Introduction to impression materials Flashcards
what is a dental impression?
means of recording intra-oral structures
uses of casts produced by taking impression?
study casts
construction of dental appliances
use of stidy casts?
diagnosis
tx planning
Why use a cast to construct dental appliances?
orthodontic appliances and retainers
removable dentures
- complete
- partial
other appliances such as implants, mouthguards, bleaching trays etc
fixed prosthesis
- inlays
- onlays
- inlay/onlays
crowns
- partial
- full
bridges
implant retained prostheses
requirements of an impression tray?
rigid and non-flexible under load when taking impression
extend sufficiently to support the impression material in the region being reproduced
- if want a crown the full sulcus depth not paramount
fit loosely around the dental arch and not touch the soft tissues
have adequate means of retaining the impression material in the tray
have a robust (integral) handle - facilitate clinical manipulation of tray
be able to be adequately decontaminated (if not meant for single use)
types of stock trays?
taken for first impression
plastic or metal
what are plastic stock trays made from?
high impact styrene
2 types of stock trays?
metal - very rigid, no distortion of material
plastic
retention for material in metal impression tray?
metal rim lock tray
adequate extension?
no - not good sulcus extension
what is wrong about the tray here?
tray close to teeth - not allow sufficient bulk material in that area
overextended on the buccal sulcus and underextended n the lingual
Why do we need rigidity in impression tray?
stresses form within material
tray reverts to original shape when removed
distorted imp material
cast inaccurate
methods for material retention in tray?
1) tray design
- perforations
- rim lock (periphery where material can get locked in there)
2) tray adhesive
- types
- usage
ideally both
perforation as a retentive denture?
material extrudes through perforations so retaining the material in the tray mechanically
How to extend peripheries of imp tray?
1) periphery wax
- contains beeswax, paraffin and other soft waxes
2) greenstick
Why use peripheral extension of stock tray?
helps to contain the fluid imp material in the stock impression tray
what is greenstick used for?
extend peripherals of stick tray
is greenstick better than wax for tray extension?
better alternative to wax to contain the fluid impression material in the impression tray
rigid (non-elastic) impression material
chemical constituents of greenstick?
resins (such as shellac)
waxes (such as bees wax)
- used to control consistency
calcium carbonate
limestone
talc
- used as fillers, control consistency and rigidity and colouring agent
Is greensick a thermoplastic material?
yes, transition temp 55-60 dc
softened by exposure to hot water or naked flame
Where in the tray is greenstick most used?
post-dam region
what tray adhesive is used for alginate?
toluene alcohol
isopropyl alcohol is volatile, leaving the toluene behind
2 presentataions of tray-adhesives?
spray
- can get pooling but is good for contamination
pain-on
- cross contamination and crusty bits around neck of bottle, get poor seal and volatile substance can evaporate
why make special trays?
accuracy in impression
permits equal thickness of impression material
equal in all dimensions - minimal distortions of material
get proper extensions into sulcus
what are special trays made from?
polymethylmethacrylate - acrylic
shellac
appropriate space for alginate impression material for second imp?
3mm
appropriate space for C-silicone impression material for second imp
2mm
appropriate space for impression paste and impression plaster impression material for second imp
1mm
primary cast material?
plaster of paris
secondary cast material?
dental stone aka gypsum
refractory model?
material which can be heated up - it is chemically and physically stable at different temps
desirable properties of impression materials?
easy to handle
compatible with oral fluids
reproducible detail accuracy
have good tear resistance
have no adverse effects on pt
have pleasant taste
easily removable from the mouth especially from undercut area
be able to be adequately decontaminated
remain dimensionally stable after removal
compatible with all model construction
impression decontamination?
