Making Polyvinyl Impressions: Success Lies in the Details

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Making Polyvinyl Impressions: Success Lies in the Details
July/August 2007
Leendert (Len) Boksman,DDS, BSc, FADI, FICD and Robert R. Cowie, DDS, FAGD
Contemporary Dental Assisting

Abstract
A multitude of variables can affect the accuracy and predictability of making polyvinyl siloxane (PVS) impressions for crown-and-bridge applications. Currently, PVS materials are the most commonly used materials for crown-and-bridge impressions because of their inherent stability. However, as with any dental material, a thorough knowledge of handling properties is critical for consistent reproduction of detail. This article will discuss how to choose the proper tray, the importance of using PVS tray adhesive, handling parameters, and avoiding contaminants, such as sulphur from latex gloves.

Learning Objectives
After reading this article, the reader should be able to:

* select the correct impression tray for different clinical situations.
* understand the importance of work and set times in situations where 2 impression materials are used simultaneously.
* seat, align, and remove the impression tray to make the most accurate impression possible.


Introduced in the 1970s, polyvinyl siloxane (PVS) impression materials are addition-reaction silicone elastomers that have gained a large share of the impression material market and have become the most widely used impression materials in restorative dentistry.1 More than 95% of the impressions sent to commercial dental laboratories are PVS, and more than 80% of prosthetic cases submitted to the laboratories are single-unit.2 PVS materials have relatively short intraoral set and work times, and usually come in regular set and fast set. PVS materials have no taste or odor and, therefore, do not create excess salivation during impression procedures. They can reproduce exact detail with excellent stability over time. PVS materials offer high elastic recovery to prevent distortion and enough flexibility to allow easy removal of the impression from the mouth. Most are available in automix cartridges, are easy to use, have adequate shelf life, are cost-effective, and can be disinfected without affecting their dimensional accuracy.3,4 However, as with all products and techniques used in dentistry, clinical success lies in the details.
Polymerization Inhibition

The set of PVS impression materials can be inhibited by a number of chemical compounds. Precautions need to be taken any time an impression will be made. Latex gloves contain a sulfur compound, zinc diethyl dithiocarbamate, which can completely inhibit polymerization of PVS in concentrations as low as 0.005%.5 This inhibition occurs even if gloves are washed before contact with PVS or tooth structure, and even if the surface is washed after contact with the gloves.6 Therefore, the use of latex gloves during crown-and-bridge procedures is contraindicated; wear vinyl or nitrile gloves when working with PVS materials.

Particulate sulfur and sulfur chloride compounds also inhibit polymerization of PVS materials.7 Residues from temporary crown materials, such as acrylics and methacrylates, and the use of petroleum jelly lubricants, such as Vaseline, may interfere with the setting reaction of PVS materials, lessening their ability to pick up fine details. If using a lubricant, apply a water-soluble jelly, such as K-Y Jelly, which can easily be washed off the preparation site.
Tray Selection

Types of Trays

When choosing a tray to make the final impression, the tray should compliment the clinical case. Full-arch trays, whether metal, plastic, or fiberglass, are usually used for impressioning multiple units. These can be multiple single units, bridges, full-mouth reconstructions, or implant crowns. When restoring a dentition that has group function, one in which multiple teeth touch on lateral movement, a full-arch impression is indicated to allow the laboratory technician to develop the prosthesis or prostheses to harmonize with these lateral excursions. Dual-arch impression trays make the upper, lower, and bite registration impressions all at once. They are indicated for 1 or 2 units of crown-and-bridge restorations if a terminal tooth is present (ie, the second molar) and if cuspid rise is present. Cuspid rise causes immediate separation of the teeth when the patient moves laterally, so the laboratory technician can create centric stops in the prosthesis or prostheses without worrying too much about lateral interferences. With dual-arch impressions, make an extra bite registration before injecting local anesthetic. It is difficult for anesthetized patients to accurately bite together. Quadrant trays, which make impressions of a small segment of the dentition, either upper or lower, can be used for crown-and-bridge procedures, but mainly are used to fabricate temporary crowns.

Stock trays do not fit as well as custom trays. Custom trays fit each patient's dentition, offering several benefits. Custom trays create uniform distribution of the impression material around the teeth and minimize the amount of impression material used. Custom trays made from an acrylicÒliquid mix should never be used immediately after fabrication. These trays inherently distort over time. Fabricate acrylicÒliquid trays at least 2 to 3 days before use to ensure that the trays distort fully before you make the impression, preventing the impression from distorting during the time between making the impression and pouring at the laboratory. Fiberglass impression trays have recently been introduced (Border-Lock Trays, Clan Dental Products, Maarheeze, Netherlands, www.borderlock.com). These rigid trays are designed to minimize the amount of impression material used and to drive the impression material apically for better details of the gingival margins.

