Impression Material Flashcards
What are 3 synthetic elastomers of impression materials
Polysulphides
Silicones
Polyethers
What are the 2 hydrocolloids impression materials
Reversible (agar)
Irreversible (alginate)
What are the 4 non elastic impression materials
Impression plaster
Impression compound
Zinc/oxide eugenol pastes
Impression waxes
What are the 2 elastic impression material categories
Synthetic elastomers
Hydrocolloids
What are 7 requirements of impression materials
Short workable time
Non toxic
Easily removed when set
Record anatomy accurately
Dimensionally stable
Palatable taste
Cost efficient
What are 5 properties of irreversible hydrocolloids
Based on alginic acid which is derived from marine plants
When mixed with water a reaction occurs where the polymer chains cross link
Main component of alginate powder is in fact fillers to control the consistency
The setting process in alginate is irreversible
It is an endothermic reaction
What are 5 tray properties
Rigid
Extension
Retention
Handle position
Sterilisation
What are special impression trays
Custom made usually from acrylic provide a uniform gap between the tray and the dental structures being recorded
What are 2 types of impression trays
Stock trays plastic or metal
Special trays
What are 5 considerations for choosing an impression tray
Anatomy led
Sufficient depth, width and length
Consider the distribution of any teeth remaining
Have enough space for 3mm of alginate (minimum)
Perforations and adhesives
What is the reason for a use of adhesive on the impressions tray
Aim is to overcome displacing force during impression removal
What are 3 materials for tray modification/extension
Wax
Stick impression compound
Heavy body silicone
How do you mix powder alginate by hand
Invert sealed alginate powder container before using measuring spoon to dispense powder into flexible bowl
Use room temperature water at the specific ration to the powder
Spatula used to mix
Paste needs to smooth and free of air bubbles
Spatula then used to load tray
What are the 3 different ways of mixing powder alginate
By hand in a flexible bowl
Alginator
Automatic centrifuge mixing machine
Which impression should you take first
Lower impression first as a patient is less likely to gag
What is gagging with an impression normally caused by
The compression of the soft palate but not necessarily so you may encounter gagging on a lower impression too
How do you load the tray with impression material
Spatula used to scoop up mixed impression material and placed into prepared tray
For lower tray may be easier to split mix into two parts
May wish to smooth alginate in tray with wet fingered glove
Any excess impression material may be kept by nurse to check its setting speed - will set faster in mouth due to heat
Where should the operator stand when taking a maxillary impression
Behind the patient just over the patients shoulder
Where should the operator stand when taking a mandibular impression
Infront, to the side of the patient
What are 5 patient mouth preparations which may be required prior to impression taking
Clean mouth
Dentate patients need teeth dried with 3 in 1 syringe
Pre packing of mixed alginate before tray insertion may be beneficial in certain situation such as in sulci
Areas of large undercuts may need waxing out to prevent alginate tearing on removal
Some patients may prefer to have petroleum jelly placed on lips to minimise trauma
What can happen if tooth much force is placed on the tray when taking the impression
If teeth touch the tray then inadequate alginate thickness results in dental stone can work its way between the tray and alginate during casting, resulting in distortion
How do you seat the impression tray
Insert into mouth without displacing the loaded alginate
One side of lip retracted with finger or mirror and other side retracted with impression tray
Mild pressure placed in an axial direction
Once seated properly keep fingers on tray to maintain position
How should you remove the alginate impression
Gentle working of tray buccally to help break seal use of gloved finger or 3 in 1 syringe may be beneficial
Once loosened tray should be removed from patients mouth taking care with patients lips
How should you review the impression
Under good light
Have all necessary anatomical features been accurately recorded
What 4 aspects are you looking for when reviewing an impression
Air blows
Tears in material
Perforations through to tray
Soft tissue displacement
What does the health and social care act 2012 stated about impression disinfection
Dentist has legal obligation to disinfect all material sent to the lab
What guidance as written in HTM 01-05 publication from the departments of health state about impression disinfection
Rinse impression until visibly clean
Disinfected as per manufacturers instructions
Specific cleaning materials will be in CE-marking instructions
Must be labelled that disinfection has been carried out
What is a hydrocolloid
A substance which forms a gel in the presence of water
What are colloids
Mixtures in which one or more substances are dispersed as relatively large solid particles or liquid droplets through a solid, liquid or gas
How are reversible impressions reversed
As the bonds are easily destroyed by heating
On re heating the gel the material reverts to a sol stage
Why is an irreversible impression irreversible
Strong cross links are formed and will not revert to the sol form
What is a sol form
Fluid
Lower viscosity
Random arrangement of poly saccharide chains
What is a gel form
Thicker
Higher viscosity
Poly saccharide chains become more aligned
Elasticity develops
What are 7 constituents of alginate impression materials
Alginate salt (sodium alginate)
Calcium salt (calcium sulphate - gypsum)
Trisodium phosphate
Diatomaceous earth (amorphous silica)
Flavourings
Colourings
Chromatic phase indicator
What is the bulk constituent of an alginate impression material
Diatomaceous earth (amorphous silica)
What constituent controls the working time of an alginate impression material
Trisodium phosphate
What are 5 advantages of alginate
Non toxic and non irritant
Good surface detail
Ease of use and mix
Cheap and good shelf life
Setting time can be controlled with the temperature of water used
What are 4 disadvantages of alginate
Poor dimensional stability
Incompatibility with some dental stones
Setting time is very dependent on operator handling
Needs to be kept damp or dries out
What is the setting reaction equations for alginate
3Ca2+ + 2Na3PO4 = Ca3 (PO4)2 + 6NA+
What happens in the setting reaction of alginate
Trisodium phosphate reacts with calcium sulphate until used up producing calcium phosphate and maintaining sol state then the calcium ions can react with sodium alginate
What is the pH of alginate in a setting reaction
pH 11 to pH 7
What happens to setting time when temperature is increased
Reduces setting time by increasing the uptake of Trisodium phosphate and increasing the rate of cross linkage formation
