Clinical Materials Review: Impression Material Basics

While there are 5 types of impression materials commonly used in dentistry today (e.g., hydrocolloids, polysulfide, addition silicone, polyether, and vinyl-polyether hybrids), only 3 are commonly used for final impressions: polyether, vinyl-polyether hybrids, and polyvinyl siloxane. In this brief update, the chemistry and important properties of these 3 impression materials will be compared and contrasted. In addition, relevant clinical features of selected properties will be addressed.

The newest class of impression material is the vinyl-polyether hybrids that include SENN (GC America, Alsip, IL; soon to be introduced in the U.S.). This new class of impression material combines properties from addition silicone and polyether impression materials. SENN is supplied as a 2-paste automixing system and contains a polymer with polyether and siloxane (e.g., addition silicone) groups that will combine elements and benefits of both impression materials. With the polyether groups, a hydrophilic material is provided without the use of a surfactant. With the siloxane groups on the polymer chain, a material that is dimensionally stable and recovers from deformation is combined with polyether properties. The material has a platinum catalyst, and the setting reaction is contaminated when powdered gloves are used to mix the material.

Little independent evaluation is available on the hybrids, but manufacturers’ data suggests that these products are hydrophilic during setting and after polymerization. They are supplied as putty, heavy, medium, and wash materials. An additional benefit is that they do not have the bitter taste of polyether materials and have a pleasant spearmint flavor. These hybrid materials may represent the blend of hydrophilicity and hydrophobicity necessary to improve impression making by wetting the tooth well and pouring easily for cast fabrication.

Polyether impression materials are supplied as a base (containing moderate length polyether macromonomer with terminal ethylene imine rings, silica filler and a plasticizer) and a catalyst (2,5 dichlorobenzene sulfonate, thickening agents and colorants).¹ The polymer is formed during the cationic polymerization and opening of the imine rings. The open rings become cations opening rings of adjacent polyether chains, producing a cascade reaction that proceeds until polymerization is complete. The catalyst attaches to the end of the opened ring, lengthening the chain. The backbone of the set polymer is a copolymer of tetrahydrofuran and ethylene oxide. No by-products are formed, and the materials have a successful clinical history. Machine mixing was introduced in 1993 (Figures 2; 3; 4) and, in 2000, an improved-taste, more flexible Impregum™ Penta™ Soft (3M™ ESPE™, St. Paul, MN) was introduced. At this time, the rigidity (i.e., strain in compression) of the set material was decreased to allow easier removal of the impressions from the mouth and improve cast removal without breaking teeth. In 2000, polyvinylsiloxane putty impression materials became the most rigid impression material (i.e., highest strain in compression). In 2005, the newest polyether impression materials were introduced: fast-setting 3M ESPE Impregum™ Penta™ Soft Quick (e.g., a monophase or medium viscosity) and Impregum™ Penta H (e.g., a tray or heavy bodied material) and DuoSoft Quick/Impregum™ L DuoSoft Quick (e.g., a low viscosity or wash material).

Polyvinylsiloxane (PVS), or addition reaction silicones, were introduced in the 1970s as a 2-paste system (i.e., a base and a catalyst). The base paste contains a polydimethylsiloxane polymer in which some terminal methyl groups are replaced by silane groups, coloring, and silica filler. The catalyst paste contains a pre-polymer in which some terminal methyl groups are replaced by vinyl groups, a chloroplatinic acid catalyst, fillers, and colorants. When the 2 pastes are mixed, an addition reaction occurs between the silane and the vinyl groups, producing a cross-linked silicone rubber. During polymerization, there is minimal dimensional change. Platinum or palladium is added to scavenge any hydrogen gas produced during the reaction. Polyvinylsiloxanes like Aquasil Ultra XLV (Dentsply Caulk, Milford, DE), Affinis™ (Coltène/Whaledent Inc., Cuyahoga Falls, OH) and Virtual™ (Ivoclar Vivadent, Amherst, NY) are accurate impression materials with excellent dimensional stability, good detail reproduction, high tear strength, adequate working time, and high recovery from deformation. Although meeting many of the criteria for an ideal impression material, polyvinylsiloxanes are hydrophobic. Cosmetic grade surfactants are now added to improve wetting of the impression material; combining this with lower viscosity wash materials has resulted in reduced remakes.

Addition silicone impression materials mixed while wearing latex gloves set slowly, if at all. The sulfur in the powder of latex (e.g., gloves or rubber dam) can inhibit the polymerization of PVS impression materials by contaminating the chloroplatinic acid catalyst,² but not all gloves produce the same effect.³ For the best results, do not mix putty impression materials while wearing latex gloves; instead, use vinyl gloves (Figure 5). Wash powderless gloves prior to beginning the preparation; this will prevent powder from passing from gloves to teeth. Any tooth surface (Figure 6) touched or retraction cord handled while wearing gloves will be contaminated and distort the impression material in that area (Figure 7). The amount of catalyst available for the setting reaction is very small and is measured in parts per million⁴ and, in these amounts, can be easily inactivated. Stone dies made from hydrophobic PVS materials are harder than those obtained from polyether and hydrophilic PVS impression materials.⁵ In spite of their limitations, new low viscosity hydrophilic PVS impression materials have better clinical success than hydrophobic PVS impression materials. In a clinical study,⁶ a hydrophilic PVS impression material (Affinis) was compared to a less hydrophilic PVS impression material. Dental students in this study made an acceptable impression on the first try 8 times more frequently with Affinis. This clearly demonstrates that while not the only variable, impression materials can affect the accuracy of the final impression.

