Dr. John O. Burgess talks about the different types of impression materials, common problems encountered when making impressions, and methods for improving impression techniques.
Dr. Dalin: Thank you, John, for taking the time to talk with me about something that I think many dentists take for granted ... impression materials. But impression materials have to be one of the most important items that we use in our day-to-day practices. I was taught that your work is only as good as your final impression. It doesn’t matter how well you prepare teeth, or what you use to cement in your final restoration. If you cannot deliver a good, clean, accurate impression to your laboratory technician, you cannot deliver a quality final restoration.
Dr. Burgess: Jeff, that is absolutely correct. Impression making is a fundamental part of tooth restoration. In fact, dental laboratories report that poor impressions are a leading factor in poor-fitting final restorations.
Dr. Dalin: Let’s start by discussing the two major types of dental impression materials: polyvinylsiloxane and polyether materials. Can you provide a capsule review of each of these, as well as their advantages and disadvantages?
Dr. Burgess: There were two, well-known classes of elastomeric impression materials. But we now have a third class. The newest class of elastomeric impression materials is a vinyl-polyether hybrid material called SENN (GC, Tokyo, Japan). SENN combines properties from addition silicone and polyether impression materials. It is a polymer with polyether and siloxane (addition silicone) groups. With the polyether groups, a hydrophilic material is produced without the use of a surfactant. With the siloxane groups on the polymer chain, a material that is dimensionally stable and has good recovery from deformation is combined with hydrophilic properties that the polyether groups produce. SENN has a platinum catalyst. The setting reaction can be contaminated if powdered gloves are used to mix the material. SENN will not polymerize in these instances. Our data shows that the tear strength is a little low, but the wetting properties of SENN are good. SENN is supplied as a putty - heavy, medium, and wash materials - in either a fast or regular set material. These hybrid materials have the intrinsic hydrophilicity necessary to improve impression making by wetting a tooth and allowing easy pouring for cast fabrication.
Polyether impression materials are represented by P2 (H Kulzer), Polygel NF (LD Caulk), and Impregum (3M ESPE). Although most clinicians in the United States use only single, medium-viscosity polyether materials (Impregum), Impregum Penta Soft Quick (a monophase or medium viscosity), Impregum Penta H (a tray or heavy-bodied material) and DuoSoft Quick/Impregum L DuoSoft Quick (a low viscosity or wash material) were introduced in 2005. At this time, three viscosities of polyether impression material are available. All polyether impression materials are hydrophilic, and do not need an added surfactant to improve wetting. As a class, the polyether impression materials have good flow, lower tear strength, and excellent wetting abilities. This is helpful for making the impression and pouring the cast.
The third class of impression materials are the polyvinylsiloxane (VPS) or addition reaction silicones like Aquasil XLV (LD Caulk), Affinis (Coltene/Whaledent), Imprint 2 (3M ESPE), and Virtual (Ivoclar). These materials are accurate, and have excellent dimensional stability, good detail reproduction, high tear strength, adequate working time, and high recovery from deformation. Although they meet many criteria for an ideal impression material, polyvinylsiloxanes are hydrophobic. Surfactants are added to improve wetting of the impression material as it contacts the oral tissue and as the cast is poured. The new low-viscosity hydrophilic PVS impression materials have better clinical success than hydrophobic PVS impression materials. We compared a hydrophilic PVS impression material (Affinis, Coltene/Whaledent) clinically to a less-hydrophilic VPS impression material (Express, 3M EPSE). An acceptable impression on the first attempt was eight times more likely with Affinis as compared to the less-hydrophilic material, Express. This shows that, while not the only variable, impression materials can affect accuracy of a final impression.
Dr. Dalin: We are bombarded by advertisements when considering an impression material. There is nothing better than an informed consumer. There are a lot of terms in ads that salespeople toss at us. Can you explain some of these terms, what they mean, and how important they are to our choice of materials?
Dr. Burgess: Sure. Here are a few:
• Shark fin test: This essentially measures flow in an impression material. The ideal material would flow until it is ready to polymerize and then snap set. Unfortunately, most addition silicones have a slow setting time rather than a snap set. So the viscosity increases soon after the material is mixed and continues to increase until the impression polymerizes. Polyether impression materials provide increased flow at the end of the setting time with a sharper set time (i.e., polymerization occurs rapidly at the end of the setting time). For example, this is especially important when making impressions of multiple abutments when you need all the working time you can get.
