Impression materials

Question 1

What is the function of impression materials?

Answer 1

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

Question 2

What material is used to fill the impression and produce a positive replica of the impression?

Answer 2

gypsum (3 types: plaster, dental stone, improved stone/densite)

Question 3

What is the function of gypsum (dental stone)?

Answer 3

to produce a positive replica of hard and soft oral tissues which represents position, shape, size, orientation of each tooth

Question 4

What is a stone cast used for?

Answer 4

• evaluating dentition when orthodontic / occlusal problem involved (e.g. monitor progression of wear)
• laboratory fabrication of restorations and prostheses

Question 5

What can impression materials be used to take impressions of?

Answer 5

single tooth, whole dentition, edentulous mouth

Question 6

What does treatment outcome depend on?

Answer 6

quality and accuracy of the initial impression

Question 7

What is required for a good quality and accurate impression?

Answer 7

impression material with right properties and a clinician with good technique

Question 8

How can impression materials be classified?

Answer 8

clinical classification (mucostatic vs mucocompressive) or properties classification (elastic vs non-elastic)

Question 9

What is a mucostatic impression material?

Answer 9

a fluid material that only slightly displaces the soft tissues to give and impression of undisplaced mucosa

Question 10

Examples of mucostatic impression materials

Answer 10

zinc oxide eugenol, low viscosity alginates

Question 11

What is a mucocompressive impression material?

Answer 11

a viscous material that records an impression of the mucosa under load to give an impression of displaced soft tissue

Question 12

Examples of mucocompressive impression materials

Answer 12

impression compound, high viscosity alginates / elastomers

Question 13

What strain does the impression material undergo when it is removed after setting?

Answer 13

elastic strain is applied (to move impression material over the bulbosity of the tooth)

Question 14

What process does the impression material undergo once the elastic strain / load is removed?

Answer 14

elastic recovery (returns to almost original shape)

Question 15

What is the ideal elastic behaviour of an impression material?

Answer 15

material has a full elastic recovery to its original dimensions after removal

Question 16

Describe the ideal elastic behaviour on a strain (%) vs time graph

Answer 16

At time=0 and strain=0%, material is length L. Once load is applied, there is instantaneous strain increase and the length is L + dL. Once load is removed, strain returns to 0% and length returns to L - not possible, in reality there is permanent strain

Question 17

What behaviour do impression materials exhibit?

Answer 17

viscoelastic behaviour (exhibit permanent strain)

Question 18

What is meant by viscoelastic behaviour?

Answer 18

exhibits permanent strain

Question 19

Describe the viscoelastic behaviour of impression materials on a strain vs time graph

Answer 19

there is a gradual increase in strain as load is applied (steep gradient -> less steep) - unlike the instantaneous strain in the ideal elastic graph, until maximum strain is reached. Stain then steeply decreases as the material undergoes elastic recovery and plateaus above 0 (permanent strain)

Question 20

Why is it advantageous to wait for some time after removing the tray before the cast is poured?

Answer 20

the impression material continues to undergo elastic recovery for a certain time period so waiting will minimise permanent strain (deformation)

Question 21

What is the effect of a shorter load time?

Answer 21

maximum strain is lower and there is therefore less overall permanent strain (deformation)

Question 22

How can you achieve a shorter load time clinically?

Answer 22

pull the impression off in a faster motion

Question 23

What are the 2 types of elastic impression materials?

Answer 23

hydrocolloids and elastomers

Question 24

What are the different types of hydrocolloids?

Answer 24

agar (reversible - no longer used) and alginate (irreversible hydrocolloid)

Question 25

What are the different types of elastomers?

Answer 25

- polysulphides (not used in GDH) - polyethers - silicones (conventional and addition cured)

Question 26

Are impression materials perfectly elastic?

Answer 26

no - still undergo a small degree of permanent strain (permanent deformation)

Question 27

Which type of elastic impression material is better - hydrocolloids (alginate) or elastomers?

Answer 27

elastomers > alginate (irreversible hydrocolloid) because they are more elastic and therefore undergo less permanent strain

Question 28

What would happen if a non-elastic impression material was used?

