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Impression Material Selection for Indirect Restorations

An accurate impression is one of the primary determinants for a precise-fitting indirect restoration and the key to the longevity of the restoration.1 Dental impression materials are utilized to make reproductions of oral structures, and these materials are in a fluid or plastic condition during the replication. The fluid materials are converted through physical change, chemical reaction, or polymerization into a negative replica of the architecture of the dentition, and this impression can be poured with a high-strength stone to duplicate the oral structures.2 This three-part discussion will discuss the most critical elements in the impression process: selection, application, and technique.

The clinical success of the indirect restoration requires a precise working model and thus depends upon the accuracy of the final impression. Numerous in vitro studies have evaluated impression materials through the accuracy of the working model or the final restoration. Several elastomeric impression materials with polymer matrices are recommended for precise replication of the tooth preparation and surrounding tissues because of their ability to provide dimensional stability and highly accurate working dies. The ideal elastomeric impression material should have the following characteristics:

•           Is Hydrophillic;

•           Has dimensional stability;

•           Provides a high level of surface detail reproduction;

•           Disinfects without loss of surface detail and accuracy; and

•           Possesses elasticity to prevent permanent distortion during removal.

            At present, no impression material fulfills every prerequisite. A thorough working knowledge of the physical properties of different materials can, however, direct the practitioner in the selection of an impression material for a specific clinical situation. Elastomeric impression materials have achieved acceptance for a myriad of indirect procedures. These materials provide applications in operative, prosthodontic, and implant dentistry.3,4  

 

Criteria for Selection 

Numerous studies have been reported on the rheological properties of elastic impression materials.5,6Themost significant physical characteristics comprise the basis of this discussion. A working knowledge of these properties can be beneficial in the selection of the proper impression material for a specific clinical situation while providing answers to restorative challenges related to impression taking. 

 

Viscosity 

Viscosity describes the flowability of impression materials (ie, those with low viscosity flow better). The viscosity of the material increases with the proportion of filler present.7 Viscosity is affected by the shear force exerted on the material. The impression material can exhibit a decrease in viscosity in response to high shear stress, which is called shear thinning. Thus, the viscosity of the impression material will vary in accordance to shear stress (ie, the greater a material’s viscosity, the more evident the effect of shear thinning). This phenomenon is likely due to the small filler particle size.8 Lower-viscosity materials can flow more easily; however, they are often used in conjunction with a second more viscous material to hydraulically push and support the lower-viscosity material.

 

Hydrophilicity  

Impression materials are characterized by their degree of hydrophilicity.9 Surface wetting describes the relative affinity of a liquid for a solid and can be quantified by measuring the contact angle. A zero contact angle would indicate complete wetting of the surface.2,4 Moisture compatibility significantly impacts the material’s ability to accurately record surface detail in the intraoral environment. Hydrophilic materials have a high affinity for moisture (ie, a low contact angle), provide good surface wetting, and allow for greater surface detail. Hydrophobic impression materials have a low affinity for moisture (ie, high contact angle), provide poor surface wetting, and a lower degree of surface detail.9Hydroactive impression materials are impression materials that are normally hydrophobic and are rendered hydrophilic through the addition of surfactants. These materials provide excellent surface wetting (ie, low contact angle) as well as a high degree of surface detail.9 When discussing the wetting capability of impression materials, however, it is necessary to distinguish the materials’ wetting ability to soft and hard tissues and to a gypsum slurry.4 

 

Setting Time 

The setting time of an impression material is the total time from the start of the mix until the impression material has completely set and can be removed from the oral cavity without distortion. The working time is measured from the start of the mix until the material can no longer be manipulated without introducing distortion or inaccuracy in the final impression.9 Elastomeric impression materials have a working time of approximately two minutes and a setting time of between two and six minutes. Generally, working time corresponds to setting time. Consequently, a fast-setting material will usually have a short working time and a slow-setting material will have a long working time. Since the setting time of all elastomeric impression materials is affected by temperature, one method for extending the working time is to refrigerate the materials before mixing. Increases of up to 90 seconds have been reported when the materials are chilled to 2° C.4,10,11 Chilling the material, however, should be approached with caution when using automix tips or dynamic mixing units. Furthermore, reducing the temperature below 18.3° C will affect the flow of the pastes and result in altered base/catalyst ratios.

