When it comes to impression materials, the goal of a fast-set product is to limit the amount of time the impression is in the mouth, both for patient comfort and to limit the opportunity for the patient to move and distort the impression while it is setting.³ Although the concept is admirable, many clinicians experience drags, pulls (Fig. 4), inaccuracies (Fig. 5) and distortion in their impressions simply because they don’t understand how much working time they really have.

Terry, in his article on the impression process, gives us two definitions: “The setting time of impression materials is the total time from the start of the mix until the impression material has completely set and can be removed from the oral cavity without distortion, and the working time is measured from the start of the mix until the material can no longer be manipulated without introducing distortion or inaccuracy in the final impression.”⁴ These two processes are, of course, intimately related by the chemistry of the impression material. Many clinicians think they know the working time of their light-body and heavy-body impression materials, but we can pretty much guarantee that most do not! One of the disadvantages of PVS impression materials is their relatively short working time.⁵ If you think the working times of your light-body polyvinyl siloxanes are what is listed in the manufacturer’s instructions, then you, too, may not understand the true “intraoral” working times of your material.

By specification, the working times of impression materials are calculated at 23 degrees Celsius and at 50% relative humidity. Unfortunately, the oral cavity is much warmer and significantly wetter. In the ADA Professional Report on Elastomeric Impression Materials, the ADA found that times measured at 23 degrees Celsius were 66% to 77% longer than those measured at 35 degrees Celsius (intraoral temperature range).⁶ Some PVS impressioning materials such as Genie Ultra Hydrophilic (Sultan Healthcare Inc.; Hackensack, N.J.) and Correct Plus (Pentron Clinical Technologies LLC; Wallingford, Conn.), whose instructions claim working times of 135 and 90 seconds respectively, actually have less than 10 seconds working time intraorally.⁶ This makes it difficult for some, and impossible for others, to impress a single unit, let alone multiple units, and be able to deliver the tray prior to the light body setting.

So why is this relevant? In order to ensure a fluid blend between your light-body and heavy-body PVS impression materials, both materials must be fluid and unpolymerized at the time the tray is inserted. If not, this could lead to gaps or ledges between the different viscosities of material (Figs. 6, 7), which will lead to inaccuracies and high occlusion of your final restoration. We as practitioners also assume that upon insertion of our heavy body material, it will drive the light body into better adaptation to our preparation. Of course, this is not possible when the light body is already set (Fig. 8). This means that unless the light body is meticulously placed in the first instance, we cannot improve the impression by the hydraulics of the heavy body impression material.

Where are your impression materials stored? Are they stored in a wall cabinet with hot fluorescent lights underneath? Is your air conditioning on a timer? Do you turn the air conditioning down to save energy over the weekend?

If your air conditioning is on a timer or the temperature of your operatory or office is higher than 23 degrees Celsius over the weekend or during the day, you need to keep in mind that it takes eight hours for impression materials to acclimatize. On those hot humid summer days or nights, your impression materials can get significantly warmer than room temperature (70 degrees Celsius) and will not cool back down until eight hours after the air conditioning comes back on. This is of significant importance when it comes to your working times. For every 10 degrees above room temperature, you lose up to 50% of your working time! For some materials, this may mean less than five seconds intraoral working time. It is impossible to impress one unit of crown & bridge in this time, let alone multiple units. Hence the need for a temperature-controlled storage unit for temperature-sensitive materials or strict control of the office temperature environment.

In clinical crown & bridge cases where you must take an impression of multiple units, it can be difficult (if not impossible) with any standard impression material, due to the shortened intraoral working times, which for most materials on the market today is less than half or even a third of what is stated on the manufacturer’s instructions. However, there was a product introduced to the market a number of years ago which is designed specifically for these cases. Multi-Prep from the Affinity line of impression materials (Clinician’s Choice Dental Products Inc.; New Milford, Conn.) has the longest intraoral working time on the market today. Although not the 2:40 minutes stated in the manufacturer’s instructions, it has an intraoral working time of 90 seconds followed by a relatively short and independent intraoral set time. Figure 9 shows a full-mouth reconstruction impression taken with Multi-Prep, which shows superb detail, adaptation and marginal capture. Two other materials come close to this working time for their light bodies as tested by the ADA: Examix™ NDS (GC America Inc.; Alsip, Ill.) at 70 seconds and the polyether Impregum™ Penta™ Soft Quick Step (3M™ ESPE™; St. Paul, Minn.) at 70 seconds.