what in an impression
a negative reproduction of tissues
what is the function of an impression material
to produce an accurate replica of the surface and shape of hard and soft oral tissues
what material is used to fill the impression and produce a positive replica
dental stone
what aspects should be shown and why do we want to create a positive replica of the teeth
- position, shape, size and orientation of each tooth
used:
- for evaluating dentition when orthodontic, occlusal or other problems involved
- in lab fabrication of restorations and prostheses
what can we take impressions of
- single tooth
- whole dentition
- edentulous mouth
What is important for getting an impression with high quality and accuracy for working out treatment options
- IM with right properties
- clinician with good technique
what are the two clinical classifications of impression materials
mucostatic
mucocompressive
what does mucostatic mean and give examples
fluid materials that displace the soft tissues slightly i.e. give an impression of the undisplaced mucosa
(displace soft tissues slightly)
e.g. zinc oxide eugenol, low viscosity alginates
what does mucocompressive mean, give examples
viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissue(displace soft tissues more)
e.g. impression compound, high viscosity alginates/ elastomers
do we use non-elastic impression materials anymore
no
what are the two different types of elastic impression materials called
hydrocolloid
elastomeric
(ps not sure how this fits in with mucostatic/mucocompressive)
why is it really important that impression materials are as elastic as possible
so that they overcome the elastic strain that occurs during removal (elastic recovery)
if it was 100% elastic (none really are) then what should the strain % be following removal
0%
No IM is 100% elastic so what do we term them
viscoelastic, the lower the level of viscoelasticity the closer they are to the 0% strain we want
if the IM doesn’t recover following removal what do we end up with
permanent strain, a non-accurate impression
so we start off with an imperfect respresenation
how can we minimise the amount of strain the IM undergoes and why do we want to do this
- Apply the load quickly
- elastic recovery closer to 0% - remove impression with a sharp pull
- wait for a while after removing the tray before you pour the cast to minimise permanent strain (deformation)
What hydrocolloid do we use
alginate
What elastomers do we use
- polyethers
- silicones (conventional and addition cured)
If the materials were non elastic (no material is 100% elastic) what would happen when they are removed over the bulbous part of the tooth
either deformed or fractured
Give a summary of the impression process
- Preparation
- mixing (2 pastes; paste and liquid, powder and liquid etc)
- chemical reaction
- heat
… material flows and takes up shape/ space avaliable - Insertion
- tray type?
- need for adhesive? - Setting
- chemical
- polymerisation
- cooling
- forms solid replica - Removal
- Storage
- how long?
before dimensional changes become significant?
- should you wait to allow elastic recovery - Cast preparation
What are the ideal properties for IM
- accuracy
- patient factors
- operator factors
What does it mean for an impression material to be accurate
- surface detail recorded
- no dimensional change on setting
- when removing, no effect on oral tissues, no change dimensionally (i.e. no thermal contraction), complete elastic recovery)
- doesn’t change dimensionally during storage
- dimensions and surfaces unaltered during decontamination
- compatible with cast material
What properties affect IM accuracy
- viscocity affects the detail recorded
- setting mechanisms could cause dimensional change
- thermal expansion coefficient should be low to cope with mouth/room temp
- hydophobic/hydrophilic affects surface contact
- elasticity
- tear strength to withstand tearing when removed esp undercuts
what does it mean for an IM to have good patient factors
- non-toxic, non-irritant
- acceptable taste and smell
- short setting time
- removable without damage to oral tissues
