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he dimensional stability of impression materials and itsffect on in vitro tooth wear studies

ose M. Rodrigueza, David W. Bartlettb,∗

UCL Eastman Dental Institute, Prosthodontics, 256 Gray’s Inn Road, London WC1X 8LD, UKKings College London Dental Institute, Fixed and Removable Prosthodontics, Floor 25, Guy’s Tower, Great Maze Pond, London SE1 9RT,K

r t i c l e i n f o

rticle history:

eceived 24 February 2010

ccepted 15 October 2010

eywords:

mpression materials

imensional stability

ooth wear

a b s t r a c t

Objectives. To assess the dimensional stability of 8 impression materials over 12 weeks rele-

vant to in vitro tribology studies.

Methods. Ten impressions from eight impression materials were taken of a metal block (ADA

block) conforming to the American Dental Association specification for impression materi-

als and of another metal block (custom block) which allowed measurements over a larger

surface area. The impressions and blocks were scanned on a non-contacting laser profilome-

ter (Taicaan® – Southampton, UK) and using surface metrology software Boddies® (Taicaan®

– Southampton, UK) measurements were made at 24 h, 2, 4, 8 and 12 weeks. The impression

materials tested were [1] Aquasil®, [2] Aquasil® DECA, [3] Affinis®, [4] Express®, [5] Extrude®,

[6] Impregum®, [7] President® and [8] Take 1®.

Results. Seven addition silicones and one polyether [6] were tested. [2] and [6] were monopha-

sic, the rest were putty-wash. The results from impressions of the ADA block showed that

all materials contracted compared to measurements obtained directly from the block [1]

expanded over time (+31.5 �m) (p < 0.05). The results from the custom block showed that all

materials contracted compared to direct measurements of the block. [4] and [7] expanded

over time (+62 �m and +63.8 �m respectively). [8] contracted over time (−54.7 �m) (p < 0.05).

Significance. No material showed linear changes >1.5% and were stable for 12 weeks. Nev-

ertheless, the range of changes would affect tribology studies were cut-offs lesser than the

reported changes are selected. All impressions should be processed after similar time delays

ntrod

emy

samples of different opacity [4]. When used clinically, these

to reduce the errors i

© 2010 Acad

. Introduction

mpression materials can be used to record topographicalhanges on teeth subjected to wear episodes in vivo [1] and

n vitro [2]. The replicas can be scanned on profilometers andsing surface metrology software, tooth wear can be measureds either step height of volumetric loss [3] from unchanged

∗ Corresponding author at: Kings College London Dental Institute, Fixedond, London SE1 9RT, UK. Tel.: +44 0207 188 5390.

E-mail address: david.bartlett@kcl.ac.uk (D.W. Bartlett).109-5641/$ – see front matter © 2010 Academy of Dental Materials. Puoi:10.1016/j.dental.2010.10.010

uced by dimensional changes.

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

reference areas. Reproducing the surfaces using impressionmaterials rather than directly scanning them overcomes theproblem of scanning materials of different colour and reflec-tivity which can introduce a source of error when scanning

and Removable Prosthodontics, Floor 25, Guy’s Tower, Great Maze

impressions are normally cast within a few days; however, formany laboratory investigations there is less urgency to castor digitise the impressions so there can be delays between

blished by Elsevier Ltd. All rights reserved.

254 d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 253–258

Table 1 – Impression materials tested.

Trade name Manufacturer Type Consistency Lot/Expiry date

Affinis® Coltene/Whaledent Poly-vinyl siloxane Heavy Light 0096188/July 2008 0090562/May 2008Aquasil® Dentsply Caulk Poly-vinyl siloxane Putty Light 0607000513/Dec2008 060303/Mar 2009Aquasil DECA® Dentsply Caulk Poly-vinyl siloxane Medium 060406/Apr 2008Express® 3M ESPE Poly-vinyl siloxane Putty Light 6JM21/Oct 2008 5HLF2HR/Aug 2008

® ane

aneane

Extrude Kerr Hawe Poly-vinyl siloxImpregum® 3M ESPE PolyetherPresident® Coltene/Whaledent Poly-vinyl siloxTake 1® Kerr Hawe Poly-vinyl silox

impression taking and scanning. Also, in most in vitro studiesmeasuring tooth wear, samples are subjected to several cyclesof wear and the completion of these cycles may take from afew days to a few months, hence there may be a delay betweenimpression taking and processing.

