1
Q
Definition of Impressions [1]
A
Impressions are negative reproductions of dental structures
2
Q
Requirements of an impression material [10]
A
- Capable for plastic to rigid transformation
- Acceptable for patient (Non-toxic,-irritant, setting time up to 5-7 minutes)
- Good handling properties (Easy to mix, thixotropic, adequate working and setting time)
- Accuracy and details production (Ability to replicate the intraoral surface details)
- Dimensionally stable (Ability to retain its absolute dimensional size over time)
- Resistance to mechanical stress (Elastic and rigid)
- Tear resistance ( ability to resist tearing in thin sections such as through the feather-edged material within the gingival sulcus)
- Compatible with model materials
- Resistance to disinfectants
- Cost effective
3
Q
Classification of impression material [2]
A
- Rigid/inelastic (Old, not important)
2. Flexible/elastic (Elastomers & Hydrocolloids)
4
Q
When are rigid/inelastic materials used? [1]
What is their problem? [1]
A
Used when there are no undercuts and for edentulous patients
Problem is any significant deformation produces a permanent deformation
5
Q
What are the four rigid/inelastic materials?
A
- Impression compound
- Impression plaster
- Zinc oxide eugenol
- Impression waxes
6
Q
What are impression compound made of? [3]
A
- resins (wax),
- filler (soapstone)
- lubricants (stearic acid)
7
Q
Properties of impression compound [9]
A
- Poor surface detail
- High coefficient of thermal expansion (contracts up to 0.3% when removed from mouth to room temp.)
- Distorts when removed over undercut areas
- Mucodisplacive
- Poor dimensional stability
- Can be modified by re-heating
- Non-toxic and non-irritant
8 Good shelf life
8
Q
Advantages of impression compound? [5]
Disadvantages of impression compound? [3]
A
Ad: o Non-toxic/ irritate o Reusable o Can be reheat and re-adapted o Support other materials for wash impressions o Mucocompressive
Disad:
o Poor surface details
o Poor dimensional stability
o Will distort if remove from undercuts
9
Q
What is impression plaster’s composition? [1]
What does it mix with to set? [1]
What does it become when set? [1]
A
Calcium sulphate hemihydrate
Mix with water (Often adds anti-expansion solution)
Becomes dihydrate
10
Q
Properties of impression plaster [8]
A
- Excellent at recording fine detail (because very fluid when inserted in the mouth)
- Dimensionally stable if anti-expansion solution is used
- Fractures if undercuts are present
- Mucostatic
- Needs to be treated with a separating medium (e.g. soap/varnish) before being cast in stone or plaster
- Exothermic setting reaction
- On storage dimensionally stable but a small amount of shrinkage may occur
- Non-toxic but may be unpleasant due to dryness and heat evolved during setting (exothermic)
11
Q
Advantages of impression plaster? [4]
Disadvantages of impression plaster? [3]
A
Ad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
Disad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
12
Q
What is ZOE composed of? [3+3]
A
Base paste:
- Zinc oxide
- Inert oils (plasticiser)
- Hydrogenated resins(increases setting time and improves cohesion)
Reactor paste
- Eugenol
- Zinc acetate (accelerator)
- Fillers (talc or kaolin)
13
Q
Properties of ZOE [5]
A
- Recording edentulous ridges in a close fitting special tray or the patients existing dentures
- Mixed in a 1:1 paste ratio
- Used in thin sections only (2-3mm) as a wash impression
- Vaseline is used as a separating agent on those areas requiring protection (soft tissues, teeth, etc)
- Firm pressure is used during impression taking
14
Q
Problem of ZOE? [1]
A
The set material contains both some unreacted zinc oxide and eugenol. Therefore any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression
15
Q
What does ZOE setting time depend on? [3]
What is the normal setting time for ZOE? [1]
A
- Accelerator additives (Zinc acetate/acetic acid)
- Exposure to moisture on mixing or the addition of water will accelerate the reaction
- Increasing temperature causes a fasting setting time
Setting time is normally 4-5 minutes
16
Q
Advantages of ZOE? [3]
Disadvantages of ZOE? [3]
A
Ad:
o Good dimensional stability
o Good surface detail
o Mucostatic/ Mucodisplacive
Disad:
o Only sets quickly in thin section
o Eugenol allergy
o Cannot be used in deep undercuts
17
Q
What is the composition of impression waxes? [2]
What is the ratio? [1]
A
- Low melting paraffin wax and beeswax
- 3:1 ratio
18
Q
When are impression waxes used? [1]
What precaution should one take after taking the impression? [1]
A
Used to correct small imperfections (Airblows) in other impressions such as ZOE impressions
Cast should be poured immediately (avoid distortion)
19
Q
What are flexible elastic materials? [1]
When are they used? [2]
A
These materials can be stretched and bent to a fairly large degree without suffering any deformation
- They are used for recording the patient’s mouth where undercuts are present.
