What makes dental impression materials work?

March 26, 2020

All impression materials are not created equal. They aren’t even the same chemistry. Understanding how impression materials work will help make the best possible conventional impression.

Impression materials are a staple in many dental practices. Choosing impression material well requires a strong grasp of each material’s characteristics and how they influence the way they work. The following review of how impression materials work will help you achieve the best possible results for conventional impressions.

The significance of impression materials dates back to the 18th century. In 1787, as the Philadelphia Constitutional Convention delegates drafted the US Constitution and Mozart put the finishing touches on “Eine kleine Nachtmusik,” a dentist in New York was using beeswax to make molds for prosthetics. He advertised in the New-York Daily Advertiser that “persons at any distance may be supplied with artificial teeth by sending an impression, taken in wax, of the place where wanted …”1

Today, dental impression materials involve chemistry and materials far more complex than beeswax. These materials record intraoral structures that serve as a road map for building restorations. Their accuracy is essential for the casts used to build prosthetics, diagnose and correct malocclusions. Most of all, quality impressions lead to satisfying patient outcomes and the desired level of clinical excellence.

However, the accuracy obtained from materials depends on matching the impression material’s characteristics to the circumstances of a given case.1 The choice of materials is usually based on personal preference, so here is a closer look at the various materials and how they work.

Different Types of Impression Material

Impression materials come in a variety of types. According to the publication Marine Drugs, the first elastic materials used for dental work were hydrocolloids.2 However, there are more elastic dental impression materials, including reversible and irreversible hydrocolloids-agar-agar and alginate, respectively-and synthetic elastomers, which include polyether (PE), vinyl polysiloxane (VPS), and vinyl polyether siloxane (VPES) products.1 Elastic impression materials improved the accuracy and dimensional stability of dental impressions, which also raised the quality of the restorations they helped produce.1

Alginates: Dentists began using alginate made from marine algae for dental impressions in 1947. By the 1950s, the material’s dimensional accuracy was well demonstrated.1 Back then, dentists took impressions the same way as now, and those taken with alginates were as accurate as the ones made with previously used materials.2

PEs: By the 1960s, PEs were challenging alginates’ domination of the impression materials market.1 PEs provided improved mechanical properties and decreased the shrinkage that occurs.2

VPS: The 1970s brought VPS.3 Its popularity is attributed to the material’s high dimensional stability.

VPES: Materials stayed in the above lanes until 2009, when dental manufacturers introduced a new impression material that combined the properties of PE and PVS.1 The features of VPES suggested easy removal, like PVS, and hydrophilicity, like PE.2

How the Different Materials Work

Each material works in a particular way. Here is a quick overview of each:

Alginates

Alginates are often powders that are mixed with water to form a gel. Per the Journal of Conservative Dentistry, the powder contains sodium alginate, calcium sulfate, trisodium phosphate, diatomaceous earth zinc oxide and potassium fluoride.3

Mixing the powder with water produces a hydrosol. This causes a chemical reaction between the sodium alginate and the calcium sulfate, which creates sodium sulfate and calcium alginate and forms the gel used for impressions. However, the reaction between sodium alginate and calcium sulfate happens fast during mixing, so trisodium phosphate helps slow the process, buying time for mixing and loading in the tray with no change in viscosity.1

Regarding set times, there are two types of alginates: fast set, typically one to two minutes, and normal set, around 2½ to four minutes. Many times, the manufacturer includes a reaction indicator that signals when the material is set. Alginates come not only in powder but also in paste-type material with tray and syringe viscosity options.1

PEs

PEs are polymers, materials made up of repeating chains of molecules. In the case of PEs, the monomers have ether linkages.1 PEs have a long chain of molecules alternating between oxygen atoms and methylene groups and reactive terminal groups in the base, along with fillers, plasticizers and triglycerides.1 A catalyst paste with a cross-linking agent and fillers and plasticizers opens the rings and causes chain lengthening and cross-linking to form the rubber material.1

Many of PE’s characteristics make it popular for dental practices and labs. First, it can adhere to itself, making it possible to border mold and create correctable impression techniques. The newer versions of PE are softer than previous generations, so they are easier to take out of the patient’s mouth but stay rigid enough for a wide variety of applications. A PE material can capture detail even when moisture control is an issue. Moreover, it does not set until the work ends, allowing plenty of time to capture the necessary detail. Then, when PE does set, it does so quickly.1

PVS

PVS is a type of silicone, a material made up of polymerized siloxanes.1 Siloxanes have chains of molecules with alternating silicon and oxygen atoms. They are known for their chemical inertness and resistance to oxidation and water. They also are stable at high and low temperatures. PVS is an inorganic polymer because the chain of atoms that make up the backbone of the polymer does not contain carbon.1

PVS links vinyl siloxane with hydrogen siloxane though a platinum catalyst in a base material.1 The viscosity differences are achieved through changing the amount filler included to produce either a putty or a wash. These are popular impression materials for fixed prosthetics.1

VPES

In 2009, dental materials manufacturers introduced the newest of the impression material types, VPES, which combines the polyether polymer with VPS’ vinyl groups.1 The World Journal of Dentistry describes VPES as a rubbery polymer that targets one-step impression users.1

The material has many advantages. It can flow into narrow crevices while maintaining stability, and it is hydrophilic, so it is useful when isolation is a problem. VPES can also hold its properties throughout the working time, which is one minute, 20 seconds for all the viscosities. Like PE, it is easy to remove because of its optimized elastomeric properties, and like PVS, it has double-snap effect to balance the setting time, coming in at five minutes, 30 seconds.1

There are numerous choices for materials for dental impression materials-and all are better than beeswax. Understanding the chemistry of each type makes it easier to match the material to the case in the chair and make a good impression that leads to excellent patient outcomes.

References

1. Glenner RA. How it evolved: dental impressions. Chairside Magazine. May 17, 2014. glidewelldental.com/education/chairside-dental-magazine/volume-9-issue-1/how-it-evolved-dental-impressions/. Accessed March 5, 2020.

2. Rubel BS. Impression materials: a comparative review of impression materials most commonly used in restorative dentistry. Dent Clin North Am. 2007;51(3):629-642. doi: 10.1016/j.cden.2007.03.006.

3. Silicone. Britannica Encyclopedia.com website. britannica.com/science/silicone. Updated March 4, 2020. Accessed March 6, 2020.