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BDS3 > BDS2 DMS > Flashcards

BDS2 DMS Flashcards

1
Q

Types of error in PMMA dentures (3)

A

Flawed initial impression
Error during curing process
Usage

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2
Q

Potential usage problems in PMMA denture (5)

A

Fractures
Uncomfortable
PMMA becomes warped
Surface wear
Patient anatomy changes

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3
Q

3 requirements of ideal denture

A

Replaces function of natural dentition
Fits comfortable
Good aesthetics

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4
Q

Ideal properties for denture base material 7

A

Can accurately create dimensions that will fit well and be stable in use
High softening temperature
Unaffected by oral fluids
High thermal conductivity
High Young’s modulus and elastic limit
Low density
Non toxic/irritant

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5
Q

What does high Young’s Modulus mean?

A

High stress causes small strain

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6
Q

What does high elastic limit mean?

A

Only large stresses will cause permanent deformation

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7
Q

Free radical addition polymerisation

A

Chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
(involves C=C bonds)

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8
Q

4 steps of acrylic polymerisation reaction

A

Activation
Initiation
Propagation
Termination

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9
Q

What happens in activation stage of acrylic polymerisation?

A

Free radicals are formed

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10
Q

What happens in initiation stage of acrylic polymerisation?

A

Free radicals break C=C bonds in each monomer, and transfer free radicals

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11
Q

What happens in propagation stage of acrylic polymerisation?

A

Polymer chain grows

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12
Q

What must be done to enamel surface before bonding a composite resin?

A

Acid etch

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13
Q

Advantages and disadvantages of porcelain

A

Aesthetically good
Hard and rigid - more easily fractured

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14
Q

What is stress?

A

Force/ unit area in N/metres squares - PASCALS

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15
Q

Difference in retention between amalgam and composite

A

Amalgam uses mechanical retention - undercuts
Composite uses adhesive retention - minimal cavity prep

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16
Q

Creep

A

Gradual dimensional change due to repetitive small force

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17
Q

Fatigue

A

Repetitive small stresses cause fracture

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18
Q

Deformation

A

Repetitive small stresses cause permanent change in material dimensions without fracture

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19
Q

Elasticity

A

Ability of a material to recover its dimensions follow application of stress

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20
Q

Properties of ideal dental adhesive (6)

A

High bond strength
Immediate bond
Durable bond
Impermeable bone
Easy to use
Safe

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21
Q

Why is enamel easy to bond to?

A

Heterogenous - densely packed prismatic structure

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22
Q

Acid etch process

A

Roughens the surface, long enamel prisms are filled with imperfectly packed hydroxyapatite crystals

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23
Q

Why acid etch?

A

Modified roughened surface produced by etch is easier to bond to resin restorative materials, increases enamel surface energy by removing surface contaminants - better wettability, allowing the resin to adapt to the roughened enamel surface

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24
Q

Why must enamel be dry for composite?

A

Moisture contamination prevents flow of resin into etched surface

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25
Q

What is used for etch?

A

30-50% aqueous phosphoric acid

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26
Q

Dentine composition

A

20% inorganic (mostly collagen)
70% inorganic (mostly hydroxyapatite)
10% water

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27
Q

Why is bonding to dentine difficult?

A

Dentine is full of permeable tubules, fluid pumps up from pulp to dentine floor making cavity wet
Hydrophilic
Low surface energy
Smear layer

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28
Q

Required properties for dentine bonding agent (4)

A

Ability to flow
Potential for intimate contact with dentine surface
Low viscosity
Adhesion to substrate by mechanical, chemical and Van der Waals forces

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29
Q

Describe mechanical bonding to dentine

A

Dentine bonding agent and dentine surface mesh and interlock together with minimal gaps

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30
Q

Chemical adhesion of dentine

A

Mineralised dentine forms ionic bonds
Organic dentine forms covalent bonds

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31
Q

What happens to dentine as it ages?

A

Becomes more mineralised

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32
Q

Relevance of surface energy in bonding

A

A liquid will only spread on a surface with higher surface energy than it

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33
Q

What is amalgam?

A

An alloy formed by the reaction of mercury (liquid) and silver, tin, copper and other metals (powder)

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34
Q

Purpose of copper in amalgam

A

Increases strength and hardness

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35
Q

Amalgam particle types

A

Lathe cut
Spherical/spheroidal

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36
Q

What are the gamma, gamma1 and gamma2 phases of amalgam?

