How impression materials have changed over time

Dentists have been using impression materials to capture the detail of patient's mouths for restorative work since the early 1700s. However, more than 200 years later, we still use impression materials for the same purpose, but not the same stuff. We spoke to experts to learn more about how these materials have changed over the years and why. 

Peter Meyerhof, DDS, Ph.D., has a private practice in Sonoma, Calif., and is on the board of the American Academy of the History of Dentistry. He says the earliest known use of impression materials was in the mid-1700s in Prussia. One dentist, Dr. Philip Pfaff, who was the King of Prussia’s dentist, used sealing wax softened in hot water, which he then molded to the teeth and then poured in plaster to make a cast for a partial denture. 

Some dentists claim that the use of wax goes back much earlier, but there is no remaining evidence for how they took impressions. For example archaeological evidence shows us that, as early as 600 BC, the Etruscans in making a fixed bridge would solder together bands around both abutments and pontics that were made of ox bone. This prosthesis would have required a model, Dr. Meyerhof explains. However, there is no surviving evidence of how the Etruscans did this. 

Wax was commonly used for impressions, particularly beeswax. George Washington, who had several sets of dentures in his lifetime, would have had a dental impression made with beeswax. Dr. Meyerhof says President Washington's dentist, Dr. John Greenwood, used the same method; softening the wax, molding it to the teeth, and cooling it with water while it hardened a little before removing it. Then, Dr. Greenwood would have poured the cast with plaster of Paris. 

Shirin Khoynezhad, DDS, a prosthodontist and the Director of Second-Year Pre-Clinical Dentistry at the University of Alabama, says another dentist advertising out of New York in the late 1700s also was taking impressions. The ad ran in 1787 in the New York Daily Advertiser and read, "persons at any distance may be supplied with artificial teeth by sending an impression, taken in wax, of the place where wanted…"¹

"So, people could take a wax impression at home and send it to them, and then he would send a denture back to them," Dr. Khoynezhad says. 

Dr. Khoynezhad teaches her students how to set teeth in wax. Wax changes dimensions in different room temperatures. She says the wax changes handling properties as the students work and raise the temperature in the room. She can only imagine the state of those impressions traveling in that mail during the 1700s with that much handling. However, it was revolutionary back then to have impressions taken and dentures made. 

By the middle of the 1800s, dentists began using plaster as their impression material. The earliest techniques used a tray made of wax. However, Dr. Meyerhof explains, plaster sets up quickly, so the dentist would have to work quickly. Moreover, since plaster is not elastic, the dentist would tap it with a mallet to break it out and then reassemble outside the mouth. Dr. Meyerhof says this technique persisted until the early 1900s. 

“That’s a pretty brutal method,” Dr. Meyerhof says. 

Material advancements
At the turn of the twentieth century, dentists began using Impression Compound, which is wax with a talc filler and a lubricant that softens when heated. Impression Compound softens when heated to 150 degrees Fahrenheit and then the dentist would let it set up.

“Once again, it will lock into undercuts, and would have to be broken and reassembled on the bench. But it could be used for dentures for people who have no teeth, for instance, quite well,” Dr. Meyerhof explains.

Around 1930, dentists began using flexible impression materials. Hydrocolloid, which contains agar made from a special type of seaweed, is a reversible material that gels at 90 degrees Fahrenheit. Dr. Meyerhof says when it is hotter, hydrocolloid is a liquid, and you can put in trays to set up. Dentists commonly used hydrocolloids as impression materials until the 1940s. However, since the seaweed came from Japan, the Allied countries switched to using alginate during World War II. Unlike hydrocolloids, alginates set up irreversibly and are made from a common brown algae. Alginates were most popular until the 1950s, although they still have many uses today.  

Dr. Meyerhof says the mid-twentieth century was a major turning point for dentistry in many ways, and this included new impression materials. In the mid-1950s, dentists began using Polysulfides, an elastic material made of synthetic rubber that is renowned for its bad smell and taste. Polysulfides had some great qualities, but they also had a long setting time and were not as accurate as some of the impression materials that came before it. Next, the dental industry introduced Polyether (PE), which was hydrophilic, making it easy to use when isolation was a problem. However, it was also stiff and tore easily. Moreover, the bad taste persisted with these early formulations, Dr. Meyerhof says. 

By the 1980s, dentists began using Polyvinyl Siloxanes (PVS). These materials were more hydrophobic than PE materials. 

