impression materials Flashcards
impression
Negative imprint of hard and soft tissues in the mouth from which positive reproduction can be formed
tray
container designs to fit over the dental arches
impression materials
- designed to be liquid or semi solid when first mixed
- quickly set to solid to form an imprint in the mouth
things to consider patient perspective
- neutral taste and odour
- short setting time
- small tray
- easy to remove
- no retakes
- non toxic
things to consider dentist
- easily mixed
- short working times
- good quality impressions
- low cost
- easily disinfected
what makes a good impression
1) accurate reproduction of surface detail
- viscosity
- wettability
2) Dimensional accuracy and stability
- shrinkage on setting
- cooling contraction
- permanent set
- storage stability
- type of tray
factors involved in choosing application and properties
impression material
impression trays (metal/plastic)
areas of application
(full denture, partial dentures, fixed prosthodontics)
what can be used for full dentures
plaster of paris
ZnO- eug compo
Zno- eug
partial denture mtierals
alginate elastomers
fixed prosthodontics impressiojmaterials
comp/wax elastomers
types of impression materials
rigid
flexibale
rigid materials
- waxes
- impression composition
- Zinc oxide/Eugenol
- Plaster of Paris
flexible materials
- Hydrocolloids (agar and alginate)
- Elastomers (polysulphide, polyether, silicone polymers)
oligonmer
long chain molecule with simple structure
polymer
monomer repitiion
sources of wax
mineral (paraffin wax)
animal (beeswax)
vegetable (carnauba and candelilla wax)
properties of dental waxes
1) softening temperature
- just above mouth temperature
2) high coefficient of thermal expansion
- cooling contraction
3) Rigid
4) Poor thermal conductor
- surface cools faster than inside
5) Low viscosity
what affects dimensional accuracy
Shrinkage - bigger model - slightly bigger is good as there is more space for the cement Expansion - smaller model is bad
thermal expansion coefficient
- the change in length, when determined per unit length, for a 1 degree change in temperature
dental waxes thermal expansion coefficient is
high TEC
expand a lot
not good as impression materials
use of dental waxes
1) Modelling waxes
2) Inlay waxes
3) Sheet casting wax
4) Sticky wax
5) Carding and boxing wax
modelling waxes
registration blocks and try in dentures
inlay waxes
inlay patterns in the mout
sheet casting wax
construction of partial dentyres
sticky wax
joining metal parts prior to soldering
joining dentures prior to repair
carding and boxing wax
boxing in impressions
mounting teeth in sets
composition complex
1) Natural or synthetic resins
- Shellac, dammar, Colophony, sandarac
2) Plasticisers
- Stearic acid or gutta percha (avoid brittleness)
3) Fillers
- Talc, calcium carbonate or limestones (helps avoid tackiness)
properties of impression composition
1) Softening temperature 55-60 degreesC
2) Rigid
3) High co efficient of thermal expansion
4) High viscosity
- mucocompressive
5) Poor thermal conductivity
- subject to stress relief
applications of compound
1) full or partial impressions
2) sticks for copper band impressions
3) for, adding to the periphery of an impression tray
mucocompression
- material is viscous and is able to compress the oral tissues on insertion in the patients mouth
mucostatic
Impression material is fluid enough to flow and does not displace oral tissues
- impression plaster
- Agar impression material
- ZnEug impression pastes
- Light body elastomers
mucocompression materials
Impression compound
Viscous alginate
Some rigid elastomers
Rheology
study of flow materials
how is liquid flow measured
viscosity s
how to calculate viscosity
sheer stress/shear rate
unit Pa.s
types of viscous behabiour
newtonian
dilatant
pseudoplastic
newtonian
constant viscosity across all shear rates
dilatent
- shear thickening fluids increase in apparent viscosity at higher shear rates (i.e. apply a force and it gets harder)
- rarely encountered
pseudoplastic
- shear thinning fluids have a lower apparent viscosity at higher shear rates eg silicone impression materials (apply force and it gets easier)
what is thixotropic behavior
time dependant shear thinning property
thixotropic behaviour meaning
Progressive decrease in viscosity with time for a constant applied shear stress, followed by a gradual recovery when the stress is removed
- fluids that are viscous under static conditions will flow over time when they are stressed
- take a fixed time to return to a more viscous state
what pastes are found in ZnO Eu and what makes them up
base paste - zinc oxide inert ols hydrogenated resins reactor paste - eugenol zinc acetate fillers
advantages of ZnO
dimensional stability
good surface detail
stable on storage and good shelf lige
disadvantages of ZnO
cannot be used in very deep undercuts
only sets quick in thin sections
eugenol allergy in some patients
properties of ZnO eug
1) Mucostatic
2) hydrophilic
- accurate reproduction of surface details
3) dimensionally stable
4) causes burning sensation of lips
5) tendacy to stick to skin
6) stable on storagage/shelf life
wetabiltiy
ability of a liquid to cover the surface of the substrate completely
smaller contact angle (wetaqbility)
more hydrophilic
why do we want good wetting
- fewer voids
- less entrapment of oral fluids
- bubble free dyes and models
- fewer retakes
surface tension what is higher
solids have greater than liquids
plaster of Paris (gypsum) composition
crystalline material of hydrates calcium sulphase
gypsum
- form as the partly dehydrated form of plaster of paris
composition of impression plaster
- calcium sulphate B- hemihydrate
- borax (slow setting rate)
- potassium sulphate (reduces expansion/accelerates setting)
- starch (aid disintegration of impression on separation from model)
- liquid (water)
what happens upon setting of plaster
expands
- results in stress
How to prevent expansion of plaster
mix with anti expansion solution (potassium sulphate 4% an borax 0.4%)
borax is a retarder
potassium sulphate accelerates
advantages of impression plaster
- easy to mix
- low viscosity (mucostatic)
- good dimensional stability and accuracy
- cheap
Disadvantages
disadvantages
- low strength
- rough surface finish
- rigid once set
- dry sensation in mouth
applications for impression plaster
- useful for making impressions of patients with excessively mobile soft tissue
- commonly used as the model material set in impression moulds
