following confirmation that an interaction between the twofactors was significant.

Results. The results showed that: (1) there was a significant interactioneffect between materials and storage times on the accuracy of the impressions.(2) Two addition type silicone materials, Aquasil and Exaflex, had the greatestaccuracy and stability. (3) The experimental material KE106A had the leastaccuracy in the first and second rounds and the alginate impression materialCAVEX had the least accuracy in the third round. (4) The stabilities of CAVEX andJeltrate were the least consistent of the 10 materials and decreased significantlywith storage time. (5) When the experimental material had a low proportion offiller (KE106A), there was a significantly greater dimensional discrepancycompared to the same material with a higher proportion of filler (KE106B).

0300-5712/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2004.04.002

Journal of Dentistry (2004) 32, 603–609

www.intl.elsevierhealth.com/journals/jden

*Corresponding author. Tel.: þ886-4-2055674; fax: þ886-4-2014043.E-mail address: saychen@mail.cmu.edu.tw (S.Y. Chen).

Conclusions. The accuracies varied among the 10 impression materials overthree rounds. Of all the materials, the addition type silicone materials, Aquasiland Exaflex, had relatively greater accuracy and stability. The discrepancy of thealginate impression materials increased with storage time. The large loading offiller showed less discrepancy.q 2004 Elsevier Ltd. All rights reserved.

Introduction

Making impressions to duplicate oral conditions andtooth morphology is an integral part of prostheticdentistry. There are many reports concerning howto improve the accuracy of impression techniques,including controlling the room temperature,1,2

using single or double impression techniques,3,4

using individual or custom trays,5,6 etc. Although allof above factors affect the outcome, the choice ofimpression material is possibly the most importantfactor.7,8

Clinically, there are many kinds of elasticimpression materials available for dental use.Generally, they can be divided into two largegroups: (1) synthetic elastomeric impressionmaterials that include polysulfide, condensationsilicone, addition silicone and polyether. Siliconeimpression materials are the most acceptable inthis group.9,10 (2) Hydrocolloid impressionmaterials. This group includes agar agar andalginate impression materials, the latter beingmore popular. Clinicians are not agreed which ofthese two groups is better but, because alginate ischeaper than impression materials, it is hoped thatit may become the material of choice. In 1989,Peutzfeldt et al. compared the accuracy of alginateand elastometric impression materials. They foundone of the alginate impression materials had adegree of accuracy comparable with other elasto-metric impression materials.11 In 1990, Craig et al.compared over 39 types of commercial elasto-metric impression material and found the additiontype silicone impression material was more stablethan polysulfide, condensation type silicone andpolyether impression materials at 1 day.12 In 1997,Federic and Caputo compared some of the agar agarand elastometric impression materials and foundthere was no significant difference in the accuracyof the mould made from the polyether and from twoagar agar impression materials.13 All the abovestudies suggested that alginate impressionmaterials had the potential to replace elastometricimpression materials. In 1988, Lin et al. comparedthe accuracy of elastometric impression materials.They found that polyether was the most accurate,followed by silicone, polysulfide, alginate and agar

agar.14 At present, elastometric impressionmaterial remains the most popular and acceptedmaterial among dentists. Therefore, comparison ofthese two groups of recently developed commercialproducts is very important to investigate theiraccuracy and stability.

The purpose of this study was to evaluate theaccuracy of impression materials by comparing thediscrepancies between the master dies and stonecasts. Sometimes, in laboratory work, it is necess-ary to make an accessory cast, so the effect ofdifferent storage times on re-pouring also wasinvestigated. Inorganic filler is a component of thenon-constricting part of the impression material.Therefore, we investigated the effect of differentproportions of inorganic filler on the accuracy ofimpressions.

