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Ageing > Impression materials > Flashcards

Impression materials Flashcards

1
Q

Impression definition

A

Negative imprint of hard (teeth) & soft tissues in mouth from which positive reproduction (or cast) can be formed

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2
Q

Tray definition

A

Container which is designed to roughly fit over dental arches

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3
Q

Process of impression materials

A

First mixed: liquid or semi-solid
Set to solid in a few minutes
Leaving imprint in mough

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4
Q

Properties wanted in impression material by px

A 
Neutral taste and colour
Short setting time
Small tray
Easily removed impression
No retakes
Non-toxic
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5
Q

Properties wanted in impression by dentist

A 
Easily mixed
Short working times
Good quality impressions 
Low cost
Easily disinfected
Simple procedure
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6
Q

What makes a good impression?

A 
Accurate reproduction of surface detail
-viscosity
-wettability
Dimensional accuracy and stability
-shrinkage on setting
-cooling contraction
-permanent set
-storage stability
-type of tray
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7
Q

Factors

A

Impression material
Impression tray
Impression technique

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8
Q

Impression trays

A 
Metal
-stainless steel
Plastic
-nylon-based
-polystyrene-based
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9
Q

Dentists need to side

A

application –>

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10
Q

Types of impression materials

A 
Rigid
-waxes
-impression composition
-zinc oxide/ eugenol
plaster of Paris
Flexible
-hydrocolloids (agar and alginate)
-elastomers (polysulphide, polyether and silicone polymers)
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11
Q

How do impression materials set?

A

Waxes, impression composition, agar: thermoplastic
zinc oxide/ eugenol, plaster of Paris: chemical reaction
Alginate, elastomers: crosslinking

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12
Q

Dental waxes

A

Lab-based material
Used in clinic only to limited degree
Not really for taking impressions –> high TEC

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13
Q

Composition of dental waxes

A 
Oligomer: long-chain molecule with simple strucutre
Ch3-[CH2]n-CH3 n=15-42
Sources:
-mineral
-paraffin wax
-animal
-beeswax
-vegetable
-carnauba and candelilla wax
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14
Q

Properties of dental waxes

A

Softening temp just above mouth T ~42 degrees C
High coefficient of thermal expansion (cooling contraction)
Rigid (good for single tooth)
Poor thermal conductor (surface cools faster than inside (stress))
-subject to stress relief
Low viscosity

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15
Q

Dimensional accuracy

A

Shrinkage –> bigger model, good
-microns, space for your cement
Expansion –> smaller model, bad news

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16
Q

Thermal expansion coefficient (TEC)

A

The change in length, when determined per unit length, for a 1 degree C change in T (α)

  • low: ceramic (8)
  • high: resin (80)
  • enamel and dentine in between
  • dental waxes expand a lot: not good as impression materials
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17
Q

Compo impression composition

A 
Thermoplastic
Sheet or stick --> water bath
Shape it in px's mouth (special trays)
Does not flow enough
Sometimes does not provide very good fit
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18
Q

Components of Compo

A

Combination of
Natural or synthetic resins
-shellac, dammar, colophony, sandarac
Plasticisers
–stearic acis or gutta percha (avoid brittleness)
Fillers
-talc, calcium carbonate or limestone (avoid tackiness)

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19
Q

Properties of Compo

A

Softening T (Tg) 55-60 degrees C
Rigid
High coefficient of thermal expansion - cooling contraction
High viscosity - mucocompressive/ mucodisplacive
Poor thermal conductivity - subject to stress relief

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20
Q

Mucostatic impression technique

A

Impression material is fluid enough to flow and does not displace oral tissues
e.g. impression plaster, agar, zinc oxide eugenol

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21
Q

Mucocompressive impression technique

A

Impression material is viscous and is able to compress oral tissues on insertion in px’s mouth
e.g. impression compound, viscous alginate

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22
Q

Rheology

A

Very useful tool for study of dental materials (‘handling’)
Study of flow of materials
For liquids flow is measured by viscosity (η)
-shear stress/ shear rate
Units of viscosity: Pa.s

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23
Q

Newtonian viscous behavoiur

A

Constant viscosity, μ, across all shear rates and include many of most common fluids e.g. water