Fit

Elastomeric impression materials contract slightly because of their chemical reaction of cross-linking during polymerization. PVS impression materials have the smallest dimensional change with a contraction range between 0.05% and 0.1%.8,9 Even though this amount seems minor, it is imperative that the impression tray fit accurately to the arch, with an even spacing of the impression material around the preparation site. To evaluate whether the teeth will be in the center of the tray, place the distal part of the tray over the molar area. An even spacing of the impression material around the prepared tooth minimizes the effect of the polymerization contraction. Therefore, impressions made from custom trays are more accurate and consistent in reproducing intraoral details than those made from stock impression trays.10

Adhesion

It is critical to always use a PVS-dedicated adhesive on the tray,11 even if the tray is perforated. A heavy body PVS by itself will not engage the tray perforations sufficiently to retain the PVS against the tray, causing separation either during removal from the mouth or on separation of the poured dies (Figures 1 and 2). When an adhesive is used, the polymerization contraction is pulled toward the tray rather than toward the center of the impression material, creating a more accurate impression.12 The adhesive also will ensure that the PVS material does not pull away from any of the inside surfaces of the tray, such as the lateral walls or palatal area. When any part of the impression is pulled from the tray while the impression is being removed from the mouth, the resulting dies will be distorted, and the crown(s) or bridge(s) will not precisely fit the prepared teeth.

PVS adhesives do not work with PVS putties. Putties do not chemically attach to tray adhesive and require mandatory mechanical retention in the impression tray.13 This mechanical retention can be created by placing large round bur holes in the tray. Adhesive strength to acrylic resin (custom trays) is significantly lower than to polystyrene or metal stock trays14 and, therefore, when using these types of trays, auxiliary retention holes placed with a slow-speed round bur, as well as adhesive, should be placed.

Single-arch Technique

Successful full- or single-arch impressions are dependent on the timing of delivery of the tray by the dental assistant to the dentist. Ideally the filled tray should arrive at the dentist the moment the dentist completes syringing the light body material. Therefore, it is critical to remember 2 things. (1) Filling the tray with the heavy body material must be timed according to the number of teeth being impressed. Because it takes longer for the dentist to syringe around multiple preparations, the dental assistant must begin filling the tray with heavy body material later for multiple preparations than for a single preparation. (2) The light body impression material being syringed around the prepared site starts to set immediately because of the temperature in the mouth, while the heavy body material will set slower because it is at room temperature. The dental team should practice this procedure until the timing is refined.

Quadrant Technique

Currently, the quadrant technique with PVS materials is usually used for making impressions of the teeth before preparation to create a matrix for fabrication of the temporary restoration. This technique eliminates the time and expense of fabricating alginate-driven stone models to create a suck-down matrix. If an alginate is used to create the temporary, the alginate only can be used once because it dehydrates and distorts over time. The PVS matrix is stable and, therefore, if a patient should break the temporary, the matrix can be reused to fabricate a new one. By using a quadrant tray with a fast set PVS impression material, such as Template Ultra Quick Matrix Material (Clinician's Choice Dental Products, Inc, New Milford, Conn, www.clinicianschoice.com), the matrix can be created in 30 seconds.

Dual-arch Technique

A dual-arch impression, which impresses the preparation site, opposing arch, and bite registration all at once, is often used for 1 or 2 adjacent preparations. The more rigid the tray and the more rigid the impression material, the more accurate the final replicated stone die cast.15 Crowns fabricated from dual-arch impressions are equivalent in marginal accuracy and superior occlusally to crowns fabricated from a single upper- or lower-arch impression.16 The casts made from dual-arch impressions provide more accurate maximal intercuspal relationships than mounted casts from single-arch impressions.17 Accuracy, however, is reduced when dual-arch trays flex when the patient closes his or her mouth.18 Impressions made with flexible plastic trays produce consistent discrepancies of 180 µm to 210 µm.19 Heavy body PVS materials can distort the trays laterally, creating an elastic recoil after being removed from the mouth. In addition, many dual-arch plastic trays have side walls that are too high, creating tissue impingement on closure that distorts the trays (Figures 3 and 4). To prevent tissue impingement and distortion, a new metal tray without side walls (Quad-Tray Xtreme, Clinician's Choice Dental Products, Inc) and a new PVS material specifically designed for the dual-arch technique (Affinity Inflex Clinician's Choice Dental Products, Inc) have recently been introduced. It is also critical when dispensing the heavy body to coat the lateral walls of the dual-arch tray so that the upper and lower impressions are joined together (Figures 5 and 6). This unitized impression will facilitate multiple pours in the laboratory, ensuring that the upper and lower portions do not separate.