Materials and methods

Materials

The impression materials used in this study includedthree alginate impression materials: Algiace Z(Sankin Kogyo KK, Japan), CAVEX (CAVEX, Holland)and Jeltrate (DENTSPLY ASIA, Hong Kong), fivecommercial silicone impression materials: AquasilLV (GC America Inc., Chicago, IL, USA), Exaflexregular type (GC America Inc., Chicago, IL, USA),Express (3M Dental products, USA), Coltex fineColtene/Whaledent Inc., Mahwah, NJ, USA), andRapid liner (Coltene/Whaledent Inc., Mahwah, NJ,USA), and two experimental silicone impressionmaterials KE106A and KE106B (Shinetu ChemicalCo., Japan). MG Crystal rock (MARUISHI GYPSUMCo., Ltd, Tokyo, Japan) was the type IV stone usedin this study. Detailed information on the commer-cial impression materials is listed in Table 1. Thecompositions of the experimental materials aregiven in Table 2.

Methods

Sample preparationTen simulated stainless tooth models were pre-pared with a diameter of 9 mm, height of 10 mm

S.Y. Chen et al.604

and with a bevel to produce an occlusal surface8 mm in diameter (Fig. 1(A)). Each model waspaired with a metal ring 20 mm in height, 20 mm ininternal diameter and with a puller as theimpression tray (Fig. 1(B)). Both the model andtray were numbered from 1 to 10 for eachimpression test.

Three kinds of alginate impression materialswere used following the manufacturers instructionsfor powder/water mixing ratios and working timesto make the impressions. The impression materialswere mixed using an electric mixer (AIGIMAXAM505, GC Co., Japan) for 10 s, then part of thematerial was used to fill a plastic syringe andinjected onto the model, the remainder was putinto the tray and used to make the impression. Theimpression materials and metal dies were separatedafter 5 min. The stone was mixed by hand in apowder/water ratio of 10:3 (w/w) within 1 min andpoured as soon as the metal dies and impressionmaterials were separated. Ten stone casts weremade for each impression material. All the stoneswere allowed to set in a plastic storage box. After1 h the stone casts were separated from theimpressions and the second round stone castswere made using the same conditions and storedin the same way. They also were separated 1 hlater. The impression materials were stored in thebox for a further 24 h, then the third round stonecasts were made in the same way.

Five kinds of commercial silicone impressionmaterials were mixed following the manufacturersinstructions and the impression materials wereseparated from the dies 5 min after beginningmixing. The impressions were put into the storage

box for 30 min to allow the recovery of elasticdeformation. Stone was poured into theimpressions and allowed to set in the storage boxand the second and third round stones were madeusing the same conditions as for the alginateimpression materials.

Two experimental silicone impression materialswith different proportions of filler on the sub-micron (0.02–0.04 mm) colloidal silica (Aerosil R972) were prepared in this laboratory and used toevaluate the effect of the inorganic filler. Thesetwo materials included the same amounts of allcomponents (Table 2), except for Aerosil R 972,where KE106A had less Aerosil R972 (5 g) thanKE106B (20 g).

Testing methodsThe diameters of the occlusal surfaces of metal diesand stone casts were determined from photographsof the occlusal surfaces taken with a metal scaleusing a Kodak DC 290 digital camera, followed bymeasurement using a photomicrography digitizedintegration system (Photomicrograph MGDS-260,Taiwan) on a personal computer. The system used

Table 1 Basic description of impression materials.

Material Types of material Manufacturer Lot No.

Algiace Z Alginate Sankin Kogyo KK, Japan 317011CAVEX Alginate CAVEX, Holland 991202Jeltrate Alginate DENTSPLY ASIA, Hong Kong SL319Aquasil LlV Addition

type siliconeGC America Inc.Chicago, IL,USA

990831

Exaflex regular(medium body)

Additiontype silicone

GC America Inc. Chicago, IL,USA

Base: 052196A;Catalyst: 052396A

Express Additiontype silicone

3M Dental Products, USA OGLY3D1

Coltex fine (light body) Condensationtype silicone

Coltene/Whaledent Inc., Mahwah,NJ, USA

Base: FH23;Catalyst:FE95

Rapid liner (light body) Condensationtype silicone

Coltene/Whaledent Inc., Mahwah,NJ, USA

FL81

KE106A (light body) Additiontype silicone

Shinetu Chemical Co. Japan Base: 705257;Catalyst: 706250

KE106B (regular body) Additiontype silicone

Shinetu Chemical Co. Japan Base: 705257;Catalyst: 706250

Table 2 The composition of the experimental materials,KE106A and KE106B.

Component (unit: g) KE106A KE106B

Rubber base (KE106) 60 60Catalyst 15 15Accelerator 12 12Aerosil R972 5 20Talc extra-fine powder 6 6Pigment 1 1

Accuracy of impression materials 605

the Image-Pro Plus version 4.1 software (MEDIACYBERNETICS, USA) as image analyzing instrument,it can trace the margin of the circle of the occlusalsurface automatically and gives the average diam-eter. The discrepancies between the metal dies andstone casts were determined by dividing theabsolute value of the differences between thediameters of the metal dies and stone casts bythe diameter of the metal die and converted intopercentages. Greater values indicate higherdiscrepancies.

Statistical analysis

SPSS 10.0 software was used for the statisticalanalysis. The two-factor mixed factorial ANOVA wasused to evaluate the effect of materials and storagetimes on the accuracy of impressions. The depen-dent variable was the accuracy of impressions. Thebetween-impressions factor was the 10 kinds ofimpression material (three alginates, five commer-cial silicones and two experimental silicones) andthe within-impressions factor was storage time atthree levels (30 min, 1 1

2 and 24 h). The simpleeffects analysis combined with multiple compari-sons considering the per family type I error rate wasperformed following confirmation that an inter-action between the two factors was significant.Following up the significant interaction in the two-factor mixed factorial ANOVA, the simple effect ofthree different storage times for each material wastested by performing three paired-samples t-tests.The simple effect of 10 different materials for eachround was tested by performing one-way ANOVAs.The Bonferroni test was then used to carry out PostHoc pairwise comparisons when the ANOVA showeda significant main effect of the material factor.

The same procedure was used to evaluate the effectof different proportions of inorganic filler.15

Results

From the results of two-factor mixed factorialANOVA, the interaction effect between impressionmaterial and storage time on the accuracy wasaffected significantly ðp ¼ 0:001Þ: As such, theimpression material was analyzed separately foreach round of storage time, and the storage timewas analyzed separately for each impressionmaterial. Table 3 and Fig. 2 show the means andstandard deviations of the accuracy of all combi-nations of 10 impression materials and threestorage times. In addition, the results of simpletests for the effect of one factor condition on aspecific level of the other factor also are shown.

The results of the effect of material factor wereas follows (Table 3). For the first round ðT1Þ; Aquasilwas the most accurate material [0.70 (0.45%)],followed by Express [82 (0.64%)] and Exaflex [0.89(0.66%)], and KE106A was the worst [1.81 (0.77%)].However, there were no significant differencesamong the materials ðp ¼ 0:095Þ: For the secondround ðT2Þ; Rapid liner was the most accuratematerial [0.60 (0.42%)] followed by Exaflex [0.78(0.62%)] and Aquasil [0.98 (0.84%)], and KE106A wasthe worst [2.46 (1.69%)]. In this round, only KE106Awas significantly better than CAVEX. For the thirdround ðT3Þ; Coltex fine was the most accurate [0.95(0.53%)] material followed by Aquasil [1.00 (0.79%)]and Exaflex [1.02 (0.86%)], and CAVEX was theworst [3.38 (1.36%)]. The results from the thirdround showed that CAVEX was significantly lessaccurate than all of the other materials, except for

Figure 1 The stainless steel model used for impression. (A) Metal die. (B) Impression tray.

S.Y. Chen et al.606

KE106A and Jeltrate, and KE106A was significantlyless accurate than Coltex fine (Table 3).

The results of the effect of storage time for eachmaterial were as follows. For Algiace Z, Aquasil,Exaflex, Coltex fine and KE106B, there was nosignificant effect of storage time. For CAVEX, thedifferences in accuracies between time 1 and time 3and between time 2 and time 3 were significant. ForJeltrate, Express and KE106A, the accuracies fromtime 1 to time 3 were reduced significantly and forRapid liner, the accuracy from time 2 to time 3 wasreduced significantly. Overall, the stability of allalginate impression materials, Algiace Z, CAVEX andJeltrate, reduced when the storage times increased(Table 3 and Fig. 2).

