In this month’s dental student guide, Hannah Hook explores the different types of impression materials and the pros and cons of each.
Impression material is encountered in various aspects of dentistry. From study models for diagnosis and treatment planning to prosthetics. From restorative to orthodontic and more.
Impressions allow a simple method of obtaining an exact copy of a patient’s teeth.
Here we will look at the different types of impression materials encountered in general practice along with their uses, advantages and disadvantages.
Impression materials
We can categorize impression materials into non-elastic or elastic. From this, we can further divide elastic materials into synthetic elastomers and hydrocolloids.
Table 1 shows the various types of impression materials available and their classification. We will cover those in italics later in this article.
| Non-elastic materials | Elastic materials | |
|
Hydrocolloids | Synthetic elastomers |
|
|
|
Table 1: Based on the classification of impression material by Wassell, Barker and Walls (2002)
Non-elastic materials
These materials are rigid when fixed and therefore exhibit very little elasticity.
Due to the stiffness of the materials, any significant deformation in the impressions results in permanent deformation.
1. Plaster
Plaster is an ideal mucostatic impression material. However, its use has now been superseded by light body silicones and low viscosity alginate.
- Clinical use: occasionally for mucostatic impressions for flaccid ridge
- Time setting: two to three minutes
- Advantages: hydrophilic, good detail reproduction, good dimensional stability
- Disadvantages: fragile, without recovery from deformation, excess saliva adversely affects the reproduction of details.
2. Impressive compound
There are two main types of impression compounds used in dentistry. Either the green is used as a green stick for contour casting and tray expansion or the red is used for full dentures.
Compound for complete denture impressions requires heating to soften the compound before use.
- Clinical use: primary impressions for complete dentures, contour casting/expanding trays
- Time setting: changed with water temperature, use colder water for shorter working time
- Advantages: can be reheated and the impression repeated/modified. Mucopressant, good for full impressions without a flabby ridge
- Disadvantages: mucocompressive (avoid use on flabby ridges), too stiff (avoid deep cuts), poor dimensional stability, poor surface detail reproduction.
3. Zinc-oxide eugenol
It consists of a base paste (zinc oxide, inert oils, resins) and a catalyst paste (eugenol, accelerator, filler).
- Clinical use: secondary impressions for complete dentures
- Time setting: four to five minutes (can shorten the time by increasing the temperature or humidity)
- Advantages: mucostatic, good surface detail, good dimensional stability
- Disadvantages: setting time is variable, too stiff (deep cuts may cause final impression to distort, split or break), eugenol allergy
Hydrocolloids
Agar and alginate are two hydrocolloid impression materials.
Agar is a reversible hydrocolloid. It has the ability to repeatedly switch between different viscosities simply by heating and cooling (Wassell, Barker and Walls, 2002). It was originally introduced in 1937, but is not commonly used today (Sears, 1970).
Alginate on the other hand is an irreversible hydrocolloid. Once set, it cannot return to its previous state.
They are both relatively hydrophilic and do not undergo much displacement from wet surfaces. However, their tendency to readily absorb water results in poor dimensional stability.
The accuracy of these impressions is based on rapid casting.
1. Alginate
Alginate is probably the most commonly used impression material in dentistry. It’s cheap and can impress with reasonable surface detail.
- Clinical use: primary impressions for dentures, opposing arch impressions for crowns/bridges, study models
- Working time: 1:45 minutes – 2:45 minutes (adjustable with water temperature)
- Time setting: 30 seconds +
- Advantages: easy flow, cheap, reproduction detail adequate, fast set time, minimal drift
- Disadvantages: poor dimensional stability, poor tear strength, warping if not supported, requires good mixing otherwise air bubbles, minimum thickness of 3mm required.
Synthetic elastomers
They can undergo deformation, but quickly recover their original dimensions when the pressure is released.
There are three distinct groups of synthetic elastomers. These include: polysulfides, polyethers and silicones (addition and condensation). They come in two parts, which are mixed before taking the impression: a base and a catalyst.
1. Polysulfides
Supplied as base paste and catalyst. With the release of newer impression materials, polysulfides are becoming increasingly unpopular due to their list of disadvantages.
The reaction is a condensation polymerization, which releases H2THE.
- Clinical use: full denture, long working time required
- Working time: five minutes
- Time setting: 10 minutes +
- Advantages: high tear resistance, good accuracy, good dimensional stability, more flexible elastomer, different viscosities, long working time, long service life
- Disadvantages: unpleasant taste and smell, long setting time, messy use, distortion during setting, which is aggravated by delay in casting.
2. Polyethers
These are the most hydrophilic of the synthetic elastomer impression materials. Therefore we often use them when humidity control is difficult.
However, because of this they tend to swell in high humidity conditions. Therefore, they need dry storage (Wassell, Barker and Walls, 2002).
The reaction is addition-curing polymerization, which has no by-product resulting in a material with very good dimensional stability.
- Clinical use: final impressions for crowns, bridges, partial dentures and implants
- Working time: 2:30 minutes
- Time setting: five minutes
- Advantages: dimensionally stable, good accuracy, single-phase imprinting, adequate tear resistance
- Disadvantages: stiff when set (avoid deep undercuts and bridges), water absorbent (do not store with alginate impressions), low tear strength, short working time.
3. Adding silicone
This material is platinum catalyzed and undergoes an addition polymerization reaction that has no byproducts.
The lack of by-product increases the dimensional stability and accuracy of the material, allowing impressions to remain unchanged for a long time.
It is the most widely used impression material in dentistry. Also called PVS or VPS. He can use it as a one or two stage impression.
- Clinical use: crowns, bridges, partial dentures and implants
- Working time: 2:15 minutes
- Time setting: five minutes
- Advantages: good detail reproduction, excellent dimensional stability (no shrinkage during setting), fast setting, high patient acceptance, adequate tear resistance, better dimensional stability than condensation silicone
- Disadvantages: hydrophobic, dry teeth to avoid wind blows. Poor tear resistance, expensive, platinum catalyst reacts with powder gloves/new composites/GIC and rubber barrier, temperature sensitive.
4. Condensing silicone
During the coagulation of this material a condensation reaction occurs, hence its name. This reaction results in the production of ethyl alcohol. Loss of this byproduct results in impression shrinkage.
Silicones are very hydrophobic and therefore require drying of the teeth or bumps will form on the impressions.
- Clinical use: crowns, bridges, partial dentures and implants
- Working time: 2:30 minutes
- Time setting: nine minutes
- Advantages: accurate, high patient acceptance, easy to use, can be used in patients with deep incisions
- Disadvantages: hydrophobic, unreliable dimensional stability, severe shrinkage during coagulation with by-product release.
Summary of impression materials
- Impression materials are broadly categorized into ‘elastic’ and ‘non-elastic’
- There are various advantages and disadvantages for each of the above mentioned materials. Keep these in mind when choosing which material to use for each case.
Glossary of terms
- Hydrophilic: affinity with water
- Hydrophobic: adverse interaction with water
- Tear strength: the amount of resistance a material has to tearing
- Working time: the time from mixing until the elastic properties of the material develop. It is usually measured at room temperature
- Time setting: the time from mixing the material until it completely sets. It is usually measured at the temperature of the mouth
- Dimensional stability: the degree to which a material retains its original dimensions when subjected to changes in temperature and humidity. Resistance to deformation under various storage conditions1
- Polymerization: a chemical reaction that joins small molecules (monomers) into large chain structures (polymers)
- Viscosity: resistance to flow or “thickness”
- Elasticity: the ability of a material to regain its normal shape after being stretched or ‘stretched’
- Mucostatic: material does not displace the mucosa. Good for producing an impression of tissues while not under load, with the tissue in a non-functional state. The most common use is with flabby ridges
- Mucopressor: material causes mucosal displacement, allowing recording of oral tissue in a functional state.
bibliographical references
Martins F, Branco P, Reis J, Navarro I and Mauricio P (2017) Dimensional stability of two impression materials after a 6-month storage period. Acta Biomater Odontol Scand 3: 84-91
Sears AW (1970) Hydrocolloid impression technique for inlays and fixed bridges. Sci Educ Bull 3: 55-62
Wassell RW, Barker D and Walls AWG (2002) Crowns and other extracoronal restorations: Materials and impression technique. Br Dent J 192: 679-90
Check out our previous student guide
Follow Dentistry.co.uk on Instagram to keep up with all the latest dental news and trends.