• Contact angle: This is a measure of how well the material wets another. Surface wetting is the ability of a liquid to spread over a surface. The wetting of a solid by a liquid can be measured by the contact angle. A contact angle of 0 degrees indicates complete wetting (hydrophilic). Higher contact angles (greater than 900) mean lower wetting (more hydrophobic). Impression materials with low contact angles have good wetting and intimate adaptation of the impression material to the tooth surface. Surfactants lower the contact angle of the set PVS impression material and reduce voids in the recovered cast. In impression materials, two contact angle measurements are important. One is the contact angle on the unset material, which measures the ability of the impression material to wet or adapt to the oral tissues. The other is the contact angle on set material, which measures the ability of stone to wet the impression. The first measurement provides good surface detail of the prepared tooth while the second measurement shows how many bubbles will be present in the cast. However, the contact angle made against unset impression material is generally higher than against set material. The surfactant in the freshly mixed VPS impression material must migrate to the surface to make that surface hydrophilic. Since it takes time for the surfactant to migrate, silicone impression materials are not actually hydrophilic on initial contact with the moist oral tissue. Recent studies have focused on measuring the contact angle of the unset impression material. When addition silicone and polyether impression materials are compared, the initial contact angle is lowest with polyether impression materials. In contrast to addition silicone impression materials, polyether impression materials are hydrophilic with a high affinity to moist surfaces, such as tooth and moist, soft tissue. Polyether materials have intrinsic hydrophilicity compared to the hydrophilicity produced by adding surfactants to the impression material. The hydrophilicity of polyethers is due to the polarity of the polyether molecule.
• Moisture displacement: Recently, a number of articles have examined the hypothesis that a hydrophilic impression material can displace moisture in the sulcus or on metal dies. Some studies have reported improved wetting with newer polyvinylsiloxane impression materials; however, a clinically dry field produces more predictable results with any impression material. Also, hemorrhage control is essential for making excellent impressions.
• Tear strength: When an impression is removed from the mouth, it is stretched in some areas and compressed in others. Particularly vulnerable to stretching are the thin fins of impression material in the sulcus, where the impression material is stretched as the impression is removed. Initially, these areas stretch and if the pressure is released, the material returns to its original size and shape. Unfortunately, if stretching continues, it is stretched to a point of “no return” (the yield point) in which permanent distortion occurs. If stretching continues, the impression tears (tear strength). Impression materials that deform and do not tear may look as though all the margins have been captured with even flash past the margins. But they may be inaccurate due to the distortion produced during impression removal. Rapid impression removal tends to maximize recovery from deformation. Thus, the set impression should be removed from the mouth with a rapid pull (as far as is practicable), and not “teased” out slowly. Ideally, the space remaining after the retraction cord is removed is 0.3-0.4 mm. No differences in tear strength occur between elastomeric impression materials when the impression material is 0.2 mm; however, when the sulcus is retracted less than 0.2 mm, greater distortion is produced in the marginal area of the impression. Tear strength depends upon several factors: the amount of gingival retraction since thin areas of impression material are more likely to distort and tear, the depth of the subgingival margin, the amount of hemorrhage that produces flaws in the impression thus lowering tear strength, and sharp edges and rough preparations that increase the force required to release the impression material from the surface.
• Viscosity: Impression materials are supplied in four viscosities ranging from low (syringe or wash material), medium or monophasic, high (tray or heavy body), and very high (putty). As the filler content of the impression material increases, the flow of the material decreases. Viscosity also is reduced by shearing forces, allowing a medium-body impression material to have lower viscosity when injected into the sulcus through a syringe tip, but still have a high enough viscosity to avoid slumping when loaded into an impression tray. Being able to capture a subgingival margin is difficult, and requires the impression material to be placed apical to the prepared tooth margin. Being able to capture this area in the impression requires adequate retraction and placement of the low-viscosity material into the sulcus. Allowing the cord to remain in the sulcus for 10 minutes produces maximum retraction. But most clinicians do not allow this much retraction time. If little retraction is produced, then a thin fin of impression material is expressed into the sulcus. This fin is susceptible to distortion. If a high-viscosity impression material is used with less retraction, then the impression material will not flow to the base of the sulcus and incompletely capture the prepared margin. Low-viscosity materials flow into the sulcus easier than high-viscosity materials. So the ability of the impression material to duplicate the marginal area is dependent upon the viscosity of the impression material and the retraction time. If the prepared tooth margin is subgingival, you can improve your chances for clinical success by increasing retraction time and using a low-viscosity syringe material.
• Working and setting time: Working time is measured from the start of mix and includes time to manipulate and syringe the material around the tooth. Working time allows the impression material to be placed in the tray and syringe and injected into the sulcus. The setting time for impression materials is the time the material needs to be in place in the oral environment to polymerize. Elastomeric impression materials have a working time of approximately two minutes and a setting time of two to six minutes (fast set vs. regular set). The setting time of all elastomeric impression materials is affected by temperature. To increase working time, refrigerate impression materials before use.
Dr. Dalin: I feel like I am sitting back at my alma mater, Indiana University School of Dentistry, listening to Ralph Phillips in my dental materials class! We now can interpret these terms and features that we are bombarded with and can become better dental consumers. I know you think that tissue handling and moisture control is as important as the material used. What methods can we follow to improve our technique when it comes to this?
