Dental Research

Effectiveness of intraoral delivery tips in reducing voids inelastomeric impressionsYea-Hwe Chong* / George Soh**

Voids in impressions made by five aiitomixed addition silicone elastomers wilh andwithout intraoral delivery tips were investigated. The number of voids were counted ata predetermined site on 3 days. Hand-mixed impressions of the automixed malerialswere made for comparison. There was no .statistically significant difference in the meannumber of voids produced in each automixed silicone dispensed with the use ofintraoral delivery tips and the number of voids produced without the tise of the tips.However, there were significant differences in the number of voids among differentmaterials. The rank order of the automixed silicone elastomers was fairly consistent,with Express producing the highest and Reprosil the lowest number of voids. Automixedsilicone materials should only be prepared by automixing for optimal results.(Quintessence Int 1991,22:901-910.)

Introduction

The occurrence of surface defects at critical areas of atooth preparation, such as at margins or line angles,often frustrates the clinician's attempt at producing agood impression. Addition silicone elastomers are thenewest of the elastomeric impression materials. Theyexhibit excellent dimensional stability and low perma-nent deformation'- and produce more accurate stonecasts when compared with other impression mate-rials. '̂* However, void formation remains an inherentproblem of elastomeric impressions.''""

Much effort has been made to reduce formation ofdefects in these impressions. These include modifyingthe method of mixing impression materials^'^'" as wellas varying the type of syringes" and syringe-tipdiameters.^ Smear-mixing generally produces less voidsthan stir-mixing,^'" and mechanical spatulation undervaeuum produces even fewer impression voids, butthere are doubts about its apphcation for elastomericimpression materials.'

Lecturer, Department of Restorative Dentistry, Faculty of Den-tistry, National University of Singapore. National UtiiversityHospital, Lower Kent Ridge Road. Singapore n51t.Senior Leeturer, Department of Preventive Dentistry, Facultyof Dentistry, Natiotial Utiiversity of Sitigapote.

The automatic mixing system for addition siliconeswas introduced to overcome the problems associatedwith hand spatulation of the impression materials.It provides more uniform ratios of base and catalystimpression pastes and less waste of materials.'*'•''Automixing substantially reduces the number of voidsformed in impressions.**'""^

Intraoral delivery tips that ean be attached to theautomatic mixing tips for ease of dispensing impres-sion tnaterial have also been introduced by one manu-facturer. The use of these intraoral delivery tips elimi-nates the need to load the impression material into animpression syringe for dispensing, which theoretieallywill reduce incorporation of air in the impressionmaterial. The aim of this study was to evaluate theeffectiveness of these intraoral delivery tips in reduc-ing voids in the automixed addition silicones whentested under simulated clinical conditions.

Method and materials

Five medium-viscosity, automixed addition silicone im-pression materials were selected for the study (Table 1).The impression materials were extruded from theircartridges according to recommended ratios, butmixed and then dispensed in three ways:

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Dental Research

Table J Automixed addition silicone elastomers evaluated

Fig 1 (A) Disposable automatic mixing tip and (B¡ intraoraldelivery tip.

Fig 2 Model of stainless steel cylinders witb lid.

Fig 3 (a)Tl\e cut surface of an impression sample with (b}tbe plastie template demarcating tbe 2 x 5-mm area ofmeasurement.

Product

ExpressImprintMirror 3 ExtrudeOmnisilReprosil

Code

EXfMMEOMRP

Manufacturer

3M Dental Products Div3M Dental Products DivKerr/Sybron CorpCoe Laboratories, IneLD Caulk/Dentsplyfnternational

1. Extruded througb tbeir respective disposable auto-matie mixing tips and back-loaded directly into areusable metal syringe (ESPE GmbH)

2. Extruded witbout the automatic mixing tips onto amixing pad and stir-mixed witb tbe edge of a rigidmetal spatula for f5 seconds, then smear-mixed forf5 seconds; the mixed material was tben loadedinto the syringe

3. Extruded directly througb their respective dispos-able automatic mixing tips with attached intraoraldelivery tips (3M Dental Produets Div) (Fig 1)

Eaeb of the mixed impression materials was tben dis-pensed in its respective manner onto a model con-sisting of six stain I ess-steel cyhnders (samples) 8 mm indiameter, 5 mm higb, and mounted 8 mm apart (Fig2), The impression material was also loaded into tbePerspex lid and seated over tbe cylinders.

All impressions were made in a random manner. Siximpressions were made for eacb impression material.Each impression produeed six samples, giving a totalof 540 samples. After the recommended setting time,tbe polymerized impression material was earefuUyremoved from tbe model to avoid tearing, A speciallydesigned cutting device (made up of a weighted assem-bly holding a microtome blade) was used to sectioneach sample longitudinally into symmetric halves.