- For RPD, overdentures, implants, crowns and bridge work.
20
Q
2 types of flexible elastic materials? [2]
A
- Elastomers
2. Hydrocolloids
21
Q
What is sol state? [1]
A
Relatively fluidic state when the solute particles present are dispersed thoughout the medium (usually liquid)
22
Q
What is gel state? [1]
A
Viscous and jelly-like state when the particles become attached to each other, forming a loose network with the medium which restricts movement of the solute molcules.
23
Q
What are the phases between sol, gel and sol-gel states? [3]
A
Before gel point (sol) –>fluid
At gel point –> loss of fluidity
After gel point –> gel
24
Q
When are elastomers used? [3]
What are the 2 main properties over hydrocolloid? [2]
A
- Used when high degree of accuracy is needed
- Bridge
- Crown
Ad:
- Good tear resistance
- Dimensional stability
25
Main composition of elastomers [2]
| One of them contains a few other things
1. Flexible matrix
- Multifunctional prepolymer, oligomer or polymer (not from monomer)
- Crosslinking agent
- Curing agent ('catalyst or initiator)
- Modifiers (accelerators, retarders, plasticizers, flavoring agents, colorants)
2. Filler/extender
26
What are the four main groups of elastomers? [4]
1. Polysulphide
2. Polyether
3. Polysiloxane (silicone rubber: Condensation and addition silicone)
4. 'Hybrid'
27
Composition of polysuphide? [6]
Base paste:
1. Polysulphide prepolymer
2. Filler (give body) (TiO2)
3. Plasticizers (control viscosity)
Activator paste:
1. Lead oxide (cause polymerization)
2. Sulphur (facilitates reaction)
3. Inert oil (forms a paste)
28
What type of reaction is the setting reaction for polysulphide? [1]
Condensation polymerization
29
What is the normal setting time for polysulphide? [1]
10 minutes
30
How to use polysulphide? [6]
1. 2 equal lengths (1:1) are mixed together with a spatula for about 1 min
2. The tray needs to be treated with an adhesive (rubber solution in acetone) to provide retention for the polysulphide
3. Wait for 5 mins, then place in patients mouth
4. Final setting time is usually about 10 mins from the start of mixing
5. A 1 or 2 stage impression technique may be used
6. Although dimensionally stable, the impression should be cast within 24 hours
31
Advantages of polysulphide? [5]
| Disadvantages of polysulphide? [5]
```
Ad:
o Dimensional stable
o Accuracy
o Long working time
o Long shelf life
o Comes with number of different viscosities
```
```
Disad:
o Dimensional stable
o Accuracy
o Long working time
o Long shelf life
o Comes with number of different viscosities
```
32
Properties of polysulphide [7]
1. Dimensionally stable
2. Excellent surface detail (only used in special trays)
3. Viscosity depends on the brand used
4. Very small setting contraction (0.3-0.4% over the firs 24 hrs)
5. Contract on cooling from mouth to room temperature
6. Very good tear resistance
7. Good shelf life
8. Viscoelastic
33
What is the type of setting reaction for polyether? [1]
Ionically initiated additional polymerization
34
Composition of polyether? [8+4]
```
Base:
1. Polyether prepolymer
2. Colloidal silica
3. Plasticizer (high and low viscosity)
4. Pigments
6, Flavorings
7. Triglucerides
8. Accelerators
```
```
Activator
1. Alkyl benzene sulphonate ester
2. Fillers