A

Gamma - unreacted particles Ag3Sn
Gamma 1 - Ag2Hg3 and Gamma 2 - Sn7Hg9
make up the amalgam matrix

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37
Q

Setting reaction for amalgam

A

Ag3Sn +Hg -> Ag2Hg3 +Sn7Hg9

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38
Q

Set structure of gamma particles in amaglam

A

Gamma 1 holds together particles of Gamma and Gamma 2

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39
Q

In modern amalgam what are 2 setting dimensional changes?

A

Small contraction (<0.2%)
Solid solution of Hg in Ag3Sn

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40
Q

Why is zinc avoided in lots of dental materials?

A

When zinc reacts with water it forms ZnO and H2 causing bubbles of H2, causes pressure which can cause expansion, pulpal pain or cause restorations to sit above the surface

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41
Q

4 factors affecting amalgam’s properties

A

Handling
Cavity design
Corrosion
Variation between products

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42
Q

5 advantages of spherical amalgam particles

A

Earlier high tensile strength
Higher tensile strength
Less sensitive to condensation
Easier to carve
Less mercury required

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43
Q

Describe early (1hr) and late (>24hr) strength of amalgam

A

Early - poor
Late - fairly good

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44
Q

Abrasion resistance of amalgam

A

High, suitable for posterior teeth
Too high for deciduous

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45
Q

5 factors decreasing strength of amalgam

A

Undermixing
Too high Hg content after condensation
Too low condensation pressure
Slow rate of packing - increments don’t bond
Corrosion

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46
Q

3 factors affecting marginal integrity of amalgam

A

Creep
Cavity design
Corrosion

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47
Q

What is creep?

A

When a material flows slightly as a result of repeated low level stresses. This is a kind of permanent deformation

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48
Q

Thermal conductivity of amalgam and relevance

A

High thermal conductivity so in deep cavities it may be necessary to use a liner

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49
Q

How is amalgam bonded to tooth?

A

It is not bonded. It uses mechanical retention

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50
Q

What is added to amalgam to resist corrosion?

A

Copper

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51
Q

Is it easier to bond a material to enamel or dentine?

A

Enamel

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52
Q

What acts as a “wetting agent” when carrying out a restoration?

A

Bonding agent

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53
Q

4 benefits of copper enriched amalgam

A

Higher early strength
Less creep
Higher corrosion resistance
Increased durability of margins

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54
Q

Advantages of amalgam (5)

A

Strong
Hard
Durable
Radiopaque
User friendly

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55
Q

Disadvantages of amalgam (4)

A

Corrosion
Leakage
Poor aesthetics
Mercury - potential toxicity and environmental impact

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56
Q

5 potential restorative materials

A

Composite
GIC
Amalgam
Precious metal
Ceramic

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57
Q

Why is release of chemicals an undesirable property in restorative materials?

A

Could be pulpal irritants and lead to pain or pulpal damage

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58
Q

What is the purpose of intermediate restorative materials (cavity liners)?

A

Prevent gaps
Act as protective barrier

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59
Q

What is a cavity base?

A

Thick mix placed in bulk
Replaces dentine to minimise restoration material used or block out undercuts

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60
Q

Which type of restorative material more often requires a cavity base?

A

Metal

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61
Q

What is a cavity lining?

A

Thin coating <0.5mm over exposed dentine
Able to promote pulp health by adhering to the tooth structure or by anti-bacterial action

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62
Q

Pulpal protection by lining material (3)

A

Chemical stimuli from unreacted chemicals in the filling material or initial pH of filling
Thermal stimuli e.g. composite exothermic setting reaction
Bacteria and endotoxins, microleakage of oral fluids and bacteria between restoration and cavity walls

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63
Q

Therapeutic effect of lining materials

A

Calm down pulp inflammation and promote healing

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64
Q

3 properties to make cavity liner easy to use

A

Easy to mix
Long working time
Short setting time/command set

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65
Q

3 thermal properties of ideal lining material

A

Low thermal conductivity
Thermal expansion coefficient similar to dentine
Thermal diffusivity similar or lower than dentine

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66
Q

Ideal thermal conductivity of denture base and reasoning

A

High, otherwise pt may ingest too hot liquid/food, causing burns to oesophagus

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67
Q

What is thermal expansion coefficient?

