Unique VPS Impression Material Uses an Advanced Temperature-Sensitive Formulation

Howard E. Strassler; Travis Weeks

In the routine practice of restorative dentistry, impression materials are one of the key materials that make our practices easier and more predictable than in the past. In the past decade, vinyl polysiloxane (VPS) impression materials have become the standard for routine fixed prosthodontic treatment. While there are many choices in VPS impression materials, there are characteristics of these products that are important to consider in the decision-making process.^1,2 Some of these features include:

• Clinically relevant working time; reasonably fast setting time
• Ease of handling
• Hydrophilic (will work in a moist environment)
• Multiple viscosities for different clinical techniques
• Color contrast for impression readability
• Accuracy and dimensional stability
• Excellent tear strength when removed from sulcus
• Elastic recovery
• Absence of an unpleasant taste or odors
• Convenient delivery (automixing cartridges, putty tubes, magnum cartridges).

While there are many excellent VPS impression materials for use in fixed prosthodontic applications that fulfill many of these characteristics, there is one product that helps provide for control of working and setting time through a unique chemical formulation. Flexitime^®; (Heraeus-Kulzer, Inc, Armonk, NY) is a VPS that has created a cooperative relationship of fast set and short working time while providing for most of the ideal characteristics of an impression material. In the past, if a material was fast-setting, the clinician faced a shorter working time. For a single unit that is not generally a problem. However, when an impression needs to be made for multiple units, the clinician would many times have to change to another slower- or regular-setting VPS to provide the extended working time needed. With Flexitime VPS, the intraoral condition of temperature is the defining measure of setting time. Flexitime has solved that problem by chemically enhancing the traditional VPS formula with the addition of what the manufacturer calls ATS (Advanced ThermaSense™), a chemical modification that provides the impression material a greater sensitivity to the warmer intraoral temperatures inside the patient’s mouth. Flexitime’s viscosities of light body, Correct Flow (low viscosity, syringe consistency) with ATS, helps to eliminate a clinician’s worry that the material will set pre-maturely. Flexitime, with its ATS setting control, will work well for single-unit impressions including crowns, inlays, and onlays, but it can be used equally well for implants and other fixed partial denture applications. Its high tear strength also makes it an ideal material to use when cast posts and cores are being fabricated indirectly in the laboratory from an impression of the tooth and root canal.

Using the heavier viscosities, Flexitime can be used for removable prosthodontic impressions as well. Flexitime has better elastic recovery once the impression is removed from the mouth when compared to the polyether impression materials.³ Also, it has been shown that the accuracy of Flexitime is scarcely affected by storage time or by ambient humidity when compared to polyether impression materials.⁴ This kind of accuracy in a multitude of different conditions eliminates concerns about sending impressions to the laboratory for cast and die fabrication.

As every clinician knows, timing is critical for impression making. The working time of Flexitime with ATS is a window that ranges from a minimum of 30 seconds up to 2.5 minutes (for loading the impression tray and seating it into the mouth). Once seated, Flexitime has a consistent, defined “in mouth holding time” of 2.5 minutes. Timewise, the clinician has about 1.5 minutes to syringe the light body before seating the tray material over it. So for a quick, single-unit impression, a clinician could syringe the Correct Flow wash material in as little as 30 seconds, seat the impression tray loaded with Flexitime Heavy Tray, and hold it in place for the 2.5 minutes of “in mouth” time. This yields a fast, 3-minute setting time from the start of the mix. But for a more complicated case, such as a multiunit bridge or implant case, a dentist can take a full 2.5 minutes to load the impression tray and syringe the wash material into the mouth. Once seated, the tray is stabilized for the required 2.5 minutes in-mouth time. This provides for a longer 5-minute setting time from the start of the mix. So for almost all fixed prosthodontic applications, this timing flexibility provides an extended working time with a fast setting time. For those desiring an even faster setting time, one can choose Flexitime Xtreme, which has an in-mouth holding time of only 1.5 minutes and a pleasant wild berry aroma for higher patient acceptability.

Choices in Impression-Making Techniques

Flexitime and Flexitime Xtreme are packaged with the most popular viscosities. Depending on the clinical technique a practitioner chooses to use, Flexitime can be used in a putty/wash technique, single-stage (monophase VPS or heavy/light body) technique, and double-arch bite impression (monophase or heavy/light body) technique. One has the choices of a putty (Flexitime Easy Putty), regular-medium body viscosity (Flexitime Monophase), heavy-body tray viscosity (Flexitime Heavy Tray), and light-body syringeable viscosity (Flexitime Correct Flow). With Xtreme the choices include both heavy-body tray and wash material, Xtreme Correct Flow (Figure 1). Both products can be dispensed with the standard cartridges and the Flexitime monophase is available in Magnum 360 cartridges (Figure 2).

