AbstractPolyvinyl siloxane impression materials haveapplications in a variety of indirect procedures inp r o s t h o d o n t i c s and restorative dentistry. Favourablehandling properties, good patient acceptance andexcellent physical properties have resulted in theirpopularity in today’s practice. In this review, thechemistry and important physical properties ofpolyvinyl siloxanes are summarized, and recentclinical questions of improved hydrophilics, trayadhesives, disinfection, and glove-inducedpolymerization inhibition are addressed.

Key words: Polyvinyl siloxane, elastomer, impressionmaterials, hydrophilic, polymerization inhibition.

(Received for publication November 1997. RevisedMarch 1998. Accepted April 1998.)

Introduction

The polyvinyl siloxane impression materials areaddition reaction silicone elastomers which werefirst introduced in the 1970s. Since that time andespecially in the past decade, these materials havegained in their acceptance and account for a largeshare of the impression material market. Polyvinylsiloxanes have applications in fixed prosthodontics,operative dentistry, removable prosthodontics andimplant dentistry.1,2 The materials are presented inthe form of two pastes (a base and an accelerator)which can be hand spatulated or autodispensed froma dual cartridge, and mixed in equal quantities foruse. They have achieved a high level of dentist andpatient acceptance as they are clean, odourless andtasteless.

Chemistr y

Polyvinyl siloxane materials are a modification ofthe original condensation silicones. Both are basedon the polydimethyl siloxane polymer, however thepresence of differing terminal groups accounts for

Australian Dental Journal 1998;43:(6):428-34

Polyvinyl siloxane impression materials: An updateon clinical use

Michael N. Mandikos, BDSc, MS*

their different curing reactions.3 As the dimensionalstability of the polyvinyls is so much improved andthe setting reaction is sufficiently different from thec o n d e n s ation curing silicones, the polyvinyl silox a n e swa rrant classification as a separate cat e g o ry ofmaterial.4

The base material contains a polymethyl hydrogens i l oxane copolymer, which is a moderately lowmolecular mass polymer with silane term i n a lgr o u p s.3 , 5 The accelerator mat e rial contains thevinyl-terminated polydimethyl siloxane. This is alsoa moderately low molecular mass polymer but hasvinyl terminal groups.3,5 The accelerator materialalso contains chloroplatinic acid as a homogeneousmetal complex catalyst.6-8 On mixing, an additionreaction occurs between the silane and vinyl groups(Fig. 1). There is minimal dimensional change duri n gthis polymerization and there are no by-products.

S e veral authors have reported hydrogen gasbubble formation on the surface of gypsum diespoured immediately from polyvinyl silox a n eimpressions.1,2,6 A side reaction of the hydrides onthe base polymer can produce hydrogen gas ifmoisture or residual silanol groups are present.Manufacturers have now eliminated the possibilityof this side reaction by proper purification andaccurate proportioning of the materials, and by theaddition of palladium to the pastes as a hydrogenabsorber.7,8 It is no longer necessary to wait for onehour before pouring these impressions.

The base and accelerator pastes also containfillers. Amorphous silica or fluorocarbons are usedas fillers to add bulk and improve the properties ofthe paste. The filler is also normally silanated toincrease the bond strength between filler andpolymer, which better allows it to function as ac r o s s - l i n k e r.6 C o l o u ring agents are added todistinguish the base and catalyst pastes and to aide va l u ation of mixing. More recently, intri n s i csurfactants have also been added in an attempt tonegate the hydrophobicity of these materials.9

428 Australian Dental Journal 1998;43:6.

*Advanced Education Program in Prosthodontics, State Universityof New York at Buffalo, New York, USA.

b. Working and setting times

Modern polyvinyl siloxanes have a working timeof two minutes and a setting time of six minutes(with slight variation).1,2 These times are consideredto be adequate if not ideal. Occasionally, situationswill present which require extended working timesand some methods of altering working and settingtimes have been reported in the literature. Alterationof the proportion of catalyst is to be avoided as thisleads to variable results and has been suggested tofacilitate the side reaction which produces hydrogengas.

