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Fixed Prosthodontics I > L1b-Impression materials > Flashcards

L1b-Impression materials Flashcards

1
Q

What are impression materials used for?

A
  • To record the details of intraoral structures in order to fabricate a reproduction of hard (teeth) and soft tissues for the construction of dental prosthesis.
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2
Q

Ideal properties of impression materials:

A
  • Produce an accurate replica or the intra oral structure.
  • Prevent deformation.
  • Be atraumatic when removing from undercuts.
  • Have proper setting time and biocompatibility.
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3
Q

What is an impression?

A

A negative reproduction of the tissues.

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4
Q

How is a positive cast made?

A

By filling the impression with dental stone or other model material, a positive cast is made.

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5
Q

impression materials Classification:

A
  • elastic or
  • non-elastic
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6
Q

Which are the non-elastic impression materials?

A
  • Impression plaster
  • Impression compound
  • Impression waxes
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7
Q

elastic impression-subclassification:

A
  • Synthetic elastomers
  • Hydrocolloids
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8
Q

Hydrocolloids:

A
  • Reversible (Agar)
  • Irreversible (Alginate)
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9
Q

Synthetic elastomers:

A
  • Polysulphides
  • Polyether
  • Silicones
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10
Q

Subgroups of silicones:

A
  • Additional
  • Condensation
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11
Q

Desirable qualities of an impression material:

A
  • A pleasant odor, taste, and acceptable color.
  • Absence of toxic or irritant constituents.
  • Adequate shelf life for requirements of storage and distribution.
  • Economically commensurate with the results obtained.
  • Easy to use with the minimum of equipment.
  • Setting characteristics that meet clinical requirements.
  • Satisfactory consistency and texture.
  • Readily wets oral tissues.
  • Elastic properties that allow easy removal of the set material from the mouth and good elastic recovery.
  • Adequate strength to avoid breaking or tearing upon removal from the mouth. Good dimensional stability over temperature and humidity, long enough to permit the production of a cast or die.
  • Compatibility with cast and die materials.
  • Accuracy in clinical use.
  • Readily disinfected without loss of accuracy.
  • No release of gas or other byproducts during the setting of the impression or cast and die materials.
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12
Q

What is a colloid?

A
  • Colloid is a mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance.
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13
Q

Which is the form in which colloids can exist?

A

In the form of:

  • viscous liquid known as a sol
  • or a jelly like elastic semi-solid described as a gel
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14
Q

When is a colloid called hydrocolloid?

A

If the particles are suspended in water, the suspension is called hydrocolloid.

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15
Q

How can a sol change into gel?

A
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16
Q

Characteristic of hydrocolloids:

A
  • They are hydrophilic with very good wettability
  • have very limited dimensional stability because they are composed of 85% water.
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17
Q

What is agar?

A
  • It is a polysaccharide (a sulfuric ester of a linear polymer of galactose) extracted from certain types of seaweed.
  • Although it is an excellent impression material and yields accurate impressions, presently it has been largely replaced by alginate hydrocolloid and rubber
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18
Q

What can agar be used for?

A
  • For cast duplication (during fabrication of cast metal removable partial denture)
  • For full mouth impressions without deep undercuts
  • For crown and bridge impressions (before elastomers came to the market)
  • As tissue conditioner
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19
Q

What is alginate?

A
  • It is a mucous extract yielding from certain brown sea weeds.
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20
Q

Alginate-chemical reaction:

A
  • They change from the sol phase to the gel phase because of a chemical reaction.
  • Once gelation is completed, the material cannot be re-liquefied to a sol.
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21
Q

Alginate-Setting reaction:

A
  • Sodium/Potassium alginate powder (soluble) dissolves in water to form asol.
  • The sol reacts with calcium sulfate (reactor) to form calcium alginate (insoluble gel).
  • This reaction is delayed by addition of a retarder (sodium phosphate), to give more working time.
22
Q

Alginate impression powder-Ingredients

A
23
Q

Alginate-Advantages & disadvantages:

A

Advantages:

  • Easy to mix and manipulate, need minimum equipment
  • If properly handled, gives accuracy and good surface details even in presence of saliva.
  • Low cost
  • Hygienic and comfortable to the patient

Disadvantages:

  • Poor tear strength and dimension stability (poured with in 15 minutes).
  • Not recommended when high level of accuracy is required (crown and bridge).
  • May occur distortion if the material is not properly handled.
  • It cannot be corrected.
24
Q

Properties of Alginate:

A
  • Syneresis
  • Imbibition
25
Q

What is Syneresis?

A
  • Alginate impressions may lose water by expulsion of liquid molecules.
26
Q

What is Imbibition?

A
  • Gain water when immersed into it.
27
Q

What can both phenomenon of Syneresis and Imbibition cause?

A

Both the phenomenon can cause dimensional changes and produce inaccurate casts.

  • To avoid this, impression should be poured immediately (within 15 min).
28
Q

Alginate indication:

A
  • fabrication of diagnostics casts
29
Q

What is Polysulfide?