Fig. 3 shows the mean and standard error for thetwo experimental materials KE106A and KE106B.

Overall, the accuracy of KE106B is superior to thatof KE106A and KE106B had greater stability thanKE106A. The accuracy of KE106B was significantlybetter than that of KE106A in the third round ðp ¼

0:007Þ:

Discussion

An accurate model is indispensable for the fabrica-tion of a crown or bridge and the choice ofimpression material is vital. In 1989, Eriksson et al.evaluated one agar agar and seven alginates andtwo addition silicones.16 They detected the dis-crepancies in the diameters of the occlusal surfacesand cervical areas and measured the height of

Table 3 Comparisons of mean and standard deviation of the accuracy in each group ðn ¼ 10Þ based on 10 impression materials andthree storage times.

Materials Storage times Comparisons of storage time effecta

T1 T2 T3 Significant pairsb

Accuracies of impressions (%)Alg 1.15(0.84)c 1.36(1.20) 1.65(0.96)Cav 1.09(1.03) 1.30(0.68) 3.38(1.36) T3 . T2; T3 . T1Jel 1.23(0.75) 1.57(1.01) 2.11(1.18) T3 . T1Aqu 0.70(0.45) 0.98(0.84) 1.00(0.79)Exr 0.89(0.66) 0.78(0.62) 1.02(0.86)Exp 0.82(0.64) 1.26(0.85) 1.35(0.77) T3 . T1Col 1.03(0.94) 1.33(1.89) 0.95(0.53)Rap 1.15(0.84) 0.60(0.42) 1.88(1.41) T3 . T2KEA 1.81(0.77) 2.46(1.69) 2.46(0.95) T3 . T1KEB 1.52(0.84) 1.78(1.43) 1.36(0.64)

Comparisons of material effecta

(1) F test of 1-way ANOVA p ¼ 0:095 p ¼ 0:041 p , 0:001(2) Significant pairsb None KEA . Rap Cav . all except KEA

and Jel; KEA . Col

Alg, Algiace Z; Cav, CAVEX; Jel, Jeltrate; Aqu, Aquasil; Exr, Exaflex; Exp, Express; Col, Coltex fine; Rap, rapid liner; KEA, KE106A;KEB, KE106B; T1; T2; T3; first round, second and third rounds of experiments.a Simple test was evaluated following up the significant effect of interaction between materials and storage times ðp , 0:001Þ from

two-way mixed ANOVA.b All results of Post Hoc tests are controlled for family type I error rate ¼ 0.05.c Mean (standard deviation), n ¼ 10:

Figure 2 Comparison of accuracy based on materials and storage times. Note: different characters, a and b, representsignificant difference within each material.

Accuracy of impression materials 607

the stone and stainless steel model. They foundthat the occlusal surface of the stone cast wassmaller than that of the master die, and that theheight of the stone cast was less than that of themaster die. In addition, the cervical portion waslarger than that of the master die. This showed thatthe constriction area of the impression materialvaried in different parts of the stone cast. Thisphenomenon is complicated and difficult to explain.In order to simplify the comparison in this study, wemeasured only the diameters of metal dies andstone casts. Some of the stone casts had smallbubbles or obscure areas on the outer margin of theocclusal surface, so we measured only the innerdiameters of the occlusal surfaces.

The discrepancies between the stone dies andmetal casts had positive and negative values, whichalso was reported by Eriksson et al.16 In order toavoid false results due to the positive and negativevalues canceling each other out, the data wereconverted to absolute values and the accuracieswere calculated in percentages. The smaller valuesof the percentages indicate greater accuracy.Because the mechanical properties of the stonematerial are influenced predominantly by thewater/powder ratio,17 all the specimens in thisstudy were made using the ratio 10:3 (w/w).