Some manufacturers supply a retarder that can beincorporated into the mix to provide additionalworking time without compromising other propert i e s.2

The retarder is a small, reactive, tetracyclic vinylmolecule that polymerizes preferentially to thesiloxane copolymers. This small molecule is cyclicand does not form a chain. It is thus a chain stopper,and temporarily prevents polymeri z ation of thelinear siloxane molecules. The retarder continues topolymerize until it is completely consumed and thenthe linear siloxane molecules polymerize causing theimpression material to set. The retarder was used bytwo dental schools in the USA to provide studentswith extended working times in a teaching

Australian Dental Journal 1998;43:6. 429

Properties

a. Viscosity

Po l y v i nyl siloxanes are available in viscosities rangi n gfrom very low (for pouring, syringing or wash use),to medium, high and very high. The viscosity of them at e rial increases with the proportion of filler present.Viscosity is also affected by the shear force placed onthe material. The mixed base and catalyst pastesexhibit a decrease in their relative viscosities inresponse to high shear stresses. This is termed shearthinning. Thus a medium body impression materialcan possess sufficient viscosity to avoid excess flow ifloaded into an impression tray, yet it can also exhibitan apparent lowered viscosity suitable for intrasulcularimpressions, when it is expressed through animpression syringe tip.5,10 The higher the viscosity ofthe material, the more pronounced is the effect ofshear thinning. This phenomenon is suggested to bedue to the extremely small filler particle size.10

Thixotropy is the property of certain gels toliquefy when subjected to vibrating forces (forexample, ultrasonic waves) and then to solidify whenleft to stand again. Po l y v i nyl siloxanes behavesimilarly to this but have not been sufficientlyinvestigated to be classified as thixotropic materials.

Fig. 1. – The accelerator polymer is terminated with vinyl groups which cross-link to the silaneterminal groups on the base polymer when activated by a platinum salt catalyst. Note that this is an

addition reaction and there are no by-products.

environment. It has become less commonly usedsince the polyvinyl siloxanes became available inautomix dual cartridges.

The most convenient and widely advo c at e dmethod for extending working time is to refrigeratethe materials before mixing. Gains of up to 90seconds have been reported when the materials arechilled to 2°C.2,11

c. Reproduction of detail

Polyvinyl siloxanes are currently considered toreproduce the greatest detail of all the impressionmaterials. The international standard for dentalelastomeric impression materials1 states that a typeIII (light body) impression material must reproducea line 0.020 mm in width. With the exception of thevery high viscosity putty materials, all polyvinylsiloxanes (light, medium and heavy body) achievethis. Very low viscosity materials can reproduce lines1-2 µm wide.5,12,13

It should be noted that the literature does not tendto support the use of putty and wash impressiontechniques for greatest accuracy in impressions.Wassell and Ibbetson14 reported that heavy body andwash impressions were more accurate than putty a n dwash impressions. Frederick and Caputo1 5 f u rt h e rshowed that the putty and wash technique wassignificantly less accurate than polyether (heavy andlight body) or reversible hydrocolloid impressions.

d. Dimensional stability

The accuracy of an impression mat e rial isdependent on the dimensional stability. There are anumber of possible causes for dimensional changesin elastomeric impression mat e ri a l s. All ru b b e rp o l y m e rs must contract slightly during polymeri z at i o nas a result of reduction in spatial volume as theycross-link. If the setting reaction produces a by-product or if accelerator components are liberated,then the set volume is further decreased. Someimpression materials like the polyethers are lesshydrophobic and can absorb and then even losewater if they are placed in wet or varying humidityenvironments. Changes in temperature can also leadto varying changes in the final dimensions.