A

Change in length per unit length for temp rise of +1 celsius measured in ppmC-1

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68
Q

What is thermal diffusivity?

A

Similar to conductivity, measured in cm2/sec

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69
Q

Thermal conductivity units

A

W/m-1/(degrees)C-1

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70
Q

Benefit of radiopaque cavity lining material

A

Easy to see difference between lining and tooth, makes it easier to see secondary caries or leakage

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71
Q

Why is it important that lining materials are cariostatic?

A

Prevent secondary caries

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72
Q

Two potential cariostatic properties of cavity liners

A

Fluoride releasing
Antibacterial

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73
Q

3 required characteristics for biocompatibility of cavity liners

A

Non toxic
Not damaging to pulp (pH neutral and no excessive heat on setting)
Low thermal conductivity

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74
Q

Examples of cavity lining materials

A

Setting calcium hydroxide - liner
Zinc oxide based cements - base
GlC/RMGIC - base or liner

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75
Q

Two components of setting calcium hydroxide

A

Base
Catalyst

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76
Q

Setting reaction of calcium hydroxide

A

Chelation reaction between ZnO and butylene glycol disalicylate

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77
Q

What is the reactive element in the catalyst used in calcium hydroxide?

A

Butylene glycol disalicylate

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78
Q

Initial pH of calcium hydroxide cement

A

around 12

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79
Q

What effect does CaOH have on dentine?

A

It stimulates production of reparative tertiary dentine by causing irritation to odontoblast layer

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80
Q

Relevance of CaOH pH

A

pH 12
Bactericidal to cariogenic bacteria as they survive in acidic conditions

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81
Q

3 useful properties of CaOH

A

Quick setting
Radiopaque
Easy to use

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82
Q

Before mixing how does CaOH appear?

A

Two pastes

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83
Q

2 undesirable properties of CaOH liner

A

Low compressive strength
Soluble

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84
Q

Zinc oxide based cement examples (5)

A

Zinc phosphate
Zinc polycarboxylate
Zinc oxide eugenol ZOE
Resin modified ZOE
Ethoxybezoic acid (EBA)ZOE

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85
Q

What type of reaction is used to create zinc phosphate cement, between which two types of material?

A

Acid base, between a powder and a liquid, followed by a hydration reaction

86
Q

Components of zinc phosphate cement powder and their purpose

A

Zinc oxide >90% - main reactive ingredient
Magnesium dioxide <10% - gives white colour and increases compressive strength
Other oxides (alumina and silica) - improve physical properties

87
Q

Components of zinc phosphate cement liquid

A

Aqueous phosphoric acid - approx 50%
Oxides which buffer the solution - aluminium oxide ensures the consistency of set material and zinc oxide slows the reaction to give better working time

88
Q

Disadvantages to zinc phosphate (7)

A

Low initial pH
Exothermic setting reaction
Not adhesive to tooth or restoration
Not cariostatic
Final set takes 24hours
Brittle
Opaque

89
Q

Difference between zinc phosphate cement and zinc polycarboxylate cement

A

Polyacrylic acid instead of phosphoric acid

90
Q

Advantages of zinc polycarboxylate compared with zinc phosphate

A

Bonds to tooth surfaces (similarly to GIC)
Less heat on reaction
pH returns to neutral more quickly
Cheap

91
Q

Disadvantages of zinc polycarboxylate

A

Difficult to mix
Difficult to manipulate
Soluble in oral environment at lower pH
Opaque
Lower modulus and compressive strength that zinc phosphate

92
Q

Uses for ZOE

A

Linings
Temporary restorations (resin modified or EBA)
Root canal sealer (slow setting 24hours)
Periodontal dressings (fast setting 5 min)

93
Q

Reaction type ZOE and elements involved

A

Acid base
ZnO is base
Eugenol is acid
Base + acid -> salt and water
On setting, chelation of zinc oxide with eugenol forms zinc eugenolate matrix, which bonds the unreacted ZnO

94
Q

5 properties of ZOE

A

Adequate working time
Relatively fast setting
Low thermal conductivity
Low strength (unsuitable under amalgam)
Radiopaque
High solubility

95
Q

Effects of ZOE constantly releasing eugenol

A

Free eugenol has obtundant effect on pulp and can reduce pain
Free eugenol can inhibit set of resin based filling materials, softening and discolouring them (not suitable under composite)

96
Q

Resins added to ZOE

A

Polymethylmethacrylate
Polystyrene

97
Q

What is the purpose of modifying ZOE by adding resins?

