Impression Materials

Question 1

what are the two clinical classifications of impression materials

Answer 1

mucostatic e.g alginate - fluid materials that only slightly displace soft tissues so gives impression of undisplaced mucosa
mucocompressive - viscous materials that displace soft tissues

Question 2

what are the 2 property classifications of impression materials

Answer 2

elastic and non elastic
no impression material is fully elastic (experiences full elastic recovery after strain removed)
majority display visco elastic behaviour which is an inbetween

Question 3

how can we reduce the permanent deformation experienced by viscoelastic materials

Answer 3

reduce load time experienced i.e remove impression tray with a quick sharp pull

Question 4

hydrocolloid

Answer 4

fine particles (colloid) suspended in water (hydro) 
alginate - irreversible hydrocolloid

Question 5

what is alginate composed of

Answer 5

inert filler
salt of alginate - reacts with Ca ions e.g sodium alginate
calcium sulfate - provides calcium ions
trisodium phosphate - delays gel formation so can adjust working time

Question 6

what is the simple equation for the formation of alginate and why is this significant

Answer 6

NaAlg + CaSO4 - NaSO4 + CaAlg

univalent sodium ions are displaced for bivalent calcium ions which allow for cross linking

Question 7

what temp should water be for alginate and what effect does temperature have

Answer 7

18-24 degrees

increaseing temp speeds up setting - this also means that the product nearest the tissues will set first

Question 8

how must alginate be stored once an impression has been taken

Answer 8

should be cast asap

until then stored in a plastic bag with damp paper towel

Question 9

what is alginates accuracy like

Answer 9

okay

has poor tear strength and must be stored carefully

Question 10

what causes dimensional changes in alginate

Answer 10

syneresis - release of water

imbibition - uptake of water

Question 11

examples of elastomers

Answer 11

poly ether and addition silicon

Question 12

what is the ISO standard for impression materials for depth of groove

Answer 12

they must be able to record grooves /indentations at least 20 or 50 micrometres

Question 13

ideally what should impression materials wettability be like

Answer 13

wettability - surface wetting = how well an elastomer makes contact with dental tissues
better surface wetting = less chance of bubbles / voids

Question 14

how do working times and setting times compare between addition silicones and polyether

Answer 14

generally polyether working times = half that of addition silicones
setting time for polyethers is also less