Polyvinyl siloxanes show the smallest dimensionalchanges on setting of all the elastomeric impressionmaterials. The majority of this shrinkage is due tocontinued polymerization occurring within the firstthree minutes of removal of the impression from them o u t h .2 Reductions in volume due to polymeri z at i o nhave been reported to be as low as 0.1 to 0.05 percent.5,7 This linear contraction is well matched to thesetting expansion of modern type III and type IV diestones and results in a slightly larger replication ofthe preparation.5,13,16

Long term dimensional stability of polyviny lsiloxanes is reported in the literature. This is becausethey are not susceptible to changes in humidity, andthey do not undergo any further chemical reactionsor release any by-products.1-3,5,16-18 Polyvinyl siloxaneimpressions may be repoured to produce stone dieswhich are as accurate as the original, as many asseven days later.1,16,19

The linear coefficient of thermal contraction isr e l at i vely high for all elastomers. When an impressionis removed from the mouth, there is an element ofshrinkage due to the decrease in temperature thatoccurs as the material moves from the mouth to thebench. Lower viscosity materials show the greatestchange (0.02-0.05 per cent shrinkage) due to theirlower filler content.16,17 Reheating an impression to37°C before pouring has been demonstrated toimprove the accuracy of the resultant die; however itis doubtful that this is clinically significant.11,17

e. Tear energy, elastic recovery and deformation

Impression materials must have sufficient strengthto allow removal from a gingival sulcus withouttearing. Tear energy is that energy required tosustain a tear through a material, and is of obviousi m p o rtance in thin intrasulcular or interp r ox i m a lareas.

Elasticity is inherent to all the elastomeri cimpression materials as they are polymers withhighly flexible kinked segments that allow freedomof movement. Under a load, the flexible kinkedsegments of these polymers will uncoil allowingmovement. Upon removal of the load, an idealelastomer will exhibit complete elastic recovery andreturn to its prestressed configuration. The degreeto which this occurs is a measure of the elasticrecovery of the material. Permanent deformationoccurs when a polymer is elongated beyond thepoint where elastic recovery is possible. Permanentdeformation is related to the degree of cross-linkingof the polymer strands, temperature, and the rate ofapplied stress.20

The ideal impression mat e rial should exhibitmaximum energy absorption with minimal distort i o n .However, it is also desirable that the material tearsrather than deforms past a critical point such as amargin. Polyvinyl siloxanes deform at much slowerr ates and tear at points of less permanent deform at i o nthan do the other elastomeric materials.20 Polyvinylsiloxanes are frequently reported to be the mostideally elastic impression materials because theyexhibit better elastic recovery and less permanentdeformation than the other elastomers. They canabsorb over three times more energy up to the pointof permanent deformation than other elastomers,and if elongated to over 100 per cent (strain at tear),

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Australian Dental Journal 1998;43:6. 431

they rebound to only 0.6 per cent perm a n e n tdeformation.16,20

f. Creep compliance

All elastomers are viscoelastic materials, implyingt h at deform ation and elastic recove ry are timedependent as well. Therefore, the longer the mat e ri a lis deformed (as occurs when impressions areremoved slowly from the mouth or separated slowlyfrom a poured model), then the longer time it takesfor elastic recovery and the possibility of permanentdeformation becomes higher.5 Polyvinyl siloxaneshave the least viscoelastic qualities thus requiring theleast time for recove ry from viscoelastic deform at i o n .Blomberg et al.21 reported that polyvinyl siloxanesh ave sufficient elastic recove ry to allow an impressionto be poured only six minutes after removal from themouth.

g. Radiopacity

Radiopacity of impression materials is importantfor radiographic identification of excess materialwhich may be accidentally swallowed, aspirated orleft in gingival tissues. Presently, only the poly-sulphide materials exhibit significant radiopacity dueto their lead dioxide content.