A

Increase compressive strength to >40MPa and greatly reduce solubility, making it suitable for cavity lining

98
Q

Properties of EBA ZOE

A

Stronger than ZOE and resin modified ZOE, around 60MPa
Less soluble

99
Q

Most commonly used lining material

A

Glass ionomer

100
Q

Advantages of GIC

A

Cariostatic, releases fluoride over time
Can bond to and seal dentine
Can bond to composite resin
Easy to use
Light cured - long working time short setting
Thermal conductivity lower than dentine
Thermal expansion similar to dentine
High compressive strength
Radiopaque

101
Q

What is the only type of material to seal dentinal tubules?

A

Glass ionomer

102
Q

Least soluble dental cement

A

RMGIC

103
Q

Cytotoxic material released during RMGIC polymerisation reaction, and their effect

A

Benzoyl iodides and benzoyl bromides, can destroy residual cavity bacteria

104
Q

Why is it important that RMGIC is completely cured?

A

Unreacted HEMA could damage pulp

105
Q

What type of bonding occurs between tooth surface and GIC, and RMGIC?

A

Micromechanical and chemical bonding to the resin in RMGIC

106
Q

When to use lining material, and which?

A

Use RMGIC for amalgam and large cavities to be filled by composite
Use CaOH only when pulp is exposed, then cover with RMGIC

107
Q

4 reasons a direct restoration would be required

A

New dental caries
Abrasion/erosion
Failed restoration/secondary caries
Trauma

108
Q

Ideal properties of a direct filling material

A

Mechanical - strength, rigidity, hardness
Bonding to tooth/compatible with bonding systems
Thermal properties
Aesthetics
Handling/viscosity
Smooth finish/polishable
Low setting shrinkage
Radiopaque
Anticariogenic
Biocompatible

109
Q

5 components of composite resin

A

Filler particles
Resin
Camphorquinone
Low weight dimethacrylates
Silane coupling agent

110
Q

Monomers used in the resin component of composite resin

A

BIS-GMA
Urethane dimethacrylates

111
Q

2 key characteristics for resin monomer in composite resin

A

Contains C=C
Undergoes free radical addition polymerisation

112
Q

What is the purpose of camphorquinone in composite resin?

A

Produces free radical molecules when activated by blue light, which go on to initiate free radical addition polymerisation of BIS-GMA

113
Q

What is the purpose of silane coupling agent in composite resin?

A

Helps create good bond between filler particles and resin, stops water adhering to glass filler which would prevent resin bonding to glass

114
Q

What type of material is used in composite resin as filler particles?

A

Glass

115
Q

Effects of adding filler particles

A

Improved strength/hardness/rigidity/abrasion resistance
Lower thermal expansion
Lower polymerisation shrinkage
Less heat of polymerisation
Improved aesthetics
Some radiopaque

116
Q

Advantages of light curing

A

Extended working time
Less finishing
Immediate finishing
Less waste
Higher filler levels
Less porosity

117
Q

Typical depth of cure of composite resin

A

2mm

118
Q

5 potential problems of light curing

A

Light/materials mismatch - overexpose
Premature polymerisation under dental lights
Optimistic depth of cure - use 2mm increments
Recommended setting times too short
Polymerisation shrinkage

119
Q

Conventional v microfine v hybrid composites

A

Conventional - strong but problems with finishing and staining due to soft resins and hard particles
Microfine(smaller particles) - better aesthetics, smoother surface, inferior mechanical properties
Hybrid - most modern composites, improved mechanical properties

120
Q

Abrasion definition

A

Removal of surface layers when two surfaces make friction contact

121
Q

3 effects of surface roughness caused by abrasion

A

Appearance
Plaque retention
Sensation when in contact with tongue

122
Q

Material factors affecting the wear of a composite resin (5)

A

Filler material
Particle size distribution
Filler loading
Resin formulation
Coupling agent

123
Q

Clinical factors affecting wear of a composite resin (6)

A

Cavity size and design
Tooth position
Occlusion
Placement technique
Cure efficiency
Finishing methods

124
Q

How long to acid etch enamel and with what?

