For most of the first half of the 20th century, dental impression choices were simple: dental compound (a thermodynamic material) and plaster. The procedure didn’t matter much as both materials were often used together.
For example, for crown and bridge impressions, individual copper tubes used compound to take the impression of the prepared tooth and then a “pick up impression” was taken with plaster to link the copper tube to the adjacent teeth or preparations. Considering the lack of “give” in either of these materials, the consequences of undercut were lethal. Compound also was used as a preliminary impression material for full denture cases, and then modified for use as a tray. A wash impression of plaster was used as the final impression material.
In these early days, the plaster was not fast setting, and thus it was a messy, time-consuming process. In my prosthodontic residency, I had the privilege of using plaster once or twice as an educational exercise, and what I quickly learned was that fast-setting or not, I never wanted to use plaster as an impression material again! However, I did learn to use compound both in its cake and stick formulations and its various colors.
The choices we make in the materials we use need to result in efficient, predictable impressions, yielding well-fitting prostheses. In addition, we must consider the economics of material management, and not have boxes of unused or seldom-used materials expiring on our shelves. The materials we choose must be predictable and provide efficiency. Materials that don’t work result in remakes of our prosthetics and increase our costs in both material used and that finite commodity, time. When possible, it is better to have materials with multiple uses, thus lowering our holding and ordering costs. But in any event, the material must be workable in a dentist’s hands.
Although there are many multiple use materials today (that is, materials that can be used for a variety of functions, such as removable prosthetics and crown and bridge and implants) I have found some single use materials to be better suited to one’s practice. Materials such as compound and zinc oxide eugenol pastes are useful solely in removable prosthetics, primarily full dentures; the polyethers and polyvinyl siloxanes, while usable in removable prosthetics, are better suited to fixed prosthetics and implants.
Dental Compound
The different colors of compound reflect slightly different thermodynamic behavior, i.e. they soften and harden at different temperatures and thus provide more or less tissue detail. Red compound tends to soften at lower temperatures but isn’t as accurate as grey stick compound, which softens a slightly higher temperature but provides for greater detail for border molding purposes.
I still use compound today for removable impression purposes. The fact that the material is thermodynamic means that corrections and additions are easily made. Also, the viscosity of the material is controlled with temperature, so structures such as the tongue, cheeks, floor of the mouth, and lips can be pushed aside, avoiding improper influence on the impression—or borders can be developed into their proper extensions without concern over material slump. This makes compound the obvious material for preliminary full denture impressions.
In my residency, I also had experience with trimming and removing the compound material from the stock trays and then employing the compound as the final impression trays using a wash material, usually a zinc oxide and eugenol material. This technique is not recommended since the compound trays are weak and subject to breakage, but it was a great learning experience. However, compound is not just for edentulous impressions; it can facilitate partially dentulous or full dentulous impressions. For patients with deep palates, compound can help customize the stock tray and provide support for alginate or polyvinyl siloxane materials. Similarly, compound can be employed to support edentulous areas or saddle areas, and facilitate either preliminary or final impressions for partial dentures or better study models. Lastly, compound is cheap. In short, I find compound indispensable in my practice.
Final Impression Material for Edentulous Cases
There are essentially five types of materials used as final impression materials for edentulous cases: alginate, polyvinyl siloxanes, polyethers, rubber base, and zinc oxide eugenol pastes.
Probably, for the old timers out there, the first image that rubber base conjures is brown, foul-smelling, foul-tasting runny material that took an eternity to set. While that material is still available, the rubber base material of today has none of those properties. However, given the relatively singular use of this material in removable cases, this material can probably no longer be justified in that most offices employ either polyethers (think Impregum) or polyvinyls for crown and bridge uses that can also be used for removable cases as well. A major advantage of the polyvinyl siloxanes is long-term dimensional stability and the ability to repour impressions without loss of accuracy. Polyethers also possess the property of long-term dimensional stability but cannot be repoured. Rubber base material must be poured within an hour of the impression and this is a major disadvantage given the time constraints of pouring an impression.
In my opinion, the gold standard for edentulous materials are zinc oxide eugenol pastes. This is because in edentulous impressions, elimination of pressure on the ridges or buccal shelves is essential, or the result will be a denture that will cause an overextended prosthesis, resulting in multiple sore spots for the patient, multiple office visits for the dentist, and likely a very unhappy patient.
Zinc oxide eugenol pastes are “dead soft”—that is, they do not cause any pressure on the oral tissues. The material I like to use is Krex, which sets quickly, is not runny, and yet provides all of the properties of a dead soft material. A secondary advantage is that any voids of less than 4 mm in diameter in the final impression can be corrected with “Iowa Wax,” which has the property of flowing at mouth temperatures. Iowa Wax can be placed in voids, the impression then replaced in the mouth and held in place for several minutes, and the wax will fill in the void and record the impression of the tissue in that void area thus eliminating the need for another impression. Another advantage of Krex is that the set time can be accelerated by increasing the amount of catalyst used.
Polyvinyl siloxanes can also be used; however, there are many viscosities available and in reality, only the light body material can be considered dead soft, but is probably too runny to be of much use. I am not saying you cannot use Polyvinyls for final denture impressions, but I am saying that they are less accurate than zinc oxide eugenol pastes. Combining a medium body tray material and a light body wash material can be, and is, used successfully for implant cases employing fixed removable or fixed prosthetic options. The difference is that in these cases, the majority of the stress-bearing load is born by the implants, not tissue or ridge. Pressure spots can be relieved without loss of retention. This is not the case for full denture scenarios where maximal coverage is essential to maximize retention.