The dimensional stability of impression materials has beenstudied widely in the literature [5–7]. Most studies follow pro-tocols described by the American Dental Association (ADA)which replicates a clinical scenario [8]. These guidelines adviseusing a cylindrical metal block allowing measurements overtwo horizontal coordinates in an area of less than five mil-limetres in length. Some researchers have recognized that thisstandard may not be sufficient to account for changes over thethree coordinates [9] or over larger surface areas.

Surprisingly little is known about the long term stability ofimpression materials and how this may affect in vitro toothwear studies where measurements are reliant on accurateand stable reproductions of the tissues being studied. Theaim of this study was to assess the dimensional stability ofeight commonly used impression materials over a period of12 weeks (84 days) using two blocks; the one specified on the

ADA specification no. 19 for elastomeric impression materials[8] and another custom block. The null hypothesis was thatthe materials showed linear dimensional changes <1.5% forthe duration for the study.

Fig. 1 – The ADA block.

Putty Light 549546/May 2008 6-1137/May 2009Medium 262218/Feb 2009

Putty Light 0109448/Jan 2009 0095975/Jul 2008Heavy Light 6-1230/Aug 2009 6-1094/Apr 2009

2. Materials and methods

Seven addition cured silicone and one polyether impressionmaterials (Table 1) were assessed for dimensional changesover 12 weeks using two blocks. One block conformed to theADA specification no. 19 for elastomeric impression materials(ADA block – Fig. 1) and the other (custom block – Fig. 2) allowedmeasurements to be taken in the ‘X’ and ‘Y’ axes. The ADAblock was a 30 mm diameter metal cylinder with three paral-lel grooves of 20, 50 and 75 �m deep repectively; the maximumdistance between the outermost grooves was five millimeters.The custom block was a 50 mm by 50 mm square with multi-ple features which allowed measurements to be obtained overmore than one coordinate.

Ten impressions of each block were taken with eachimpression material following their manufacturer’s instruc-tions. Impregum® and Aquasil DECA® were mixed on aPentamix® machine. The putty materials were weighed (5 g ofbase and catalyst each) on a set of laboratory scales (Mettler-Toledo AT250 - accuracy 1 mg, repeatability 3 mg) and werehand mixed until no streaks were visible to the naked eye. The

rest of the materials were mixed using a gun-cartridge systemand dispensed after thorough purging. Putty-wash impres-sions were taken with all materials except Impregum® andAquasil DECA®, which were monophasic. To ensure an even

Fig. 2 – The custom block.

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 253–258 255

Table 2 – Direct measurements from the ADA block; mean and (SD) measurements of impressions of the ADA block withthe eight impression materials over time (mm).

Impression material Direct measurements from the block

5.11 mm

1 day 2 weeks 4 weeks 8 weeks 12 weeks

Aquasil® 5.07 (0.02) 5.08 (0.02) 5.08 (0.02) 5.07 (0.02) 5.08 (0.01)Aquasil® DECA 5.06 (0.01) 5.06 (0.01) 5.09 (0.03) 5.08 (0.02) 5.09 (0.02)Affinis® 5.08 (0.02) 5.09 (0.02) 5.08 (0.02) 5.08 (0.02) 5.08 (0.02)Express® 5.08 (0.01) 5.08 (0.02) 5.09 (0.02) 5.06 (0.01) 5.08 (0.02)Extrude® 5.10 (0.03) 5.07 (0.02) 5.07 (0.02) 5.08 (0.02) 5.09 (0.03)

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Impregum 5.08 (0.03) 5.08 (0.02President® 5.09 (0.03) 5.09 (0.02Take 1® 5.09 (0.02) 5.08 (0.02

hickness of the impressions a glass slab and a weight of oneilogram were positioned on top of either the block. All mate-ials were used at least six months before their expiry date.he impressions were left to set undisturbed for 10 min andfter removal checked under an optical microscope (EMZ TR,eiji, Japan) for the accuracy of the impression. The impres-

ions were carefully trimmed, labelled and their orientationxes marked.

The impressions were stored for 24 h and then scanned onnon-contacting laser profilometer (Xyris 2000TL, Taicaan®

echnologies, Southampton, UK). The scanner had an axisnd sensor resolution of 0.1 �m and a 785 nm laser sensorith a spot size diameter of 30 �m. Accuracy was 0.1 �m

nd repeatability 0.3 �m. The impressions were scannedsing a step-over distance of 50 �m. They were then re-canned at two weeks, four weeks, eight weeks and 12 weekssing the same techniques. All testing and storage was per-

ormed in a dry environment (20 ± 1 ◦C). Data analysis waserformed with Boddies v1.81 surface metrology software

Taicaan®Technologies, Southampton, UK). For the ADA block,wo measurements were obtained on the ‘X’ axis. For the cus-om block two measurements were obtained on the ‘X’ axis,wo on the ‘Y’ and two cross sectional (from one corner of thelock to the opposite corner).

Finally, the two blocks (ADA and custom) were scannedirectly on the non-contacting laser profilometer using theame settings as described above; this allowed comparison ofeasurements obtained directly from the block to measure-ents obtained from the impressions. One-way analysis of

ariance (ANOVA) and post ANOVA Scheffe tests were used tossess differences in measurements obtained from differentmpression materials over time. Statistical significance wasnferred where p < 0.05.

. Results

he measurements obtained from directly scanning the sur-ace of the ADA block and the mean and (standard deviation)

easurements of impressions of the ADA block using the

ight impression materials at each time interval are shown onable 2. The measurements obtained from directly scanninghe surface of the custom block and the mean and (standardeviation) measurements of impressions of the custom block

5.08 (0.02) 5.09 (0.03) 5.08 (0.02)5.08 (0.02) 5.08 (0.02) 5.08 (0.03)5.10 (0.05) 5.08 (0.04) 5.09 (0.03)

using the eight impression materials at each time interval areshown in Table 3. Data showed there were variations for allimpression materials compared to direct measurements fromthe blocks and that there were differences in measurementswith time.

3.1. ADA block

Table 4 shows the range of mean change and (percent-age change) in measurements from impressions at all timeintervals compared to the directly scanned surface of theADA block. All impression contracted in size comparedto the direct measurements of the ADA block. The meancontraction range was between 14 �m and 54.2 �m (mean per-centage change range 0.27–1.07%). The maximum change wasrecorded by Aquasil® DECA and the least change was recordedby Extrude®. Data showed that Aquasil® DECA showed statis-tically significant differences in measurements between dayone and 12 weeks (p = 0.01); the mean expansion was 31.5 �m(0.62% change). None of the other impression materials of theADA block were statistically significant different in measure-ments on the ‘X’ axis over 12 weeks (p > 0.05).

3.2. Custom block

Table 4 shows the range of mean change and (percentagechange) in measurements on the ‘X’, ‘Y’ and cross sectionalaxes at all time intervals compared to measurements fromdirect scans of surface of the custom block. There was a gen-eral trend towards materials contracting when compared tothe directly scanned surface of the custom block but this wasless clear when compared with data from the ADA block. Onthe ‘X’ axis, the least change was shown by Express® (1.8 �m –0.01% change) and the most by President® (111.3 �m – 0.35%).On the ‘Y’ axis, the least change was shown by Affinis® (0.2 �m– 0%) and the most by Aquasil® (183.6 �m – 0.5%). Cross-sectionally, the least change was shown by Aquasil® (8.5 �m –0%) and the most by Impregum® (214.6 �m – 0.5%).

Data showed that Express® showed statistically significantdifferences in measurements on the ‘X’ axis between week

8 and week 12 (p = 0.02); the mean expansion was 62.0 �m(0.19% change). President® showed statistically significant dif-ferences in measurements on the ‘X’ axis between week 2and week 4 (p = 0.01); the mean expansion was 63.8 �m (0.20%

256 d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 253–258

Table 3 – Direct measurements from the custom block; mean and (SD) measurements in all axes (X, Y and cross-sectional(C-S)) of impressions of the custom block with the eight impression materials over time (mm).

Impression material Direct measurements from the block

X axis: 31.90 mm; Y axis: 31.86 mm; cross-sectional: 39.64 mm

Axis 1 day 2 weeks 4 weeks 8 weeks 12 weeks

Aquasil®

‘X’ 31.83 (0.02) 31.84 (0.02) 31.83 (0.05) 31.86 (0.05) 31.84 (0.03)‘Y’ 31.84 (0.04) 31.68 (0.38) 31.84 (0.05) 31.83 (0.02) 31.84 (0.02)‘C-S’ 39.56 (0.14) 39.62 (0.14) 39.47 (0.17) 39.64 (0.12) 39.58 (0.07)