A
  • elastomer that is also known as mercaptan, or simply as rubber base
30
Q
  1. What is Polysulfide packaged in?
  2. What does this result into?
  3. What is the reaction of the two pastes?
A
  1. in two tubes, a base and an accelerator (reactor)
  2. When the two pastes are mixed equally, it results in an increased viscosity and finally in an elastic material.
  3. exothermic
31
Q

Polysulfide:

Which is the composition of base paste and reactor paste?

A
32
Q

Polysulfide-Properties:

A
  • Stability.
  • Low contraction (shrinks 0.3- 0.4% during first 24h).
  • High tear strength.
  • Hydrophobic.
  • Excellent reproduction of details.
  • Excellent shelf life.
  • Highly compatible with stone and plaster.
33
Q

Polysulfide:

Working time - Setting time:

A

Mixing and working time:

  • 4-6 min.

Setting time:

  • 12-16 min.
34
Q

Polysulfide indication:

A
  • Mostly for partial and complete dentures
35
Q

Polysulfide:

Advantages and disadvantages:

A

Advantages:

  • Good stability and accuracy
  • It comes in different viscosities.
  • Long working (4-7 min) and setting time (7-10 min)

Disadvantages:

  • Lead oxide in the base paste can be toxic.
  • Mixing procedure can become messy.
  • Unpleasant odor and color
  • Requires the use of custom tray.
36
Q

Condensation silicones-Components:

A

Base:

  • Polydimethyl siloxane (Hydroxy terminated).
  • Colloidal silica or micro-sized metal oxide (filler).
  • Color pigments.

Accelerator:

  • Ortho-ethyl silicate (cross linking agent).
  • Tin (Sn) Octoate (catalyst).
37
Q

Condensation reaction-polymerization reaction:

A
  • Ethyl alcohol eliminates from the reaction, the evaporation of which is responsible for shrinkage and dimensional instability.
38
Q

Condensation silicones-Properties:

A
  • High permanent deformation (1-3)% due to shrinkage.
  • Tear strength is lower than polysulfides.
  • Hydrophobic.
  • Requires dry areas of mouth.
39
Q

Condensation silicones:

Working time-Setting time:

A

Working time:

  • 2-4 min.

Setting time:

  • 6-8 min.
40
Q

Condensation silicones-Indications:

A

Mostly for lab work (duplication of casts and wax –ups)

41
Q

Condensation silicones:

Advantages and disadvantages:

A

Advantages:

  • Clean and pleasant.
  • Custom tray not required.
  • Good working time.
  • Ease of use.
  • Relatively economic.

Disadvantages:

  • High polymerization shrinkage.
  • Low tear strength.
  • Second pour is not so accurate.
  • Volatile (evaporation of by-product). Hydrophobic.
42
Q

Additional silicones-Composition:

A

Base paste:

  • Terminal silane hydrogen groups
  • Fillers

Catalyst paste:

  • Terminal vinyl groups
  • Platinum salt (catalyst)
  • Palladium (hydrogen absorber)
  • Filler

release hydrogen gas

43
Q

additional silicones-Properties:

A
  • Pleasant odor and color.
  • Excellent reproduction of surface details.
  • Good tear strength.
  • Hydrophobic material.
  • Wettability can be improved with surface active agents.
44
Q

Additional silicones:

Working time-Setting time:

A

Working time:

  • 1-4 min.

Setting time:

  • 4-6 min.
45
Q

Additional silicones- Indications:

A

Fixed prosthesis and implant restorations.

46
Q

Additional silicones-Advantages & Disadvamtages

A

Advantages:

  • Ease of use.
  • Fast setting.
  • Excellent dimensional stability.
  • Can be re-poured
  • Good shelf life (1-2 years).

Disadvantages:

  • Hydrophobic (although improved). No flow if sulcus is moist.
  • Sometimes is difficult to remove from mouth (undercuts).
  • High cost.
47
Q

Polyether-Composition:

A

Base paste:

  • Poly ether polymer.
  • Silica (filler).
  • Glycol ether or phthalate (plasticizer)

Catalyst paste:

  • Aromatic sulfonate ester (cross-linking agent).
  • Silica (filler).
  • Phthalate or glycol ether (plasticizer).
48
Q

Polyether-Properties:

A
  • Pleasant odor and color.
  • Very good dimensional stability.
  • Adequate tear resistance and elastic properties, approaching those to addition silicones.
  • Hydrophilic material.
49
Q

Polyether:

Working time-Setting time:

A

Working time:

  • 3 min.

Setting time:

  • 6 min.
50
Q

Polyether- Indications:

A

Fixed prosthesis and implant restorations.

51
Q

Polyether:

Advantages and Disadvantages:

A

Advantages:

  • Ease of use.
  • Excellent dimensional stability.
  • Accuracy.
  • Multiple pouring.
  • Can be poured hours after impression is taken.

Disadvantages:

  • May cause allergic reaction due to the sulphonic acid.
  • Stiff set material (could be difficult to remove from mouth).
  • High cost.

Fixed Prosthodontics I (12 decks)

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