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Impression materials Flashcards

1
Q

What are the direct and indirect procedures of working with impression materials?

A
  • Direct procedures – all the work constructing the device is done chairside (filling/ cavity liner etc)
  • Indirect procedures – items made in lab then applied at dentist
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2
Q

Why are 2 colours of impression materials used?

A

Combination used to get best possible reproduction of arch

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3
Q

Why is impression material initially fluid?

A

Material deforms around all the detail giving an impression

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4
Q

What is the use of an impression tray?

A

To support fluid material and preventing the material from overflowing

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5
Q

How does material impression set to make a permanent record?

A

o Chemical - polymerisation/ acid-base/ hydration

o Physical – flow on heating/ stop on cooling

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6
Q

When are the plastic and elastic stages of impression materials?

A

Plastic (deformed) initially when placed in mouth but elastic upon removal

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7
Q

What are the requirements of a well constructed model of impression materials?

A

o When it sets, don’t want it to be too rigid
o Accurate
o Dimensionally stable

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8
Q

List some clinical applications of using impression materials.

A

Crown & bridge, complete denture or implant

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9
Q

What are the 5 chemical nature classifications of impression materials?

And give examples from each.

A

• Elastomers – synthetic rubber
o Silicone, polyether, polysulphide (not used in UK anymore)
• Hydrocolloids – based on extracts of seaweed (polysaccharides)
o Alginate, agar
• Pastes
o Zinc Oxide (cavity liner)
• Gypsum
o Impression plaster
• Thermoplastics – when heated it is permanently deformed
o Compound, wax

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10
Q

What are rheological properties related to?

A

Viscosity

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11
Q

What are rheological properties effects on tissues?

A

o Mucostatic – does not displace soft tissue
- Taking denture impression at rest when non-compressed
o Mucocompressive – displaces soft tissue
- Very viscous
- Provides fine detail of surrounding oral mucosa

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12
Q

Why are loose fitting trays better than close ones?

A

Loose - more flow possible

Close - less flow possible

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13
Q

What is the combo of mechanical properties of impression materials?

A

o Elasticity – non-permanent deformation

o Stiffness – ease of deformation

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14
Q

Why are impression materials elastic, flexible and rubbery and not non-elastic and rigid?

A

• Elastic, flexible, rubbery
o Can be removed past undercuts
o Will ‘spring-back’ to correct shape
• Non-elastic, rigid
o Cannot readily be removed past undercuts
o Will fracture or be distorted if subjected to stress
o Could leave to extraction of teeth accidentally

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15
Q

Decide out of the list which impression materials are elastic and non-elastic

  • Elastomers
  • Gypsum
  • Pastes
  • Thermoplastics
  • Hydrocolloids
A

Elastic - elastomers and hydrocolloids

Non-Elastic - pastes, gypsum and thermoplastics

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16
Q

What are the desirable properties you want from an impression?

A
  • Accuracy –> dimensions, detail (e.g. edentulous ridges/ sulcus depth)
  • Dimensional stability –> i.e. change in accuracy with time such as delay as impression sent to lab or how soon can a model be poured after taking an impression
  • Ease of use –> mixing, technique sensitivity, working time & setting time
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17
Q

What changes in the detail of an impression when there is lower viscosity?

A

Finer detail

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18
Q

Why do hydrophobic impression materials need a dry field?

A

Water doesn’t spread evenly and may start to get small air holes

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19
Q

What is a dry field in the mouth?

A

Completely dry oral environment that impression is being taken of

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20
Q

How does the setting reaction affect accuracy of the impression material?

A

Changes dimensionally
 Polymerisation – shrinkage – if occurs after setting die will be oversized
 Crystal growth – expansion – if occurs after setting die will be undersized
 Thermoplastic transformation

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21
Q

Thermal contraction affects impression accuracy? Where and how?

A
  • Mouth to room —> ~ 32°C to ~ 23°C

- Thermoplastic warming/cooling —-> ~ 55°C to ~ 23°C

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22
Q

What type of impression material is required for undercuts?

