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Enhancements in Impression Materials Aimed At Boosting Predictability, Accuracy
Howard E. Strassler, DMD
Available in a variety of types to meet a range of needs, elastomeric impression materials continue to evolve, with the latest generation of products directed at better meeting the needs of both clinicians and patients. When selecting an impression material, clinicians have numerous characteristics to consider to achieve an optimal outcome, including working time, hydrophilic properties, delivery technique, and more.
In the practice of dentistry, impression materials play a key role in that they allow clinicians to capture details of teeth, implants, and soft tissue for a variety of purposes, such as the fabrication of restorations, orthodontic diagnosis and treatment, occlusal analysis and treatment, and more. These materials are among those dental products that continue to evolve. The current generation of impression materials offer a wide assortment of choices to accommodate the variety of different clinically successful techniques used in dental practices. Vinyl polysiloxane (VPS) followed by polyether (PE) are the most popular choices of impression materials, and, recently, a new-generation hybrid impression material—vinyl polyether silicone (VPES)—was introduced that combines the most desirable characteristics of VPS and PE materials.
Characteristics to Consider
In the past decade, in an effort to push back against the onslaught of digital imaging for impressioning, makers of impression materials have incorporated changes and improvements to make using these materials more predictable without compromising accuracy. Some recent VPS materials allow for direct in-lab digital scanning, and all impression materials have acceptable accuracy for scanning of casts to fabricate CAD/CAM restorations.1,2 When making a decision which impression material to choose, clinicians have a number of critical characteristics to consider,3-7 including:
• clinically relevant working time; reasonably fast setting time handling for ease of use
• hydrophilic (for accuracy in a moist environment)
• multiple viscosities for applications with different clinical techniques
• color contrast for impression readability
• accuracy and dimensional stability over time
• optimized flow characteristics for better wettability and adaptation to tooth preparations and implants and for penetration in a deep gingival sulcus
• excellent tear strength when removed from the sulcus elastic recovery
• absence of an unpleasant taste or odors (for better patient acceptance)
• convenient delivery (eg, automixing cartridges, putty tubes, magnum cartridges)
The latest generation of elastomeric impression materials allow clinicians to choose the impression-making technique they prefer that will not be tied to the thickness of the impression material within the tray; this enables the clinician to choose from a variety of techniques, including the use of custom trays, prefabricated full-arch and quadrant trays, or bite impression trays. All impression materials will provide better accuracy and minimal distortion when used with a paired tray adhesive that is allowed to dry following manufacturer’s recommendations.8 When the current generation of impression materials are removed from the mouth, today’s chemistries of elastomeric materials allow for the capture of surface detail along with the ability of the material to be removed over natural undercuts without distortion. The surface details provided are sufficient for the fabrication of a well-fitting laboratory-fabricated restoration.
Challenges of a Wet Environment
VPS, PE, and VPES impression materials are extremely accurate and dimensionally stable over time.5-7 Because the mouth is like a jungle—full of plaque, bacteria, saliva, and blood products—the challenge to consistently make an impression in a wet environment can be overwhelming. By chemical composition, PE and VPES impression materials are hydrophilic and will accurately wet the surface of a tooth even in the presence of slight moisture or bleeding. The term typically used when referring to wetting ability of an impression material against surfaces is contact angle. Water beading up on Teflon has a contact angle of 100 degrees. PE and VPES impression materials usually have contact angles ranging from 60 to 75 degrees. This translates into accurate impression making in a slightly moist field of the gingival sulcus and the capturing of excellent detail of the tooth.