A

20 secs
30% phosphoric acid

125
Q

Why is bonding to tooth surface necessary for a well placed composite resin? (2)

A

Reduce microleakage
Counteract polymerisation shrinkage

126
Q

Thermal properties of composite

A

Thermal conductivity - low - good
Thermal diffusivity - low, similar to dentine - good
Thermal expansion coefficient - high - poor

127
Q

How does self cure composite appear before use?

A

Two pastes to be mixed

128
Q

Elastomeric impression material types (2)

A

Polyether
Addition silicone

129
Q

Advantages of alginate as impression material

A

Fast setting for patient comfort
Easy to mix
Accurate impressions
High elasticity for severe undercuts etc

130
Q

Important properties in impression material

A

Flow
Surface detail
Wettability
Elastic recovery
Stiffness
Tear strength
Mixing time
Working time

131
Q

What is a study model?

A

Positive replica of dentition produced from an impression, used to record position, shape and dimensions of teeth to aid assessment of dentition and enable design and manufacture of dental prostheses

132
Q

What is gypsum used for?

A

Dental study casts

133
Q

Manufacture equation for dental stone or plaster

A

CaSO4(calcium sulphate dihydrate) + 2H2O -(heat)-> (CaSO4)2H2O (calcium sulphate hemihydrate) + H2O

134
Q

Types of gypsum (3)

A

Plaster (beta-hemihydrate)
Dental stone (alpha-hemihydrate)
Densite (improved stone)

135
Q

Gyspum plaster structure and production

A

Large porous irregular crystals, created when heated in open vessel

136
Q

Dental stone structure and production

A

Non-porous, regular crystals, less water molecules produced when heated in an autoclave

137
Q

Structure and production of densite

A

Compact smoother particles produced when heated in presence of Ca and Mg chloride

138
Q

Compare gypsum setting reaction to it’s manufacture

A

Reverse reaction
(CaSO4)2H2O (hemihydrate) + 3H2O —-> 2CaSO4 (dihydrate) + 2H2O

139
Q

Mixing ratios of dental plaster and stone

A

Plaster - 50-60ml/100g
Stone - 20-35ml/100g

140
Q

Strengths compared of dental plaster, stone and densite

A

Densite - strongest
Stone - middle
Plaster - least strong

141
Q

What is the effect of increasing powder in a gypsum mix?

A

Faster set and greater expansion

142
Q

Advantages of gypsum (3)

A

Dimensionally accurate and stable
Low expansion
Good colour contrast

143
Q

Disadvantages of using gyspum (5)

A

Low tensile strength
Poor abrasion resistance
Very brittle
Surface detail less than elastomer impression
Poor wetting of some impression materials

144
Q

Two types of glass ionomer cement

A

Resin modified GIC
Conventional GIC

145
Q

Two types of RMGIC

A

Self cure
Light cure

146
Q

4 uses of GIC

A

Restoratione
Core build up
Lining
Luting

147
Q

What are the two components of GIC and what reaction do they undergo?

A

Liquid - polyacrylic acid and tartatic acid
Powder - 30-40%SiO2, 15-30%Al2O3, 15-35%CaF2, 2-10%AlF3, 4-20%AlPO4, 4-10%NaF

Undergo an acid base reaction

148
Q

What are the materials for anhydrous GIC?

A

The acid component (polyacrylic acid and tartaric acid) is freeze dried and added to the powder (silica, aluminium oxide, calcium fluoride, aluminium fluoride, aluminium phosphate, sodium fluorider), to mix, distilled water is added.

149
Q

What is the main advantage of anhydrous GIC?

A

Easier mixing/handling

150
Q

What are the main advantages of using encapsulated GIC?

A

Consistent powder/liquid ratio, easier to use, more consistent properties of mixed material

151
Q

What are the effects of having smaller particles in a GIC?

A

Faster setting
More opaque

152
Q

Powder particle size of GIC for luting cement?