Alginate also has been used as both a border molding and final impression material. But in order to be workable, the alginate must be especially viscous and also must have sufficient bulk. This usually requires employing a type of stock tray and the combination can result in overextensions and an improper fit of the prosthesis. Alginate is very useful for partial denture cases and very satisfactory for partial denture framework construction.
Modern rubber base materials such as Omniflex set quickly, and are useful in situations where undercuts are a concern. Zinc oxide eugenol pastes are not usable in moderate to severe undercut situations.
Fixed and Implant Impression Materials
The most common fixed and implant material used today are the polyvinyl siloxanes, followed by the polyethers. It is impossible to open a dental magazine advocating either a new or new and improved polyvinyl purporting to deal with the wet environment of the mouth better than the next. In reality, most of the materials are very similar, varying in viscosities and set time. One unique material is Panasil, which employs a putty wash system.
Once upon a time, the most common form of polyvinyl siloxane material employed a putty tray material that was mixed by hand into a ball by combining a base and a catalyst, and then placed into a tray. Wash material was syringed around the preparation and putty material pressed over the preparation. The success rate was very high because the viscosity of the putty was so high that blood, saliva, and crevicular fluid was not a concern. The problem was that latex acts as an inhibitor to the material setting, and even if the hands were scrubbed, latex residue still remained, causing the impression to fail. Panasil eliminates the hand mixing and instead uses an auto mix machine, but retains the properties that make the putty system so attractive. This material can also be used in wash impressions if one desires.
One tip for this material and all other polyvinyls—latex is still a problem and you come into contact with the preparation either during preparation or in packing cord. This can lead to voids and pulls in your impression. You can eliminate this problem by thoroughly scrubbing the preparations with 3% hydrogen peroxide, and in addition, this will remove blood and blood debris, which can affect the impression outcome.
I have probably used in excess of a dozen types of other polyvinyl siloxanes, and have been dissatisfied with the majority of them. My dissatisfaction lies in the fact that I believe the impression should be obtained on the first take. My acceptable level is 90%, i.e., 90% of the impressions taken should yield an accurate, void-free impression usable for the desired prosthesis. Most of the polyvinyls I have used, including Penta, Accusil, President, etc. all yielded a far lower success rate. I found inconsistencies from batch to batch and even experienced incomplete setting or no setting of the material and variations in set time. Apart from the occasional use of Panasil, I employ two materials, Honigum and PerfectIM.
Honigum is a polyvinyl siloxane; I have found it to be very predictable with a precise set time of exactly three minutes in the mouth. The tray material can be used with an automix machine and the wash uses an impression gun. The wash material is not runny and the number of retakes is low. My lab loves the material because it yields very clean margins. This material works well in either dual phase or monophase impressions. The downside to this material, like most polyvinyl siloxanes, is that it requires some sort of retraction method. Most of us pack cord to obtain retraction and this is a time consuming process, usually requiring a wait of 10 minutes or more from the time the cord is placed before adequate retraction is achieved. This technique is most effective with dual packing, which takes more time. Of course, there is the added risk of damaging the epithelial attachment or the gingival tissues. I have tried the paste retraction systems and found them wanting in both my hands and the pictures they publish in our magazines. In some cases, the ills of cord packing can be avoided with a material called PerfectIM.
PerfectIM is also a polyvinyl siloxane material, but it employs a tray material that when set, has a durometer hardness that exceeds plaster! Because of its density, the tray material pushes the gingival tissues out of the way and enables the wash material to capture detail. Another advantage is speed. The material is available in a set time of 30 seconds in the mouth. However, when you add working time the actual total set time is about 45 seconds. In addition, since there is no need for retraction, if the tooth is not vital, then no anesthesia is required—saving more time. The technique works best with triple trays and it is a two-stage process. After tooth preparation and cleansing with hydrogen peroxide, an impression is taken with the tray material. After waiting about 45 seconds, the patient is guided open, keeping the tray attached to the opposing arch. The tray and preparation are air dried or rinsed as needed, and then dried. A small amount of wash material is placed into the impression of the preparation in the tray material. The patient is then asked to close and bite tightly together for 45 seconds. I like to keep my hand on the patient’s chin to ensure closure. The tray can then be read for accuracy and should demonstrate show through of the blue tray material. The wash material is yellow and the impressions are easily read.
I find that PerfectIM is best used in the posterior regions of the mouth and can be used for three unit bridges or single crowns. In my opinion the key component is the amount of occlusal force the patient generates, and this is why it is best used posteriorally. In some female patients there may be insufficient occlusal force to provide for a good impression. I have also found that PerfectIM does not work well for impression for porcelain veneers or longer span bridges, however, since the majority of crown and bridge procedures are likely single unit or three unit bridges, this material has found a very useful place in my practice and can benefit you as well.
Conclusion
Taking impressions is an important part of every dental practice; it’s often overlooked in both its importance and economic impact on our practices. Certainly there is the raw cost of purchasing supplies, and then there is the cost of retaking impressions. There is also the impact on our reputations. Repeated impressions can initially impart a feeling in the patient that the dentist is paying attention to detail. After several impressions, the patient may start to question your ability. In short, the impression process must be an efficient, predictable part of your practice not to be overlooked.