Impression Materials

Question 1

what are impression materials used for

Answer 1

produce a replica of the patients hard and soft tissues, this is a negative impression, poured into with gypsum to produce a positive replica - used for designing dentures and orthodontic appliances

Question 2

what does the treatment using a cast hinge on

Answer 2

accuracy - of impression material, quality of impression - by operator

Question 3

how can impression materials be classified

Answer 3

clinically - mucostatic or mucocompressive

properties - elastic or not

Question 4

what are the ideal properties of an impression material

Answer 4

viscoelastic, low thermal expansion, storage

Question 5

why is viscoelasticity important

Answer 5

can flow around undercuts etc, into all areas, but must also be able to be removed and then once removed bounce back to the shape it was in the mouth once set

Question 6

describe the ideal viscoelasticity

Answer 6

when a load is applied, it is applied immediately, as the load remains, the strain is continuous, once the load is removed, the material goes back to the dimensions it was at the start with and the strain is removed

Question 7

what actually happens with the viscoelasticity

Answer 7

the load is applied over time, meaning the strain gradually increases, the longer the load is applied, the worse the strain, as the load is removed, the strain doesnt go back perfectly, some permanent deformity

Question 8

how can the permanent deformity be reduced when taking an impression

Answer 8

if the load is applied quickly, less strain is induced. also the less time the load is held for the less permanent deformity seen. have to do it quickly and take it off quickly

Question 9

what properties affect accuracy

Answer 9

elasticity, viscosity, thermal expansion, tear strength

Question 10

what is tear strength and what should it be

Answer 10

the strength it can withstand without distortion, want this to be high so it can be removed from undercuts without tearing the material

Question 11

what is alginate

Answer 11

a hydrocolloid

Question 12

what is the setting reaction of alginate

Answer 12

calcium sulphate with sodium alginate - forms calcium alginate as calcium induces cross linking in the alginate

Question 13

what particles are involved in the setting of alginate

Answer 13

calcium sulphate, sodium alginate, tri-sodium phosphate

Question 14

what is the role of tri sodium phosphate

Answer 14

calcium preferrentially bonds to this over sodium alginate, prevents calcium cross linking immediately so the operator can place the material before set

Question 15

what are the properties of alginate

Answer 15

good viscosity, elasticity is okay, poor tear strength - shouldnt be used in areas with large under cuts

Question 16

why should alginate be stored with damp gauze

Answer 16

prevents it taking on moisture or releasing moisture and drying out
taking on moisture - syneresis, releasing - imbibition

Question 17

give examples of elastomers

Answer 17

addition silicones, polyether

Question 18

what are some properties of elastomers

Answer 18

good flexability not rigid, low viscosity, surface reproduction and wetability, high tear strength, good elasticity

Question 19

what are ISO standards

Answer 19

the standards required to be met for a safe material, doesnt compare the best on the market just safe enough to be used

Question 20

give an example of an ISO standard

Answer 20

surface reproduction of an elastomer, has to be able to flow into small areas, gap size is dependant on the viscosity of the material - low viscosity - 20um, high viscosity - 50um

Question 21

what are the 3 different requirements for ideal properties of an elastomer

Answer 21

able to flow over the surface and into gap
obtain accurate detail
removal from undercuts

Question 22

what properties are important for the material to flow over hard and soft tissues

Answer 22

viscosity - cant be too thick that it doesnt flow into small gaps, wetability - have intimate contact with surface

Question 23

what properties are important for the material to obtain accurate detail

Answer 23

elasticity - want it to be a low number - high suggests large strain for a fixed load and thus permanent deformation, surface reproduction - dependant on viscosity

Question 24

what properties are important for the material to be removed from undercuts

Answer 24

flexability - want a low YM, not rigid

tear strength - want this to be high, not fractured with high stress on removal