Technique for Accurate Impression-Making

VPS impression materials will work well with a custom tray, fitted stock tray, or bite impression tray.² One of the most commonly used impression techniques is a single-stage impression using the heavy viscosity in a single-arch tray or double-arch bite tray while syringing a light-body wash to capture the details of the prepared tooth. After the try-in of the tray at the beginning of the appointment, place the VPS adhesive on the tray and allow it to dry fully for 5 to 10 minutes.⁵ Continue the procedure. When you are ready to make the impression, use your tissue-management technique of choice to expose the margins of the preparation(s).

When using the automix cartridges, assemble the automix cartridges and correct mixing tips in the respective applicator guns. A good hint is to make sure to always use the specific mixing tips provided or recommended by the manufacturer with each cartridge. For example, the yellow mixing tip provided for use with the Flexitime wash material will not work properly if it is used with the cartridge of the Flexitime tray material. Using mixing tips from other brands of material or other manufacturers is also likely to cause problems. When using the heavy-body/light-body technique, placing the small intraoral syringe tip on the mixing tip of the Correct Flow cartridge eliminates the clean-up of a separate syringe.

Case Presentations

Case 1
For this case, a light-body injection material with a heavy-body tray technique with a fast-setting PVS (Flexitime Xtreme) in a custom tray was used. After the cord was removed from around the preparations, Correct Flow was injected around the tooth preparations, starting with the tip in the sulcus, always keeping the tip embedded in the impression material (working time 30 seconds to 1.5 minutes) (Figure 3). To ensure a good bond between the wash and tray materials, it is a good idea to always have the tray material loaded first before starting to syringe the wash material. When the assistant has nearly completed placing the heavy-tray material into the impression tray, he or she should indicate to the dentist that it is time to begin placing the wash. While syringing the material, do not blow air to thin the light-body material—this can lead to the incorporation of air bubbles and does not enhance the impression.⁶ Some practitioners prefer that some of the Correct Flow also be placed in the tray over the heavy-tray material in the area being impressed.

When the VPS was dispensed as desired, the tray was seated in the mouth and stabilized for a holding time of 1.5 minutes. When the fast-setting VPS was set, the tray was removed and the final impression was evaluated for how it captured the detail of the margins and tooth preparation (Figure 4). The color contrast of the heavy-body and wash material made reading and evaluating the impression easy. The working cast revealed the details of the margins captured in the impression (Figure 5).

Case 1
In this case, a putty/wash hydraulic technique was used for making the impression. After the crown preparations were completed for tissue management, a double-cord technique was used with the place-ment of a first cord flush within the sulcus followed by a larger-diameter cord that would be removed before capturing the impression (Figure 6). Using a prefitted stock tray, a putty impression was made. The putty impression was removed after 2.5 minutes of intraoral time so that the putty was not fully set (Figure 7). The top cord was removed and the teeth were thoroughly cleaned and dried at the same time the dental assistant placed the Correct Flow wash material into the putty imprint (Figure 8).

The unset Easy Putty within the tray with the Correct Flow wash material was reinserted into the mouth over the mandibular arch. Using this hydraulic technique and with the thixotropic properties of the impression material, the light body was forced to the base of the sulcus to the primary gingival retraction cord; injection was not necessary. The impression material was allowed to set for 2.5 minutes and then removed. Inspection of the impression revealed excellent detail with no tearing (Figure 9). The primary cords were removed from the impression by cutting them with a fine-tipped surgical suture scissor. Note: never pull the cord from the impression material, as this can lead to tearing of the fine details that have been captured of the margins. The cast demonstrated excellent marginal detail (Figure 10).

Conclusion

Using Flexitime with ATS provides the practitioner with an impression material that no longer compromises between fast set and longer working time. The ThermaSense formulation allows for an extended working time so that the clinician does not have to rush during the important phase of capturing the details of the margins and tooth preparation, while providing a timely setting of the entire impression once the tray is placed in the mouth. Throughout the procedure, the dentist stays in control.

References

1. Impression Materials. The Dental Advisor. 2003;20(10):1-4.

2. Lee EA. Impression material selection in contemporary fixed prosthodontics: technique, rationale, and indications. Compend Contin Educ Dent. 2005;26(11):780-789.

3. Endo T, Finger WJ. Evaluation of elastic recovery of polyether impression materials. Am J Dent. 2005;18(6):355-360.

4. Kanhira M, Finger WJ, Endo T. Volatization of components from and water absorption of polyether impressions. J Dent. 2006;34(2):134-138.

5. Peregrina A, Land MF, Wandling C. et al. The effect of different adhesives on vinyl polysiloxane bond strength to two tray materials. J Prosthet Dent. 2005;94(3):209-213.

6. Strassler HE. Hints from Howard: is your impression technique as good as your materials. Incisal Edge. 2005;1(2):38-40.

About the Author

Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics
and Operative Dentistry
University of Maryland Dental School
Baltimore, Maryland

Travis Weeks, DDS
General Practice
Baltimore, Maryland