The number of voids presenting at a predeterminedsite (demarcated by a plastie template with a 2 x 5-niiiiwindow) on one half of each sample was countedunder a binocular microscope (Stereo Star, AmericanOptical Company) at X7 magnification (Fig 3), Coun-ting was carried out on 3 days by one operator. To stan-dardize perception of voids, any break in tbe surfacecontinuity, regardless of its size or shape, was includedin the count.

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Fig 4 Mean number of voids in theaddition silicone impressions producedby (Hand) hand mixing and (Auto-I/O)automixing with and (Auto) automixingwithout the use of intraorai deiiverytips 'Standard deviation.

Tukey's Studentized range test was used to establishdifferences between the mean number of voids in thefive autotnixed silicones, dispensed with and withoutthe intraorai delivery tips. Student's t test was used tocompare differences in the mean number of voids forthe automixed materials prepared by automixing andhand mixing, and between each automixed materialdispensed with and without the intraorai delivery tips.

Results

The mean numbers of voids for each impression mate-rial, type of mixing, and dispensing method werederived by dividing the total void count with thenumber of sample counts made over the 3 days. Therank order of mean void counts by the type of auto-mixed silicone elastomer was fairly consistent for themethod of dispensing. Automixed EX produced themosi voids when dispensed with or without the intra-orai delivery tips, while automixed RP had the fewestin both categories (Fig 4). Three of the automixedmaterials (EX, ME and IM) had significantly [P ^.05) more voids than OM and RP when they were dis-pensed using the intraorai delivery tips, but there wereno significant differences among those three materials.The standard deviations of the means in some mate-rials were relatively high.

There were no statistically significant differences inthe number of voids between each antomixed siliconewhen dispensed with or without the use of intraoraidelivery tips. However, all the impression samples pro-

duced by automixing had significantly fewer voids {?^ .0001) than did those produced by hand mixing.

Discussion

There were no significant differences in the numbersof voids in automixed silicone impressions dispensedby intraorai dehvery tips and those dispensed conven-tionally, indicating that the intraorai dehvery tips wereno more effective than a conventional impressionsyringe for the reduction of voids. This finding dif-fered from that of a recent study''' which claimed that,for certain materials, the intraorai tips produce lessvoids than do a conventional syringe.

However, intraorai delivery tips may offer other ad-vantages. Greater emphasis is now placed on infectioncontrol in the dental operatory, and many steps havebeen recommended to prevent the transmission of in-fectious diseases." The use of disposable automaticmixing tips, together with intraorai delivery tips, todispense impression materials may safeguard againstcross contamination. Moreover, the use of intraoraidelivery tips enables the clinician to dispense the im-pression material directly onto the tooth preparationwithout loading the impression material into a conven-tional impression syringe, thus saving time andeliminating the need for assistance.

Hand-mixed impressions of the automixed materialswere made for comparison so that the voids producedin the impressions could be attributed to the mixingtechnique rather than to the materials. All the auto-

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mixed materiais prepared through automixing pre-sented with significantly fewer voids than did similarmaterials prepared throttgh hand mixing, indicatingthat automixed materials may only be suitable for mix-ing and dispensing through their automatic mixing tips.

Possible extraneous influences were controlled byusing the same syringe-tip diameter, type of impres-sion syringe, and method of loading and syringingthroughout the study. Hand mixing of impressionmaterials was carried out hy an experieneed dentai as-sistant and syringing of materials done by one inves-tigator to reduce between-operator variability. How-ever, despite standardizing these procedures, highstandard deviations of the means were observed insome of the materials. This could be attributed to theproduction of by-products commonly associated withaddition silicone elastomers.'̂ ^

A previous investigation showed that there is noconsistent pattern in the location or size of voidsformed in impressions,^ A predetermined site on thecut surfaee of the impression sample was thus chosento facilitate and standardize the counting process. Thechosen site also enabled detection of subsurface voidsthat might not otherwise have been visible on the im-pression surface. A magnification of x 7 was chosen tocount the voids because a recent study showed that, atthis magnification, there is a significant reduction inoperator variability for the examination of surface de-fects in impressions,"'

Conclusions

Under the simulated clinical conditions of this study,there was no statistically significant difference in themean nuinber of voids produced in each automixedsilieone dispensed with the use of intraoral deliverytips and the number of voids produced without the useof the tips. However, there were significant differencesin the number of voids among different materials.Automixed silicone materials should only be preparedby automixing for optimal results.

References

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12. Craig RG: Evaiuation of an automatic mixing system for anaddition silicone impression material. I Am Dent Assoc1985;110:213-215.

13. Dougherty JC, Wilson MA: Voids in silicone impressionmaterials ir relation to mixing and syringing techniques. JDent Res 199Ü;69:97O {abstr No. 126}.

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15. Niehoison JW, Medlock JW, Norling BK: Quantifying gasevolution from addition silicone elastomers. J Dent ResI985;64:243 (abstr No. 617).

16. Chong YH, Soh G: Determination of the optimal magnifi-cation for examining voids in elastomerie impressions. / DenlRes 1990:69:970 (abstr No. 124). D

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