A dental student’s guide to…impression materials

impression materialsIn this month’s dental student’s guide, Hannah Hook explores the different types of impression materials and the pros and cons to each.

Impression materials are encountered throughout various aspects of dentistry. From study models for diagnosis and treatment planning to prosthodontics. From restorative to orthodontics and much more.

Impressions allow a straightforward method of obtaining an accurate replica of a patient’s teeth.

Here we will look at the various types of impression materials encountered in general practice along with their uses, advantages and disadvantages.

Impression materials

We can categorise impression materials as non-elastic or elastic. Of that, we can divide the elastic materials further into synthetic elastomers and hydrocolloids.

Table 1 shows the various types of impression materials available and their classification. The ones in italics we will cover later in this article.

Non-elastic materials Elastic materials
  • Plaster
  • Impression compound
  • Zinc-oxide eugenol.
Hydrocolloids Synthetic elastomers
  • Agar – reversible
  • Alginate – irreversible.
  • Polysulphide
  • Polyether
  • Addition silicone
  • Condensation silicone.

Table 1: Based on the classification of impression material by Wassell, Barker and Walls (2002)

Non-elastic materials

These materials are rigid when set and therefore exhibit very little elasticity.

Due to the rigidity of the materials any significant deformation to the impressions results in permanent deformation.

1. Plaster

Plaster is an ideal mucostatic impression material; however, its use has now been superseded by light bodied silicones and low viscosity alginate.

  • Clinical use: occasionally for mucostatic impressions for flabby ridge
  • Setting time: two to three minutes
  • Advantages: hydrophilic, good reproduction of details, good dimensional stability
  • Disadvantages: brittle, no recovery from deformation, excess saliva negatively effects detail replication.

2. Impression compound

There are two main types of impression compound used in dentistry. Either green used as greenstick for border moulding and tray extension or red used for complete dentures.

Compound for complete denture impressions requires heating to soften the compound before use.

  • Clinical use: primary impressions for complete dentures, border moulding/extension of trays
  • Setting time: altered with temperature of water, use cooler water for a shorter working time
  • Advantages: can reheat and impression retaken/modified. Mucocompressive, good for full impressions with no flabby ridge
  • Disadvantages: mucocompressive (avoid use on flabby ridges), very rigid (avoid deep undercuts), poor dimensional stability, poor reproduction of surface detail.

3. Zinc-oxide eugenol

Consists of a base paste (zinc oxide, inert oils, resins) and a catalyst paste (eugenol, accelerator, filler).

  • Clinical use: secondary impressions for complete dentures
  • Setting time: four to five minutes (can shorten time with an increase in temperature or humidity)
  • Advantages: mucostatic, good surface detail, good dimensional stability
  • Disadvantages: setting time is variable, very rigid (deep undercuts can cause distortion of the final impression, separation or breakage), eugenol allergy

Hydrocolloids

Agar and alginate are two hydrocolloid impression materials.

Agar is a reversible hydrocolloid. It has the ability to repeatedly pass between different viscosities simply by heating and cooling (Wassell, Barker and Walls, 2002). It was initially introduced in 1937, however it is not commonly used today (Sears, 1970).

Alginate on the other hand is an irreversible hydrocolloid. Once it is set, it cannot return to its former state.

They are both relatively hydrophilic and do not undergo major displacement from wet surfaces. However, their tendency to readily absorb water results in poor dimensional stability.

The accuracy of these impressions relies on casting quickly.

1. Alginate

Alginate is probably the most commonly used impression material in dentistry. It is inexpensive and can produce impressions 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 temperature of the water)
  • Setting time: 30 seconds +
  • Advantages: easy flow, cheap, reproduction detail adequate, fast setting time, minimal displacement
  • Disadvantages: poor dimensional stability, poor tear strength, distortion if unsupported, requires good mixing otherwise air bubbles, minimum 3mm thickness required.

Synthetic elastomers

These are a group of synthetic polymer-based impression materials, which chemically cross-link when setting.

They can undergo deformation, yet rapidly recover to their original dimensions when the stress is released.

There are three distinct groups of synthetic elastomers. These include: polysulphides, polyethers and silicones (addition and condensation). They come in the form of two parts, which are mixed before taking the impression: a base and a catalyst.

1. Polysulphides

Supplied as a base and catalyst paste. With the release of newer impression materials, polysulphides have become increasingly unpopular due to their list of disadvantages.

Reaction is a condensation polymerisation, which releases H2O.

  • Clinical use: complete dentures, long working time required
  • Working time: five minutes
  • Setting time: 10 minutes +
  • Advantages: high tear resistance, good accuracy, good dimensional stability, most flexible elastomer, different viscosities, long working time, long shelf life
  • Disadvantages: unpleasant taste and smell, long setting time, messy to use, distortion on setting, which worsens with delay in casting.

2. Polyethers

These are the most hydrophilic of the synthetic elastomer impression materials. Therefore we often use them when moisture control is difficult.

However, because of this they possess the tendency to swell in conditions of high humidity. Therefore they need storing dry (Wassell, Barker and Walls, 2002).

Reaction is addition cured polymerisation, 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
  • Setting time: five minutes
  • Advantages: dimensionally stable, good accuracy, monophase impression, adequate tear resistance
  • Disadvantages: rigid when set (avoid deep undercuts and bridges), absorbs water (don’t store with alginate impressions), low tear strength, short working time.

3. Addition silicone

This material is platinum catalysed and undergoes an addition polymerisation reaction that has no by-products.

The lack of by-product increases the dimensional stability and accuracy of the material enabling the impressions to remain unchanged for a long period of time.

It is the most widely used impression material in dentistry. Also called PVS or VPS. Can use it as a one or two-stage impression.

  • Clinical use: crowns, bridges, partial dentures and implants
  • Working time: 2:15 minutes
  • Setting time: five minutes
  • Advantages: good detail reproduction, excellent dimensional stability (no shrinkage on setting), fast setting, high patient acceptance, adequate tear resistance, better dimensional stability than condensation silicone
  • Disadvantages: hydrophobic, dry teeth to avoid air blows. Poor tear resistance, expensive, platinum catalyst reacts with powdered gloves/new composites/GIC and rubber dam, temperature sensitive.

4. Condensation silicone

During the setting of this material a condensation reaction occurs, hence its name. This reaction results in the production of ethyl alcohol. The loss of this by-product leads to shrinkage of the impression.

Silicones are very hydrophobic and therefore require drying of the teeth otherwise blows will form in the impressions.

  • Clinical use: crowns, bridges, partial dentures and implants
  • Working time: 2:30 minutes
  • Setting time: nine minutes
  • Advantages: accurate, high patient acceptance, easy to use, can use on patients with deep undercuts
  • Disadvantages: hydrophobic, unreliable dimensional stability, marked shrinkage on setting with release of by-product.

Summary of impression materials

  • Impression materials are broadly categorised into ‘elastic’ and ‘non-elastic’
  • There are various advantages and disadvantages for each of the aforementioned materials. Take these into consideration when choosing which material to use for each case.

Glossary of terms

  • Hydrophilic: affinity towards water
  • Hydrophobic: unfavourable interaction with water
  • Tear strength: the amount of resistance a material has to tearing
  • Working time: the amount of time from mixing until the elastic properties of the material have developed. Usually measured at room temperature
  • Setting time: the amount of time from mixing the material until it is fully set. Usually measured at the temperature of the mouth
  • Dimensional stability: the degree to which a material maintains its original dimensions when subjected to changes in temperature and humidity. Resistance to distortion under various storage conditions1
  • Polymerisation: a chemical reaction joining small molecules (monomers) into large chainlike structures (polymers)
  • Viscosity: resistance to flow or ‘thickness’
  • Elasticity: the ability of a material to resume its normal shape after stretching or ‘stretchiness’
  • Mucostatic: material does not displace the mucosa. Good for producing an impression of the tissues whilst not under load, with the tissue in a non-functional state. Most common use is with flabby ridges
  • Mucocompressive: material causes displacement of the mucosa, allowing the recording of the oral tissue in a functional state.

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 extra-coronal restorations: Impression materials and technique. Br Dent J 192: 679-90


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