Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What 2 materials are trays usually?

A

Metal: Stainless steel

Plastic: Nylon-based and Polystyrene-based

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Px factors with impression materials

A
  • Neutral taste and odour
  • Short setting time
  • Small tray
  • Easily removed impression
  • No retakes
  • Non-toxic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Dentist factors with imp materials

A
  • Easily mixed
  • Short working times
  • Good quality impressions
  • Low cost
  • Easily disinfected
  • Simple procedure
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What makes a good impression?

A
  • Accurate reproduction of surface detail

- Dimensional accuracy and stability

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What affects accuracy of reproduction of surface detail?

A

viscosity, wettability

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What affects dimensional accuracy/stability?

A 
shrinkage on setting, 
cooling contraction, 
permanent set, 
storage stability, 
type of tray
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

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

A
  • Rough/uneven surface
  • Air bubbles
  • Irregular shaped voids
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What might cause a rough/uneven surface?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What might cause air bubbles?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What might cause irregular shaped voids?

A

 Surface contamination (moisture)

 Premature movement

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Examples of poor fit?

A
  • Distortion
  • Casting too big
  • Casting too small
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is mucostatic impression technique?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

4 examples of mucostatic imp materials

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

3 examples of mucocompressive imp materials

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is TEC?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Name 3 viscous behaviours

A
  • Newtonian
  • Dilatant
  • Pseudoplastic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Name some flexible imp materials

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Sources of wax

A
  • Mineral
  • Parraffin wax
  • Animal beeswax
  • Vegetable
  • Carnauba and candelilla wax
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

Impression compound composition

A

Natural/synthetic Resins + Plasticisers + Fillers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions