impression materials Flashcards
what is an impression
negative reproduction of tissues
what are the 3 necessary functions of impression materials
produce an accurate replica of the surface and shape of
hard and soft oral tissues
negative reproduction
only useful if accurate
- material property and
technique sensitive
what is the job dental stone
used to fill the impression and produce a positive replica (stone cast)
which is used :-
- for evaluating dentition when orthodontic, occlusal or other problems involved
- in laboratory fabrication of restorations and prostheses
dental stone aka
gypsum
impression materials can take an impression of….
single tooth
whole dentition
edentulous mouth
what factor does the tx outcome of impression hinge on
quality & accuracy of the initial impression
what is required to achieve a good quality and accurate initial impression
IM with right properties
- Alginate can be good enough but not the best
clinician with good technique
ways to classify IM
clinical
- mucostatic
- mucocompressive
properties
- elastic
- non-elastic
mucostatic
fluid materials that displace the soft tissues slightly
i.e. give an impression of the undisplaced mucosa.
zinc oxide eugenol, low viscosity alginates)
mucoccompressive
viscous materials that record an impression of the mucosa under load
ie give impression of displaced soft tissue.
eg impression compound, high viscosity alginates/elastomers
ideal elastic behaviour
recover to original shape after flaring over bulbosity of tooth
viscoelastic behaviour
closer to reality, none truly elastic, lower the viscoelasticity,
gradually stretches and gradually recovers,
- doesn’t recover to original dimension – deformed, start with imperfect representation
It’s advantageous to wait for a time (Tf –TL ) after removing the tray before you pour the cast – so as to
- minimise permanent strain (deformation)
how is applying the load quickly beneficial when taking impressions
minimises permanent strain
maximum amount of strain experienced is less than first time round, recovers to close to 0 strain (pull quickly)
If LOAD time is less - and impression removed with a sharp pull, there is less overall permanent strain (ie lower deformation)
elastic materials (2)
Hydrocolloids
elastomers
2 hydrocolloids
Agar
- reversible (lab duplicating) - no longer used!
Alginate – irreversible
3 elastomers
Polysulphides
Polyethers
Silicones (conventional & addition cured)
what material is more ideally used hydrocolloid or elastomers?
elastomers
- not truly elastic but greater
non-elastic materials
recover at all well
- Fractured, torn
impression process 6 stages
preparation
insertion
setting
removal
storage
cast preparation
preparation stage of impression process
mixing (2 pastes; paste + liquid, powder + liquid etc).
chemical reaction
heat
need a material which flows and take up shape
insertion stage of impression process
tray type selected
adhesive required?
setting stage of impression process
chemical
polymerisation
cooling
forms solid replica
storage stage of impression process considerations
how long
before dimensional changes become significant (of storage, need to do correctly, store for no longer than needed)
should you wait to allow elastic recovery ?
6 factors on accuracy of impression material
flow
- surface detail recorded, good close intimate contact
setting changes
- no dimensional change - shrink or expand (remain same dimension)
removal
- no effect on oral tissues - tear
- no change dimensionally
(i. e. no thermal contraction, between mouth and bench) - complete elastic recovery
storage
- does not change dimensionally over time (moisture) – weak point for alginates
decontamination
- dimensions & surfaces unaltered
compatible with cast material (gypsum, dental stone)
6 properties which affect accuracy of impression material
viscosity
- determines ability to flow over surface & so level of detail recorded
setting mechanism
- does physical or chemical process cause dimensional contraction
(expansion or contraction)
thermal expansion coefficient
- ideally low (to cope with mouth to room temperature transition)
hydrophobic / hydrophilic
- affects surface contact - need to remove saliva ?
elasticity
- elastic / viscoelastic behaviour
tear strength
- withstand tearing when removed esp. from undercuts in mouth
- —-flow into undercut regions, small strips in undecruts – vulnerable to tearing and fracture when remove the tray
4 factors of pt comfort that need to be satisfied by impression material
non-toxic, non-irritant
acceptable taste and smell
short setting time (unpleasant experience for pt)
removable without damage to oral tissues
4 operator considerations for impression materials
quick, simple technique
convenient working/ setting times
must be able to be decontaminated
cost is a consideration (not a property) -inexpensive
2 non-elastic impression matierals
impression compound
impression paste
2 types of elastic impression materials
hydrocolloids
elastomers
what is a hydrocolloid
A colloid is a 2 phase system of fine particles (1-200nm) of one phase dispersed in another phase (normally a liquid)
if dispersing medium is water then HYDROCOLLOID
2 hydrocolloid states
sol
gel
sol state of hydrocolloid
viscous liquid state
gel state of hydrocolloid
jelly like consistency
agglomeration of particles
entangled framework of solid particles enclosing liquid by capillary forces
undertakes reaction
example of an irreversible hydrocolloid
alginate
5 components of alginate and their roles (irreversible hydrocolloid)
Salt of alginic acid (e.g. Na alginate) 12%
- Reacts with Ca ions
Calcium sulphate 12%
- Provides Ca ions
Trisodium phosphate 2%
- Delays gel formation
Filler (chalk like powder) 70%
- Cohesion, strength,
Modifiers, flavourings, chemical indicators (sol to gel state)
- Improve surface, taste, pH colour change
role of salt of alginic acid in alginate
reacts with Ca ion
calcium ions interact with trisodium phosphate – pause things
remaining Ca ions react with sodium alginate
role of calcium sulphate in alginate
provides Ca ions
role of trisodium phosphate in alginate
Delays gel formation
calcium ions interact with trisodium phosphate – pause things
remaining Ca ions react with sodium alginate
role of filler in alginate
cohesion
strength
role of modifies, flavourings and chemical indicators in alginate
Improve surface, taste, pH colour change
alginate setting reaction
2 Nan Alg + n CaSO4 –> nNa2SO4 + Can Alg
long cross-linked fibril entangling undissolved particles
- calcium alginate (gel)
cross linking makes it more viscous
what allows the alginate delay
Tri-Sodium phosphate preferentially react with Ca ions in CaSO4
2 Na3 PO4 + 3 CaSO4 –> Ca3 (PO4)2 + 3Na2 SO4
Then:
2 Na Alg reacts with Ca ions
how to use alginate correctly
use correct powder/ liquid ratio
- shake container
use water at 18 - 24°C
use perforated tray and adhesive
remove impression with a sharp pull
- elastic recovery
- maximum tear strength
increased temperature speeds up setting
sets nearest tissue first (in - contact with warm soft tissue – reaction faster)
crosslinking continues after apparent set
- greater elastic properties if you wait further minute or two
- need set throughout whole volume not just extremities
how can you minimise an impression materials tendency to tear
permanent distortion and tearing slightly reduced by using large bulk of materials
- (Typ. 5mm)
Minimizes tendency to tear – if tears need to retake, no saving time
how to store alginate
avoid moisture
- use a damp gauze/tissue to sit over it
- —-guess as to how moist
prepare cast ASAP to avoid dimensional change, can do both:
- syneresis - release of water
- imbibition - uptake of water
both distorts so renders impression useless
syneresis
release of water e.g. from alginate
imbibition
uptake of water e.g. from alginate
accuracy evaluation of alginate
flow - fine detail - OK
setting changes - OK
but movement leads to internal stresses leads to distortion
nearly elastic – not 100% elastic recovery
poor tear strength – avoid deep undercuts
storage – syneresis or imbibition may affect dimensions /accuracy
