impression materials Flashcards
what is the function of an IM?
to produce an accurate replica of the surface and shape of hard and soft tissues
what is dental stone used for?
to fill the impression and produce a positive replica which represents the position, shape, size and orientation of each tooth
what is a dental cast used for?
evaluating dentition when orthodontic, occlusal or other problems involved, in lab fabrication of rest. prostheses etc.
what can IM capture?
single tooth
whole dentition
edentulous mouth
how is a good quality/accurate impression taken?
IM with right properties
clinician with good technique
what is mucostatic?
fluid materials that displace the soft tissues slightly ie. give an impression of the undisplaced mucosa
eg. zinc oxide eugenol, low viscosity alginates)
what is mucocompressive?
viscous materials that record an impression of the mucosa under load ie. give impression of displaced soft tissues
eg impression compound, high viscosity alginates
what is elastic?
material set=solid by using force removed from mouth-bulges elastic strain during removal IM temporarily changed shape after removal elastic removal final form matches closely but not 100%
what is viscoelastic?
not fully elastic
-some deformity
how should you remove an impression tray?
short, sharp pulling force
load time less
=less permanent strain
give 2 examples of hydrocolloids
agar- no longer used
alginate
-irreversible
give 3 examples of elastomers
polysulphides
polyethers
silicones
describe prep of IM
mixing chemical reaction heat material flows -setting reaction -as IM flows into tray it begins setting reaction, when inserted it surrounds hard/soft tissues and setting continues
how is accuracy achieved?
flow -surface detail recorded setting changes -no dimensional change removal -no effect on oral tissues -no dimensional change -complete elastic recovery storage -no dimensional change over time eg moisture decontamination -dimensions/surfaces unaltered compatible w/ cast material
which properties affect accuracy?
viscosity -determines flow over surface so detail setting mechanism -cause dimensional contraction thermal expansion coefficient -low -mouth to room temp hydrophobic/hydrophilic -surface contact elasticity tear strength -withstand tear on removal
what are the ideal properties for px comfort?
non-toxic, non-irritant
acceptable taste and smell
short setting time
removal w/o 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
what are the (2) non-elastic IM
impression compound
impression paste
what is a hydrocolloid?
colloid= 2 phase system of fine particles of 1 phase dispersed in another phase
dispersing medium=water
what are the (2) states of hydrocolloid?
sol -viscous liquid state gel -jelly like consistency -agglomeration of particles -entangled framework of solid particles enclosing liquid by capillary forces
what are the components of alginate?
salt of alginic acid calcium sulphate trisodium phosphate filler modifiers, flavours, chemical indicators
what is the purpose of trisodium phosphate?
preferentially reacts with ca ions released by ca sulphate
only when all used ca ions released reacts with na alginate
delaying formation of alginate gel
used to adjust working time
what does the ca alginate formed consist of?
long cross-linked molecules surrounding undissolved particles
describe how to use alginate correctly
correct powder/liquid ratio
water 18-24 c
perforated tray and adhesive
remove with sharp pull
why should you wait for a further minute after apparent set?
crosslinking continues after apparent set
describe alginates px comfort factors
non-toxic
non-irritant
acceptable taste, smell
setting time ok
describe alginates operator conventience
easy to use setting time ok storage -avoid moisture -prepare casts asap to avoid dimensional change
what is syneresis?
release of water
what is imbition?
uptake of water
describe alginates accuracy factors
flow- fine detail ok setting changes ok nearly elastic poor tear strength storage -syneresis/imbition may occur
