A- Materials Flashcards
(89 cards)
1
Q
What is an impression?
A
Accurate reproduction of the oral cavity. A negative imprint of hard (teeth) and soft tissues in the mouth from which a positive reproduction (or cast) can be formed.
2
Q
What 2 materials are trays usually?
A
Metal: Stainless steel
Plastic: Nylon-based and Polystyrene-based
3
Q
Px factors with impression materials
A
- Neutral taste and odour
- Short setting time
- Small tray
- Easily removed impression
- No retakes
- Non-toxic
4
Q
Dentist factors with imp materials
A
- Easily mixed
- Short working times
- Good quality impressions
- Low cost
- Easily disinfected
- Simple procedure
5
Q
What makes a good impression?
A
- Accurate reproduction of surface detail
- Dimensional accuracy and stability
6
Q
What affects accuracy of reproduction of surface detail?
A
viscosity, wettability
7
Q
What affects dimensional accuracy/stability?
A
shrinkage on setting, cooling contraction, permanent set, storage stability, type of tray
8
Q
Examples of Poor reproduction of surface detail of imps (3)
A
- Rough/uneven surface
- Air bubbles
- Irregular shaped voids
9
Q
What might cause a rough/uneven surface?
A
Incomplete set (premature removal, improper mixing, contamination)
Rapid set (humidity, temp, wrong mix)
10
Q
What might cause air bubbles?
A
Rapid set
Improper mixing
Surface contamination (moisture)
11
Q
What might cause irregular shaped voids?
A
Surface contamination (moisture)
Premature movement
12
Q
Examples of poor fit?
A
- Distortion
- Casting too big
- Casting too small
13
Q
What might cause distortion?
A
Adhesive failure between tray and impression material
Tray not sufficiently rigid
Excessive seating pressure resulting in too much permanent set
Movement of tray during setting
14
Q
What might cause a too big cast?
A
Inappropriate impression technique
Model poured too late (excessive shrinkage on storage)
Impression stored under wrong conditions
15
Q
What might cause a too small cast?
A
Inappropriate impression technique
Model poured too early (insuffient time for elastic recovery)
Impression stored under wrong conditions
16
Q
Why are bad imps bad!!!?
A
- Extra chair-side time
- Repeat visits for pt
- Pt distress
- Waste of expensive materials
- Remake bill from the lab
- Spoil relationship with lab
17
Q
What is mucostatic impression technique?
A
Imp material is fluid enough to flow and does not displace the oral tissues.
18
Q
4 examples of mucostatic imp materials
A
1- impression plaster
2- agar imp material
3- zinc oxide eugenol
4- Light body elastomers
19
Q
What is mucocompressive imp technique?
A
Imp material is viscous and is able to compress the oral tissues on insertion in the px mouth
20
Q
3 examples of mucocompressive imp materials
A
1- impression compound
2- viscous alginate
3- some rigid elastomers
21
Q
What is TEC?
A
change in length, when determined per unit length for a 1’C change in temperature (∝)
22
Q
What is Rheology?
A
study of flow of materials, for liquids: flow is measured by viscosity
Viscosity= n = shear stress/ shear rate
Units of viscosity= Pa.s
23
Q
Name 3 viscous behaviours
A
- Newtonian
- Dilatant
- Pseudoplastic
24
Q
Describe each viscous behaviour (3)
A
Newtonian: constant viscosity, u, across all shear rates; many common fluids e.g. water.
Dilatant: Shear-thickening fluids increase in apparent viscosity at higher shear rates.
rarely encountered e.g. silly putty
Pseudoplastic: Shear-thinning fluids have a lower apparent viscosity at higher shear rates e.g. silicone impression materials
25
What is Thixotropic behaviour?
time-dependent viscosity
Thixotropy is defined as the progressive decrease in viscosity with time for a constant applied shear stress, followed by a gradual recovery when stress is removed.
- Characteristic of many elastomeric impression materials and improves handling properties
- Some degree of molecular rearrangement caused by mixing
26
Why is hydrophilic a good property for impression materials?
good wetting is the ability of a liquid to cover the surface of the substrate completely.
smaller contact angle.
