Hydrocolloid Impression materials

Question 1

what kind of surface detail do high viscosity materials give?

Answer 1

less detail but can displace the tissues

useful for denture extensions but not flabby ridges

Question 2

what kind of surface detail to low viscosity materials give?

Answer 2

more detail but tray wont contain

Question 3

can viscosities change?

Answer 3

viscosity can change with time

Question 4

what repels the impression material?

Answer 4

moist surfaces

hydrophobicity

Question 5

what do flexible trays do?

Answer 5

distorts the material

must have effective bonding strength

Question 6

what does an impression material need to be?

Answer 6

no changes when exposed to disinfectants
no interaction with model/die materials
non toxic
easily manipulated
palatable taste
realistic working/setting time

Question 7

types of impression materials?

Answer 7

non elastic - impression plaster/impression compound/zinc oxide and eugenol pastes
elastic - hydrocolloids/ synthetic elastomers

Question 8

types of synthetic elastomers?

Answer 8

synthetic elastomers - polysulphides/silicone rubbers/polyethers
hydrocolloid - agar - reversible, alginate - irriversible

Question 9

what do both types of hydrocolloids have in common?

Answer 9

both are colloidal suspensions of polysaccharides in water

Question 10

what do both types of hydrocolloids exist in?

Answer 10

soluble - fluid/low viscosity/random chain arrangement

gel - increased viscosity/parallel chain arrangement, elastic properties

Question 11

how to turn sol to gel?

gel to sol?

Answer 11

temperature/chemical

temperature

Question 12

what is Agar?

supplied as a gel with?

Answer 12

complex polysaccharide made from seaweed
borax - to give more body
potassium sulphate - counteract retardant effect of borax on dental plaster

Question 13

agar requires what?

what temperatures?

Answer 13

requires water bath conditioning
100degrees = gel to sol
65degrees = holding temp
45degrees = clinical usage temperature
clinical use requires water cool trayed

Question 14

advantages of agar?

Answer 14

take up moisture

re usable

Question 15

disadvantages of agar?

Answer 15

special equipment
cost
cross infection concerns re use
low tear strength

Question 16

why are agar materials dimensionally unstable?

Answer 16

water content
syneresis - loss of water from squeezed chains
imbibition - uptake of water - swelling due to chain seperation

Question 17

clinical and laboratory uses of agar?

Answer 17

clinical - p/dentures, crowns, bridges

lab - duplicating material

Question 18

how are alginates supplied?

Answer 18

powder to be mixed with water

Question 19

composition of alginate?

Answer 19

• sodium of potassium salt of alginic acid - 11-16%, forms sol with water and in set becomes a cross linked gel
• calcium sulphate dihydride - 11-17%, calcium for cross linking
• sodium phosphate - 1-3% - controls working time
• inert filler 65-75% - body for manipulation
• reaction indicator in some to signify set

Question 20

how does sodium phosphate control the working time?

Answer 20

sodium phosphate limits availability of calcium ions.

Question 21

properties of alginate?

Answer 21

low viscosity - mucostatic
adequate working time - operator control with water temp
rapid set - at soft tissue first - dont disturb tray
elastic
must use no less than 3-5mm to prevent distortion on removal
susceptible to syneresis and imbibition - pour soon

Question 22

what are modified alginates?

Answer 22

incorporation of silicone polymers
2 pastes
better tear resistance and better detail production
poor dimensional stability

Question 23

what is impression disinfection?

Answer 23

immersion in freshly made solution
time immersion
rinse after

Question 24

how much achtichlor to use?

Answer 24

10 tabs in 1000ml

Question 25

alternatives to impressions?

Answer 25

CAD CAM

intraoral scanner