June, 1958 157

Impressions using rubber-based and silicone materials Richard H. Roydhouse, B.D.S. (N.Z.)

University of Otago Dental School, Dunedin, N.Z.

The use of these new materials depends upon the reproduction of detail and dimensions and both materials a r e highly satisfactory in this respect. However, whether the materials a r e used, and their applications, will depend upon their behaviour on the t r a y and in the mouth ; the flowing characteristics of these impression materials must be considered.

The Flow of Impression Materials

Terminology can cause confusion, for thick- ness and thinness a r e colloquial relative terms and descriptions of viscosity in centipoises a re oversimplifications. Thickness here denotes similarity in effects to softened impression compound. Thinness will be associated with the s ta te of dental cements when used to cement crowns or inlays.

Thick materials will affect the soft tissues during the seating of a n impression. This is advantageous when tissues lie over gingival floors of inlay cavities ; displacement of soft tissues may not be desired in prosthetic im- pression techniques, however.

The size of a film produced by squeezing a material between two surfaces is related to the viscosity of the material and the force bringing the surfaces together. So thick mixes may be unsuitable for impressions using the dentures or blocks as a t ray , for the increase in thickness of the denture or block may affect the recording of jaw separation, or excessive seating force on the t r a y may be necessary.

In muscle-trimming techniques, resistance to flow must be low enough to permit the mould- ing of rounded peripheries on the impression and yet grea t enough to prevent flow due to gravity. Muscle-trimming requires thick materials.

Air bubbles in the impression occur fre- quently with all grades of rubber-based and silicone impression materials. The explanation would be based on cohesion and surface ten- sion, both of which a r e related to viscosity. The materials may flow around a i r trapped in crevices or fail to displace a i r held by saliva on soft tissues. Little can be done about this.

The thin materials, on the other hand, do not displace soft tissues, will form thin lin- ings, but will not allow muscle-trimming. Air

Received for publication December, 1957.

bubbles a r e small in size and less frequent. For the same reason, t h a t is, low viscosity, handling is more difficult.

The thickness of the material used will depend upon the particular technique and upon personal choice. The viscosity of the material is established by the manufacturer and users a re warned against altering the proportions of the base and catalyst t o pro- duce a thicker mix. The ra te of setting is altered and distortians occur. These polymeri- sing materials slowly become thicker, t h a t is, the polymerisation registers as increase in viscosity; they a r e rubber-like in nature and should the impression be taken when the material has begun to set, invisible distortions a r e likely. Similarly, to allow i t to thicken before seating the impression is unwise. If a change in the viscosity of the mix is desired, one must choose another variety. Hence those materials with what might be called a n “inter- mediate viscosity” with a correspondingly wide range of uses will doubtless become the most popular.

Because the set materials a r e rubber-like in nature, additions to a n impression in the same material must be made with caution. During the re-seating of the impression to obtain fur ther detail o r missing regions, the original rubbery impression may distort; on withdrawal, the recovery of the pr imary impression may distort the addition.

The Materials and Applications

The rubber-based materials come in various grades of consistency. By altering the nature of the catalyst and base some manufacturers have produced so-called heavy-bodied varieties. Those based on natural rubbers seem very fluid.

The silicone materials have a different con- sistency from the above materials and seem to act under pressure in a different fashion to the rubber-based materials. I n behaviour the silicones resemble alginate-types whereas similarity exists between the oil-resin pastes and rubber-based materials.

The slight differences in behaviour, arising from the varying constituents and the methods of mixing, a r e as yet unanalysed but the uses of these materials have become clear.

158 Australian Dental Journal

Gold and porcelain restorations In the preparation of such restorations, the

thin materials, t h a t is, the rubber-based varieties, a r e used ; penetration into pinholes and the like is desired. But such ready flow has two disadvantages. The material drips off the vehicle used to convey it and the fluidity combined with a lack of elasticity in the unset mixture makes i t difficult to exert pressure upon the material; such elasticity is present in silicone materials partly because some a i r is incorporated during mixing. Perforation of the copper bands or trays, if used to retain the set material on withdrawal, also makes “pressure” impressions impossible. A sticky latex or glue painted on the inside of the band or t r a y is perhaps better. Patent syringes a r e marketed so tha t these materials may be injected into the cavity ; this complication and the lack of tissue displacement by these materials makes their use only a slight im- provement over tha t of a high quality impres- sion compound in such situations.

Thus the thin rubber-based materials a r e used in copper bands. The thicker, heavy- bodied grades and the silicones may be used where abutment preparations a r e well ex- posed, and where pinholes a r e absent or catered for by other means; in these circum- stances half or full t rays may be used. Perhaps the major advance in technique is t h a t such impressions, along with directions for design and shade, may be posted to a distant dental laboratory.

Partial dentures Full impressions of teeth a r e possible only

with silicone materials and such a n impression provides a n unrivalled start for any tech- nique. As the effects of expansion of ordinary dental stones have not been determined and therefore a r e not controlled, the cast should be made from a hard durable stone, such as is used in crown and bridgework, t h a t does not expand. The accuracy is such that in the remaking of a cast metal partial a silicone impression poured in a good stone is used to produce a cast on which the framework may be placed accurately; alginate impressions do not permit this.

The use of a silicone impression and a crown and bridgework stone in chrome-cobalt

work would represent a n increase in cost of five to ten per cent.; this is negated by the resulting accuracy which will be visible clinically.

Par t ia l dentures made of acrylic resin warp ; combined with a slightly inaccurate cast, such inaccuracies cause discomfort over the period when the “consumer resistance” is greatest. A silicone impression and a good stone avoids this. Appliances made on such casts f rom self-curing acrylic resin show a fur ther improvement in fit; caution is neces- sary, for the fatigue strength and the colour stability a r e less than tha t of heat-cured acrylic resin.

Full dentures The silicone material may be used with

excellent results in techniques such as full impressions, lining in special t rays trimmed with “compo”, and linings in existing dentures or blocks or in scraped “compo” impressions. Whether the practitioner uses this material, the alginate-type material or oil-resin paste depends on the accuracy required.

Rubber-based materials a r e restricted in use to linings ; heavy-bodied varieties permit muscle-trimming.

Conclusion The new materials produce accurate impres-

sions and so will increase the fit, comfort and retention of gold and porcelain restora- tions, partial and full dentures. Such impres- sions deserve a high quality stone. Accurate casts for prosthetic restorations, hitherto unavailable by simple methods can be made without a n y difficulty.

The rubber-based materials a r e of such viscosity tha t their use is confined to crown and bridgework and linings for full dentures. Applicable to all phases of prosthetic im- pression techniques the silicone impression materials provide accurate substitutes for the alginates; for use in crown and bridgework some operators will prefer the rubber-based materials.

Accuracy obtained makes the extra cost negligible.

318 Great King Street

New Zealand Dunedin

, Education The highest form of education consists in encouraging a man to ask a relevant question. This is the aim of a university education. It implies t h a t one has the knowledge and mental receptivity to f rame a question t h a t is meaningful and relevant; i t implies t h a t one has the skill and intellectual desire to solve a question should i t remain unanswered; i t implies t h a t one has a real respect for knowledge irrespective of its pure or applied value. Sydney D. Rzibbo, Discovery, November, 1957.