Dr. Burgess: Adequate hemorrhage control is essential when making impressions and can be obtained using several techniques. A two-cord retraction technique is an excellent method for retracting tissue when the prepared tooth margin is subgingival or when the gingival tissue is thick. Two cords provide additional retraction in these situations. The first cord inserted into the sulcus is the smallest and should be positioned below the prepared tooth margin. The second, larger cord is placed over the first cord. To be effective, the first cord should be positioned below the prepared tooth margin. If this cord pops up during packing, then a single-cord technique should be considered. When only one small cord can be packed, or if a shallow sulcus is encountered, the single-cord technique should be used. Alternate retraction systems also are available, such as the Magic FoamCord from Coltene/Whaledent and Expasyl from Kerr. The Magic FoamCord is an expanding addition silicone impression material that is injected into the sulcus and held in place with Comprecaps, which prevent the expanding FoamCord from escaping the sulcus. This is a nice system for moderate retraction and eliminates packing retraction cord. One helpful hint is to give a syringe and some chlorhexidine to the patient, and have them irrigate the tissue daily for a week prior to making the impression. This reduces soft-tissue bleeding.
Dr. Dalin: I remember seeing a great trouble-shooting guide from 3M ESPE that is useful in recognizing problems as an impression is taken. Would you discuss the problems that can be encountered with impressions, and how to prevent them?
Dr. Burgess: Certainly. Here are few problems to note:
■ Delamination: Separation or delamination of the impression material occurs with contamination of the impression material. A two-step putty wash technique can produce this delamination if significant saliva remains on the set putty in the tray. Delamination from the tray can occur if the wrong adhesive is used with the impression material. Do not use polyether adhesives with addition silicone impression materials. If a stock tray is tried into the mouth to select the proper size tray, and either saliva or disinfectant remains on the tray after try in, then the impression material will peel from the tray. Before you take an impression, be sure to clean trays thoroughly after trying them in.
■ Ledges, drags, and folds: Ledges, drags, and folds occur when the working time of the impression material is exceeded. This produces a rapid increase in the impression material’s viscosity and low flow. Folds occur between the heavy or putty impression material and the lower-viscosity syringe material. This produces inaccuracies in the impression, especially if they appear on the marginal areas. You can correct this by using an impression material with longer working time or by refrigerating the impression material. Another possible reason for this ocurring is that the putty material was mixed too early. Assistants often will mix the putty impression material early, then mix the wash material. Since the setting time of these materials are matched, the putty is setting. Thus, it is easy to exceed the working time of the putty that is producing the folds and ledges.
■ Rough margins: This occurs with poor control of bleeding from the soft tissue, especially when margins are subgingival.
■ Surface inhibition: Surface inhibition is produced when polyvinylsiloxane impression materials are used. The catalyst in these materials is inhibited by small amounts of sulfur found in the powder on latex gloves. Touching teeth during preparation leaves a residue of the powder on the surface. This inhibits the set of the impression material. Likewise, when a retraction cord is handled while wearing gloves, powder from the gloves is transferred to the retraction cord. This powder residue inhibits the set of the impression material in the worst possible area - the sulcus. Avoid handling the retraction cord with gloved hands. Use instruments or wear vinyl gloves instead.
Dr. Dalin: Is there anything else you would like to discuss with the readers of Dental Economics®?
Dr. Burgess: Recently, we compared a monophasic single-step Impregum Penta Soft and Aquasil XLV in a one- step putty wash technique. Two impressions were made of each preparation using one of each material. The same retraction method, tray type and size, were used. One laboratory technician completed all Empress 2 restorations. After the restorations were tried in and adjusted, an impression was made of the exposed margins. The impressions were poured and a model was separated. The marginal opening obtained for each material was measured and compared. No significant difference was seen for either impression material. Careful attention to detail and an exacting technique produced outstanding but equal results. An excellent impression may be obtained by controlling bleeding, carefully retracting the sulcus, removing the cord, rinsing thoroughly, and carefully syringing the impression material.
The bottom line of this discussion is: Technique is most important, but materials are helpful in difficult situations.
Dr. John O. Burgess is a graduate of Emory University School of Dentistry. He received his M.S. in biomedical sciences from the University of Texas Health Science Center in Houston, where he completed a one-year general practice residency and a two-year general dentistry residency.
Currently, Dr. Burgess is assistant dean for clinical research at the University of Alabama at Birmingham. A former military consultant in general dentistry to the Air Force Surgeon General, Dr. Burgess has written more than 300 articles, abstracts, and textbook chapters. He is active in clinical evaluations of dental materials, and is an investigator on clinical protocols evaluating adhesives, ceramic restoratives, fluoride releasing materials, posterior composites, impression materials, and bleaching agents.
Jeffrey B. Dalin, DDS, FACD, FAGD, FICD, practices general dentistry in St. Louis. He also is the editor of St. Louis Dentistry magazine, and spokesman and critical-issue-response-team chairman for the Greater St. Louis Dental Society. Contact him by e-mail at [email protected], by phone at (314) 567-5612, or by fax at (314) 567-9047.