Common properties / failures of elastomeric impression materials

Question 1

How to describe the viscosity of the material?

Answer 1

most viscous

• putty
• heavy bodied
• medium (or regular/universal) bodied
• light-bodied
• extra light bodied

least viscous

Question 2

thixotropy?

Answer 2

how the material flows and stays where it is without dripping

• important from a handling perspective

Question 3

thixotropy aka?

Answer 3

structural viscosity

Question 4

how to measure viscosity?

Answer 4

45 seconds after start of mixing

Question 5

why measure viscosity at 45 seconds?

Answer 5

approx the time at which the material is being injected into the mouth after mixing by hand but before substantial cross-linking has occurred

Question 6

does viscosity increase of impression material in the mouth?

Answer 6

yes

Question 7

effect of viscosity on impression?

Answer 7

syringing of the light-bodied material has to be quick

material must flow all over the preparations

failure of impression material to flow will cause irregularities in the impression and cast

Question 8

standard impression technique?

Answer 8

putty of heavy bodied ad light-bodied wash in a stock tray

putty/heavy body provides support for the light-bodied material which on its own would distort under load during pouring of the model

light-bodied material is sufficient fluid to reproduce the fine details where required - it doesn’t have high level filler in it

different viscosity material bind together when set

Question 9

advantages of heavy-bodied presentation over putty?

Answer 9

not quite so viscous as a putty –> no displacement of the teeth

no deformation of a non-rigid tray

no displacement of the light bodied material from around the preparation

no folding of the putty leaving a defect like a seam in the impression

Question 10

universal/regular viscosity material in a stok tray?

Answer 10

universal viscosity material is injected around the preparation and also fills the tray

loaded tray is then seated over the syringed material

detail reproduction of the universal paste must be sufficient to provide an adequate representation of the preparation

monophase impression

use a non-perforated tray

usually used with the polyethers

Question 11

what can affect the working time of a material?

Answer 11

ambient temp and humidity

Question 12

what material has the longest working and setting time?

Answer 12

polysulphide

different consistencies of the same material exhibit diff setting times

Question 13

dimensional change on shrinkage?

Answer 13

degree of shrinkage primarily determined by their setting reaction

Question 14

what causes the dimensional change of setting of the material?

Answer 14

any by-product of the setting reaction

Question 15

what causes the extreme shrinkage of c-silicones?

Answer 15

due to production of ethanol

Question 16

what material shows the second greatest shrinkage?

Answer 16

polysulphide

Question 17

what material show the least shrinkage?

Answer 17

polyether and addition silicones sow minimal

Question 18

when are polyethers dimensionally unstable?

Answer 18

if stored in water

when decontaminating - leave in bath for correct period of time

dry thoroughly and send to lab to be cast

Question 19

dimensional change in setting depends on what?

Answer 19

depends on filler loading

more filler means less rubber to shrink

putty has low shrinkage

Question 20

can distortion be reversed after removing from the mouth?

Answer 20

no

Question 21

rate the elastomers and their distortion?

Answer 21

least

addition silicones
polyether
c-silicones
polysulphides

most

Question 22

how to describe the ‘snap out’ of elastomers?

Answer 22

viscoeleastic

remove as quickly as possible to material is strained for shortest time

Question 23

what happens if you remove elastomer slowly from the mouth?

Answer 23

flow and strain from which it may not recover

Question 24

define strain?

Answer 24

flexibility of the material

Question 25

what is the most flexible material?

Answer 25

polysulphides

Question 26

stiffest to most flexible material?

Answer 26

most flexible polysulphides c-silicones addition silicones polyether least flexible

Question 27

define hardness?

Answer 27

measure of the depth of an indentation in the material created by a given force under standard load

Question 28

do polysulphide and addition silicones change hardness with time?

Answer 28

no

Question 29

what features of an impression material make it hard to remove from the mouth?

Answer 29

low flexibility and high hardness values use a more loose fitting tray with more material which allows more flexure within the body

Question 30

tear strength?

Answer 30

Ability to withstand tearing when removing

Question 31

do elastomers produce detail accurately?

Answer 31

yes light-bodied materials contain less filler

Question 32

why does light-bodied materials make detailed impressions?

Answer 32

contain less filler flow more readily over preparation and there is less likelihood of air being trapped filler size also determines the ability of the impression material to record fine detail smaller the filler particles the finer the detail the material can record

Question 33

elastomers hydrophobic or hydrophilic?

Answer 33

hydrophobic need good moisture control

Question 34

effect of moisture contamination on impression?

Answer 34

moisture acts as a separating medium so preventing the impression material from binding to the tray delamination may occur some haemostatic agents used to stop gingival bleeding have an adverse effect on the setting of tall impression elastomers

Question 35

ratio of addition silicone putty?

Answer 35

1:1

Question 36

When would you use elastomers for impressions?

Answer 36

required when precision is of utmost importance, like crown and bridge preparation and implants

Question 37

Types of elastomers?

Answer 37

addition silicone, also known as polyvinyl siloxanes (PVS), condensation silicone, polyether, and polysulfide.

Question 38

How to mix elastomers?

Answer 38

Mechanical/machine Mix the 2 pastes together 1:1

Question 39

Problems with imp?

Answer 39

Thin lingual area Not much light-body Plastic tray - question rigidity, could have distorted imp Size of tray, not fully captured distal aspect Tear strength of material - between 2nd and 1st molar, canine and first premolar, 1st molar and 2nd premolar

Question 40

What has occurred on the left image?

Answer 40

Drag Insufficient I’m material there Or When tray has been inserted, not inserted axilly but wiggled into place Also folding

Question 41

What is wrong with imp on right

Answer 41

Drag on upper anterior (put in place and dragged dramatically) Light body not on all teeth Tray is ok - rigid Not enough imp material in tray - sulcus on upper right canine region

Question 42

What is wrong with the imp on left?

Answer 42

Void When cast it will be distorted Mixing machine can reduce voids

Question 43

What is wrong with image on right?

Answer 43

Inaccurate representation of margins - traction not great Drag Poor tear strength - imp distorted Light bodied material displaced to sulcus and palate - putty material pushed it out

Question 44

What is wrong with image on left?

Answer 44

Imp not been looked after roper Overly mixed Reaction between I’m material and cast (can see chalkiness) Need compatibility between imp and cast material