* Fewer voids
* Less entrapment of oral fluids
* Bubble-free dyes and models
* Fewer retakes
27
Surface tensions and perfect wetting...
For perfect wetting the critical surface tension of the SOLID must be greater than the surface tension of the liquid...
28
NAme some rigid Types of impression materials: (4)
1- Waxes
2- impression compound
3- Zinc Oxide- Eugenol
4- Plaster of Paris (gypsum)
29
Name some flexible imp materials
1- Hydrocolloids e.g. Agar, alginate, colloidal silica in resin
2- Elastomers
30
Sources of wax
- Mineral
- Parraffin wax
- Animal beeswax
- Vegetable
- Carnauba and candelilla wax
31
Properties of wax
- Softening temp (Tg~ 42 degrees’C), just above mouth temp
- Rigid- good for a single tooth
- Poor thermal conductor- surface cools faster than inside (stress), subject to stress relief
- Low viscosity
- High Thermal Expansion Coefficient (TEC) - cooling contraction, Waxes tend to expand a lot: not a good property
32
Types of dental waxes
- Modelling waxes: registration blocks and try-in dentures
- Inlay waxes: inlay patterns in the mouth (direct) or on a model or die (indirect)
- Sheet casting wax: construction of partial dentures
- Sticky wax: joining metal parts prior to soldering, joining broken denture prior to repair
- Carding and boxing-in wax: boxing-in impressions, mounting teeth in sets
33
Properties of impression compound
- Softening temp (Tg~ 55-60 degree’C)
- Rigid
- High viscosity- mucocompressive
- Poor thermal conductivity, subject to stress relief
- High TEC
- Thermoplastic
34
Impression compound composition
Natural/synthetic Resins + Plasticisers + Fillers
35
Common applications of impression compound
- Full or partial impressions
- Sticks for copper band impressions
- For adding to the periphery of an impression tray
36
What is in the base and reactor paste of ZoE?
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)
37
Adv and Disadv for ZoE?
Mucostatic
Advantages:
- Dimensional stability
- Good surface detail- accurate reproduction due to hydrophilic nature
- Stable on storage and good shelf life
Disadvantages:
- Cannot be used in very deep undercuts
- Only sets quickly in thin section
- Eugenol allergy in some patients- burning sensation of lips
- Tendency to stick to skin
38
What is plaster of paris?
Crystalline mineral of hydrated calcium sulphate with chem formula= CaSO4 • 2H2O
39
Properties of plaster of paris?
* Colourless or white
* Not highly water-soluble
* Not at all hard
* Gypsum + water => the gypsum hardens as the water evaporates
* Gypsum is mainly used in the partly dehydrated form of plaster of Paris= 2CaSO4 • H2O
40
Composition of impression plaster:
• Powder=
- Calcium Sulphate Beta-Hemihydrate
- Borax (for a slower setting rate)
- Potassium sulphate (to reduce expansion/ accelerates setting)
- Starch (aid disintegration of impression on separation from model)
• Liquid= - Water
41
Why does Plaster expand on setting?
Interaction between growing gypsum crystals results in formation of stresses and ultimately in the expansion of the material (consider dimensional accuracy, shrinkage of model?).
Expansion of 0.3-0.6%.
42
Anti-expansion solution?
Anti-expansion solution=
4% potassium sulphate + 0.4% Borax
Potassium Sulphate reduces expansion to 0.05%, but also accelerates setting reaction.
Borax is added as a retarder to give more time to take the impression.
43
Advantages of plaster of paris
- Easy to mix
- Low viscosity; mucostatic
- Good dimensional stability and accuracy
- Cheap
44
Disadvantages of plaster of paris
- Low strength
- Rough surface finish
- Rigid once set
- Dry sensation in the mouth
45
Common applications: of plaster of paris
- Useful for making impressions of patients with excessively mobile soft tissue
- Model material set in impression moulds
46
What is a colloid?
Heterogenous mixture of 2 phases, where the 2 phases are not readily differentiated
- Agar (reversible)
- Alginate
47
Uses of Agar- reversible hydrocolloid?
used for complex impressions for advanced restorative work.
| Used in labs to duplicate models bc can be reused many times.