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24
Q

Dilatant viscous behaviour

A

Shear-thickening fluids increase rates in apparent viscosity at higher hear rate
Rarely encountered e.g. silly putty

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25
Q

Pseudoplastic

A

Shear-thinning fluids have lower apparent viscosity at higher shear rates e.g. silicone impression materials

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26
Q

Thixotropic behaviour

A

Time-dependent viscosity

  • characteristic of many elastomeric impression materials and improves handling properties
  • some degree of molecular rearrangement caused by mixing
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27
Q

Constituents of typical zinc oxide/ eugenol

A 
Base paste
-zinc oxide
-inert oils (plasticiser)
-hydrogenated resins (increases setting time and improves cohesion)
Reactor paste
-eugenol (or can be carboxylic acid)
-zinc acetate (accelorator)
-fillers (talc or kaolin)
Mixed in 1:1 ratio until paste of even colour
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28
Q

Properties of zinc oxide/ eugenol

A 
Mucostatic (depending on brand)
Hydrophilic: accurate reproduction of surface details
Dimensionally stable
Causes burning sensation of lips
Tendency to stick to skin
Stable on storage and good shelf life
29
Q

Why do we want good wetting?

A

< voids
< entrapment of oral fluids
Bubble-free dies and models
< retakes

30
Q

Wettability

A

Good wetting is ability of liquid to cover the surface of the substrate completely
Small contact angle –> more hydrophilic

31
Q

Surface tensions

A

mJ/ m^2

For perfect wetting critical surface tension of the solid has to be greater than surface tension of liquid

32
Q

Advantages of zinc oxide/ eugenol

A

Dimensional stability
Good surface detail
Stable on storage and good shelf life

33
Q

Disadvantages of zinc oxide/ eugenol

A

Cannot be used in very deep undercuts
Only sets quickly in thin section
Eugenol allergy in some pxs

34
Q

Plaster of Paris (Gypsum)

A

Crystalline mineral of hydrated calcium sulphate
Colourless or white, is not highly water-soluble and is not at all hard
Mixture of gypsum and water can be poured - gypsum hardens as water evaporates
Plaster of Paris is partly dehydrated form of gypsum

35
Q

Composition of impression plaster

A

Powder
-calcium sulphate β-hemohydrate
-borax (slow setting time)
-potassium sulphate (reduce expansion/ accelerates setting)
-starch (aid disintegration of impression on separation from model)
Liquid - water
Expands on setting

36
Q

Working and setting times of plaster

A

Working time 2-3min
Setting time 2-3min
Final set 4-6min

37
Q

Mechanism of setting expansion plaster

A

Interaction between growing gypsum crystals results in formation of stresses and ultimately in expansion of the material

38
Q

Advantages of impression plaster

A 
Easy to mix
-working time 2-3min
-setting time 2-3min
Low viscosity
-mucostatic
Good dimensional stability and accuracy
Cheap
39
Q

Disadvantages of impression plaster

A 
Low strength
Rough surface finish
Poor abrasion resistance
Rigid once set
Dry sensation in mouth
40
Q

Basic concepts of hydrocolloids

A

Colloid is heterogeneous mixtuer of two phases, where the two phases are not readily differentiated

  • colloidal silica in resin
  • agar and alginate impression materials
41
Q

Sol

A

Viscous liquid

42
Q

States of hydrocolloids

A

Sol –> gelation –> gel (jelly-like material)

43
Q

Agar impression materials

A

Reversible hydrocolloids

Gel –> heating –> sol –> cooling –> gel

44
Q

Advantages of agar impression materials

A

Good surface detail

Reusable and relatively easily steralised

45
Q

Disadvantages of agar impression materials

A

Need specials equipment (water bath) and special technique

Dimensional instability

46
Q

Alginate impression materials

A

Irreversible hydrocolloids

Sol –> chemical reaction –> gel

47
Q

Composition of alginate impression materials

A

Sodium alginate: hydrogel former
Calcium sulphate dihydrate: provides calcium ions
Calcium ions displace sodium ions in sodium alginate polymer: crosslinking reaction

