hydrocolloid impressions

what does high viscosity material do

less detail but can displace soft tissues

what does low viscosity do

more detail but tray wont contain

what happens to viscosity as time goes on

it increases

how can type of tray affect accuracy

bendy tray can distort

adhesive needed, essential

what affects the accuracy and stability of impressions

type of tray

flexibility and recovery of material

storage prior to casting eg needs sent to laboratory

what are the classifications of materials

elastic and non elastic

what are the synthetic elastomers

hydrocolloids;

reversible agar

irreversible alginate

poylysulfides

silicone rubbers (condensation or addition curing)

polyethers

What are hydrocolloids?

colloidal extensions of polysaccharides in water

exist in

sol - low viscosity

gel high viscosity

what are reversible hydrocolloids made of

agar

what is agar made of

complex polysaccahride made from algae

has borax to give more body

pottasium sulphate used to counteract retardant effect on the afar from the borax

how is reversible hydrocolloids stored and used

100C gel to sol

60C holding temp

40C clinical use

what are the advantages and disadvantages of agar

advantages

take up moisture

re use

disadvantages

special equipment

cost

cross infection concerns

low strength

what are the uses of agar

clinical - partial dentures, crowns and bridges

lab - duplicating material

what are irreversible hydrocolloids

alginate

what is alginate made of

NA or K salts for alginic acid

calcium sulphate dihydrate - source of ca to cross link chains of alginic acid

sodium phosphate to control working time

inert filler for body

what are the properties of alginate

low viscosity

can control working time with water temp

rapid set

elastic

why is it important to disinfect impression trays

to stop the spread of blood borne viruses to patients clinicians and postal workers

what are alternatives to impressions

CADCAM

intraoral scanner