Chemically, VPS materials are hydrophobic. Many manufacturers have modified the chemistry of VPS materials by adding surface surfactants and hydrophilic modifiers that change the ability of the vinyl polysiloxanes so they can accurately record detail on slightly moist surfaces. One manufacturer has introduced a pre-impression tooth surface optimizer. Typically, the contact angle of vinyl polysiloxane without modifiers is around 140 degrees. With the addition of chemical modifiers, vinyl polysiloxane has an improved wetting ability. PE impression materials have lower tear strengths than their VPS and VPES counterparts, which can impact accuracy for subgingival preparations.3,5
Working and Setting Times
Depending on the technique selected and patient tolerance to impression making clinicians have choices in the working and setting times they can select. While for one or two crowns/inlays/onlays for an impression the working and setting time can be shorter, for multiple units, especially for a full-arch impression, an extended working time will be needed for placement of the impression material. Recently, some manufacturers have made modifications to the chemistry of VPS materials so that the relationship of setting and working time is altered using temperature-sensitive chemical initiators. Without compromising the other important physical properties of the material, the thermosensitive VPS has greater sensitivity to the warmer intraoral temperatures inside the patient’s mouth, accelerating the setting reaction. With thermosensitive impression materials this setting reaction acceleration reduces the risk of distortion and deformation that can occur when holding and stabilizing a tray in place during the setting reaction.
Automixing continues to be the most popular impression material technique for dispensing. For high-volume dental practices, larger bulk cartridges can be used in electric automatic impression mixing machines that allow for faster extrusion for tray and syringe loading.
There have been a number of cordless gingival retraction and hemorrhage control pastes introduced aimed at simplifying soft-tissue management during impression making; however, this still requires the use of a multi-step technique—ie, placing and then removing the retraction paste, drying the tooth, then performing the steps for making the impression. One recent innovative system combines the benefits of cordless retraction and soft-tissue management with the accuracy of VPS impression materials. After applying a hemostatic agent to control bleeding, the clinician applies a pre-impression tooth surface optimizer to the tooth preparation. Using a novel air-driven handpiece/dispenser and the VPS wash material cartridge, the wash material retracts the soft tissue and captures the details of the impression. The higher-viscosity VPS material in a tray is then seated.
One Size Does Not Fit All
PVS, VPES, and PE impression materials have demonstrated improvements in the past decade as manufacturers have responded to clinician and patient needs. Like most dental materials, one size does not fit all. Clinicians have choices to use the acceptable techniques that are most comfortable to them and reproducible in their practice. If a clinician is interested in expanding into CAD/CAM restorations without purchasing a digital scanning device, he or she should evaluate the direct scannable PVS impression materials. As dentistry pushes further into the digital realm, impression materials are still a highly viable clinical option for dental practices.
1. Seelbach P, Brueckel C, Wöstmann B. Accuracy of digital and conventional impression techniques and workflow. Clin Oral Investig. 2013;17(7):1759-1764.
2. Cho SH, Schaefer O, Thompson GA, Guentsch A. Comparison of accuracy and reproducibility of casts made by digital and conventional methods. J Prosthet Dent. 2015;113(4):310-315.
3. Elastomeric impression materials. The Dental Advisor. 2003;20(10):1-4.
4. Lee EA. Impression material selection in contemporary fixed prosthodontics: technique, rationale, and indications. Compend Contin Educ Dent. 2005;26(11):780-789.
5. Re D, De Angelis F, Augusti G, et al. Mechanical properties of elastomeric impression materials: an in vitro comparison. Int J Dent. 2015;2015:428286. doi:10.1155/2015/428286.
6. Nassar U, Oko A, Adeeb S, et al. An in vitro study on the dimensional stability of a vinyl polyether silicone impression material over a prolonged storage period. J Prosthet Dent. 2013;109(3):172-178.
7. Walker MP, Rondeau M, Petrie C, et al. Surface quality and long-term dimensional stability of current elastomeric impression materials after disinfection. J Prosthodont. 2007;16(5):343-351.
8. Peregrina A, Land MF, Wandling C, Johnston WM. The effect of different adhesives on vinyl polysiloxane bond strength to two tray materials. J Prosthet Dent. 2005;94(3):209-213.
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