The Impression Process--Application

Douglas A. Terry, DDS

An accurate impression is a primary determinant for the precise fitting of an indirect restoration and the key to clinical success. The impression process depends upon material selection as well as proper application and technique. The three most popular elastomeric impression materials include polyvinyl siloxanes (PVS), polyethers, and vinyl-polyether hybrids.¹ Part I of this article discussed the most significant physical properties of these materials and how a working knowledge of these properties can be beneficial in the selection of the proper impression material for a specific clinical situation. This discussion will review the unique characteristics of each type of elastomeric impression material and will illustrate the advantages and disadvantages for clinical application; Part III will describe the clinical criteria and technique utilized for attaining an accurate impression.

Polyether Impression Materials

Polyethers are supplied as two-paste systems (ie, base, catalyst) that result in a reaction, converting the paste to a polymer when the base and catalyst are mixed.  This polymer is formed through ionic polymerization and an opening of the terminal imine rings. The open rings become cations, opening rings of adjacent polyether chains and producing a cascade reaction that continues until polymerization is complete. Since no volatile byproduct is formed, the reaction results in unusually low polymerization shrinkage, which improves long-term dimensional stability.² The most significant attributes of polyethers are their hydrophilic nature, enhanced wetting ability,⁵  and ability to reproduce surface detail.³ Because of their ability to absorb water, however, the storage and disinfection of polyethers is critical. Distortion can occur if stored in water or high humidity. Thus, polyether impressions should be disinfected with glutaraldehyde for 10 minutes and will remain dimensionally stable for up to 7 days if kept dry.^3,4 Some examples of polyethers include Impregum F/Penta, Polygel NF, and P2 (Table 1).^1,2,4,5

Polyvinyl Siloxane (PVS) Impression Materials

Addition-reaction silicones, or PVS, constitute the most popular category of impression material. These are supplied as two-paste systems (ie, base, catalyst) and upon mixing, an addition-reaction occurs between the silane and the vinyl groups, producing a cross-linked silicone rubber. In contrast to condensation-reaction silicones, during polymerization no volatile by-products are produced from the set impression, resulting in less polymerization shrinkage and, therefore, minimal dimensional change.^1,4 Furthermore, the addition-reaction is fast and the conversion goes almost to completion--unlike the stepwise reactions of the polysulfide, condensation silicones, and polyethers that require several hours.^6,7 This characteristic provides dimensional stability once the impression is removed, which allows the impression to be poured at any time after removal from the mouth, whereas other elastomeric impression materials require 20 to 30 minutes for viscoelastic recovery.

Several consideration factors with addition-reaction silicones include the release of hydrogen gas, the inhibition of polymerization from latex gloves, and their limitation with moisture control. The release of hydrogen gas from a side reaction unrelated to the setting, a condition known as outgassing, can result in voids in the cast.^5,8 Most of the contemporary PVS materials, however,contain scavengers, small amounts of platinum or palladium, which prevent the release of hydrogen gas that could cause bubbles in the final model.⁶ The mechanism of inhibition of polymerization is a sulfur-containing compound released from certain latex gloves that contaminates the platinum catalyst (ie, chloroplatinic acid) in the PVS impression material.^9,10 The use of vinyl or nitrile gloves, or newer generation gloves that do not use sulfur as the accelerator, is recommended during the impression process.¹¹ Residues from methacrylates, acrylics, and petroleum jelly lubricants can also interfere with the setting reaction of PVS and may interfere with the material’s ability to capture detail. It is thus important to clean the preparation and surrounding structures prior to final impression taking.¹² The final concern with PVS impression materials is how their hydrophobic nature limits control of moisture. Although some manufacturers claim that the addition of surfactants has rendered these materials hydrophilic, it has only made them less hydrophobic.¹³ Surfactants lower the contact angle and improve the wetting characteristics against set material; however, in freshly mixed material the surfactant has not completely migrated to the surface and thus is not hydrophilic upon initial contact with intraoral moisture.¹ Therefore, the hydrophobic nature of these materials can compromise their application in areas where moisture cannot be controlled.¹⁴ Addition-reaction silicone impressions can be easily disinfected without loss of accuracy and may remain dimensionally stable for up to 7 days (Table 2).^1,4-11

Vinyl-Polyether Hybrid Impression Materials

Vinyl-polyether hybrids represent the newest class of elastomeric impression materials. The vinyl-polyether hybrid impression material combines the characteristics from the addition-reaction silicones and polyether materials. The vinyl-polyether impression material is supplied as a two-paste system (ie, base, catalyst). When the base and catalyst are mixed, it becomes a polymer chain with polyether and siloxane groups through the use of two addition-reactions. The first involves a reaction similar to that of the PVS, while the second reaction involves the addition of a vinyl-terminated polyether to a siloxane.¹⁵ In this system, the polysiloxane and polyether become linked as copolymers and the hybrid material combines elements of each of these elastomeric systems. Thus, the polyether groups on the polymer provide a hydrophilic material that can be attained without the addition of surfactants while the siloxane groups on the polymer provide a material that is dimensionally stable and recovers from deformation. These transformations in the hybrid’s properties, however, are dependent upon the ratio of polyether/polysiloxane in the elastomer. Therefore, the copolymer will take on the characteristics of the elastomer that is greater in the copolymer. The material has a platinum catalyst and the polymerization reaction can be contaminated with sulfur from latex gloves. These hybrid materials are available in different viscosities (eg, putty, heavy, medium, wash materials) to accommodate different impression techniques. The potential advantages of this system are that it combines the excellent elastic characteristics associated with PVS with the hydrophilic characteristics of the polyethers, although further independent evaluation is required to determine these benefits. The most significant reported attributes of vinyl-polyether hybrid impression materials include hydrophilicity during the setting and after polymerization, dimensional stability, and elastic recovery.^1,15

Conclusion

The impression process is the most challenging multifaceted procedure in dentistry due to the number of variables that can affect the success and long-term outcome of the indirect restoration. This process begins with knowledge of the properties of the impression materials and must be integrated with the proper application of materials. The polyether and polysiloxane impression materials provide the standard for achieving an accurate impression in operative and prosthetic dentistry. While future hybrids may complement these materials, their potential benefits will require further investigation. Whereas this part provided an overview of the most commonly used elastomeric impression materials and compared their advantages and disadvantages, the third and concluding part of this discussion will describe the final element of the impression process: technique.

*Assistant Professor, Department of Restorative Dentistry and Biomaterials, University of Texas Health Science Center Dental Branch, Houston, TX; private practice, Institute of Esthetic and Restorative Dentistry, Houston, TX.

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