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DEN203 > A- Materials > Flashcards

A- Materials Flashcards

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.

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2
Q

What 2 materials are trays usually?

A

Metal: Stainless steel

Plastic: Nylon-based and Polystyrene-based

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3
Q

Px factors with impression materials

A
  • Neutral taste and odour
  • Short setting time
  • Small tray
  • Easily removed impression
  • No retakes
  • Non-toxic
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4
Q

Dentist factors with imp materials

A
  • Easily mixed
  • Short working times
  • Good quality impressions
  • Low cost
  • Easily disinfected
  • Simple procedure
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5
Q

What makes a good impression?

A
  • Accurate reproduction of surface detail

- Dimensional accuracy and stability

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6
Q

What affects accuracy of reproduction of surface detail?

A

viscosity, wettability

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7
Q

What affects dimensional accuracy/stability?

A 
shrinkage on setting, 
cooling contraction, 
permanent set, 
storage stability, 
type of tray
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8
Q

Examples of Poor reproduction of surface detail of imps (3)

A
  • Rough/uneven surface
  • Air bubbles
  • Irregular shaped voids
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9
Q

What might cause a rough/uneven surface?

A

 Incomplete set (premature removal, improper mixing, contamination)
 Rapid set (humidity, temp, wrong mix)

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10
Q

What might cause air bubbles?

A

 Rapid set
 Improper mixing
 Surface contamination (moisture)

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11
Q

What might cause irregular shaped voids?

A

 Surface contamination (moisture)

 Premature movement

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12
Q

Examples of poor fit?

A
  • Distortion
  • Casting too big
  • Casting too small
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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

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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

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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

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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
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17
Q

What is mucostatic impression technique?

A

Imp material is fluid enough to flow and does not displace the oral tissues.

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18
Q

4 examples of mucostatic imp materials

A

1- impression plaster
2- agar imp material
3- zinc oxide eugenol
4- Light body elastomers

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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

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20
Q

3 examples of mucocompressive imp materials

A

1- impression compound
2- viscous alginate
3- some rigid elastomers

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21
Q

What is TEC?

A

change in length, when determined per unit length for a 1’C change in temperature (∝)

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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

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23
Q

Name 3 viscous behaviours

A
  • Newtonian
  • Dilatant
  • Pseudoplastic
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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

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25
Q

What is Thixotropic behaviour?

A

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
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26
Q

Why is hydrophilic a good property for impression materials?

A

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
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27
Q

Surface tensions and perfect wetting…

A

For perfect wetting the critical surface tension of the SOLID must be greater than the surface tension of the liquid…

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28
Q

NAme some rigid Types of impression materials: (4)

A

1- Waxes
2- impression compound
3- Zinc Oxide- Eugenol
4- Plaster of Paris (gypsum)

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29
Q

Name some flexible imp materials

A

1- Hydrocolloids e.g. Agar, alginate, colloidal silica in resin
2- Elastomers

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30
Q

Sources of wax

A
  • Mineral
  • Parraffin wax
  • Animal beeswax
  • Vegetable
  • Carnauba and candelilla wax
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31
Q

Properties of wax

A
  • 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
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32
Q

Types of dental waxes

A
  • 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
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33
Q

Properties of impression compound

A
  • Softening temp (Tg~ 55-60 degree’C)
  • Rigid
  • High viscosity- mucocompressive
  • Poor thermal conductivity, subject to stress relief
  • High TEC
  • Thermoplastic
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34
Q

Impression compound composition

A

Natural/synthetic Resins + Plasticisers + Fillers

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35
Q

Common applications of impression compound

A
  • Full or partial impressions
  • Sticks for copper band impressions
  • For adding to the periphery of an impression tray
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36
Q

What is in the base and reactor paste of ZoE?

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)

37
Q

Adv and Disadv for ZoE?

Mucostatic

A

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
Q

What is plaster of paris?

A

Crystalline mineral of hydrated calcium sulphate with chem formula= CaSO4 • 2H2O

39
Q

Properties of plaster of paris?

A
  • 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
Q

Composition of impression plaster:

A

• 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
Q

Why does Plaster expand on setting?

A

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
Q

Anti-expansion solution?

A

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
Q

Advantages of plaster of paris

A
  • Easy to mix
  • Low viscosity; mucostatic
  • Good dimensional stability and accuracy
  • Cheap
44
Q

Disadvantages of plaster of paris

A
  • Low strength
  • Rough surface finish
  • Rigid once set
  • Dry sensation in the mouth
45
Q

Common applications: of plaster of paris

A
  • Useful for making impressions of patients with excessively mobile soft tissue
  • Model material set in impression moulds
46
Q

What is a colloid?