48
Properties of agar
- Good surface detail
- Can be used on undercuts, but liable to tear on deep undercuts
- Non-toxic and non-irritant
- Slow setting time
- Poor tear resistance
- Adequate shelf life
- Can be sterilised by an aqueous solution of hypochlorite
49
Advantages of agar
- Good surface detail
- Reusable
- Relatively easily sterilised
50
Disadvantages of agar
- Need special equipment (water bath) and special technique
| - Dimensional instability
51
Composition of alginate?
- Sodium alginate; hydrogel former
- Calcium sulphate dihydrate; provides calcium ions
- Sodium phosphate; controls working time
- Potassium sulphate; enhances setting of model
- Fillers; controls consistency
- Sodium silicofluoride; controls pH
52
Give 3 examples of elastomers
polysulphide,
polyether
silicone polymers (condenstation or addition poly)
53
Properties of elastomers
- Dust free powder
- Cheap
- Limited Shelf life
- Well controlled working and setting times
- Mucostatic
- Hydrophillic
- Poor surface reproduction
- Poor storage stability
- Imbibition (water sorption); water is absorbed by solid-colloids causing an increase in vol
- Syneresis (water loss); shape change as loss of liquid from a gel causing a reduction in vol
- Low tear strength
- Excessive permanent deformation
54
Uses of elastomers
Mainly used for – crown, bridge work
Also used for- partial dentures, overdentures, implants
55
Ease of mixing of elastomers
PS and CCS difficult- diff amounts of base and catalyst paste required.
Gun delivery system for ACS! easy!
56
Working and setting times of elastomers
PS long times.. 6 and 12 mins respec.
CCS- erratic setting, moisture contamination
PE= consistent times.
ACS- good times but setting inhibited by latex gloves.
57
Mechanical properties (sitffness, permanent set and tear strength of elastomers)
Stiffness: PS < CCS < ACS < PE
Permanent set: PS > PE > CCS > ACS
Tear strength: PS >> PE > CCS = ACS
58
Surface detail of elastomers?
- All show excellent reproduction of surface detail on dry surfaces
- Polyethers are generally best as these are more hydrophilic than the other impression materials
- Surfactants have been added to the addition-cured silicones to improve their wettability
59
Ideal properties of denture-based materials
* Biocompatible
* Aesthetic
* Hygienic (resistant to bacterial contamination)
* Dimensionally stable
* High strength, stiffness, hardness and toughness
* Low density
* Cost
* Radiopacity
• Ease of:
o processing,
o Repair/adjust
o Reproduction of surface detail
60
Commonly used denture-based materials
- Polymethyl methacrylate (PMMA)
- Polyethereherketone (PEEK)
- Nylon: Flexible dentures (Valplast)
- Polyamide (Bredent)
- Cobalt Chrome alloy
- Titanium
61
What is Polymethyl methacrylate (PMMA)
```
Plastic
Organic polymer
Acrylic: really acrylate
PMMA tradenames= Acrylite and Perspex
Thermoplastic, Tg ranges from 85 to 165’C
```
Long chain molecule of repeated units of Methyl Methacrylate
62
To start the polymerisation process of PMMA we use the initiator called:
Benzoyl Peroxide
63
Processing problems of PMMA
- Porosity
- Polymerisation shrinkage
- Processing strains
64
Why might porosities occur in PMMA?
- Temp raised too quickly during curing
- Reaction if EXO and monomer has a relatively low temp!
AVOID- Slow heating cycle when curing and under pressure
65
Problems with Polymerisation Shrinkage?
o Can result in denture not contacting the hard palate or porosity
AVOID=
o Use polymer beads to reduce the necessary polymerisation to a minimum
o Incorporate a post dam on to the denture to compensate
66
Problems with Processing strains?
arise due to dissimilar materials being used (e.g. ceramic teeth or CoCr componenets)
o Also if polymer is cooled too quickly
67
Composition of PMMA:
Powder:
- Polymethyl methacrylate granules
- Initiator
- Pigments, dyes and opacifiers
- Plasticisers
- Synthetic fibres (nylon)
Liquid:
- Methyl methacrylate monomer
- Initiator (hydroquinone)
- Cross-linking agent
68
How is cold cure (autocure) PMMA different?
o Mainly used for repairs or attaching teeth to a CoCr RPD
o Lower molecular weight more porosity
o Therefore, is less dense, softer, weaker and is prone to discolouration
o High impact
o Contains copolymer of butadiene and styrene
o Results in a dispersion of rubber inclusions
69
What are denture teeth made out of?