Aquasil®

DECA

‘X’ 31.82 (0.03) 31.82 (0.05) 31.88 (0.04) 31.86 (0.03) 31.86 (0.12)‘Y’ 31.87 (0.05) 31.96 (0.08) 31.90 (0.06) 31.83 (0.02) 31.89 (0.10)‘C-S’ 39.49 (0.09) 39.61 (0.15) 39.62 (0.12) 39.78 (0.16) 39.64 (0.18)

Affinis®

‘X’ 31.85 (0.00) 31.86 (0.02) 31.87 (0.03) 31.89 (0.04) 31.85 (0.00)‘Y’ 31.85 (0.04) 31.87 (0.07) 31.85 (0.03) 31.82 (0.05) 31.85 (0.03)‘C-S’ 39.63 (0.14) 39.62 (0.24) 39.63 (0.09) 39.47 (0.12) 39.54 (0.18)

Express®

‘X’ 31.90 (0.03) 31.89 (0.05) 31.91 (0.04) 31.88 (0.03) 31.94 (0.03)‘Y’ 31.87 (0.05) 31.88 (0.03) 31.88 (0.05) 31.89 (0.05) 31.91 (0.06)‘C-S’ 39.58 (0.10) 39.67 (0.09) 39.64 (0.15) 39.64 (0.08) 39.58 (0.09)

Extrude®

‘X’ 31.89 (0.11) 31.88 (0.07) 31.88 (0.10) 31.88 (0.07) 31.89 (0.04)‘Y’ 31.87 (0.09) 31.85 (0.06) 31.86 (0.07) 31.86 (0.09) 31.86 (0.06)‘C-S’ 39.55 (0.15) 39.56 (0.14) 39.58 (0.19) 39.55 (0.11) 39.55 (0.13)

Impregum®

‘X’ 31.87 (0.05) 31.88 (0.09) 31.87 (0.02) 31.87 (0.09) 31.81 (0.05)‘Y’ 31.77 (0.02) 31.85 (0.03) 31.86 (0.04) 31.73 (0.38) 31.82 (0.06)‘C-S’ 39.43 (0.12) 39.55 (0.20) 39.56 (0.16) 39.56 (0.22) 39.51 (0.07)

President®

‘X’ 31.81 (0.04) 31.79 (0.08) 31.86 (0.06) 31.84 (0.05) 31.83 (0.07)‘Y’ 31.77 (0.04) 31.78 (0.08) 31.83 (0.07) 31.80 (0.06) 31.80 (0.06)‘C-S’ 39.51 (0.11) 39.45 (0.08) 39.52 (0.09) 39.53 (0.10) 39.55 (0.10)

(0.02(0.03(0.12

Take1®

‘X’ 31.91 (0.05) 31.89‘Y’ 31.85 (0.06) 31.85‘C-S’ 39.58 (0.19) 39.70

change). Take 1® showed statistically significant differencesin measurements on the ‘X’ axis between day 1 and week 12(p = 0.04); the mean contraction was 54.7 �m (0.17% change).None of the other impression materials showed statisticallysignificant difference in measurements on any axes (p > 0.05)at any time interval over 12 weeks.

4. Discussion

The results from this study showed that all impressionsmaterials contracted when compared to the directly scanned

Table 4 – Range of mean change and (percentage change) of meto measurements from direct scans of the surface of the ADA baxes at all time intervals compared to measurements from dire

Axis 1 day 2 weeks 4 we

ADA Block (�m)‘X’ axis 14.0–54.2 (0.27–1.07%) 21.5–51.8 (0.42–1.02%) 15.1–4

Custom block (�m)‘X’ 5.0–94.6 (0.02–0.3%) 15.4–111.3 (0.05–0.35%) 1.81–‘Y’ 0.7–98.4 (0.00–0.31%) 0.2–183.6 (0.00–0.58%) 5.1–3

Cross sectional 13.8–214.6 (0.03–0.54%) 22.8–194.4 (0.06–0.49% 11.6–1

) 31.86 (0.02) 31.89 (0.02) 31.86 (0.00)) 31.85 (0.07) 31.88 (0.04) 31.84 (0.05)) 39.71 (0.10) 39.50 (0.13) 39.73 (0.12)

surface of the blocks and that there were variations in mea-surements over time. In the case of the ADA block the range ofchanges were between 14 �m and 54.2 �m and in the case ofthe custom block the range of changes were between 0.2 �mand 214.6 �m. All changes were within the limits reported bythe American Dental Association specification (<1.5%). TheADA block was a simple test which only showed changes inthe X axis whereas the custom block gave more data but inter-

pretation was more complex.