A

Elastic

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23
Q

What are the factors of undercuts?

A 
o Depth of undercut
o Thickness of material
o Proper setting of material
o Elastic properties of material
        Viscoelasticity
        Technique of removal of impression
        Snap technique - sudden movement
        Time for recovery
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24
Q

Why is tearing likely to happen?

A

Thin sections of the impressions

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25
What is the snap technique of an impression material?
Sudden movement
26
Where is the accuracy a factor?
Impression material, tray, model, wax pattern, investment mould, alloy casting or polymeric structure
27
What are the methods of use of impression materials?
• Setting characteristics --> WT and ST • Type of tray needed • Method of mixing components o Powder/liquid, 2-paste, paste/liquid o Hand mix, cartridge, auto-mix • Ease of mixing and handling o Viscosity & tackiness (sticky --> difficult to place in pt mouth correctly)
28
What is the temp during the WT and ST?
WT --> 23 degrees in the mouth | ST --> 32-37 degrees outside the mouth
29
What is the difference between cleaning metal and plastic trays?
Metal trays --> autoclaved and reused | Plastic trays --> single use
30
What are special impression trays?
Individual to patient - Ideal spacing - Constructed on model, e.g. acrylics - Putty/wash systems, e.g. elastomers
31
What are the properties of impression trays?
• Provide support for material (rigidity) o Flexible materials distort under pressure o Tray should withstand any distortion to allow only the material to become deformed • Should be the correct size o If too short/ narrow, this can alter viscosity • Bond to material o Some materials shrink on setting
32
How do impression materials bond to impression trays?
 Tray adhesives – needs to bond strongly enough to ensure the material stays bonded as it shrinks  Rim lock – overhang of tray which prevents material staying on tooth upon removal of tray  Perforations
33
What are the limitations of non-elastic impression materials in relation to undercuts?
- Too rigid/stiff to be removed past undercuts | - Insufficiently elastic to resist permanent deformation when withdrawn past undercuts
34
Why would plaster (gypsum) not go past undercuts?
Sets very rigid
35
Why would impression compound not go past undercuts?
Does not go elastic = too plastic to go past undercuts; would be deformed
36
What is plaster of Paris chemically called?
β-calcium sulphate hemihydrate (calcined)
37
What plaster is used in dentistry and how is it formed?
Gypsum: - (CaSO4)2•H2O + 3H2O  CaSO4•2H2O - Powder rehydrated by adding water to give salt
38
What anti-expansion agents are used in plaster of Paris and why?
- K2SO4 (accelerates setting) and Borax (retarder) - -> Need to counteract the potassium sulphate = add Borax to prevent WT/ST being too short and material setting too quickly - More water = greater expansion potential therefore add anti-expansion agents
39
When making plaster of patio what is the gypsum, water ratio?
100g powder to 50-60ml water
40
What is the initial property of plaster of Paris? and therefore what trays are required?
• Very fluid initially --> mucostatic - No displacement of soft tissues • Special tray needed due to fluidity
41
What is the set property of plaster of Paris?
- V. ridgid - Brittle - removal past undercuts hard - Compatibility --> Separating agent required e.g. resin (allows separation of gypsum out after setting)
42
What is the expansion of gypsum on setting?
<0.1%
43
How is impression compound material thermoplastic?
* Hard when cool | * Soft when warm
44
What is the softening temp of impression compound? and how is it softened?
55-60°C in warm water
45
What form is impression compound supplied in?
- Sticks/sheets | - Hard
46
What are the stages of use for impression compound to take an impression?
* Supplied hard * Soften (by warming) – polymer chains and plasticisers separate allowing deformation * Insert * Harden (by cooling) – difficult to deform * Remove
47
What is impression compound made up of? and each of their functions.
• Resins and waxes (viscous organic materials) - Characterise melting characteristics - Traditional – natural materials - Modern – synthetic resins and waxes (better consistency) • Plasticizer e.g. stearate - Waxes and resins brittle - Lubricate more due to chain separation - Improve flow • Filler - Control viscosity, rigidity, thermal contraction - Pigments added e.g. green stick, red sheet to identify properties