“The first ones were very hydrophobic, meaning that moisture control was extremely important. If there was any liquid, they weren’t that good,” Dr. Meyerhof says. “They certainly evolved over time to become quite hydrophilic, which is important because even if you try to dry a tooth as much as you can, there's still this sulcular fluid that emerges adjacent to the tooth and often near the location of the margin of a preparation. It can certainly interfere with accuracy unless you have a hydrophilic impression material.”

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PVS is very accurate. It is also flexible, which means you can pull it out of the patient's mouth over bulging teeth. 

“It has excellent resilience and tear strength,” Dr. Meyerhof says, “and for the last 20 years or so, it can be put into an auto-mix dispenser, so it’s extremely easy to use and very uniformly consistent.”

PVS also has excellent dimensional stability. You can pour a cast from an impression long after you take it, which isn't true of many of the older impression materials. It can also be disinfected quickly because of its dimensional stability, which is essential in light of current infection control protocols. The dentist disinfects the impression and ships it to the lab, which then disinfects it again, often with CaviCide from Metrex. 

"You can't disinfect a material like an alginate, because if you spray something on it, it will absorb it and distort," Dr. Meyerhof says. 

In the 2000s, PEs persist, as well as alginates and alginate substitutes in some practices. However, PVS is the most commonly used material today. 

“I think it’s the ideal material,” Dr. Meyerhof says. 

Dr. Khoynezhad says silicone-based impression materials like PE and PVS became the gold standard. 

"It's worth mentioning that how you handle impression material is done by impression trays," Dr. Khoynezhad says. "Trays changed a lot over these times, too, from the 1800s to now. Today we have a lot of good trays, thermoplastic trays dipped in hot water, and then put into the patient's mouth to mold. If you don't have an accurate impression tray, you can't get an accurate impression."

Moreover, impression materials work best when manipulated in the best conditions, which includes the best tray fit, Dr. Khoynezhad says. For example, with alginate, you need to have room between the teeth and the tray so the material can be thick enough to capture the oral tissues. So, you use a looser tray with alginate than other materials. PVS materials, on the other hand, Dr. Khoynezhad explains, need less space because they are accurate in thinner layers. 

Dr. Khoynezhad says that after the invention of PVS materials and toward the end of the twentieth century, all these advances in impression materials slowed down a lot. Synthetic elastomeric materials are what dentists use today, although the formulas have improved since their introduction. 

"These synthetic materials were created because we needed dimensional stability," Dr. Khoynezhad says. "Alginate impressions are accurate if they are poured within 10 to 15 minutes of taking the impression. Alginates should not get dried out because their dimensional stability is very low, and they change. When you pour distorted material, you don't get an accurate cast."

With today's synthetic impression materials, Dr. Khoynezhad explains, you can take an impression and pour it the next day, and it will be the exact position that it was when removed from the patient's mouth. Also, the tear strength improved with elastomeric materials. She prefers PVS over PE because it's easier to handle and has excellent dimensional stability. Moreover, she thinks it produces accurate prostheses. 

Digital dentistry
Dr. Meyerhof sees dental offices turning to impressions using a digital scanner and computed tomography rather than impression material. However, there are some disadvantages to digital impressions which Dr. Meyerhof says are often overlooked in the rush to the latest technology. In particular, Dr. Meyerhof gives the example of subgingival margins. When trying to get a good impression of the margin so you can have a smooth emergence profile, he recommends PVS because it usually will be atraumatically pushed slightly below the prepared surface. The same is not true for digital impressions.

"If you can't see it, it won't be captured in a scan," Dr. Meyerhof says of digital impression technology. 

However, Dr. Khoynezhad prefers digital impressions and describes herself as a digital person. She also thinks the future of impressions is digital and that dentistry will no longer use impression materials in the upcoming decades. But, for now, she still uses both because a lot of cases still require impression materials to deliver the best treatment and care for a patient. 

"A lot of time, the digital scan doesn't capture the prep, especially if it is subgingival. We have to go back to impression materials to capture the fine details to fabricate more accurate crowns,” Dr. Khoynezhad says. 

"So, we still use them, and they are excellent materials."

Reference

1. Glenner, DDS, Richard A. "How It Evolved: Dental Impressions." Chairside Magazine. May 7, 2014. Volume 9, Issue 1. Accessed via the web. 5 March 2020. <https://glidewelldental.com/education/chairside-dental-magazine/volume-9-issue-1/how-it-evolved-dental-impressions/>.