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Dental Materials > Impression materials > Flashcards

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
Q

What is the snap technique of an impression material?

A

Sudden movement

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26
Q

Where is the accuracy a factor?

A

Impression material, tray, model, wax pattern, investment mould, alloy casting or polymeric structure

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27
Q

What are the methods of use of impression materials?

A

• 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)

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28
Q

What is the temp during the WT and ST?

A

WT –> 23 degrees in the mouth

ST –> 32-37 degrees outside the mouth

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29
Q

What is the difference between cleaning metal and plastic trays?

A

Metal trays –> autoclaved and reused

Plastic trays –> single use

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30
Q

What are special impression trays?

A

Individual to patient

  • Ideal spacing
  • Constructed on model, e.g. acrylics
  • Putty/wash systems, e.g. elastomers
31
Q

What are the properties of impression trays?

A

• 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
Q

How do impression materials bond to impression trays?

A

 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
Q

What are the limitations of non-elastic impression materials in relation to undercuts?

A
  • Too rigid/stiff to be removed past undercuts

- Insufficiently elastic to resist permanent deformation when withdrawn past undercuts

34
Q

Why would plaster (gypsum) not go past undercuts?

A

Sets very rigid

35
Q

Why would impression compound not go past undercuts?

A

Does not go elastic = too plastic to go past undercuts; would be deformed

36
Q

What is plaster of Paris chemically called?

A

β-calcium sulphate hemihydrate (calcined)

37
Q

What plaster is used in dentistry and how is it formed?

A

Gypsum:

  • (CaSO4)2•H2O + 3H2O  CaSO4•2H2O
  • Powder rehydrated by adding water to give salt
38
Q

What anti-expansion agents are used in plaster of Paris and why?

A
  • 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
Q

When making plaster of patio what is the gypsum, water ratio?

A

100g powder to 50-60ml water

40
Q

What is the initial property of plaster of Paris? and therefore what trays are required?

A

• Very fluid initially –> mucostatic
- No displacement of soft tissues
• Special tray needed due to fluidity

41
Q

What is the set property of plaster of Paris?

A
  • V. ridgid
  • Brittle - removal past undercuts hard
  • Compatibility –> Separating agent required e.g. resin (allows separation of gypsum out after setting)
42
Q

What is the expansion of gypsum on setting?

A

<0.1%

43
Q

How is impression compound material thermoplastic?

A
  • Hard when cool

* Soft when warm

44
Q

What is the softening temp of impression compound? and how is it softened?

A

55-60°C in warm water

45
Q

What form is impression compound supplied in?

A
  • Sticks/sheets

- Hard

46
Q

What are the stages of use for impression compound to take an impression?

A
  • Supplied hard
  • Soften (by warming) – polymer chains and plasticisers separate allowing deformation
  • Insert
  • Harden (by cooling) – difficult to deform
  • Remove
47
Q

What is impression compound made up of? and each of their functions.

A

• 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
Q

What is the typical composition of impression compound?

A
  • Rosin, Copal or Shellac
  • Carnauba wax
  • Stearic acid
  • Talc
  • Pigment
49
Q

What is said about the viscosity of the organic material resin?

A

Highly viscous

50
Q

What is said about the viscosity of the organic bases, waxes?

A

Viscous but more solid

51
Q

What is the main difference of type 1 and 2 impression compound?

A

Type 1 lower temp fusing (hard to flow at low temp)

Type 2 high temp fusing

52
Q

What temp does type 1 impression compound have high flow at?

A

Just above mouth temp

53
Q

Type 1 impression compound is an insulator, how and why?

A

Low thermal coefficient and low conductivity

54
Q

What is Type 1 impression compound sheet form used for? and how is it used?

A
  • 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
Q

What is Type 1 impression compound stick form used for?

A
  • 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
Q

What is the flow of Type 2 impression compound at or just above mouth temp? therefore what is its use?

A

Low flow value

Rigid at mouth temp - use as tray material

57
Q

What temp is impression compound cooled to?

A

50 degrees

58
Q

What happens if the impression compound if:

  1. Too cool
  2. Too hot
  3. Time too short
  4. Time too long
A
  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
Q

How is impression compound softened and what precautions need to be taken?

A

Sticks - soften in Bunsen flame = easier to handle
• Careful of plasticiser leaching
• Catch fire

60
Q

What are the properties of impression compound due to its high viscosity?

A
  • Mucocompressive

* No fine detail - margins for denture can be seen

61
Q

What are the properties of impression compound due to its thermal contraction?

A
  • 55°C – 23°C
  • Stress relief
  • Dimensionally unstable without stress relief
62
Q

Why can mobile teeth be extracted during an impression with impression compound?

A

Extraction of mobile teeth during impression for a partial denture due to rigidity

63
Q

What are the applications of impression compound?

A
  • Edentulous areas
  • Tray extensions
  • Beading in to take full sulcus impression
64
Q

What colour is impression compound?

A

Brown

65
Q

What are the 2 pastes used for zinc oxide eugenol? and how are they mixed?

A
  • Paste 1: ZnO, Zn acetate, oils
  • Paste 2 Eugenol, filler (e.g. kaolin)
  • Mixed by hand
66
Q

What is produced from the ZOE pastes mixed together?

A

Zinc eugenolate salt produced

67
Q

What is the ZOE reaction that takes place once the cements are mixed?

A

• Chelation reaction

  • Accelerated by: water, accelerator
  • Setting in the mouth quick
68
Q

What is the viscosity of the ZOE pastes? and when is it used?

A
  • 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
Q

What do narrow spaces do to viscosity?

A

Narrow space increases viscosity therefore NEEDS to be low viscosity

70
Q

How accurate is ZOE? and how about undercuts?

A
  • Small dimensional change on setting (thin section)
  • Cannot record undercuts – due to plastic when setting
  • Detail achieved of arches
71
Q

How dimensionally stable is ZOE?

A
  • Small dimensional change on storing (thin section)

* Small dimensional change on setting (thin section)

72
Q

Why is eugenol an irritant?

A
  • Burns can be caused by spillages e.g. on the lip
  • Allergies
  • Eugenol free versions available (similar properties but not the same)
73
Q

What are the impression stages for edentulous arches?

A

• 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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