S h i l l i n g burg et al.2 2 tested an experi m e n t a lpolyvinyl siloxane material containing 20 per centbarium sulphate to improve radiopacity. Althoughthey were able to demonstrate qualities of radiopacityequal to that of the polysulphides, there we r eassociated physical property drawbacks includinghydrogen bubble formation in dies poured from theimpressions, and evidence of long term breakdownlimiting the shelf life.

h. The hydrophilic question

According to O’Bri e n ,8 wetting describes therelative affinity of a liquid for a solid. It is the degreeto which a drop will spread on a solid surface, andcan be quantified by observing the contact angle.High angles (greater than ninety degrees) indicatepoor wetting, whilst a zero angle would indicateperfect wetting of the surface. When discussing thewetting characteristics of impression materials, it isimportant to distinguish between the ability of thematerial to flow around the soft and hard tissues ofthe mouth, and the ability of the material to be wetby a gypsum slurry.

Polyvinyl siloxanes are inherently hydrophobic.3,7

However in recent times, new ‘hydrophilic’ polyvinylsiloxanes have been introduced with manufacturerclaims that they better wet moist dental surfaces.These new formulations have intrinsic surfactantsadded. Typically these are non-ionic surfactants ofnonylphenoxypolyethanol homologues.7,23 Research

in this area has involved observations of the wettingof set surfaces of polyvinyl siloxanes. The conclusionthen is that the ‘hydrophilic’ statements refer to thenewer materials being more readily poured up witha gypsum-based die stone than the previousgeneration materials. There is no scientific evidenceto indicate that polyvinyl siloxanes advertised as‘hydrophilic’ can be syringed into a wet sulcus for anaccurate impression.2,9,12,23

This has been confirmed by Takahashi andFinger,24 who demonstrated that under a simulatedclinically dry field, both the hydrophilic and originalf o rm u l ations of polyvinyl siloxane wet tooth stru c t u r ewith equal results. It has also been shown that thenewer hydrophilic materials perform no better thanthe original formulations of polyvinyl siloxane inwettability for pouring dies, if a compatible, extri n s i c,spray-on surfactant is applied before pouring.9,23

The application of an extrinsic surfactant to thesurface against which an impression is to be madehas also been suggested. Millar and co-workers25

reported a significant reduction in the number ofvoids and an overall increased quality of polyvinylsiloxane impression when a modified polydimethylsiloxane wetting agent was applied to the preparedtooth surfaces before impressions were made.

Impression trays and adhesives

Tray spacing and tray design have often been citedas potential sources of error in impressions for fixedprosthodontic procedures. When only polysulphidem at e rials were available, even tray spacing wa scritical to minimize distortion due to uneven poly-merization shrinkage in areas of greater impressionmaterial bulk. The improved physical properties ofm o d e rn elastomers and particularly polyviny lsiloxanes have diminished this concern and now theuse of stock trays for impressions has become commonpractice for reasons of cost and convenience.26

Common stock trays made of polystyrene orchromium plated brass are reported to be suitablystiff to prevent flexure or distortion, although thereremains some possibility of tray wall flexure withpolystyrene trays.27 Wassell and Ibbetson14 and Payneand Pereira28 identified poor fitting stock trays andconsequent tray wall flexure as the greatest concernfor inaccuracy.

Whether custom or stock trays are used forimpressions, another potential source of error mayarise if the material is not adequately retained in theimpression tray when it is removed from the mouth.The use of adhesives in trays has been shown toachieve higher material bond strengths for polyvinylsiloxanes than has mechanical retention.28,29 Theadhesives used are usually polydimethyl siloxane andethylsilicate. The adhesive reacts with the surface of

432 Australian Dental Journal 1998;43:6.

the tray material and forms a chemical bond to thetray and to the impression material. It is generallyrecommended to wait ten to fifteen minutes aftera p p l i c ation of the adhesive before making theimpression.29 This allows time for the solvent toreact with the tray material.

The polyether systems have the greatest bondstrengths while the polyvinyl siloxanes show greatvariability between manufacturers with some beingve ry poor, and others ri valling the polye t h e rsystems.27 Bond strengths will also vary dependingon the tray material. In contrast to the polyethersand polysulphides, Chai et al.2 7 r e p o rted thatadhesive strength to acrylic resin (custom trays) wassignificantly lower than polystyrene or metal stocktrays for the polyvinyl siloxanes.