Wettability is the ability of a liquid to spread over a surface. The wetting of a solid by a liquid can be measured by the contact angle. A contact angle of 0° degrees indicates complete wetting (i.e., hydrophilic). Higher contact angles (e.g., greater than 90°) mean lower wetting (i.e., more hydrophobic). Low contact angles mean good wetting and intimate adaptation of the impression material to the tooth surface. Surfactants lower the contact angle of the set PVS impression material and reduce voids in the recovered cast. However, the contact angle made against unset impression material is generally higher than against set material. The surfactant in the freshly mixed PVS impression material must migrate to the surface to make that surface hydrophilic. Since it takes time for the surfactant to migrate, silicone impression materials are not actually hydrophilic upon initial contact with moisture in the oral cavity (e.g., when syringing or inserting the tray). Recent studies have focused on measuring the contact angle of the unset impression material. When addition silicone and polyether impression materials are compared, the initial contact angle is lowest with polyether impression materials.⁷ In contrast to addition silicone impression materials, polyether impression materials are hydrophilic, with a high affinity to surfaces such as tooth and soft tissue. Polyether materials have intrinsic hydrophilicity compared to the hydrophilicity produced by adding surfactants to the impression material. The hydrophilicity of polyethers results from polarity of the polyether molecule. Recently, a number of articles have examined the hypothesis that a hydrophilic impression material can displace moisture in the sulcus. Some studies have reported improved wetting with PVS.⁸ However, clinically, a dry field produces more predictable results with any impression material.

Impression materials are supplied in 4 viscosities. American Dental Association (ADA) Specification 19 determines viscosity by measuring the diameter of 1 ml of impression material placed between 2 glass plates with a standard weight applied; the larger the diameter of the disc of impression material, the lower the viscosity.⁹ All classes of elastomeric impression materials are available in multiple viscosities ranging from low (e.g., syringe or wash material), medium or monophasic, high (e.g., tray or heavy body), and very high (e.g., putty). The viscosity or flow of impression material increases as the filler content increases. Viscosity is also lowered by shearing forces (i.e., shear thinning). For example, a medium-body impression material has lower viscosity when injected into the sulcus through a syringe tip, but has adequate viscosity to avoid slumping when loaded into an impression tray.

Surface detail is the ability of an impression material to accurately reproduce the surface of an object and is related to the viscosity of the impression material; low viscosity produces better detail.¹⁰ Detail reproduction is measured by making impressions of metal dies with 20- 50- and 75-micron grooves scribed in the metal block. A light-body impression material must reproduce a line 20 microns in width. High-viscosity putty materials have poorer detail reproduction. Smooth, rounded preparations reproduce better with all impression materials and die stones. Retraction cord medicaments—aluminum chloride, ferric sulfate, and ferric subsulfate/ferric sulfate, etc.—adversely affect the surface detail reproduction of PVS impressions.^11,12 The sulcus should be rinsed thoroughly to remove all traces of the hemostatic agent applied to the retraction cord prior to making a PVS impression.

The setting time for impression materials is the minimum time the material needs to be in place in the oral environment. Reducing the set time results in less distortion due to possible tray movement. Working time is measured from the start of mixing and includes time to manipulate and syringe the material around the tooth and into the sulcus, as well as be placed in the tray. Elastomeric impression materials have a working time of approximately 2 minutes and a setting time of between 2 and 6 minutes (i.e., fast vs. regular set). The setting time of all elastomeric impression materials is affected by temperature. The best method to increase working time is to refrigerate impression materials before use; increases of up to 90 seconds have been obtained when the material was chilled to 2° C.¹⁰

All elastomeric impression materials shrink during polymerization. Linear shrinkage varies by class of impression materials, with the addition silicone materials producing the lowest shrinkage. This shrinkage is compensated for by the expansion of the dental stone used to pour the impression.

When an impression is removed from the mouth, the material is stretched and compressed, particularly in the sulcus or interproximal area. The impression material will undergo 3 phases during removal; initially the impression material is stretched and, if the pressure is released, it will spring back to its original size and shape. But, if stretching continues, it is stretched to a point of “no return” called the yield point, where releasing the tension does not cause the impression to return to its original length, but it is instead permanently distorted. If stretching continues past the yield point, it tears (e.g. tear strength).