- Acrylic
- Highly cross-linked acrylic
- Composite
- Ceramic
70
Ideal properties: of denture teeth
- Biocompatible
- Aesthetic
- Hygienic (resistant to bacterial contamination)
- Dimensionally stable
- High strength, stiffness, hardness and toughness
- High thermal conductivity
- Low density
- Cost
- Ease of processing, repair/adjust, reproduction of surface detail
- Radiopacity
71
Mechanical properties
- Strength: tensile, compressive, flexural, impact (static)
- Strength: fatigue strength, creep strength (dynamic)
- Stiffness
- Resilience
- Toughness
- Bond strength (to teeth or repair)
72
What is stress?
the force per unit cross-sectional area, that is acting on a material
Stress= F/A =
73
What is strain?
the fractional change in the dimensions caused by the force
Strain= (L1-L0)/L0
74
What is resilience?
the amount of energy a material can absorb without undergoing any plastic deformation
75
What is ductility?
the amount of plastic strain at fracture
76
What is toughness?
the amount of energy a material can absorb up to the point of fracture, represented by the area under the stress/strain curve
77
Alternative processing routes
‘Injection molding’ of acrylic dough
Injection molding of acrylic above Tg
Milling from block
78
Processing
Two part mould and acrylic dough
79
What is Relining?
Carried out when denture becomes ill-fitting due to bone resorbtion.
80
Name a denture relining material
Conventional PMMA
Cold cure resin
Tokuso rebase (chairside reline)
Soft liners
81
How is denture relining done?
- Usually reline using conventional PMMA.
- Remove undercuts & periphery chairside
- Take an impression (ZOE or Silicone)
- Send to lab, Model is cast
- Vertical dimension recorded (articulator or reline jig)
- Impression material replaced with wax
- Usual processing to replace wax with acrylic
82
When is cold cure resin used to reline (autocure)?
Cold cure resin may be used if this is to be a temporary measure or the procedure is being carried out on an RPD
83
What is chairside reline?
Poor control over vertical dimension, therefore we would normal carry out this procedure in the lab.
Not very pleasant for the patient
```
Tokuso is pretty good but:
Remove too soon – distortion
Remove too late – exothermic reaction
Residual monomer
Colour retention
```
84
What are Softliners?
Soft liners may be used to reline (these can be incorporated at time of production or added to an existing denture
85
Property requirements for soft liners?
```
Low elastic modulus
Retain low elastic modulus
High resilience
Good adhesion to denture base
High tear strength
Biocompatible
Antibacterial
Dimensionally stable
Good surface wettability
```
86
Soft liner Materials (3)
Plasticised Heat cured acrylic
e.g Coe Super Soft
Silicone rubber
Cold Cured - e.g Flexibase, TSR
Heat cured - e.g. Molloplast-B
Silicone/Acrylic co-polymer
e.g. Flexor
87
Silicone-based materials- properties/ merits
```
Highly resilient
Retain softness
Weak bond to acrylic
Susceptible to growth of candida
Poor tear strength
No permanent deformation
Poor wettability
Needs regular replacement
```
88
Acrylate-based materials- properties/merits
```
Not as resilient as silicones
Go hard with time
Resistant to bacterial growth
Excellent bond to acrylic
Acceptable tear strength
Susceptible to creep
Good wettability
needs regular replacement
```
89
What are Tissue Conditioners (Visco-gel)?
These are temporary linings that allow traumatised tissue to recover before carrying out definitive treatments.
Polyethylmethacrylate (low Tg)
Butyl phthalyl glycolate (plasticiser)
Ethyl alcohol (solvent)