Visible light-cured tray mat e rials based on urethaned i m e t h a c ry l ate have found recent acceptance.I nve s t i g ations into their bond strengths withpolyvinyl siloxane adhesives indicate that they bondbetter than polymethyl methacry l ate mat e ri a l sprovided that the air inhibited non-polymerizedlayer is removed with isopropyl alcohol or a carbidebur.28,30 Sulong and Setchell31 demonstrated thatroughening the surface of the impression tray willsignificantly improve the effectiveness of polyvinylsiloxane adhesives.

It is important to note that although otherelastomers have stronger bonding adhesives, thesecannot be substituted for use with polyviny ls i l ox a n e s. Also, polyvinyl siloxane putties are report e dto show no chemical adhesion to their adhesive andso the use of putties requires mandatory mechanicalretention in the impression tray.27

In many clinical situations, the impression tray mustbe tried in the mouth prior to impression makingand this leads to saliva contamination. If the tray hasalready been painted with adhesive, then subsequenta p p l i c ation is recommended to maintain bonds t r e n g t h s. Contaminated adhesives have shown a dropin bond strength to one-fifth of their ori ginal amount.3 2

Disinfection

Disinfection is the inhibition or destruction ofpathogens and can be achieved by immersion of animpression into antimicrobial chemical solutions for3 to 90 minutes depending on the agent. Steri l i z at i o nis the total elimination of all micro-organisms andspores and requires immersion periods of 6 to 10 hours.Extended periods of immersion can risk distortingimpressions by liquid uptake and subsequentswelling of the material. Consequently, proceduresc u rrently used to control the transmission ofp athogens from impressions have tended to bedisinfecting and not sterilizing due to concern forthe accuracy of the impression.

Herrera and Merchant33 tested the dimensionalstability of different impression materials followingimmersion disinfection for thirty minutes. Theyobserved that polyvinyl siloxane and polysulphidewere unaffected after immersion in sodiumhypochlorite, 2 per cent glutaraldehyde, 0.5 per centp ovidone-iodine and 0.16 per cent halogenat e dphenol whilst polyethers were significantly unstable.Even after extending the immersion times to sixtyminutes, del Pilar Rios et al.3 4 a greed with the findingsof Herrera and Merchant. Johansen and Stackhouse3 5

demonstrated that polyvinyl siloxanes were able tobe immersed in 2 per cent glutaraldehyde for 16hours without any observed dimensional changes,whilst polyether mat e rials showed dramatic distort i o n sunder the same conditions.

Holtan et al .36 measured the dimensional stabilityof polyvinyl siloxanes after sterilization proceduresusing a conventional steam autoclave, and anethylene oxide gas autoclave. They determined thatsterilization in ethylene oxide gas resulted in gasinclusions into the impression material which thenformed bubbles in dies poured immediately fromthem. Waiting to pour dies for 24 hours after gasautoclaving prevented this problem. Impressionss t e rilized in the steam autoclave did undergodistortion that would have been significant enoughto prevent a casting from seating. It was concludedthat steam autoclaving was a suitable sterilizationmethod if the impressions were not to be used forfixed prostheses.

Most recently, radiofrequency glow discharginghas been advo c ated for use as a disinfectingprocedure for polyvinyl siloxane impressions.3 7

Whilst this procedure is claimed to clean andimprove the wettability of the impression surface, itis not clear if glow discharging results in steri l i z at i o n . †

Compatibility with die materials

The hydrophobicity of polyvinyl siloxanes is wellestablished, however, as mentioned previously, theintroduction of newer mat e rials with intri n s i cs u r factants or the conjunctive use of extri n s i csurfactants topically, can greatly improve wettability.This makes polyvinyl siloxanes compatible withgypsum, epoxy resin, and polyurethane resinmaterials.9,12,38

Gypsum stones cannot reproduce detail muchsmaller than 20 µm because their crystal size rangesfrom 15 to 25 µm. Epoxy and polyurethane resinscan reproduce detail down to 1 to 2 µm makingthem highly compatible with the detail capturepossible with polyvinyl siloxane impressions.1 2

Polyvinyl siloxanes can also be silver electroplated.This procedure can be facilitated by soaking the

† Baier RE. Written communication, September 1996.

