Elastomeric Impression Materials: Factors to Consider
Gregg A. Helvey, DDS
Clinicians have much to consider when selecting an elastomeric dental impression material. Before such a material can be brought to market it must first fulfill the criteria of the American Dental Association Specification No. 19, which states that the material must be able to reproduce fine detail of 25 microns or less. Other criteria for the material’s mechanical properties include consistency, permanent deformation, strain in compression, flow, shore hardness, and tear strength.1 These guidelines can be further sub-categorized into other physical and mechanical properties. What are these various properties, and what should a clinician look for in an impression material—of which there are some 600 brands2?
Hondrum3 stated that accuracy, dimensional stability, and tear strength (elastic recovery) are the most critical properties. Wettability, or wetting, which is a material’s ability to spread over a surface,4 strongly affects the accuracy of an impression material. The ability of a liquid material to wet a solid surface is measured by the contact angle—the lower the contact angle, the more hydrophilic the material. Contact angle measurements that define wettability are made mostly on impression material in a static, unchanging state; changes occur in the contact angle during the setting of impression materials.5
In the moist environment of the oral cavity, hydrophilicity is important for adaptation of the impression material to the tooth surface. The polarity of the polyether molecule provides an inherent hydrophilicity or low contact angle for polyether impression material.6 Surfactants are added, for example, to polyvinyl siloxane impression materials to lower the contact angle, thereby increasing hydrophilicity.6 As the impression material is mixed, the surfactants diffuse through the material to the surface, increasing hydrophilicity.7 As for the surfactant once it reaches the surface, some believe it remains attached to the impression material surface,8 while others think the surfactant is released into the liquid at the interface.9 Balkenhol et al7 found that the reduction of the surface tension was a result of the surfactant in contact with the liquid, and it did not affect the surface properties of the polyvinyl impression material. Thus, the ideal impression material would have the lowest possible contact angle.
All elastomeric impression materials experience shrinkage during setting.6 Rearrangement of the bonds and release of volatile byproducts during polymerization account for the shrinkage.1 The amount of shrinkage is lower with polyvinyl siloxane materials compared to polyether materials. Low shrinkage indicates that an impression material has greater resistance to distortion and is, therefore, more dimensionally stable.3
When an impression is removed from the mouth it experiences compressive and tensile forces.10 If the set impression is engaged in an undercut, a tight interproximal area, or a thin sulcus, it will stretch when removed. The stretched material may experience one of three different phases depending on the physical properties of the material and the amount of applied stress.6 During removal, if the material is stretched and then becomes unengaged or the tension is released, the material will spring back to its original size and shape. The material is thus said to have both good tear strength and elastic recovery. When stretched under similar circumstances, different impression materials will permanently distort if the tension reaches the yield point of that particular material. If the tension is increased past the material’s yield point, a tear will occur (tear strength value).6 For polyvinyl siloxane materials, the elastic recovery is dependent on the components, such as base silica, copolymer filler, and chain extenders.11 A material that is said to have 99% elastic recovery has 1% permanent deformation.1
The movement or flow of a liquid causes a resistance to the force that is trying to move it. Resistance to this movement is called viscosity.1 The flow characteristics of a material are the basis for the science of rheology.12 The correlation of shear stress and shear rate determines the behavior of the material and can be described as either Newtonian (where viscosity is not affected by shear stress)—of which water is an example—or non-Newtonian. Non-Newtonian materials (ie, most dental materials) can be subdivided further by how they are affected by stress. If stress produces shear thinning then the material is said to behave in a pseudoplastic manner, as opposed to dilatant behavior where shear thickening occurs.1 An example of a non-Newtonian material is fluid denture base resin.
The flow or viscosity of an impression material is dependent on the filler content. There are four basic categories: low (syringe or wash material), medium (one-step monophasic material), high (tray material), and very high (putty material). Viscosity is important when subgingival margins are to be captured. The ability of the impression material to reach the base of the sulcus is not only dependent on the viscosity but also the width of the sulcus. A wide sulcus allows for greater flow and thickness, which increases tear strength. Whichever method of sulcus preparation (mechanical, chemical, or surgical) is employed, it is critical that sufficient space is provided.
Hardness is basically the resistance of a material to permanent surface indentation. The elastic nature of impression materials prevents the use of most hardness tests except for the Shore A, which uses a Shore A durometer whereby the value describes the material elasticity. The value is provided with two numbers: one represents the hardness at 90 seconds after removal from the mouth; the other at 2 hours. The hardness of the impression material affects the force necessary to remove it from the mouth.1 The measurement of the flexibility or stiffness of an impression material is the strain in compression.13 The lower the strain in compression, the stiffer the material. This factor indicates whether the polymerized impression can be removed from the oral cavity without injury to the impressed tissues. It also indicates whether the polymerized impression will be stiff enough to prevent deformation when a gypsum product is poured into it and whether the set gypsum material can be removed from the impression with fracture.13 Lu et al found that strain in compression was inversely correlated to elastic recovery—the higher the elastic recovery, the lower the strain in compression.13
The time measured from when mixing the impression material begins until complete polymerization occurs is the setting time. To prevent distortion, the impression should be left undisturbed for the full setting time as indicated by the manufacturer. The time measured from the start of mixing the impression material to the point where further manipulation will introduce distortion or inaccuracy is called the working time.14 There is usually a correlation between the working and setting times. The shorter the setting time, the shorter the working time and vice versa. The setting times can be increased by decreasing the temperature through refrigeration.15
Because elastomeric impression materials have so many physical and mechanical components, selecting the proper one for a given application can be quite tedious. To simplify the selection process, clinicians should look for a smooth-flowing, hydrophilic material that has good tear strength, shrinks very little upon setting, and can be poured multiple times. The material should also have a pleasant taste for the patient.
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About the Author
Gregg A. Helvey, DDS
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry