Impression Materials
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what is the function of an impression material?
to produce an accurate replica of the surface and shape of hard and soft oral tissues
what kind of reproduction does an impression give of the soft tissue?
negative
what is dental stone used for?
to fill the impression and produce a positive replica (stone cast) which represents the position, shape, size and orientation of each tooth
what are casts used for?
evaluating dentition when orthodontic, occlusal or other problems involved, in laboratory fabrication of restorations and prostheses
what are the classifications of impression materials?
mucostatic, mucocompressive and elastic/non-elastic
what are mucostatic materials?
fluid materials that displaces the soft tissues slightly and give impression of the undisplaced mucosa
what are mucocompressive materials?
viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissues
what is the ideal elastic behaviour for an impression material?
once the strain is taken off of the material i.e. removing impression material from the tooth, it should go back to the way it was when it covered the tooth making an exact impression
what term is used for the elastic behaviour of impression materials?
viscoelastic
what does the term viscoelastic behaviour mean?
when the impression material is removed from the tooth it never fully recovers and so it has permanent strain
what helps to decrease permanent strain?
if the load time with the impression material on the tooth is less
name a hydrocolloid
alginate
name 3 elastomers
polysulphides, polyethers, silicones
how do you prepare impression materials
mixing 2 pastes either a paste and liquid or powder and liquid which produces a chemical reaction
what are the ways in which an impression material sets?
chemical, polymerisation, cooling
what are the ideal properties for accuracy for an impression material?
surface detail recorded, no dimensional change, no effect on oral tissues, no change dimensionally, complete elastic recovery, does not change dimensionally over time, dimensions and surfaces unaltered, compatible with cast material
what type of factors affect accuracy?
viscosity, setting mechanism, thermal expansion coefficient, hydrophobic, elasticity, tear strength
what should the thermal expansion coefficient be?
low
what does viscosity determine?
the ability to flow over the surface and so the level of detail recorded
what are the ideal properties for patient comfort?
non-toxic, non-irritant, acceptable taste and smell, short setting time, removable without damage to oral tissues
what are the ideal properties for operator convenience?
quick, simple technique, convenient working/setting times, must be able to be decontaminated, cost is a consideration
what are the 2 main types of elastic impression materials?
hydrocolloids and elastomers
what are the 2 types of non-elastic impression materials?
impression compound, impression paste
what is a hydrocolloid?
a colloid is a two phase system of fine particles of one phase dispersed in another phase - the dispersing medium is water