Australian Dental Journal 1998;43:6. 433

impressions in water for 24 hours before applicationof the silver powder. It is thought that this alters theconcentration of surfactants in the surface layers ofthe impression.‡ Crispin et al .39 reported that silverdies recovered from polyvinyl siloxane impressionsare the most accurate dies.

The ability to pour duplicate dies from oneimpression is highly advantageous. Morgano et al .40

reported that the removal of dies and subsequentr e p o u ring of polyvinyl siloxane impressions producedclinically accurate duplicate models.

Gloves and the inhibition of polymerization

Occasionally an inhibition or retarding effect isseen on polyvinyl siloxanes when they are used in aclinical setting. This phenomenon can occur afterdirect contact between the impression material andlatex gloves, or a region of the mucosa previouslytouched by latex gloves.41-45 It was initially suspectedthat the corn starch powder used as a lubricant inthe gloves was interacting,41 while other authorssuggested atmospheric oxygen inhibition, or inter-actions with haemostatic agents were the cause.Several haemostatic agents were tested before deCamargo et al.46 concluded that they were not theinhibitors. Jones et al.47 were able to show thatprovisional luting agents did not interfere with thepolymerization.

A sulphur compound has since been identified asbeing responsible for the retarding effect on poly-m e ri z ation. Zinc diethyl dithiocarbamate is anaccelerator used in the manufacture of the latexgloves. It reacts with the platinum catalyst in thepolyvinyl siloxane to cause a delay or total inhibitionof polymeri z at i o n .4 4 , 4 5 , 4 8 B a u m a n n4 4 r e p o rted thateven in concentrations as low as 0.005 per cent, totalinhibition of polymerization of polyvinyl siloxanecan be observed. It is also believed that thecompound can remain on a previously gloved hand,and so washing gloves or washing hands after gloveuse is not recommended as a means of avoidingcontamination.

Interestingly, not all latex gloves will cause aninhibition of set. It has been observed that syntheticlatex gloves do not produce this phenomenon, whilesome natural latex gloves do.42-44 One’s own testingand subsequent use of a non-retarding glove isrecommended.

Conclusions

The results of inve s t i g ations into polyvinyl silox a n eimpression mat e rials indicate that they producehighly accurate impressions because they reproducefine surface detail, and have excellent elastic recovery,

a d e q u ate tear strengths, and exceptional dimensionalstability. They are compatible with all common diematerials, can be disinfected or sterilized, and can berepoured after delayed periods. They are dispensedin convenient automixing dual cartridges or singletubes and are available in several viscosities. Ifhandled appropriately, polyvinyl siloxanes can beapplied in almost any indirect procedure.

Acknowledgements

The author would like to thank Dr J. MalcolmC a rter, Associate Professor Biomat e rials, Depart m e n tof Restorative Dentistry, State University of NewYork at Buffalo, for his advice in preparing thismanuscript.

References1. I n t e rn ational Standards Organization. Dental elastomeri c

impression mat e rial. ISO 4823-1992. Section 5.11 Detailreproduction. Stand alone document. Chicago: A m e ri c a nDental Association Department of Standards Administration,1992.

2. Chee WWL, Donovan TE. Po l y v i nyl siloxane impressionmaterials: A review of properties and techniques. J ProsthetDent 1992;68:728-32.

3. van Noort R. Introduction to dental materials. Spain: Mosby,1994.

4. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixedprosthodontics. 2nd edn. Chicago: Quintessence Publishing Co.,1981.

5. Craig RG. Restorative dental materials. 9th edn. St Louis:Mosby, 1993.

6. Williams JR, Craig RG. Physical properties of addition siliconesas a function of composition. J Oral Rehabil 1988;15:639-50.