3 Plasticizer
4. Pigments
```
35
Properties of polyether [8]
1. Hydrophilic (absorbs water)
2. Good shelf life of up to 2 years
3. Good elastic recovery
4. Non toxic
5. Low setting contraction
6. Low tear strength
7. Excellent surface detail
8. Good dimensional stability
36
Advantages of polyether? [6]
| Disadvantages of polyether? [4]
```
Ad:
o Good dimensional stability
o Accuracy
o Ease of use
o Good on undercuts
o Excellent surface detail
o Low setting contraction
```
```
Disad:
o Allergic reaction (sulphonate ester)
o Poor tear strength
o Short working time
o Stiff set material (hard to remove from mouth)
```
37
Instruction for use of polyether? [4]
1. Mix the polymer and sulphonate ester together, they react to form a stiff polyether rubber
2. Setting time occurs in about 6 minutes
3. Usually only comes in 1 viscosity - regular bodied, but can also come as light + heavy bodied (diluent)
4, Heat and moisture speed up the setting reactions, Plasticizer thins the mixture
38
How are polysiloxane (silicone rubber) classified? [2]
By the type of chemical reaction by which they set:
1. Condensation silicone
2. Addition silicone
39
In condensation silicone setting reaction, what is the catalyst precursor? [1]
What also plays an important role in that reaction? [1]
Precursor is Tin octoate
Atmospheric H2O play an important role
40
Composition of silicone [9+3]
Base paste:
1. Polydimethysiloxane (PDMS)
2. Micronised Talc
3. Calcium carbonate
4. Pyrogenic silica
5. Paraffin Wax
6. Micronised quartz
7. Corn Starch
8. Water
9. Colourant
Activator paste:
1. Ethyl orthosilicate (TEOS)
2. Tin Octoate
3. Tri-isooctylamine
41
Instruction for use of condensation silicone [2]
1. Mix 2 pastes to react, cross-linking occurs and setting takes about 7 minutes
2. Ethanol gas is evolved on setting which leads to surface pitting and roughened surface to the resulting model
42
What is condensation silicone used for? [3]
1. Crown and bridge work mostly
2. Partial dentures, implants and overdentures
3. Stock trays and special trays
43
Within how many hours should condensation silicone be cast? [1]
Within 24 hours
44
Properties of condensation silicone [8]
1. Hydrophobic
2. Ethanol gas evolved on setting
3. Moderate shelf life
4. Moderate tear strength
5. Good surface detail
6. Shrinking of impression over time
7. Non toxic and non irritant
8. Very elastic (near ideal)
45
Advantages of condensation siliconer? [4]
| Disadvantages of condensation silicone? [3]
Ad:
1. Accurate
2. Ease of use
3. Very elastic (near ideal), used on severe undercuts
4. Good surface detail
Disad:
1. Ethanol evolution
2. Liquid component of paste/ liquid system cause irritation
3. Shrinking of impression over time
46
What type of react is the setting reaction for condensation silicone? [1]
Condensation reaction
47
Composition of addition silicone? [6+5]
Base
1. Hydrogen siloxane
2. Silicone oil
3. Fillers
4. Pigments
5. Hydrophilizers
6. Vinyle siloxane
Catalyst
1. Vinyl siloxane
2. Silicone oil
3. Fillers
4. Pigments
5. Platinum complex
48
What type of react is the setting reaction for addition silicone? [1]
Addition polymerisation reaction
49
What is evolved when addition silicone is set? [1]
Hydrogen gas
50
Instruction for use of addition silicone [2]