Other factors that can influence the setting and working time include humidity, base to catalyst ratio, and how the material was mixed. In addition, extending the insertion time to ensure that the material has completely polymerized has shown improvement in elastic recovery and decreased permanent deformation.7 There are several factors that can influence the required working time for impression of preparations, including the number of preparations, utilization of automix or hand-mix material, viscosity of material, and the use or non-use of an auxiliary. Multiple preparations may benefit from utilizing an automix material that has a longer working time with a low viscosity syringe material with an auxiliary. 

 

 Tear Resistance and Elastic Recovery 

Impression materials should have adequate strength to allow removal without tearing. A material with a higher tear energy confers better resistance to tear for the impression.6 Elasticity allows the material to resist tearing and recover to its original prestressed configuration. The degree to which this occurs is a measure of the elastic recovery of the material. Permanent deformation can occur when the polymer is elongated beyond the point where elastic recovery is possible. It is desirable that the material tears rather than deforms past this critical point. Permanent deformation is related to the degree of cross-linking of the polymer strands, temperature, and the rate of the applied stress.4,12  

Tear resistance and elastic recovery are important in preserving the accuracy of the impression during intraoral removal and after cast separation. Materials with sufficient tear resistance and elastic recovery will withstand multiple pours, producing several accurate casts. This is a major advantage in contemporary restorative dentistry.13,14 

 

 Dimensional Stability 

An impression’s ability to accurately replicate the intraoral structures depends upon dimensional stability. The reasons for dimensional changes in elastomeric impression materials include the following: a reduction in spatial volume caused by contraction from polymerization, reduction in set volume from liberation of byproduct or accelerator components, water absorption from wet or varying humidity environments, and changes in temperature.12 Materials with sufficient dimensional stability can remain unchanged for a period of approximately seven days, and resist temperature extremes during shipping while retaining the ability to produce multiple accurate casts.14  

 

Conclusion 

Knowledge of the rheological properties of elastomeric impression materials can provide insight into proper selection and application for clinical situations.The most popular elastomeric impression materials for indirect procedures in prosthodontics and restorative dentistry include the polyethers, polyvinyl siloxanes,11 and the vinyl-polyether hybrids (Figure 1). Part II of this discussion will provide a description of each impression material and will illustrate the advantages and disadvantages of their clinical application, while part III will describe the criteria and technique for attaining an accurate impression.

 

*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.

 

References 

  1. Sorensen SE, Larsen IB, Jorgensen KD. Gingival and alveolar bone reaction to marginal fit of subgingival crown margins. Scand J Dent Res 1986;94(2):109-114.
  2. O’Brien WJ. Dental Materials: Properties and Selection. Carol Stream, IL: Quintessence Publishing; 1989.
  3. Burgess JO. Impression material basics. Inside Dent 2005;1(1):30-33.
  4. Mandikos MN. Polyvinyl siloxane impression materials: An update on clinical use. Aust Dent J 1998;43(6):428-434.
  5. Lu H, Nguyen B, Powers JM. Mechanical properties of 3 hydrophilic addition silicone and polyether elastomeric impression materials. J Prosthet Dent 2004;92(2):151-154.
  6. Chai J, Takahashi Y, Lautenschlager EP. Clinically relevant mechanical properties of elastomeric impression materials. Int J Prosthodont 1998;11(3):219-223.
  7. Vinyl polysiloxane impression materials: A status report. Council on Dental Materials, Instruments, and Equipment. J Am Dent Assoc 1990;120(5):595-600.
  8. Chai J, Pang IC. A study of the "thixotropic" property of elastomeric impression materials. Int J Prosthodont 1994;7(2):155-158.
  9. Pitel ML. Successful Impression Taking. First Time. Everytime. 1st ed. Armonk, NY: Heraeus Kulzer; 2005.
  10. Chee WW, Donovan TE. Polyvinyl siloxane impression materials: A review of properties and techniques. J Prosthet Dent 1992;68(5):728-732.
  11. Chew C, Chee WW, Donovan TE. The influence of temperature on the dimensional stability of poly (vinyl siloxane) impression materials. Int J Prosthodont 1993;6(6):528-532.
  12. Hondrum SO. Tear and energy properties of three impression materials. Int J Prosthodont 1994;7(6):517-521.
  13. Lee EA. Predictable elastomeric impressions in advanced fixed prosthodontics: A comprehensive review. Pract Periodont Aesthet Dent 1999;11(4):497-504.
  14. Lee EA. Impression material selection in contemporary fixed prosthodontics: Technique, rationale, and indications. Compend Contin Educ Dent 2005;26(11):780-789.
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