Desirable Properties of Impression Materials

• Accuracy of an impression is related to both dimensions and detail compared to original. It also depends on the application (eg denture or crown).
• Dimensional stability:

  • Change in accuracy with time
  • Is it related to how soon a model can be poured
• Convenience/ease of use:

  • Mixing, technique sensitivity, working time, setting time
  • These factors are more important to the dental profession than the patient
• Accuracy is a multi-factorial property related to:

  • Viscosity of the impression material
  • Wetting of the impression material on the oral tissues
  • Setting reaction - is there any dimensional change
  • Thermal conduction - temperature changes for mouth to room temperature
  • Adhesion to tray - is there distortion during setting and/or removal of the impression
  • Are undercuts present, and if so, are any elastic materials needed
• The impression is only the first step:

  • The accuracy of the impression will affect the accuracy of the cast
  • Any errors in the impression will translate through to the final device
• Accuracy - viscosity:

  • The lower the viscosity the finer the detail
  • eg heavy body shows less detail than light body
  • In areas where fine detail is needed - low viscosity
  • Note: fine detail not needed everywhere
  • Note: fine detail not needed for all impressions
• Accuracy - wetting:

  • Oral tissues are naturally moist
  • For hydrophilic materials this is not a problem
  • Some impression materials are hydrophobic
  • Water tends to form droplets on the surface
  • This can lead to bubbles (or blows) appearing in or on the surface
  • For hydrophobic materials you need a dry field
• Accuracy - effect of setting; setting can result in a dimensional change:

  • Polymerisation (shrinkage)
  • If impression bonded to tray - shrinkage away from too
  • Potentially leads to an oversized die
  • Crystal growth (expansion)
  • Potentially leads to an undersized die
  • Remember the setting reaction can continue after the setting time!
• Accuracy - effect of setting; temperature changes during setting:

  • Thermal contraction
  • Mouth temperature (around 32°C) to room temperature (around 23°C)
  • Higher the coefficient of thermal expansion the more contraction there is
  • Thermoplastic warming/cooling
  • Impression compound is softened at around 55°C prior to use
  • During impression taking temperature drops from around 55°C to 32°C to 23°C
  • Warping and distorting is very common
• Accuracy - undercuts:

  • Removal leads to a lot of distortion
  • Need an elastic material to recover all the distortion
• Factors affecting accuracy with undercuts:

  • The depth of the undercut - deeper means more distortion on removal
  • Proper setting of the material
  • Allow to become fully elastic - not viscoelastic
  • Removal technique - important
  • Allow sufficient time for recovery
  • Thickness of the impression material
  • Tearing can occur in thin sections
  • Leads to impression faults
  • Difficult to spot - bright colour used
• "Accuracy" is a complex factor - each stage in the device construction is important:

  • Accuracy of impression material
  • Accuracy of tray
  • Accuracy of model
  • Accuracy of wax pattern
  • Accuracy of investment mould
  • Acciracy of alloy casting or polymeric structure

  Any errors will be transferred through all subsequent stages.
• Dimensional stability - the amount that accuracy is lost over time:

  • Most dental surgeries send impressions to labs
  • Delay of at least a few hours before model poured
  • Often overnight or a few days' delay!
• Dimensional stability - the amount that accuracy is lost over time - changes can be due to:

  • Continued setting
  • Setting time does not mean 100% setting complete
  • Polymerisation or crystallisation may continue
  • Shrinkage or expansion can occur
  • Loss of volatiles
  • Materials may contain water or ethanol
  • May evaporate over time - leads to volume change
  • Stress relief - very common with impression compound
• Method of use - convenience - method of mixing components:

  • May come as: powder/liquid, 2-paste, paste/liquid
  • Can be mixed by: hand mix, cartridge, auto-mix
  • All can result in waste: cartridges and auto-mix have more waste
• Method of use - convenience:

  • Ease of mixing and handling
  • Much easier to mix low viscosity materials
  • High viscosity typically hand-mixed
  • Some materials are tacky during mixing
  • Type of tray needed - and whether tray adhesives are needed
• Setting characteristics:

  • Working time (normally measured at 23°C)
  • Change in rheological properties
  • Last time to insert in mouth
  • Setting time (normally 32-37°C)
  • May be defined by manufacturer as time to
  • Achieve sufficient stiffness
  • OR
  • Achieve sufficient elasticity
  • Safe to remove
• Setting characteristics:

  • Ideal behaviour
  • Like the black curve
  • Long period of constant viscosity during mixing - long working time
  • Rapid increase in viscosity at the end - short setting time
  • Not may materials behave like this