Impression materials

Question 1

what is the function of impression material

Answer 1

• accurate replica of the surface and shape of hard and soft oral tissues

Question 2

4 classification types of impression types

Answer 2

elastix
non elastix
mucocompressive
mucostatic

Question 3

what is the diff between mucostatic and mucocompressive

Answer 3

mucocompressive
 high viscosity alginates
 impression of displaced soft tissues

mucostatic
 low viscosity alginates
 does not displace soft tissues

Question 4

is elastic ideal

Answer 4

yes but no material is 100% elastix, there will be a slight mismatch

Question 5

why is non elastic not ideal

Answer 5

 fractures
 deforms
 not good for cuspal areas
 break after undercuts

Question 6

describe viscoelastic behaviour of impression material

Answer 6

load -> elastix strain -> load removed -> elastix recovery -> permanent strain

Question 7

how to minimize permanent strain?

Answer 7

wait for a time after removing the tray before pouring cast

less load time and sharp pull

Question 8

what does shorter load time mean?

Answer 8

remove the impression tray with a SHARP PULL

Question 9

Impression process

Answer 9

• preparation
• insertion
• setting
• removal
• storage
• casting

Question 10

Ideal properties of impression materials

Answer 10

• accuracy
• flow/viscosity and record details
• no dimensional change after setting and removal
• no effect on oral tissues upon remove
• ideally have complete elastic recovery
• no dimensional change or warping during storage
• decontaminated without changing dimensions
• compatible with gypsum
• LOW thermal expansion, so that there is no change in dimensions when going from mouth temp 37deg to room temp of 22deg
• withstand tearing when removal from undercuts
• non toxic
• taste
• short setting time
• removable without damage to oral tissues

Question 11

is hydrocolloid elastix or non elastix

Answer 11

it is elastic

can be reversible hydrocolloid or irreversible

Question 12

what is alginate classification

Answer 12

irreversible hydrocolloid

Question 13

wat are examples of elastomers

Answer 13

 polysulphides
 polyethers
 silicones

Question 14

wat is agar

Answer 14

reversible hydrocolloid

Question 15

what is a colloid and a hydrocolloid?

Answer 15

A colloid is a two phase system
1. fine particles
2. dispersing medium

Hydrocolloid
1. fine particles
2. medium is water

Question 16

• SOL

Answer 16

o viscous liquid state of hydrocolloid
o can flow around the impression tray

Question 17

• GEL

Answer 17

o jelly like consistency state of hydrocolloid
o after setting
o agglomeration of particles
o entangled framework of solid particles enclosing liquid by capillary forces

Question 18

Composition of alginate

Answer 18

• salt of alginic acid
• calcium sulphate
• trisodium phosphate
• filler
• modifiers
• flavourings

Question 19

purpose of trisodium phosphate

Answer 19

delay gel formation

1. trisodium phosphate reacts with calcium sulphate PREFERENTIALLY => calcium phosphate
2. sodium alginate reacts with calcium in the calcium phosphate

Question 20

Setting reaction of alginate

Answer 20

sodium alginate + calcium sulphate -> calcium alginate

Question 21

increasing temp does what to alginate setting time

Answer 21

speeds up

Question 22

why is it beneficial to wait after removing the tray from the mouth?

Answer 22

*CROSSLINKING CONTINUES AFTER SET SO GREATER ELASTIC PROPERTIES IF YOU WAIT BEFORE REMOVING FROM MOUTH

Question 23

temp of water to use to mix alginate

Answer 23

18-24

Question 24

Alginate properties

Answer 24

• permanent distortion and tearing reduced by bulk
• poor tear strength
• non toxic
• taste ok
• setting time ok
• easy to use
• storage must avoid moisture -> cast asap to avoid dimensional changes
• flow details ok
• little dimensional changes
• nearly fully elastic

Question 25

syneresis
imbibition

Answer 25

syneresis = release of water

imbibition = uptake of water

*** in relation to storage of alginate impression, can distort and change dimensions when in contact with moisture

Question 26

what are the 3 types of elastomers?

Answer 26

• polyether
• polysulphides
• silicones

Question 27

what are the ideal properties of impression materials?

Answer 27

• appropriate working and setting time
• flow readily
• wettability
• flexible
• 100% elastic recovery ideal
• high tear strength
• detail reproduction
• limited syneresis and imbibition

Question 28

what is the elastic recovery of impregum and virutal

Answer 28

virutal = 99.5%
impregum = 98%

Question 29

what is the ISO standard for detail reproduction

Answer 29

20um / 50um

Question 30

what is the tear strength of virtual and impregum

Answer 30

 Virtual = 9 MPa
 Impregum = 1.8 MPa

Question 31

what is the def of tear strength

Answer 31

 stress a material will withstand before fracturing

Question 32

what is the def of rigidity

Answer 32

How much a material changes shale depending on the force applied to it

Question 33

why should an impression material be flexible

Answer 33

flexible means low amounts of stress required to produce a large change in shape

flexibility allows the impression to be removed from undercuts and interdental regions without fracturing

Question 34

what is the working time of polyether and silicones

Answer 34

2 min polyether
4 min silicone

Question 35

what is the setting time of poly ether and silicone

Answer 35

polyether 5min
silicones 6 min

Question 36

what is the elastic recovery % of virutal?

Answer 36

> 99.5%

Question 37

what is the linear dimensional change of virutal?

Answer 37

less than 0.20%

Question 38

what is the fine detail reproduction of virtual?

Answer 38

20um