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sujatabyahatti's picture
Joined: 19 Dec 2010

Today’s dentists are exposed to a multitude of dental materials, with each manufacturer claiming the benefits and superiorities of its respective products. Sometimes, I feel that the “prehistoric” techniques that we learned in dental school years ago display more benefits than what is in vogue today. This is especially true for crown and bridge impression materials and techniques. One has to only look back and visualize copper band impressions taken with dental compound, and picture the definition, accuracy, margin detail, and meticulous dies that these impressions provided. Even now, we are still seeing the results of the “copper band masters” decades after the restoratons were placed.
We all know that a meticulous impression is paramount for a precision fit of the permanent indirect restoration. Unfortunately, for many clinicians, taking a crown and bridge impression is one of the most stressful procedures in restorative dentistry. The good news is that taking great impressions can be simple if a dentist chooses the correct impression tray, achieves adequate retraction with controlled bleeding, and uses a rigid impression material with a light body wash to clearly capture every detail of the preparation.

Throughout the last 2 centuries, different types of impression materials have been developed for use in dentistry. The nonelastic materials include: plaster, dental compound, and zinc oxide eugenol pastes. The elastic materials include: alginate (irreversible hydrocolloid), agar (reversible hydrocolloid), polysulfide, condensation silicone, addition silicone (polyvinyl siloxane [PVS]), and polyether.1
When evaluating an impression material, dentists tend to focus mainly on 3 factors:hydrophilicity, setting time, and cost. However, there are physical and mechanical characteristics which are far more critical to consider such as: detail reproduction, dimensional stability, ease of removal, gypsum (die and model stone) compatibility, elastic recovery/strain in compression, tear resistance, viscosity, complete conversion to an elastic solid, acceptable odor and taste, shelf life, and having the capability to be poured multiple times while still maintaining accuracy.2 Dental marketing seems to be focused on the concepts of hydrophilicity as if it were the only standard on which to judge the product.
It is also paramount for the dentist to understand that not all impression materials and impression situations are the same. As a result, one should choose an impression material accordingly. The dentist must evaluate what type of impression is being taken (whole arch, sectional, multiple, or single tooth) and chose an appropriate impression tray, method of tissue retraction, and impression material.
In my opinion, when taking sectional-arch impressions, or when picking up implant impression transfers; a thick consistency putty-like material is preferable, since its rigidity offers greater support. The putty material is a kneadable material that is used as a base material. After a low viscosity material is injected around the tooth, the impression tray filled with putty is then placed over the teeth, displacing the light body (low viscosity), into the crevices surrounding the preparation. After setting, the impression tray is removed, yielding an extremely accurate reproduction.
The choice of impression materials, coupled with the appropriate impression-taking techniques, is critical to success. In my opinion, it seems that we have traded accuracy for convenience. Most dentists are using disposable plastic trays, as opposed to metal (rigid) ones. Then, the flexible plastic trays are filled with impression materials that flow, rather than with a stiff material that would provide less chance for distortion. This is, in part, due to the manufacturers’ need to have a flowable impression material with a viscosity that works with automix delivery systems (where the base and catalyst are combined and mixed in a small mixing tip). These flowable impression materials lack the rigidity of the previous generation of putty systems that had to be hand mixed.
Despite the introduction of dynamic mixing devices (cartridges or automated), many dentists still value the consistency of the previous generation of putties and continue to mix their materials manually. Manual mixing putty systems such as Silagum (DMG America), Aquasil Putty (DENTSPLY Caulk), Express Penta Putty (3M ESPE), Flextime and Provil (Heraeus Kulzer), Panasil Putty (Kettenbach LP), and President and Affinis Putty (Coltène/Whaledent) have continued to be manufactured.
Until recently, I have not experienced an automix impression system displaying the benefits of the copper band technique or the putty systems where the dentist would mix 2 balls of catalyst and base together. (This stiff material, when placed in an impression tray, would displace a light body wash material injected into the sulcus surounding the preparation, resulting in an impression with sharp definable margins.) For clinicians like me, who value a thick consistency, an innovative putty (Honigum-MixStar Putty [DMG America]) has been recently introduced in an automix cartridge that is mixed in an automix machine (MixStar-eMotion [DMG America]) .

When a dentist places an impression material in the patient’s mouth, using many of the commercially available plastic impression trays, any flexing of the plastic tray causes the impression to be distorted.3 It is the lack of tray rigidity that leads to flexure, resulting in incaccurate dies in the buccal-lingual and occlusal dimension. Although the impression may look perfect, even the most minute bending of the tray on placement will create inaccuracies in the final restoration, causing adjustment to be necessary when placed in the patient’s mouth.
Dentists often receive cases back from their dental laboratory team that appear to be perfect. The occlusion looks accurate, the marginal ridges line up, and the contacts are tight. However, when tried in the patient’s mouth, the case has to be adjusted significantly. Sometimes a new impression must be taken and the case totally remade. There are many possible reasons for these inaccuracies. Perhaps the stone used to create the model was not poured according to the manufacturer’s instructions and specified proportions, causing too much expansion, yielding a totally inacurate model. Perhaps the stone dies were not properly prepared, preventing the restorations to fully seat in the mouth. There are many factors that can affect the fit of our restorations, so we must eliminate inaccuracies and distortions right from the beginning of the process. Using rigid impression trays is the first step to accuracy in your impressions