Our results showed that, in the first and secondrounds, the alginate impression materials hadaccuracies close to those of the elastomericimpression materials. However, after 24 h, thealginate impression materials were relativelyunstable compared to the elastomeric impressionmaterials. In addition, under magnified conditions,some of the stone cast surfaces which were madeusing alginate impression materials were rougherthan those made using rubber elastomericimpression materials. This may be caused by theinhibitory properties of hydrocolloid materials inthe plaster setting reaction. Therefore, the algi-nate impression materials may have the same

degrees of accuracy as those of elastomericimpression materials but, in reality, alginateimpression materials still perform more poorlythan rubber base impression materials. In 1989,Peutzfeldt and Amusen studied the accuracies ofalginate and elastometric impression materials.They found that one kind of alginate impressionmaterial was as good as the elastometric impressionmaterials.12 In 1997, Federic and Caputo comparedthe accuracies of two agar agar and three elasto-metric impression materials. They also reportedthat the accuracies of agar agar were the same aspolyether impression materials. Alginateimpression materials appeared to have accuraciesas good as those of the elastometric impressionmaterials.13 However, if water loss and the for-mation of surface roughness are considered, theproperties of elastometric impression materialsmay be better than alginate impression materials.

Johnson and Craig compared the accuracy of fourtypes of elastometric impression materials withdifferent storage times, and found no significanteffect of storage time for the addition siliconeimpression material.18 Our results were not con-sistent with their findings. Two of the five siliconeimpression materials had a storage time effect,including Express and Rapid, but the accuracy foreach storage time was still good compared to thealginate impression materials (Table 3).

The alginate impression materials have evapor-ation properties. If they are not placed in a tightlyclosed storage box, the impression materials con-strict considerably and lose their elasticity.19 Thisnot only causes large discrepancies but also makesit difficult to separate the model. Therefore, westored alginate impression materials under con-ditions of 100% relative humidity.

The accuracy of the elastometric impressionmaterials was relatively stable among differentstorage times and their discrepancies were causedpredominantly by the reaction of the com-ponents.20 However, the components of commer-cial products were usually difficult to investigate,hence two kinds of experimental elastometricimpression materials were prepared in this labora-tory and used to evaluate the effect of the inorganicfiller. In this study, the sub-micron (0.02–0.04 mm)colloidal silica (Aerosil R 972) was used, because itprovides increased volume with relatively littleweight. The data revealed that when the proportionof the filler increased, the accuracy increased. In1992, Fano et al. studied the dimensional stabilityof silicone impression materials. They reported thatthe higher the viscosity, the less the constriction.8

In 1988, Mandikos reported that lower viscositymaterials showed the greatest changes due to their

Figure 3 Comparison of accuracy of KE106A andKE106B. Note: time 1: p ¼ 0:422; time 2: p ¼ 0:351;time 3: p ¼ 0:007; based on two-sample t-test forcomparisons of the two materials.

S.Y. Chen et al.608

lower filler content.20 The results of this studyagree with those reports.

A larger volume of filler causes less elasticity andfluidity, which results in lower accuracy, so furtherstudies are needed to determine the optimumproportion of inorganic filler and methodology. In1992, Hung et al. compared the accuracy of a one-step versus two-step putty wash addition siliconeimpression technique and found that the one-stepimpression technique was more accurate than thetwo-step impression technique.3 In the same year,Chee and Donovan reported that the simultaneousputty-wash impression technique is the worstmethod.21 In 1995, Lee et al. compared the one-step with the two-step impression technique underconditions of minor movement, and no significantdifferences in accuracy were observed.22 In ourstudy, we investigated the effect of different ratiosof inorganic filler on accuracy, but we did notattempt to compare one-step and two-stepmethods, although this may be the subject offurther research.

Conclusion

In conclusion, the two-factor mixed factorialANOVA showed that the accuracy in three roundsvaried among the 10 impression materials ðp ,

0:001Þ: Two addition type silicone materials, Aquasiland Exaflex, had the greatest accuracy and stab-ility. Two alginate impression materials, CAVEX andJeltrate had the least stability and the accuracydecreased significantly when the storage timesincreased. It seems that higher filler componentmay increase the accuracy.

Acknowledgements

We are indebted to Dr Tim J. Harrison of the RoyalFree and University College Medical School ofUniversity College London (London, United King-dom) for critically reading the manuscript.

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