1. Mixing in equal portions.
2. Crosslinking occurs to form a silicone rubber
3. Setting time is about 6-8 minutes
51
Properties of addition silicone [6]
1. Good shelf life
2. Dimensionally stable
3. Moderate tear strength
4. Excellent surface detail
5. Hydrogen gas evolved when setting
6. Non toxic and non irritant
52
Advantages of addition siliconer? [4]
| Disadvantages of addition silicone? [4]
Ad:
1. Accurate
2. Ease of use
3. Fast setting
4. Wide range of viscosities
Disad:
1. Hard to mix
2. Sometimes difficult to remove the impression from the mouth
3. Too accurate in some circumstances (Cast produced is not sufficiently oversized)
4. Hydrogen evolved when setting
53
Hybrid's claimed advantages: [6]
| And the facts [2]
Claimed ad:
1. Optimised advantages from both addition silicone and polyether
2. Hydrophilic
3. Dimensional accuracy
4. High tear strength
5. Better elasticity
6. High resistance to deformation
Facts:
1. Unknown chemistry (Probably a kind of vinylsiloxane-polyether copolymer, with Pt catalyst)
2. Lab studies showed comparable performance with addition silicone and polyether
54
Revise properties comparison on ppt
Go revise
55
What is hydrocolloid? [2]
A colloid is a state of matter in which individual particles of one substance, are uniformly distributed in a dispersion medium of another substance
When the dispersion medium is water, it is termed a hydrocolloid
56
Which are the 2 types of hydrofcollloids? [2]
Agar (reversible)
| Alginate (Irreversible)
57
Hydrocolloids are placed in the mouth in the ___ state when it can record sufficient detail, then removed when it has reach the ___ state
sol
Gel
58
Hydrocolloids materials especially the alginates, may display _________ with some makes of dental stone. The resultant model may show reduced ______ ______ and possibly ________ _______ and _______
incompatibility
Surface hardness
surface irregularities
roughness
59
Composition of agar [4]
1. Agar
2. Borax, increases the gel strength
3. Potassium sulfate -hardener
4. Water - dispersion medium
60
At what temperature does agar sols forms gels upon cooling of a hot solution? [1]
Agar gels melt to sols upon heating to what temperature? [1]
30-40oC
90-95oC
61
Agar chemistry:
in sol agar forms ____
in Gelation I agar forms ____
in Gelation agar forms ____
Random coils
Double helices
Aggregation of helices
62
Instruction for use of agar [6]
1. Supplied in sealed tube to prevent evaporation of water
2. The tubes are heated in boiling water (in water bath) for 10-45mins
3. Once the impression is taken, the tray can be cooled with water to aid gel formation
4. A higher temperature is needed to convert the gel into a sol
5. The first material to set is that which is in contact with the tray since it is cooler than tissues
6. Thus it is the material in contact with the tissue with stays in the sol state for the longest time
63
Properties of agar [8]
1. Good surface detail
2. Can be used on undercuts, but liable to tear on deep undercuts
3. Evaporation or imbibtion
4. Non toxic and non irritant
5. Slow setting time
6. Poor tear resistance
7. Adequate shelf life
8. Can be sterilised by an aqueous solution of hypochlorite
64
Advantages of agar? [2]
| Disadvantages of agar? [2]
Ad:
1. Good surface detail
2. Reusable and easily sterilised
Disad:
1. Need special equipment (water bath) and special technique
2. Dimensionally instability
65
Composition of alginate [8]
1. Sodium/potassium alginate
2. Calcium sulfate
3. K2SO4, silicates, borates
4. Sodium phosphate
5. Diatomaceous earth
6. Colourant
7. Flavouring
8. Disinfectant
RPD
flashcards
Decks in class (2)
# Cards