Answer 28

when vertical force is applied to remove the material, it will either become deformed or fracture - does not undergo elastic recovery to original shape

Question 29

Outline the impression process

Answer 29

- preparation - insertion - setting - removal - storage - cast preparation

Question 30

What happens during the preparation of an impression?

Answer 30

mixing (of 2 pastes / paste + liquid / powder + liquid) which undergoes a chemical reaction and some heat is produced. The material can flow to take up shape / space available

Question 31

What should be considered when inserting an impression?

Answer 31

tray type and the need for an adhesive

Question 32

What happens during the setting of an impression?

Answer 32

chemical / polymerisation reaction takes place followed by cooling. Forms a solid replica

Question 33

What should be considered when storing an impression?

Answer 33

- how long you can can store it before dimensional changes become significant - how long to wait to allow elastic recovery

Question 34

What are the 3 categories that cover the ideal properties of impression materials?

Answer 34

- accuracy - patient factors - operator factors

Question 35

What are the ideal properties of impression materials to ensure accuracy (think of process)?

Answer 35

- low viscosity to allow recording of surface detail - no dimensional changes in setting - no effect on oral tissues - no thermal contraction on removal - complete elastic recovery - no dimensional changes over time (storage - moisture) - unaltered by decontamination process (dimensions and surfaces) - compatible with cast material (gypsum)

Question 36

What properties of impression materials affect their accuracy?

Answer 36

- viscosity - setting mechanism - thermal expansion coefficient - hydrophobic / hydrophilic - elasticity - tear strength

Question 37

How does viscosity impact accuracy?

Answer 37

viscosity determines ability to flow over surface and therefore the level of detail recorded

Question 38

How does the setting mechanism impact accuracy?

Answer 38

setting mechanism (physical / chemical process) may cause dimensional contraction

Question 39

How does the thermal expansion coefficient impact accuracy?

Answer 39

determines contraction of material as the impression transitions from mouth to room temperature

Question 40

What is the ideal thermal expansion coefficient for impression materials?

Answer 40

ideally low to cope with mouth to room temperature transition

Question 41

How does whether the impression material is hydrophobic / hydrophilic impact accuracy?

Answer 41

hydrophobic / hydrophilic nature affect surface contact with dentition

Question 42

Why is tear strength relevant to impression materials?

Answer 42

impression materials should withstand tearing when removed especially from undercuts

Question 43

What are the ideal impression material properties to maximise patient comfort?

Answer 43

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

Question 44

What are the ideal impression material properties for operator convenience?

Answer 44

- quick, simple mixing technique - convenient working and setting times - must be able to be decontamination - inexpensive (cost is a consideration but NOT a property)

Question 45

What are the types of non-elastic impression materials?

Answer 45

impression compound, impression paste

Question 46

What is a colloid?

Answer 46

a two phase system of fine particles (1-200nm) of one phase dispersed in another phase

Question 47

What is a hydrocolloid?

Answer 47

fine particles (1-200nm) of one phase are dispersed in water (colloid with water as dispersing medium)

Question 48

What states can a hydrocolloid exist as?

Answer 48

2 states - as a SOL and GEL

Question 49

What is the SOL state of a hydrocolloid?

Answer 49

viscous liquid state

Question 50

What is the GEL state of a hydrocolloid?

Answer 50

state with a jelly-like consistency whereby fine particles for an entangled framework which enclose liquid through capillary forces

Question 51

What is the consistency of a hydrocolloid in a gel state?

Answer 51

jelly-like

Question 52

Which forces are present in the gel state of hydrocolloid?

Answer 52

capillary forces which allow the framework of solid particles to enclose the liquid

Question 53

What is alginate known as?

Answer 53

irreversible hydrocolloid

Question 54

What are the components that make up alginate?

Answer 54

- salt of alginic acid (aka sodium alginate) - calcium sulphate - trisodium phosphate - filler - modifiers - flavourings - chemical indicators

Question 55

Which two components of alginate react with each other?

Answer 55

salt of alginic acid (Na alginate) and calcium sulphate

Question 56

What percent of alginate powder is comprised of salt of alginic acid / Na alginate and calcium sulphate?

Answer 56

12% Na alginate 12% calcium sulphate

Question 57

Function of salt of alginic acid (Na alginate) in alginate

Answer 57

reacts with Ca ions

Question 58

What is the function of calcium sulphate in the alginate powder?

