Impression Materials
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what are impression materials used for
produce a replica of the patients hard and soft tissues, this is a negative impression, poured into with gypsum to produce a positive replica - used for designing dentures and orthodontic appliances
what does the treatment using a cast hinge on
accuracy - of impression material, quality of impression - by operator
how can impression materials be classified
clinically - mucostatic or mucocompressive properties - elastic or not
what are the ideal properties of an impression material
viscoelastic, low thermal expansion, storage
why is viscoelasticity important
can flow around undercuts etc, into all areas, but must also be able to be removed and then once removed bounce back to the shape it was in the mouth once set
describe the ideal viscoelasticity
when a load is applied, it is applied immediately, as the load remains, the strain is continuous, once the load is removed, the material goes back to the dimensions it was at the start with and the strain is removed
what actually happens with the viscoelasticity
the load is applied over time, meaning the strain gradually increases, the longer the load is applied, the worse the strain, as the load is removed, the strain doesnt go back perfectly, some permanent deformity
how can the permanent deformity be reduced when taking an impression
if the load is applied quickly, less strain is induced. also the less time the load is held for the less permanent deformity seen. have to do it quickly and take it off quickly
what properties affect accuracy
elasticity, viscosity, thermal expansion, tear strength
what is tear strength and what should it be
the strength it can withstand without distortion, want this to be high so it can be removed from undercuts without tearing the material
what is alginate
a hydrocolloid
what is the setting reaction of alginate
calcium sulphate with sodium alginate - forms calcium alginate as calcium induces cross linking in the alginate
what particles are involved in the setting of alginate
calcium sulphate, sodium alginate, tri-sodium phosphate
what is the role of tri sodium phosphate
calcium preferrentially bonds to this over sodium alginate, prevents calcium cross linking immediately so the operator can place the material before set
what are the properties of alginate
good viscosity, elasticity is okay, poor tear strength - shouldnt be used in areas with large under cuts
why should alginate be stored with damp gauze
prevents it taking on moisture or releasing moisture and drying out taking on moisture - syneresis, releasing - imbibition
give examples of elastomers
addition silicones, polyether
what are some properties of elastomers
good flexability not rigid, low viscosity, surface reproduction and wetability, high tear strength, good elasticity
what are ISO standards
the standards required to be met for a safe material, doesnt compare the best on the market just safe enough to be used
give an example of an ISO standard
surface reproduction of an elastomer, has to be able to flow into small areas, gap size is dependant on the viscosity of the material - low viscosity - 20um, high viscosity - 50um
what are the 3 different requirements for ideal properties of an elastomer
able to flow over the surface and into gap obtain accurate detail removal from undercuts
what properties are important for the material to flow over hard and soft tissues
viscosity - cant be too thick that it doesnt flow into small gaps, wetability - have intimate contact with surface
what properties are important for the material to obtain accurate detail
elasticity - want it to be a low number - high suggests large strain for a fixed load and thus permanent deformation, surface reproduction - dependant on viscosity
what properties are important for the material to be removed from undercuts
flexability - want a low YM, not rigid tear strength - want this to be high, not fractured with high stress on removal