The American Dental Association specification N◦19 forelastomeric impression materials states that linear dimen-sional changes within 24 h should not exceed 1.5% [8]. A

asurements on the ‘X’ axis at all time intervals comparedlock and measurements on the ‘X’, ‘Y’ and cross sectionalct scans of the custom block (�m).

eks 8 weeks 12 weeks

6.2 (0.31–0.91%) 29.8–50.6 (0.59–1.00%) 20.6–39.1 (0.41–0.71%)

71.6 (0.01–0.22%) 14.1–66.0 (0.04–0.21%) 18.2–97.5 (0.06–0.31%)3.4 (0.02–0.10%) 7.5–137.0 (0.02–0.43%) 6.1–64.9 (0.02–0.20%)80.6 (0.03–0.46%) 8.5–174.0 (0.02–0.44%) 12.7–140.3 (0.03–0.3%)

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Journal of Prosthetic Dentistry 1979;42:159–62.[12] Williams PT, Jackson DG, Bergman W. An evaluation of the

time-dependent dimensional stability of eleven elastomericimpression materials. Journal of Prosthetic Dentistry1984;52:120–5.

d e n t a l m a t e r i a l s

hange of 1.5% and up to 100 �m may be clinically accept-ble in some situations, but in the case of laboratory studies,uch change would be detrimental and would affect datanterpretation. The importance of selecting the most accu-ate material over time is essential to guarantee robustata. Investigations measuring tooth wear often produceeport minute changes which may be within the dimen-ional changes of the materials used as reported in thistudy.

Some in vitro tooth wear studies reproduce worn surfacesith impressions in order to measure wear either with the usef surface mapping systems or measuring microscopes. Mosttudies fail to mention how impressions were handled andf similar time delays were employed between taking impres-ions and processing them. Some studies also compare beforend after wear impressions by either superimposing themsing surface matching software or comparing step heightifferences from baseline impressions to post wear impres-ions. If two sets of impressions are used, then differingime intervals between impression scanning and processingould affect the measurements obtained from these. It isossible that these changes are unavoidable but in the casef in vitro studies they must be taken into account. Stud-

es should be standardized to be consistent with the errorntroduced by the dimensional changes of the materials. Allmpressions should be scanned or poured after similar timeelays.

Previous research has assessed the dimensional stabil-ty of impression materials for periods ranging from 24–26 h10–12], two weeks [5], four weeks [13] or 30 days [6]. Thistudy assessed the dimensional stablity of the materials over2 weeks to simulate a longer delay between impression tak-ng and digitising or casting. Data showed that measurementsetween impression materials were not different but that withime there were fluctuations in measurements within impres-ion materials most probably related to hygroscopic expansionr residual stress relaxation. These variations, albeit small,ccurred and should be taken into account when planningtudies reproducing tooth surfaces using impression materi-ls.

The ADA no. 19 block is a commonly reported standardesigned to test detail reproduction, gypsum compatibilitynd dimensional stability of impression materials [10,11,13].lthough widely used it only allows measurements to be maden a small surface area. The ADA block is limited to 5 mm iniameter and is sufficient for most laboratory studies but inter-reting changes to those that may occur in the mouth, which isonsiderably larger, needs a degree of caution. Other authorsave recognized its limitations to measure dimensional sta-ility due to its size and the inability to measure changesver the three coordinates [9]. For this reason other investi-ators still use different standards to assess the dimensionaltability and accuracy of impression materials such as teethrepared for fixed restorations [11,12,14–16] and more appro-riately full arch models with teeth with reference points [6,17]hich would reflect changes relevant to the dimensions of theral cavity. This is the reason why in this study, we decidedo test the custom block which would give an indication ofhanges over more than one coordinate. The results from thistudy confirmed the limitations of the ADA block.

( 2 0 1 1 ) 253–258 257

5. Conclusions

All impression materials tested in this study met the manu-facturers’ claim being dimensionally stable up to two weeks.All materials showed linear changes within the allowed lim-its recommended by the American Dental Association (<1.5%);nonetheless, there were variations in measurements. We rec-ommend that impressions used in vitro for dental research arescanned at similar times to minimize the effect introduced bydimensional changes.

Acknowledgments

The authors would like to thank Dr Ron Wilson for his kindassistance with the statistical analysis. This study was sup-ported by the Guy’s and St Thomas’ Charity Grant no. G050202.

e f e r e n c e s

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[15] Price RB, Gerrow JD, Sutow EJ, MacSween R. The dimensionalaccuracy of 12 impression materials and die stone

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