The ideal material has a high elastic recovery, a short permanent deformation stage and high tear strength. If it doesn’t distort, it tears and is easily seen as short of the margin and must be retaken. Impression materials that deform and do not tear may have visibly intact margins—and even flash past the margins—but be inaccurate due to the distortion produced during the permanent deformation. Rapid straining tends to maximize recovery from deformation. Therefore, the set impression should be removed from the mouth with a rapid pull (as much as is practicable) and not stressed slowly.

PVS materials are frequently reported to be the most ideal elastic impression materials because they exhibit better elastic recovery and less permanent deformation than other elastomers. However, new “soft” polyether impression materials have higher strain in compression and lower tensile strength compared to new “hydrophilic” addition silicone materials.¹³ Ideally, the space remaining after the retraction cord is removed is .3 mm to .4 mm.¹⁴ No differences in tear strength occur between elastomeric impression materials when the impression material is greater than .2 mm.¹⁵ However, when the sulcus is narrower than .2 mm, greater distortion results.¹⁶

Tear strength depends upon several factors: the gingival retraction, since thinner areas of impression material are weak and more likely to tear; the depth of the subgingival margin; the amount of hemorrhage, which can produce flaws in the impression, thereby lowering tear strength; and sharp edges on preparations and rough preparations that increase the resistance required to release the impression material from the surface.¹⁶

Recently, monophasic single step impression materials (Impregum Penta Soft and Aquasil XLV) were compared in a one-step putty wash technique.¹⁷ Two impressions were made of each preparation using each impression material. The same retraction method, tray type and size were used. One laboratory technician completed all of the all-ceramic restorations. After the restorations were tried in and adjusted, an impression was made of the exposed margins. The impressions were poured, and a model was separated. The marginal opening obtained for each material was measured and compared. No significant difference was seen for either material. Careful attention to detail and exacting technique produced outstanding but equal results.

Controlling bleeding, carefully retracting the sulcus, removing the cord, rinsing thoroughly, and carefully syringing the impression material will help obtain an excellent impression (Figures 8 and 9). The bottom line is that technique is most important, but materials are also helpful in difficult situations.

2. Cook WD, Thomasz F. Rubber gloves and addition silicone materials. Current note no. 64. Aust Dent J. 1986;31(2):140.

3. Matis BA, Valadez D, Valadez E. The effect of the use of dental gloves on mixing vinyl polysiloxane putties. J Prosthodont. 1997;6(3):189-92.

4. Boghosian AA. Overcoming the complexity of impression materials: Part 1. An interview with Alan A. Boghosian, DDS. Dent Today. 1991;10(6):38-41.

5. Panichuttra R, Jones RM, Goodacre C, et al. Hydrophilic poly(vinyl siloxane) impression materials: dimensional accuracy, wettability, and effect on gypsum hardness. Int J Prosthodont. 1991;4(3):240-8.

6. Blatz MB, Sadan A, Burgess JO, et al. Selected characteristics of a new polyvinyl siloxane impression material—a randomized clinical trial. Quintessence Int. 2005;36(2):97-104.

7. Kettke T, Dauelsberg H-J, Zawta C. Properties of precision impression materials crucial to their clinical success. A study of a new fast setting polyether from 3M ESPE. Quintessence Int. In press.

8. Sorensen, JA. Video contact angle measurement of impression materials on various substrates. J Dent Res. 2001;80 (Special Issue): IADR Abstract #2082.

9. Boghosian AA. Overcoming the complexity of impression materials: Part 2. An interview with Alan A. Boghosian, DDS. Dent Today. 1991;10(7):26, 28.

10. Mandikos MN. Polyvinal siloxane impression materials: an update on clinical use. Aust Dent J. 1998;43(6):428-34.

11. de Camargo LM, Chee WW, Donovan TE. Inhibition of polymerization of polyvinyl siloxanes by medicaments used on gingival retraction cords. J Prosthet Dent. 1993;70(2):114-7.

12. O’Mahony A, Spencer P, Williams K, et al. Effect of 3 medicaments on the dimensional accuracy and surface detail reproduction of polyvinyl siloxane impressions. Quintessence Int. 2000;31(3):201-6.

13. Lu H, Nguyen B, Powers JM. Mechanical properties of 3 hydrophilic addition silicone and polyether elastomeric impression materials. J Prosthet Dent. 2004;92(2):151-4.

14. Ramadan FA. The linear effectiveness of dental tissue displacement materials. (Thesis) St. Louis: St. Louis University Dental School, 1968.

15. Laufer BZ, Baharav H, Ganor Y, et al. The effect of marginal thickness on the distoration of different impression materials. J Prosthet Dent. 1996;76(5):466-71.

16. Laufer BZ, Baharav H, Cardash HS. The linear accuracy of impressions and stone dies as affected by the thickness of the impression margin. Int J Prosthodont. 1994;7(3):247-52.

17. Burgess JO, Ripp A, Gallo J, Walker R and Mercante D. A double blind clinical study of two impression materials. IADR. 2005;84(Special Issue):Abstract #3047.