A

Heterogenous mixture of 2 phases, where the 2 phases are not readily differentiated

  • Agar (reversible)
  • Alginate
47
Q

Uses of Agar- reversible hydrocolloid?

A

used for complex impressions for advanced restorative work.

Used in labs to duplicate models bc can be reused many times.

48
Q

Properties of agar

A
  • 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
Q

Advantages of agar

A
  • Good surface detail
  • Reusable
  • Relatively easily sterilised
50
Q

Disadvantages of agar

A
  • Need special equipment (water bath) and special technique

- Dimensional instability

51
Q

Composition of alginate?

A
  • 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
Q

Give 3 examples of elastomers

A

polysulphide,
polyether
silicone polymers (condenstation or addition poly)

53
Q

Properties of elastomers

A
  • 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
Q

Uses of elastomers

A

Mainly used for – crown, bridge work

Also used for- partial dentures, overdentures, implants

55
Q

Ease of mixing of elastomers

A

PS and CCS difficult- diff amounts of base and catalyst paste required.

Gun delivery system for ACS! easy!

56
Q

Working and setting times of elastomers

A

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
Q

Mechanical properties (sitffness, permanent set and tear strength of elastomers)

A

Stiffness: PS < CCS < ACS < PE

Permanent set: PS > PE > CCS > ACS
Tear strength: PS&raquo_space; PE > CCS = ACS

58
Q

Surface detail of elastomers?

A
  • 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
Q

Ideal properties of denture-based materials

A
  • 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
Q

Commonly used denture-based materials

A
  • Polymethyl methacrylate (PMMA)
  • Polyethereherketone (PEEK)
  • Nylon: Flexible dentures (Valplast)
  • Polyamide (Bredent)
  • Cobalt Chrome alloy
  • Titanium
61
Q

What is Polymethyl methacrylate (PMMA)

A 
	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
Q

To start the polymerisation process of PMMA we use the initiator called:

A

Benzoyl Peroxide

63
Q

Processing problems of PMMA

A
  • Porosity
  • Polymerisation shrinkage
  • Processing strains
64
Q

Why might porosities occur in PMMA?

A
  • 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
Q

Problems with Polymerisation Shrinkage?

A

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
Q

Problems with Processing strains?

A

arise due to dissimilar materials being used (e.g. ceramic teeth or CoCr componenets)

o Also if polymer is cooled too quickly

67
Q

Composition of PMMA:

A

Powder:

  • Polymethyl methacrylate granules
  • Initiator
  • Pigments, dyes and opacifiers
  • Plasticisers
  • Synthetic fibres (nylon)

Liquid:

  • Methyl methacrylate monomer
  • Initiator (hydroquinone)
  • Cross-linking agent
68
Q

How is cold cure (autocure) PMMA different?

A

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
Q

What are denture teeth made out of?

A
  • Acrylic
  • Highly cross-linked acrylic
  • Composite
  • Ceramic
70
Q

Ideal properties: of denture teeth

A
  • 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
Q

Mechanical properties

A
  • Strength: tensile, compressive, flexural, impact (static)
  • Strength: fatigue strength, creep strength (dynamic)
  • Stiffness
  • Resilience
  • Toughness
  • Bond strength (to teeth or repair)
72
Q

What is stress?

A

the force per unit cross-sectional area, that is acting on a material

Stress= F/A = 

73
Q

What is strain?

A

the fractional change in the dimensions caused by the force

Strain= (L1-L0)/L0

74
Q

What is resilience?

A

the amount of energy a material can absorb without undergoing any plastic deformation

75
Q

What is ductility?

A

the amount of plastic strain at fracture

76
Q

What is toughness?

A

the amount of energy a material can absorb up to the point of fracture, represented by the area under the stress/strain curve

77
Q

Alternative processing routes

A

‘Injection molding’ of acrylic dough
Injection molding of acrylic above Tg
Milling from block

78
Q

Processing

A

Two part mould and acrylic dough

79
Q

What is Relining?

A

Carried out when denture becomes ill-fitting due to bone resorbtion.

80
Q

Name a denture relining material

A

Conventional PMMA
Cold cure resin
Tokuso rebase (chairside reline)
Soft liners

81
Q

How is denture relining done?

A
  • 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
Q

When is cold cure resin used to reline (autocure)?

A

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
Q

What is chairside reline?

A

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
Q

What are Softliners?

A

Soft liners may be used to reline (these can be incorporated at time of production or added to an existing denture

85
Q

Property requirements for soft liners?

A 
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
Q

Soft liner Materials (3)

A

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
Q

Silicone-based materials- properties/ merits

A 
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
Q

Acrylate-based materials- properties/merits

A 
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
Q

What are Tissue Conditioners (Visco-gel)?

A

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)

DEN203 (11 decks)

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