A

<20micrometres

153
Q

Acid base reaction involved in GIC

A

MO.SiO2 (glass) + H2A (acid) –> MA (salt) +SiO2 + H2O (Silica gel)

154
Q

3 phases of the GIC acid base reaction

A

Dissolution
Gelation
Hardening

155
Q

Describe dissolution phase of GIC setting reaction

A

Acid releases H+ ions into solution which attack the glass surface
Ca, Al, Na and F ions released
Leaves silica gel around unreacted glass

156
Q

Describe the initial set of GIC and the part calcium ions play in this

A

Calcium ions crosslink with the polyacid by chelation with carboxyl groups to provide the initial set
Calcium ions are bivalent so they can react with 2 molecules, joining them
Crosslinking is not perfect as the Ca can chelate with two carboxyl groups on the same molecule

157
Q

What is the gelation stage of GIC setting reaction?

A

The initial set (first few minutes) caused by the formation of calcium polyacrylate

158
Q

What ions are responsible for hardening of GIC during setting reaction and how?

A

Trivalent aluminium ions ensure good crosslinking, forming aluminium polyacrylate
(This does not start for at least 30 min and can take 1 week + to be complete)

159
Q

What is GIC protection and why is it necessary?

A

Placing over a varnish, resin or grease/gel to protect from contamination with moisture or excessive drying out as this will lead to a weaker final set of the material

160
Q

Describe adhesion of GIC

A

Can bond to enamel or dentine without intermediate material
Bond strength not high compared with composite to acid etched enamel (5MPa vs 20MPa)
Good sealing ability with little leakage around margins

161
Q

Disadvantages of GIC

A

Poor aesthetics (lack translucency)
Poor tensile strength
Lower compressive strength than composite (80-110MPa)
Higher solubility than composite

162
Q

Advantages of GIC

A

Good thermal properties
No contraction on setting
Fluoride release
Resistant to staining
Stable chemical bond to enamel and dentine

163
Q

Disadvantages of GIC

A

Brittle
Poor wear resistance
Moisture susceptible when first placed
Poor aesthetics
Poor handling characteristics
Susceptible to acid attack and drying out over time
Possible problems bonding to composite

164
Q

What makes up the powder for resin modified GIC?

A

Fluro-alumino-silicate glass
Barium glass
Vacuum dried polyacrylic acid
Potassium persulphate
Ascorbic acid
Pigments

165
Q

What is in RMGIC to increase readiopacity?

A

Barium glass

166
Q

What is the liquid in RMGIC?

A

HEMA
Polyacrylic acid with pendant methacrylate groups
Tartaric acid
Water
Photo-inhibitors

167
Q

What are the three desired advantages to RMGIC over GIC?

A

Light curing
Improved physical properties
Better aesthetics

168
Q

What is HEMA?

A

2-hydroxyethylmethacrylate, a resin monomer

169
Q

Describe the dual curing of RMGIC

A

On mixing acid-base reaction begins same as GIC
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix
Quickly light activation is complete (20s)
Acid base reaction continues within resin matrix for several hours

170
Q

What is the importance of placing RMGIC in layers?

A

Reasonably opaque material so light does not penetrate deeply, important to use layers or it may not set

171
Q

Describe the tri-curing process of RMGIC

A

Initial acid base reaction on mixing
REDOX reaction between methacrylate monomers
On light activation free radical methacrylate reaction forms resin matrix (complete in 20s)
REDOX continues for about 5 mins after initial mix
Acid base continues within the resin matrix for several hours
Final hardening with aluminium polyacrylate formation can take days

172
Q

Bonding RMGIC tooth surface conditioning

A

Variation in manufacturers advice
Vitrebond - none
Fuji cement LC - optional
Fuji II LC - Yes
Vitremer - Yes
Vitremer luting - No mention
Always follow manufacturers instructions

173
Q

Advantages of RMGIC

A

Good bond to enamel and dentine
Better physical properties than GIC
Lower solubility
Fluoride release
Better translucency and aesthetics
Better handing

174
Q

Disadvantages of RMGIC

A

Polymerisation contraction
Exothermic setting reaction
Swelling due to water uptake (HEMA extremely hydrophilic)
Monomer leaching (HEMA toxic to the pulp so must be completely polymerised)
Reduced strength if not light cured

175
Q

Uses of RMGIC (8)

A

Dressing
Fissure sealant
Endodontic access cavity temporary filling
Luting
Orthodontic cement
Restoration of deciduous teeth
Restoration of permanent teeth
Base or lining

176
Q

Uses of metal alloys

A

Partial denture framework - CoCr
Crowns - stainless steel
Denture base - stainless steel
Orthodontic appliance - NiTi
Restorations - amalgam

177
Q

What are the good mechanical properties of metals?

