Impression materials are used for a variety of purposes. This category will cover polyethers (PE) designed for impressing preps for indirect restorations.
To be able to fabricate a well-fitting, indirect restoration, capturing in detail the preparation on which it will ultimately reside is critical. This task is currently assigned to two different types: vinyl polysiloxane and polyether. Both are very accurate and possess good dimensional stability that allows impressions to be poured as much as two weeks later. But most new impression materials are VPS since they are easier to manufacture and are not restricted by the patent held by 3M that has controlled the manufacture of PE over the last 30 years or so.
The decision to use either a PE or a VPS is based mainly on personal preference and experience. Impressions made from either category of material can lead to excellent restorations. However, one of the main differences is hydrophilicity.
Ever since the first "hydrophilic" VPS was introduced a number of years ago, there has been a race among the manufacturers to create it with as much hydrophilicity as found in a PE. The thought was that the more hydrophilic a material was, the less likelihood that fluid in the sulcus would distort the impression. The hydrophilic material will merely absorb the fluid and continue with its mission. Note that hydrophilic properties in VPS products need to be additives, since these materials are not inherently hydrophilic as they are with PE.
However, one VPS is marketed as "hydrokinetic", which breaks down to simply mean "moving water". Well, you can't move water if you also love it, which is the essence of the meaning of "hydrophilic". Therefore, another way of describing "hydrokinetic" would be "hydrophobic". In other words, we have come full circle, since VPS materials start out hydrophobic and only become hydrophilic by the addition of various types of surfactants.
Does any of this matter when you are trying to take an accurate impression? Well, if the sulcus is filled with fluid including blood that is obscuring your margin, then it could definitely make a difference. If you are using a supremely hydrophilic material, you hope that the product will literally soak up the fluid similar to a sponge and, at the same time, register the impression.
On the other hand, if the material is hydrokinetic, the aim is to move the fluid out of the sulcus first and then capture the margin. Is this a better strategy? The answer is probably yes, since there is less chance that the fluid will distort the material, as it may do if it was absorbed. But if this strategy is preferred, why have virtually all manufacturers opted for the hydrophilic route?
One reason could be the mob mentality. If it works for one company, then other companies produce the same item with some slight tweaks. Another reason is that the concept flies in the face of the trend. Hydrophilic is the IN concept, from adhesives to cement to sealants. Why should impression materials be any different? And hydrophilic PE followed in the successful footprints of hydrophilic hydrocolloid. Finally, only one company thought of it.
So should you switch to a hydrokinetic impression material? Not necessarily. There are numerous other factors to consider such as working and setting time, flow, availability in different delivery systems, etc. All these criteria may be as or even more important than the hydrophilicity.
And, of course, none of this matters at all if you use proper soft tissue management BEFORE you even lay a diamond on the tooth. Preventing a bloody sulcus is much more effective than having to deal with it after-the-fact.
However, although this is an admirable goal, it doesn't always happen. Therefore, finding an impression material that will be forgiving has significant value. This is why PE continues to garner kudos from its devotees -- these products tend to be less sensitive to moisture and have a terrific ability to wet-out the preparation under adverse conditions.
Dentists are very loyal to their brand of impression material. It seems like once you find a brand that works for you, there is very little reason to switch. One of the reasons why we tend to stay with one brand is that most of the differences between products do not relate to whether one material is more accurate than another, since all of our CHOICES are capable of taking very accurate impressions. As a matter of fact, every material scored excellent when it came to accuracy. In addition, there was a marked absence of voids when we sectioned our test batches of set materials. So, if a dentist is going to switch impression materials, it will probably be due to issues such as viscosity, delivery options, working and mouth removal times, and cost.
CRITERIA FOR SELECTION/TESTS
Viscosity and Flow
This is an issue that goes back to how you prefer to take an impression. Most members of our Editorial Team prefer a light body/heavy body combination. In this instance, you would look for a light body material that syringes easily and flows well without being too runny combined with a heavy body tray material that will push the syringe material firmly against the preparation and, at the same time, not run down the patient's throat. Less popular is a monophase material for both the syringe and tray.
Note that most monophase/medium body and tray/heavy body materials have virtually the same viscosity and flow, even though a few may be somewhat stiffer, making them better choices for the closed-bite technique. However, the viscosities of syringe/light body materials still vary quite a bit. Our shark fin tests and intraoral simulation photographs will give you a vivid view of their differences.
So what is the shark fin? It is a unique testing device developed by 3M. As its name implies, it has a long thin recess shaped like the dorsal fin of a shark. It also has a reservoir at its base that simulates an impression tray. We inject the syringe material directly into the shark fin recess to simulate the same clinical procedure of injecting the material into a retracted sulcus, followed immediately by seating the base, which has been filled with tray material. After setting, these shark fins are recovered from the mold and measured to determine the extent of penetration. The results of this test should give you insight into how the various combinations of syringe/tray materials (or monophase for both) will flow when used for impressions.
NOTE: The shark fin device has a maximum height to the fin itself of 33mm. However, there is a narrow extension even beyond that depth. If a material completely filled the shark fin and even duplicated part of the extension, we listed this penetration as 33+. If you want a material that has maximum flow and probably has the best chance of capturing a deep subgingival margin, selecting from those that scored 33+ would be prudent choice.
It is presumed that harder materials would be preferred for closed mouth impressions, especially if a sideless tray is used. In these trays, the impression material itself provides the lateral support. On the other hand, a hard material may lock into undercuts, hard and soft tissue, and may not be the material of choice for a full arch impression, especially if you are using a well-fitting custom tray.
We have measured the hardness of tray materials at mouth removal time and 24 hours, the latter of which could be important during laboratory procedures. This hardness is measured using a digital durometer and reported in Shore A units.
With the electronic mixers and automix guns now on the market, mixing and dispensing is not much of an issue any longer. Some materials offer both mixing and dispensing options.
Extraoral Working Time
With all materials being automix, this criterion is only important if you are working without an assistant and only applies to the tray material, since the only extraoral working time for syringe materials is the time it takes you to squeeze the trigger of the gun or inject freshly mixed material that has been loaded into an impression syringe directly from an automatic mixer. After this mixing, all the rest of the syringe material working time is intraoral (see next criterion).
Extraoral working time could be critical when it comes to tray materials when you are working alone. In this scenario, you (the dentist) would most likely mix and load the impression tray with the heavy body (or medium body/monophase) material and then place it on the bracket table or countertop. You would then begin to mix and inject the low viscosity material (or monophase) around and over the preparation. Then the tray would be retrieved and seated in the patient's mouth. Therefore, the time between mixing, loading, and seating the tray could be critical.
To test this working time, we dispensed tray materials into a full arch stock impression tray. For the control, we injected light body material over a full arch plastic model with half the teeth prepared for indirect restorations. We then immediately placed the filled impression tray over the model to simulate its placement in a patient's mouth.
For the working time test, we repeated this procedure, but instead of injecting the light body material and seating the tray immediately, we filled the impression tray and allowed it to sit on the bracket table until one minute of working time (as designated by the manufacturer) remained. We then injected the light body material over the model. This process required almost one minute. The tray of material whose working time was about to expire was then seated over the model. The control and working time impressions were then compared visually.
If there were no differences between the two impressions, we concluded that the working time as designated by the manufacturer was accurate. If there were differences, we shortened the working time and repeated the test. Nevertheless, regardless of how much working time you have, seating a tray filled with impression material should be done as quickly as possible after mixing and loading.
Intraoral Working Time
In some instances, when you are impressing a single tooth, a fast setting material would be a definite advantage, while a 10-unit impression requires extended working time. Unfortunately, the working times provided by manufacturers may be determined at room temperature. While this provides somewhat of a comparison between products, it doesn't really give you much indication about how much time you have between the inception of syringing the material around your preparation and when you need to seat the tray.
For example, if you are taking a 10-unit impression, how much time do you have from when you syringe material around the first preparation and when you need to seat the tray? This is critical to know because the material syringed around the first of the 10 preparations is already starting to set, which is accelerated by the heat and moisture of the mouth. If it sets too fast, the tray material will not bond adequately to the syringe material and you'll most likely end up with wrinkles or other types of distortion.
To assess whether a material will be acceptable for essentially a full arch impression of numerous preparations, we placed a model of Ivorine teeth in our temperature/humidity chamber for at least two minutes to bring it close to mouth conditions. Half the teeth in this model had been prepared for indirect restorations. We then syringed light body material over all the teeth, attempting to simulate the same dispensing method that you would use clinically. This dispensing time was approximately one minute. We then seated a tray filled with heavy body material over the light body and allowed the impression to set. After removing the set impression, we examined it visually. If the replication detail of the last tooth was as good as the first and the impression as a whole appeared to be clinically acceptable, we concluded that the material was acceptable for full arch impressions.
Note that we only used regular set materials for this test. It is not prudent to use a fast set material for a full arch impression unless the patient is a gagger and removing the impression as quickly as possible is essential.
Mouth Removal Times
We mixed and seated a tray of material in a staffer's mouth as fast as we could accomplish this task. We then removed the tray when the material overflowing the border resisted penetration with an instrument and "felt" set. In many instances, we were able to remove the impression sooner than the manufacturer's recommended "in mouth" time. We then measured the hardness of the material (using the previously mentioned digital durometer reading in Shore A units) that had been placed in our temperature/humidity chamber at this mouth removal time and compared it to the hardness at the official manufacturer times and at 24 hours. Ideally, the mouth removal hardness would not be significantly different than the hardness at 24 hours.
Nevertheless, we selected the time that produced the higher hardness as our recommended mouth removal time from beginning of mixing to removing the impression from the mouth. This test further validates the fact that merely poking an impression to see if it is set may not be a wise decision. Be sure to time all impressions.
NOTE: The working and mouth removal time tables supplied by many manufacturers can be confusing, since terms such as "setting time" are used frequently, but don't really explain when you can safely remove an impression from a patient's mouth. In addition, if the syringe and tray materials have different mouth removal times, the longer time would have to rule, assuming you started mixing both at the same time.
Mouth removal times, however, can even vary based on how soon after mixing you place the impression in a patient's mouth. Nevertheless, to be on the safe side, it is always prudent to allow the impression to remain in the patient's mouth for one additional minute past the time recommended for mouth removal. Otherwise, you could end up wasting much more time and money having to retake a prematurely removed impression. Of course, if the patient is a gagger and in distress, you may need to remove the impression sooner.
If everything else is equal, then cost plays a role in the selection process.