Impression Materials

Question 1

what is the function of an impression material?

Answer 1

to produce an accurate replica of the surface and shape of hard and soft oral tissues

Question 2

what kind of reproduction does an impression give of the soft tissue?

Answer 2

negative

Question 3

what is dental stone used for?

Answer 3

to fill the impression and produce a positive replica (stone cast) which represents the position, shape, size and orientation of each tooth

Question 4

what are casts used for?

Answer 4

evaluating dentition when orthodontic, occlusal or other problems involved, in laboratory fabrication of restorations and prostheses

Question 5

what are the classifications of impression materials?

Answer 5

mucostatic, mucocompressive and elastic/non-elastic

Question 6

what are mucostatic materials?

Answer 6

fluid materials that displaces the soft tissues slightly and give impression of the undisplaced mucosa

Question 7

what are mucocompressive materials?

Answer 7

viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissues

Question 8

what is the ideal elastic behaviour for an impression material?

Answer 8

once the strain is taken off of the material i.e. removing impression material from the tooth, it should go back to the way it was when it covered the tooth making an exact impression

Question 9

what term is used for the elastic behaviour of impression materials?

Answer 9

viscoelastic

Question 10

what does the term viscoelastic behaviour mean?

Answer 10

when the impression material is removed from the tooth it never fully recovers and so it has permanent strain

Question 11

what helps to decrease permanent strain?

Answer 11

if the load time with the impression material on the tooth is less

Question 12

name a hydrocolloid

Answer 12

alginate

Question 13

name 3 elastomers

Answer 13

polysulphides, polyethers, silicones

Question 14

how do you prepare impression materials

Answer 14

mixing 2 pastes either a paste and liquid or powder and liquid which produces a chemical reaction

Question 15

what are the ways in which an impression material sets?

Answer 15

chemical, polymerisation, cooling

Question 16

what are the ideal properties for accuracy for an impression material?

Answer 16

surface detail recorded, no dimensional change, no effect on oral tissues, no change dimensionally, complete elastic recovery, does not change dimensionally over time, dimensions and surfaces unaltered, compatible with cast material

Question 17

what type of factors affect accuracy?

Answer 17

viscosity, setting mechanism, thermal expansion coefficient, hydrophobic, elasticity, tear strength

Question 18

what should the thermal expansion coefficient be?

Answer 18

low

Question 19

what does viscosity determine?

Answer 19

the ability to flow over the surface and so the level of detail recorded

Question 20

what are the ideal properties for patient comfort?

Answer 20

non-toxic, non-irritant, acceptable taste and smell, short setting time, removable without damage to oral tissues

Question 21

what are the ideal properties for operator convenience?

Answer 21

quick, simple technique, convenient working/setting times, must be able to be decontaminated, cost is a consideration

Question 22

what are the 2 main types of elastic impression materials?

Answer 22

hydrocolloids and elastomers

Question 23

what are the 2 types of non-elastic impression materials?

Answer 23

impression compound, impression paste

Question 24

what is a hydrocolloid?

Answer 24

a colloid is a two phase system of fine particles of one phase dispersed in another phase - the dispersing medium is water

Question 25

what are the two states of hydrocolloid?

Answer 25

sol and gel

Question 26

what is the sol state of hydrocolloid?

Answer 26

viscous liquid state

Question 27

what is the gel state of hydrocolloid?

Answer 27

jelly like consistency, agglomeration of particles, entangled framework of solid particles enclosing liquid by capillary forces

Question 28

in alginate, what does trisodium phosphate do?

Answer 28

delays gel formation so varying amount of this varies the clinicians working time

Question 29

what is the basic setting reaction of alginate?

Answer 29

sodium alginate to calcium alginate

Question 30

what occurs in the setting reaction of alginate?

Answer 30

sodium molecules are swapped over for divalent calcium ions which allow crosslinking to increase viscosity and strength

Question 31

how does trisodium phosphate work in the setting reaction of alginate?

Answer 31

trisodium preferentially reacts with Ca ions in CaSO4 and then sodium alginate reacts with calcium ions

Question 32

how can you manipulate alginate?

Answer 32

use correct powder/liquid ratio, use water at 18-24 degrees, use perforated tray and adhesive, remove impression with a sharp pull

Question 33

what does increased temperature do to alginate setting?

Answer 33

speeds up setting

Question 34

how do you get greater elastic properties with alginate?

Answer 34

wait further minute or two before removing from mouth

Question 35

how do you reduce permanent distortion and tearing?

Answer 35

using large bulk of materials

Question 36

how do you store alginate?

Answer 36

avoid moisture, prepare cast ASAP to avoid dimensional change

Question 37

what is syneresis?

Answer 37

release of water

Question 38

what is imbibition

Answer 38

uptake of water

Question 39

what is the tear strength of alginate?

Answer 39

poor so avoid deep undercuts

Question 40

what does movement of alginate cause?

Answer 40

internal stresses and distortion