48
What is the typical composition of impression compound?
* Rosin, Copal or Shellac * Carnauba wax * Stearic acid * Talc * Pigment
49
What is said about the viscosity of the organic material resin?
Highly viscous
50
What is said about the viscosity of the organic bases, waxes?
Viscous but more solid
51
What is the main difference of type 1 and 2 impression compound?
Type 1 lower temp fusing (hard to flow at low temp) | Type 2 high temp fusing
52
What temp does type 1 impression compound have high flow at?
Just above mouth temp
53
Type 1 impression compound is an insulator, how and why?
Low thermal coefficient and low conductivity
54
What is Type 1 impression compound sheet form used for? and how is it used?
- Preliminary impressions for edentulous patients - Sheet placed into stock tray and after heating and applied into patients mouth = adequate detail of patient’s edentulous mouth - Used to inform manufacture of the special tray
55
What is Type 1 impression compound stick form used for?
- Marginal additions to stock trays (extension to stock trays to fit patient’s anatomy e.g. extension peripherally or beading in – around the tray to show extent of patient’s sulcus) - Recording sulcus - Causes tissue compaction – full extent of where margins should go
56
What is the flow of Type 2 impression compound at or just above mouth temp? therefore what is its use?
Low flow value Rigid at mouth temp - use as tray material
57
What temp is impression compound cooled to?
50 degrees
58
What happens if the impression compound if: 1. Too cool 2. Too hot 3. Time too short 4. Time too long
1. Will not flow and conform to anatomy 2. Sticky (lose plasticiser) and painful 3. Insufficient softening (due to material being an insulator) 4. Leaching of plasticizer
59
How is impression compound softened and what precautions need to be taken?
Sticks - soften in Bunsen flame = easier to handle • Careful of plasticiser leaching • Catch fire
60
What are the properties of impression compound due to its high viscosity?
* Mucocompressive | * No fine detail - margins for denture can be seen
61
What are the properties of impression compound due to its thermal contraction?
* 55°C – 23°C * Stress relief * Dimensionally unstable without stress relief
62
Why can mobile teeth be extracted during an impression with impression compound?
Extraction of mobile teeth during impression for a partial denture due to rigidity
63
What are the applications of impression compound?
* Edentulous areas * Tray extensions * Beading in to take full sulcus impression
64
What colour is impression compound?
Brown
65
What are the 2 pastes used for zinc oxide eugenol? and how are they mixed?
- Paste 1: ZnO, Zn acetate, oils - Paste 2 Eugenol, filler (e.g. kaolin) - Mixed by hand
66
What is produced from the ZOE pastes mixed together?
Zinc eugenolate salt produced
67
What is the ZOE reaction that takes place once the cements are mixed?
• Chelation reaction - Accelerated by: water, accelerator - Setting in the mouth quick
68
What is the viscosity of the ZOE pastes? and when is it used?
* Low viscosity – therefore used in thin sections * Pseudoplastic – reduction of viscosity to flow in space * Records detail – much more detail than impression compound, mucostatic * Used in thin section (normally ~ 1mm thick) = In a close-fitting special tray due to low viscosity
69
What do narrow spaces do to viscosity?
Narrow space increases viscosity therefore NEEDS to be low viscosity
70
How accurate is ZOE? and how about undercuts?
* Small dimensional change on setting (thin section) * Cannot record undercuts – due to plastic when setting * Detail achieved of arches
71
How dimensionally stable is ZOE?
* Small dimensional change on storing (thin section) | * Small dimensional change on setting (thin section)
72
Why is eugenol an irritant?
* Burns can be caused by spillages e.g. on the lip * Allergies * Eugenol free versions available (similar properties but not the same)
73
What are the impression stages for edentulous arches?
• Stage 1 (primary impression) - -> Displace soft tissues - -> Record full depth of sulcus * Make special tray * Stage 2 (major impression) - -> Record detail - -> Maintain sulcus impression * Use compound in stock tray * Use impression paste in special tray, extended with compound

Dental Materials (1 decks)

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