Retraction Cord

To help visualize and access the crown margin more precisely, place retraction cord, unless the margin is entirely supragingival.20 A single retraction cord may be used if the sulcus is shallow or if the margins of the preparations are equigingival. If the sulcus is deep, one cord needs to be placed on top of the other so that the last cord placed is at a level equal to the height of the gingival margin. If a 2-cord technique is used, the first cord must be placed butt-end to butt-end. Placing this first cord end to end ensures that the cord can be left in the sulcus after the second cord is removed without being picked up in the impression. If the retraction cord is picked up in the impression it needs to be removed. The cord can absorb water from the die stone when the impression is poured, creating a porous die cast.

After the first cord is placed, the preparation margins can be refined by the dentist. The second retraction cord should be the same size or 1 size larger. This cord ensures complete retraction of the gingival tissue. After 3 to 5 minutes, the second cord should be removed while the preparation is moist. Then, rinse the preparation with water and thoroughly dry it.

Hemostatic agents usually are used in combination with retraction cord for sulcular control. However, these hemostatic products often contain particulate sulfur and sulfur chloride compounds, which can inhibit polymerization of PVS materials. It is extremely difficult to remove these compounds from the preparation site. Because the impression material is at its thinnest and weakest at the gingival margins, inhibition of set can have disastrous consequences.21 Lack of set will create an inaccurate margin or the material that is in the sulcular area can tear during removal of the impression. Therefore, after placing the retraction cord and before making an impression, use a surface cleansing agent, such as Detail (Clinician's Choice Dental Products, Inc), an ethylenediaminetetraacetic acid (EDTA) solution, for 7 to 10 seconds to decontaminate the surface. An EDTA solution also reduces the surface tension of the preparation making it easier for the PVS material to intimately contact the preparation. Alternatively, the prepared surface can be cleaned with an antibacterial slurry, such as Consepsis Scrub (Ultradent Products, Inc, South Jordan, Utah, www.ultradent.com).
Making an Impression

Expiration Dates

Always check the expiration dates of dental materials, and make sure not to use expired products. All products denature over time. The fresher the material, the more likely it will work properly. From date of manufacture, to shipping from the manufacturer, to arrival at the distributor, to shipping to the dental office can take as long as 18 months.

Work Time

Every PVS material has distinct work and set times. Syringing the light body material around the preparation site, syringing of the heavy body into the trays, and complete seating of the tray must occur before the end of the work time. Careful planning and coordination between the assistant and dentist are critical.22 If you exceed the work time of either the light or heavy body, impression defects will be created. Figure 7 shows a complete lack of union between the heavy body and the light body caused by exceeding the work time. If the work time of the heavy body and light body are the same, it is best to start to load the tray with material before the dentist begins syringing the light body around the tooth preparations. Because filling the tray takes longer than syringing one or more teeth intraorally, this will ensure delivery of the tray with the heavy body material the second the intraoral placement of the light body is complete. Also, because the tray is loaded at room temperature, its material will have more work time than the material syringed in the mouth, with its higher temperature.

Refrigeration of PVS materials to increase work time has been recommended in the past because for every 10?F the material's temperature is lowered, work time is doubled. However, refrigeration can cause the materials to not extrude evenly from the cartridge, creating a disproportionate mix, and polymerization may still be occurring when the impression is removed from the mouth, creating distortion. Therefore, today, refrigeration of PVS materials to gain work time is contraindicated. Further, a newly introduced PVS material (MultiPrep, Clinician's Choice Dental Products, Inc) provides a longer work time (2 minutes 40 seconds), making refrigeration no longer necessary.

Set Time

Seat the impression tray slowly, taking the time to properly align the tray. Premature syringing of the impression material intraorally before seating the tray can cause voids.23 Slower placement and insertion of the tray will prevent the drags, pulls, and distortions that can occur if the tray is realigned after seating. Hold the tray passively so that no pressure is placed on the material while it is setting. Undue pressure results in elastic recoil in the material, creating inaccuracy. Do not move or transfer the impression between the dentist and the dental assistant at this time unless extreme care is taken to hold the tray passively.21

PVS impression material must be fully set when removed or it can distort or tear at the margins. To evaluate whether the impression material is set intraorally and ready to be removed, place a tiny amount of heavy body impression material on a working surface behind the patient, after syringing it into the tray. Because this material will set slower (it is at a lower temperature than in the mouth), when it is set, the intraoral material always will be polymerized and ready for removal.
Conclusion

Clinical misadventures can be avoided by knowing the manipulation variables when using PVS materials. The dental assistant plays an invaluable role in helping to create consistent and predictable impressions. PVS materials are only one of a myriad of materials used clinically. Thorough knowledge of the properties and handling of all materials used clinically is of paramount importance, not only to the dentist, but the patient as well.
Disclosure

Dr. Boksman is a part-time consultant for Clinical Research Dental and Clinician's Choice Dental Products, Inc.
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