Classification of Impression Materials

• Basic principles of impression materials:

  • Initially fluid
  • Flow around tissues, record details
  • Need support -> impression tray
  • Undergo setting
  • Physical/chemical process
  • Significant decrease in fluidity
  • Set material removed from mouth
  • Need set material to be elastic for undercuts
  • Need set material to not be too rigid to remove
  • Model construction
  • A model is only as accurate as the impression
  • How long does the impression remain dimensionally stable/accurate
• Classification - chemical nature - there are 5 chemical classes:

  1. Pastes - zinc oxide
  2. Gypsum - impression plaster
  3. Thermoplastics - compound, wax
  4. Hydrocolloids - alginate, agar
  5. Elastomers - silicone, polyether, polysulphide

  Simply knowing these does not help in choosing the correct material - BUT knowing how the different materials behave will.
• Classification - rheological properties:

  • Related to the unset material
  • Tells you how easily the material flows
  • Low viscosity will flow easily
  • High viscosity will now flow easily
  • Lower viscosity provides higher accuracy
• Classification - rheological properties:

  • Viscosity
  • Some materials come in a variety of viscosities
  • Putty, wash, heavy bodied, light bodied
  • Different viscosities can be combined - eg putty and wash
  • Effect on soft tissues
  • Some materials can displace the soft tissues during impression taking
  • Very viscous materials displace soft tissues - termed mucocompressive
  • Low viscosity materials do not displace soft tissue - mucostatic
• Impression trays:

  • How well the impression tray fits to oral anatomy affects viscosity
  • If there is not much space between the tray and the tissues
  • Less flow possible - effectively increases viscosity
  • The more space between tray and tissues
  • More flow is possible - effectively reduces viscosity
  • Impression trays may come as:
  • Stock trays - mass produced, standard sizes
  • Custom trays - bespoke, individual sizes
• Stock impression trays:

  • Commercially available in standard sizes
  • Made from polymers (disposable) or alloys (reusable)
  • Attachment of the impression material to the tray
  • Perforations (fluid materials flow through)
  • Tray adhesive (applied to tray before dispensing impression material)
• Special impression trays (individual):

  • Designed to have ideal spacing
  • Can be either:
  • Constructed on model eg acrylics
  • Made using putty/wash systems eg elastomers
  • More time consuming but better results
• Properties of impression trays - they must:

  • Be the correct size - viscosity effects
  • Provide support for the material
  • Sufficiently rigid to withstand pressure
  • Flexible materials distort under pressure
  • Bond to material
  • Some materials shrink on setting
  • Shrinking will pull away from the tray
  • Potentially lead to distortion
  • Bonding achieved by: perforations, tray adhesives, rim lock (a lip on the tray)
• Classification - mechanical properties:

  • Related to the set material
  • Two properties important:
  • Is the set material elastic
  • Is the set material rigid
• Classification - mechanical properties:

  • Elasticity
  • Removing an impression material causes distortion
  • Amount of distortion relates to oral tissues present
  • If there are undercuts:
  • Need an elastic material to spring-back to correct shape on removal
  • Plastic and viscoelastic materials will distort - inaccurate impression
• Classification - mechanical properties:

  • Rigidity (stiffness)
  • How easy it is to deform the impression material
  • Low stiffness is easier to deform - easier to get passed undercuts
  • Too stiff a material becomes difficult to remove
  • Impression material may fracture when removing
  • May damage or remove mobile standing teeth
• Classification - combined:

  • Compound, wax
  • Chemical nature = thermoplastics
  • Mechanical properties = rigid
  • Clinical applications = areas without undercuts
• Classification - combined:

  • Zinc oxide
  • Chemical nature = pastes
  • Mechanical properties = rigid
  • Clinical applications = areas without undercuts
• Classification - combined:

  • Impression plaster
  • Chemical nature = gypsum
  • Mechanical properties = rigid
  • Clinical applications = areas without undercuts
• Classification - combined:

  • Alginate, agar
  • Chemical nature = hydrocolloids
  • Mechanical properties = elastic
  • Clinical applications = can record undercuts
• Classification - combined:

  • Silicone, polyether, polysulphide
  • Chemical nature = elastomers
  • Mechanical properties = elastic
  • Clinical applications = can record undercuts