48
Q

Properties of alginate impressionn materials

A 
Dust free powder
Cheap
Limited shelf life
Well controlled working &amp; setting times
Mucostatic
Hyprophilic
Poor surface reproduction
Poor storage stability
-imbibition (water sorption)
-syneresis (water loss)
Low tear strength
Excessive permanent deformation
49
Q

Alginate tray

A

Perforations in tray required to enable impression fix to tray securely
-alginate has poor dimensional stability

50
Q

Elastomeric impression materials

A 
Polysulphides
Polyethers
Silicones
-condensation cured
-addition cured
51
Q

Polysulphide impression material

A 
Base paste
-polysulphide
-filler (TiO2)
Activator paste
-lead dioxide
-sulphur
-plasticiser (dibutyl phthalate)
52
Q

Polysulphide molecular composition

A

Polymer with terminal and pendant mercaptan groups (-SH) –> chain lengthening and cross-linking (condense together) –> byproduct H2O

53
Q

Polyether impression material

A 
Base paste
-polyether
-filler (colloidal silica)
Activator paste
-aromatic sulphonate ester
-filler
-plasticiser (dibuttyl phthalate)
54
Q

Polyether molecular composition

A

Structure of polyether –> crosslinking reaction by addition polymerisation with imine end groups (no end product)

55
Q

Silicone impression material (psuedoplastics) - condensation cured

A 
Base paste 
-silicone polymer
-filler (colloidal silica)
-organo-tin compound
Activator paste
-silicone polymer
-filler (colloidal silica)
tetra-ethyl silicate
56
Q

Silicone impression material condensation cured molecular composition

A

Hydroxyl terminated polydimethyl siloxane –> crosslinking reaction (condensation)

57
Q

Silicone impression material: addition cured

A 
Base paste
-silicone polymer
-filler (colloidal silica)
-silanol
Activator paste
-silicone polymer
-filler (colloidal silica)
Pt catalyst
58
Q

Silicone impression material addition cured molecular composition

A

Vinyl terminated polydimethyl siloxane –> crosslinking reaction (no byproduct)

59
Q

Impression techniques

A 
Twin-mix technique
-typical for silicone impression materials
-removal can be difficult
Two-stage with spacer technique
Two-stage without spacer technique
60
Q

Relative merits of elastomeric impression materials: handing

A

Ease of mixing
-polysulphides & condensation-cured silicones somewhat difficult to mix due to different amounts of base and catalyst paste required
-gun delivery system for addition-cured silicones makes them easier to handle
Working and setting times
-polysulphides have long working and setting times
-condensation cured silicones have good working and setting times but setting may be inhibited by latex gloves

61
Q

Relative merits of elastomeric impression materials: mechanical properties

A

Stiffness: PSPE>CCS>ACS

Teat strength: PS»PE>CCS=ACS

62
Q

Relative merits of elastomeric impression materials: surface details

A

All show excellent reproduction of surface detail on dry surfaces

  • polyethers generally best as more hydrophilic
  • surfactants have been added to addition-cured silicones to improve their wettability
63
Q

Relative merits of elastomeric impression materials: dimensional accuracy and stability

A

Setting shrinkage: PE=ACS

64
Q

Failures with impressions: Poor reproduction of surface detail

A 
Rough or uneven surface 
-incomplete set
-rapid set
Air bubbles 
-rapid set
-improper mixing
-surface contamination with moisture
Irregular shaped voids 
-surface contamination with moisture
-premature movement
65
Q

Failures with impressions: poor fit

A 
Distortion
-adhesive failure on tray
-tray not rigid enough
-excessive seating pressure --> too much permanent set
-movement of tray during setting
Casting too big
-inapprppriate technique
-model poured too late
-impression stored under wrong conditions
Casting too small
-inappropriate impression technique
-model poured too early
-impression sotred under wrong conditions
66
Q

Pros and cons intraoral scanners

A 
Pros:
-px comfort
-dentist auto evaluation
-< model time
-favours clinic-lab communication
Cons
-cost investment
-training
-just surface registration
-coating
67
Q

Define imbibition

A

Shape change as water is absorbed by solid-colloids causing an increase in volume
Gel + H2O –> bigger gel

68
Q

Define syneresis

A

Shape changes as loss of a liquid from a gel causing a reduction in volume
Gel –> smaller gel + H2O

Ageing (15 decks)

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