7. Craig RG, O’Brien WJ, Powers JM. Dental materials. Propertiesand manipulation. 6th edn. St Louis: Mosby, 1996.

8. O ’ B rien WJ. Dental mat e ri a l s. Properties and selection.Chicago: Quintessence Books, 1989.

9. Panichuttra R, Jones RM, Goodacre C, Munoz CA, Moore KB.Hydrophilic poly(vinyl siloxane) impression mat e ri a l s :dimensional accuracy, we t t a b i l i t y, and effect on gypsumhardness. Int J Prosthodont 1991;4:240-8.

10. Chai J, Pang I. A study of the “thixotropic” property ofelastomeric impression materials. Int J Prosthodont 1994;7:155-8.

11. Chew C, Chee WWL, Donovan TE. The influence oftemperature on dimensional stability of poly(vinyl siloxane)impression materials. Int J Prosthodont 1993;6:528-32.

12. Derrien G, Le Menn G. Evaluation of detail reproduction forthree die materials by using scanning electron microscopy andtwo-dimensional profilometry. J Prosthet Dent 1995;74:1-7.

13. Chee WWL, Donovan TE. Fine detail reproduction of very highviscosity poly(vinyl siloxane) impression mat e ri a l s. Int JProsthodont 1989;2:368-70.

14. Wassell RW, Ibbetson RJ. The accuracy of polyvinyl siloxaneimpressions made with standard and reinforced stock trays. JProsthet Dent 1991;65:748-57.

15. Frederick DR, Caputo A. Comparing the accuracy of reversiblehydrocolloid and elastomeric impression materials. J Am DentAssoc 1997;128:183-8.

16. Phillips RW. Skinner’s science of dental materials. 9th edn.Philadelphia: Saunders, 1991.

17. Tjan AHL, Li T. Effects of reheating on the accuracy of additionsilicone putty-wash impressions. J Prosthet Dent 1991;65:743-8.

18. Lewinstein I, Craig RG. Accuracy of impression materialsmeasured with a ve rtical height gauge. J Oral Rehabil1990;17:303-10.‡Garlapo DG. Written communication, February 1996.

19. DeWald JP, Nakajima H, Bell JL. Bond strengths betweenelastomeric impression materials and disinfected preliminaryimpressions. J Prosthet Dent 1994;71:394-9.

20. Hondrum SO. Tear and energy properties of three impressionmaterials. Int J Prosthodont 1994;7:517-21.

21. Blomberg PAH, Mahmood S, Smales RJ, Makinson OF.C o m p a r at i ve elasticity tests for elastomeric (non putty)impression materials. Aust Dent J 1992;37:346-52.

22. S h i l l i n g burg HT, Wilkerson-Lyman SL, Duncanson MG.Radiopacity enhancement of an experimental vinyl polysiloxaneimpression material. Quintessence Int 1989;20:657-63.

23. Chai JY, Yeung T. Wettability of nonaqueous elastomeri cimpression materials. Int J Prosthodont 1991;4:555-60.

24. Takahashi H, Finger WJ. Dentin surface reproduction withhydrophilic and hydrophobic impression materials. Dent Mater1991;7:197-201.

25. Millar BJ, Dunne SM, Robinson PB. The effect of a wettingagent on void form ation in impressions. J Prosthet Dent1997;77:54-6.

26. Tjan AHL, Nemetz H, Nguyen LTP, Contino R. Effect of trayspace on the accuracy of monophasic polyvinyl silox a n eimpressions. J Prosthet Dent 1992;68:19-28.

27. Chai JY, Jameson LM, Moser JB, Hesby RA. Adhesiveproperties of several impression material systems: Part 1. JProsthet Dent 1991;66:201-9.

28. Payne JA, Pereira BP. Bond strength of three nonaqueouselastomeric impression materials to a light-activated resin tray.Int J Prosthodont 1992;5:55-8.