Full-Arch Impression Trays
Full-arch metal impression trays are the “gold standard.” When accurate full-arch impressions are taken of the opposing arches and poured and prepared in a consistent and precise manner, many sources of inaccuracy are eliminated resulting in restorations that should require minimal adjustment intraorally.
Rigid protocols are essential for reliability and consistency in dental procedures. All full-arch impressions in our practice (PVS, alginate) are taken with Rim-Lock Impression Trays (DENTSPLY Caulk). When I place and remove these trays from the patient’s mouth, I am confident that these trays are not flexing, thus eliminating the first source of error from the impression-taking process. It is then up to the dental laboratory team to follow the remainder of the protocol and to produce consistent restorations requiring minimal adjustment.

Dual-Arch Impression Trays
Dual-arch impressions continue to be very popular and, if done properly, will produce single-unit restorations requiring minimal or no adjustment.5,6 Dual-arch trays have several advantages: only one tray is needed for the impression of both of the opposing arches; the bite is registered simultaneously in the impression; it provides improved patient comfort versus full-arch trays; less impression material is used, resulting in lower cost; and, because 3 procedures (2 impressions, bite registration) are performed simultaeously, it takes less time, further reducing the costs associated with impression taking.
However, one must keep in mind that the dual-arch impression procedure is very technique sensitive and should only be used when the following criteria are met:

It should be used with an intact dentition (Braley Class I) and the single prepared unit should have intact teeth adjacent to it.
The occlusal surfaces opposing the prepared teeth should be ideal.
The patient should be able to close down into maxiumum intercuspation without interference.

In my opinion, plastic impression trays should never be used because they flex, creating inaccuracies. Instead, a metal dual-arch impression tray should be chosen. It should fit passively and the operator must ensure that it does not rub against any tooth/anatomic structures to avoid any potential interferences when the patient bites together. If passive fit of the tray cannot be achieved, then conventional full-arch impressions should be taken. In our practice, we use the Quad Tray Plus (CLINICIAN’S CHOICE). It is designed in its shape and dimensions to accommodate most patients’ dental arches. Its rigidity creates extremely accurate impressions when used with compatible rigid impression materials.

After selecting the appropriate dual-arch impression tray, the dentist should practice inserting the tray into position in the patient’s mouth. Also, the patient should practice biting into maximum intercuspation. After several practice runs of placing the tray and having the patient bite down, place some O-Bite (DMG America) bite registration material on the opposing arch on the contralateral arch (the nonprepped side of the patient’s mouth). Then, have the patient bite together to capture this contraleteral bite; this bite will serve as a “lock” to ensure that the patient is biting consistently into maximum intercuspation . Leaving this bite registration in place , the clinician once again practices accurate placement of the dual-arch impression tray . Next, light body Honigum (DMG America) is injected into the sulcus of the preparation by the doctor, while the dental assistant simultaneously dispenses the Honigum Putty from the MixStar-eMotion . The dual-arch tray loaded with putty is placed in the manner that was previously practiced. The bite registration will serve as a guide to ensure that the patient is in complete habitual centric occlusion closure . Using this impression technique protocol will allow the dentist to easily achieve consistently accurate impressions with precise detail.

In order to take consistently accurate impressions, we must be able to visualize the margins of our preparations clearly. If our margins are placed supragingivally, capturing them is relatively simple. However, much of the time, the margins are placed subgingivally beyond the presence of existing large restorations, or for aesthetic reasons. Dentists sometimes proceed to take subgingival impressions without being 100% able to visualize the prepared margins, “hoping” that the impression will succeed. If they miss the first impression, they often take it again, while still not being able to totally visualize their margins. They are basically again “hoping for a miracle.” They blame the lack of hydrophilicity of the material (or some other reason?) for their own fundamental errors.
Hydrophilicity is not an issue if there is no bleeding and the dentist can totally visualize the margin of the preparation. Dentists are obsessed with hydrophilicity. However, there is really no such thing as a hydrophilic PVS impression material.7 A clear field, free of blood and other oral fluids/contamination, is the most important factor in capturing a good impression. All PVS materials have the same weakness: they are unable to consistently make an impression in the presence of blood/fluids. Since 2 materials cannot occupy the same space at the same time, sulcular bleeding must be controlled prior to taking the impression. Adequate retraction must be accomplished in all subgingival areas to guarantee that the impression material will flow under the margin. Several different methods of retraction are in vogue today, the most popular of which are: retraction cords, laser or electrosurgery techniques, and paste systems.

Capturing an accurate dental impression is one of the most stressful and challenging steps in restorative dentistry! And, although digital impression and CAD/CAM technologies are being touted as a new alternative for accurate impressions and definitive restorations to the dental profession, it will likely take quite some time for these technologies to be implemented into the majority of private practices. In the meantime, with the majority of dentists still involved with taking physical (nondigital) impressions, many different protocols, techniques, and materials are being used. By understanding fundamental dental principles, following a technique protocol; carefully choosing materials, impression trays, method of retraction, and appropriate impression materials; dentists everywhere can easily achieve stress-free and consistently accurate impressions.


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