Answer 58

provides Ca ions

Question 59

What is the function of trisodium phosphate (2%) in the alginate powder?

Answer 59

delays the gel formation (allows longer working time)

Question 60

What is the main component of alginate powder?

Answer 60

inert filler - 70%

Question 61

What is the function of the inert filler in alginate powder?

Answer 61

provides cohesion and strength

Question 62

What percent of alginate is made up of modifiers, flavourings and chemical indicators?

Answer 62

small percentage - approx 4% combined

Question 63

What is the function of modifiers in the alginate?

Answer 63

improves surface quality

Question 64

What is the function of flavourings in alginate?

Answer 64

improves / controls taste

Question 65

What is the function of chemical indicators in alginate?

Answer 65

highlight changes in pH via a colour change

Question 66

What is the setting reaction of alginate?

Answer 66

sodium alginate + calcium sulphate -> calcium alginate + sodium sulphate

Question 67

Describe the structure that forms in the alginate setting reaction

Answer 67

calcium alginate undergoes crosslinking as the Ca2+ ions replace the Na+ in sodium alginate (allows development of viscosity and strength of material)

Question 68

How does trisodium phosphate delay the gel formation of alginate?

Answer 68

trisodium phosphate preferentially reacts with the calcium ions released by calcium sulphate. Once the trisodium phosphate is used up, the subsequent calcium ions released react with sodium alginate

Question 69

How can the manufacturer adjust the alginate's working time?

Answer 69

by varying the proportion of trisodium phosphate present

Question 70

What is the reaction of trisodium phosphate that results in the alginate delay?

Answer 70

trisodium phosphate + calcium sulphate -> calcium phosphate + disodium sulphate ( Na3PO4 + CaSO4 -> Ca3(PO4)2 + Na2SO4 )

Question 71

What happens once all the trisodium phosphate has reacted?

Answer 71

sodium alginate then reacts with the subsequent Ca ions released (from calcium sulphate)

Question 72

How do you use alginate?

Answer 72

- use correct powder / liquid ratio - use water at 18-24 degrees - use perforated tray and adhesive - remove impression with a sharp pull - wait a further 1-2 min after apparent set

Question 73

What temperature range of water should be used to mix the alginate?

Answer 73

18-24 degrees

Question 74

Why is it important that the alginate impression is removed with a sharp pull?

Answer 74

- allows maximum elastic recovery - maximum tear strength

Question 75

What is the effect of an increased temperature for manipulating the alginate?

Answer 75

increased temperature speeds up setting

Question 76

Which area of alginate in the mouth will set first?

Answer 76

sets nearest tissues first (greater temperature) - therefore also sets faster in operators hands

Question 77

Why should you wait further after the apparent set of alginate?

Answer 77

crosslinking continues after apparent set which would improve elastic properties

Question 78

What is one way of slightly reducing tearing and permanent distortion of alginate?

Answer 78

by using large bulk of material - more than theoretically required

Question 79

What properties of alginate make it suitable for patient comfort?

Answer 79

- non-toxic, non-irritant - acceptable taste and smell - okay setting time

Question 80

What must be considered when it comes to storing alginate?

Answer 80

- avoid moisture - prepare cast with gypsum ASAP

Question 81

Why is it important that an alginate impression has a cast prepared ASAP?

Answer 81

to avoid dimensional change because alginate can undergo syneresis or imbibition depending on environment

Question 82

What is syneresis?

Answer 82

release of water (shrink)

Question 83

What is imbibition?

Answer 83

uptake of water (swell)

Question 84

Outline the properties of alginate that affect its accuracy

Answer 84

- flow okay as it is able to record surface detail - setting changes are okay - not fully elastic - will incur slight permanent strain - poor tear strength - careful storage required to avoid dimensional changes

Question 85

What is the clinical relevance of the poor tear strength of alginate?

Answer 85

avoid deep undercuts (or block out with red wax?) or use elastomers instead (greater tear strength)

Question 86

Why is it important to avoid patient movement during the setting of alginate?

Answer 86

internal stresses can distort the shape recorded

Question 87

Summarise a series of properties that should be considered for alginates

Answer 87

- flow characteristics - setting mechanism - ease of removal - elasticity - best method of storing before producing study cast