Dental Materials - impression, trays, models Flashcards
Define impression
Negative imprint from which a positive reproduction or model can be made
Impression materials can be classified by viscosity. describe
High viscosity - impression compound
Medium viscosity = Medium bodied elastomers
Low viscosity = hydrocolloids (alginate), light bodied elastomers
Impression can be classified by elasticity. Describe
Non-elastic (rigid) - compound, zinc oxide eugenol
Elastic = elastomers or hydrocolloids Elastomers = polyether, silicones Hydrocolloids = aliginate
Differences in uses of elastic vs non-elastic impression materials?
Non-elastic cannot cover undercuts
Impression materials used for complete dentures
a) primary impression
b) major impression
a) compound
b) zinc oxide eugenol
What material should be used for major impressions if patient is allergic to zinc oxide eugenol
Silicones
How should an impression be taken of a patient with flabby ridge?
Tray is modified to have a window in area of flabby ridge. Impression taken in areas without flabby ridge
Whilst tray is still in mouth - plater is applied to flabby area by brush
Ideal impression material requirements (14)
Biocompatible
Sufficiently fluid on insertion, good wettability
Flexible
Pleasant taste, smell and aesthetic colour
Dimensionally stable
Capable of compressing soft tissues to desired degree without distortion
Set or harden at or near mouth temperature
Adequate WT and ST
Capable of additions
Simple to use
Compatible with model materials
Disinfected without loss of accuracy
Elastic properties
Tear resistant
What is wettability?
Preference of a liquid to be in contact with a solid surrounded by another fluid
Advantages of non-elastic impression compound
Muco-compressive
Can be used to make a special tray which helps make an accurate major impression
Easily trimmed, added to and re-adapted
Well-extended
Disadvantages of impression compoun
Cannot record undercuts Technique sensitive High viscosity = surface detail not accurate if not softened enough Needs water bath Shrinkage (poor dimensional stability)
Advantages of zinc oxide eugenol
Accurate detail Dimensionally stable Compatible with wet mouth (wettability) Can be added to Muco-static (but not muco-compressive)
Disadvantages of zinc oxide eugenol?
Cannot be used on undercuts Only sets rapidly in a thin layer Doesn't accurately record the periphery Allergies to eugenol Adhere to skins
Advantages of alginate?
Cheap Easy to use Non-toxic and non-irritant Can be used in stock/special trays Records fine detail and undercuts
Disadvantages of alignate?
Dimensionally unstable
Surface reproducibility not as good as elastomers
Poor tear resistance and strength
Main causes of impression inaccuracy
Polymerization shrinkage
Loss of condensation reaction by product (water)
Thermal contraction (oral-room)
Absorption of water/disinfectant over time
Taken out before fully setting
Inappropriate mixing
Technique
What type of special tray should be used for zinc oxide eugenol paste?
Close fitting tray
What type of special tray is used for elastomeric impression materials?
Spaced special tray
Why should special trays for alginate be perforated?
For excess to be pushed out and for retention
Why should special trays for elastomers not be perforated?
It adheres well to the tray
Intra-oral vs extra-oral special tray handles
Intra-oral for close fitting trays
Extra-oral for spaced
What material is used to construct special trays?
Light-cure acrylic resin - easily adaptable and uniform in thickness