29. Cho GC, Donovan TE, Chee WWL, White SN. Tensile bondstrength of polyvinyl siloxane impressions bonded to a customtray as a function of drying time. Part 1. J Prosthet Dent1995;73:419-23.

30. Dixon DL, Breeding LC, Bosser MJ, Nafso AJ. The effect ofcustom tray material type and surface treatment on the tensilebond strength of an impression material/adhesive system. Int JProsthodont 1993;6:303-6.

31. Sulong MZAM, Setchell DJ. Properties of the tray adhesive of anaddition polymerizing silicone to impression tray materials. JProsthet Dent 1991;66:743-7.

32. Chai JY, Jameson LM, Moser JB, Hesby RA. Adhesiveproperties of several impression material systems: Part II. JProsthet Dent 1991;66:287-92.

33. Herrera SP, Merchant VA. Dimensional stability of dentalimpressions after immersion disinfection. J Am Dent Assoc1986;113:419-22.

34. del Pilar Rios M, Morgano SM, Stein RS, Rose L. Effects ofchemical disinfectant solutions on the stability and accuracy ofthe dental impression complex. J Prosthet Dent 1996;76:356-62.

35. Johansen RE, Stackhouse JA. Dimensional changes of elastomersduring cold sterilization. J Prosthet Dent 1987;57:233-6.

36. Holtan JR, Olin PS, Rudney JD. Dimensional stability of apolyvinyl siloxane impression material following ethylene oxideand steam autoclave sterilization. J Prosthet Dent 1991;65:519-25.

37. Hesby RM, Haganman CR, Stanford CM. Effects of radio-f r e q u e n cy glow discharge on impression mat e rial surfa c ewettability. J Prosthet Dent 1997;77:414-22.

38. Schelb E, Cavazos E, Troendle KB, Prihoda TJ. Surface detailreproduction of type IV dental stones with selected polyvinylsiloxane impression materials. Quintessence Int 1991;22:51-5.

39. Crispin BJ, Watson JF, Frawley KR. Silver plated dies. Part II:m a r ginal accuracy of cast restorat i o n s. J Prosthet Dent1984;51:768-73.

40. Morgano SM, Milot P, Ducharme P, Rose L. Ability of variousimpression materials to produce duplicated dies from successiveimpressions. J Prosthet Dent 1995;73:333-40.

41. Duncan JD. Prevention of catalyst contamination of vinyl poly-siloxane silicone impression material during the impressionprocedure. J Prosthet Dent 1991;66:277.

42. Chee WWL, Donovan TE, Kahn RL. Indirect inhibition of poly-merization of a polyvinyl siloxane impression material: a casereport. Quintessence Int 1991;22:133-5.

43. Rosen M, Touyz LZG, Becker PJ. The effect of latex gloves onsetting time of vinyl polysiloxane putty impression material. BrDent J 1989;166:374-5.

44. Baumann MA. The influence of dental gloves on the setting ofimpression materials. Br Dent J 1995;179:130-5.

45. Kahn RL, Donovan TE, Chee WWL. Interaction of gloves andrubber dam with a poly(vinyl siloxane) impression material: ascreening test. Int J Prosthodont 1989;2:342-6.

46. de Camargo LM, Chee WW, Donovan TE. Inhibition of poly-merisation of polyvinyl siloxanes by medicaments used ongingival retraction cords. J Prosthet Dent 1993;70:44-7.

47. Jones RH, Cook GS, Moon MG. Effect of provisional lutingagents on polyvinyl siloxane impression material. J Prosthet Dent1996;75:360-3.

48. Causton BE, Burke FJT, Wilson NHF. Implications of thepresence of dithiocarbamate in latex glove s. Dent Mat e r1993;9:209-13.

Address for correspondence/reprints:Dr Michael N. Mandikos,

1st Floor, Loyal House,245 Albert Street,

Brisbane, Queensland 4000.

434 Australian Dental Journal 1998;43:6.