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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
Pseudoplastic
Shear-thinning fluids have lower apparent viscosity at higher shear rates e.g. silicone impression materials
26
Thixotropic behaviour
Time-dependent viscosity - characteristic of many elastomeric impression materials and improves handling properties - some degree of molecular rearrangement caused by mixing
27
Constituents of typical zinc oxide/ eugenol
``` 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 ```
28
Properties of zinc oxide/ eugenol
``` 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
Why do we want good wetting?
< voids < entrapment of oral fluids Bubble-free dies and models < retakes
30
Wettability
Good wetting is ability of liquid to cover the surface of the substrate completely Small contact angle --> more hydrophilic
31
Surface tensions
mJ/ m^2 | For perfect wetting critical surface tension of the solid has to be greater than surface tension of liquid
32
Advantages of zinc oxide/ eugenol
Dimensional stability Good surface detail Stable on storage and good shelf life
33
Disadvantages of zinc oxide/ eugenol
Cannot be used in very deep undercuts Only sets quickly in thin section Eugenol allergy in some pxs
34
Plaster of Paris (Gypsum)
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
Composition of impression plaster
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
Working and setting times of plaster
Working time 2-3min Setting time 2-3min Final set 4-6min
37
Mechanism of setting expansion plaster
Interaction between growing gypsum crystals results in formation of stresses and ultimately in expansion of the material
38
Advantages of impression plaster
``` Easy to mix -working time 2-3min -setting time 2-3min Low viscosity -mucostatic Good dimensional stability and accuracy Cheap ```
39
Disadvantages of impression plaster
``` Low strength Rough surface finish Poor abrasion resistance Rigid once set Dry sensation in mouth ```
40
Basic concepts of hydrocolloids
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
Sol
Viscous liquid
42
States of hydrocolloids
Sol --> gelation --> gel (jelly-like material)
43
Agar impression materials
Reversible hydrocolloids | Gel --> heating --> sol --> cooling --> gel
44
Advantages of agar impression materials
Good surface detail | Reusable and relatively easily steralised
45
Disadvantages of agar impression materials
Need specials equipment (water bath) and special technique | Dimensional instability
46
Alginate impression materials
Irreversible hydrocolloids | Sol --> chemical reaction --> gel
47
Composition of alginate impression materials
Sodium alginate: hydrogel former Calcium sulphate dihydrate: provides calcium ions Calcium ions displace sodium ions in sodium alginate polymer: crosslinking reaction
48
Properties of alginate impressionn materials
``` Dust free powder Cheap Limited shelf life Well controlled working & setting times Mucostatic Hyprophilic Poor surface reproduction Poor storage stability -imbibition (water sorption) -syneresis (water loss) Low tear strength Excessive permanent deformation ```
49
Alginate tray
Perforations in tray required to enable impression fix to tray securely -alginate has poor dimensional stability
50
Elastomeric impression materials
``` Polysulphides Polyethers Silicones -condensation cured -addition cured ```
51
Polysulphide impression material
``` Base paste -polysulphide -filler (TiO2) Activator paste -lead dioxide -sulphur -plasticiser (dibutyl phthalate) ```
52
Polysulphide molecular composition
Polymer with terminal and pendant mercaptan groups (-SH) --> chain lengthening and cross-linking (condense together) --> byproduct H2O
53
Polyether impression material
``` Base paste -polyether -filler (colloidal silica) Activator paste -aromatic sulphonate ester -filler -plasticiser (dibuttyl phthalate) ```
54
Polyether molecular composition
Structure of polyether --> crosslinking reaction by addition polymerisation with imine end groups (no end product)
55
Silicone impression material (psuedoplastics) - condensation cured
``` Base paste -silicone polymer -filler (colloidal silica) -organo-tin compound Activator paste -silicone polymer -filler (colloidal silica) tetra-ethyl silicate ```
56
Silicone impression material condensation cured molecular composition
Hydroxyl terminated polydimethyl siloxane --> crosslinking reaction (condensation)
57
Silicone impression material: addition cured
``` Base paste -silicone polymer -filler (colloidal silica) -silanol Activator paste -silicone polymer -filler (colloidal silica) Pt catalyst ```
58
Silicone impression material addition cured molecular composition
Vinyl terminated polydimethyl siloxane --> crosslinking reaction (no byproduct)
59
Impression techniques
``` Twin-mix technique -typical for silicone impression materials -removal can be difficult Two-stage with spacer technique Two-stage without spacer technique ```
60
Relative merits of elastomeric impression materials: handing
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
Relative merits of elastomeric impression materials: mechanical properties
Stiffness: PSPE>CCS>ACS | Teat strength: PS>>PE>CCS=ACS
62
Relative merits of elastomeric impression materials: surface details
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
Relative merits of elastomeric impression materials: dimensional accuracy and stability
Setting shrinkage: PE=ACS
64
Failures with impressions: Poor reproduction of surface detail
``` 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
Failures with impressions: poor fit
``` 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
Pros and cons intraoral scanners
``` Pros: -px comfort -dentist auto evaluation -< model time -favours clinic-lab communication Cons -cost investment -training -just surface registration -coating ```
67
Define imbibition
Shape change as water is absorbed by solid-colloids causing an increase in volume Gel + H2O --> bigger gel
68
Define syneresis
Shape changes as loss of a liquid from a gel causing a reduction in volume Gel --> smaller gel + H2O

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