A

Strength
Rigidity
Elastic limit
Ductility
Hardness

178
Q

What is the major disadvantage to using metals in dentistry?

A

Aesthetics

179
Q

Metal defintion

A

Aggregate of atoms in crystalline structure

180
Q

Metal alloy definition

A

Combination of metal atoms in a crystalline structure

181
Q

What is elastic limit?

A

Maximum stress without plastic deformation

182
Q

What is ductility?

A

Amount of plastic deformation prior to fracture

183
Q

Factors affecting metal alloy mechanical properties (3)

A

Crystalline structure
Grain size
Grain imperfections

184
Q

Crystal grain structures (3)

A

Equi-axed - if crystal growth is of equal dimension in each direction
Radial - molten metal cooled quickly in cylindrical mould
Fibrous - wire pulled through die (cold worked)

185
Q

What is the effect of fast cooling on metal alloys?

A

More nuclei
Small fine grains

186
Q

What is the effect of slow cooling metal alloys?

A

Few nuclei
Large coarse grains

187
Q

Which atoms act as nuclei of crystallisation?

A

Those at the edge of grains

188
Q

What are nucleating agents for alloy crystallisation?

A

Impurities or additives that act as a foci for crystal growth

189
Q

What is a grain in metal alloys?

A

Single crystal (lattice) with atoms orientated in given directions (dendrites)

190
Q

Grain boundary

A

Change in orientation of the crystal planes (impurities concentrate here)

191
Q

Advantages/disadvantages of small fine grains

A

+ high elastic limit
+ increased fracture strength, hardness and ultimate tensile strength
- decreased ductility

192
Q

What are dislocations in metal alloys?

A

Imperfections/defects in the crystal lattice. Propagation of dislocations leads to slip

193
Q

Effects of preventing movement of dislocations in metals

A

Increase - elastic limit, fracture stress/UTS, hardness
Decrease - ductility, impact resistance

194
Q

Factors that impede dislocation movement

A

Grain boundaries
Different atom sizes in atoms
Cold working

195
Q

What is cold working?

A

Work done on a metal or alloy such as bending, rolling, swaging, at a low temperature. It causes slip which makes dislocations collect at grain boundaries, making a stronger, harder material

196
Q

Effects of cold working

A

Higher EL, FS/UTS, hardness
Lower ductility, impact strength, corrosion resistance

197
Q

What is the effect of residual stress from cold working metal or alloys?

A

Causes instability in the lattice which results in distortion over time.

198
Q

How can residual stress on a metal or alloy be relieved?

A

Annealing

199
Q

Describe annealing process

A

Heating metal (or alloy) so that greater thermal vibrations allows migration of atoms

200
Q

What is recrystallisation?

A

The effect of heat on a metal/alloy, leading to new smaller equiaxed grains, lower EL, UTS, hardness, and increased ductility

201
Q

How are recrystallisation and cold working related?

A

Recrystallisation spoils the benefits of cold work, and allows further cold work
Cold work/recrystallisation is repeated until the correct shape is obtained

202
Q

What effect does cold working have on recrystallisation temperature?

A

More cold working -> lower recrystallisation temperature

203
Q

Effect of temperature increase on grain size

A

Larger grains

204
Q

What is a PHASE (metal alloys)

A

Physically distinct homogenous structure

205
Q

What is a SOLUTION (metal alloys)

A

Homogenous mixture at an atomic scale

206
Q

3 possibilities when melting together and crystallising 2 metals

A

Insoluble, no common lattice - exists as 2 phases
Intermetallic compound with specific formula e.g. Ag3Sn
Soluble and form a solid solution - common lattice

207
Q

2 types of solid solution

A
  1. substitutional - atoms of one metal replace the other metal in the crystal lattice/grain
  2. interstitial - atoms markedly different in size, smaller atoms located in spaces in lattice/grain structure of larger atom
208
Q

What are the effects of SLOW cooling a molten alloy?

A

Allows metal atoms to diffuse through lattice, ensures grain composition in homogenous, but this results in large grains

209
Q

What are the effects of RAPID cooling a molten alloy?

A

Prevents atoms diffusing through lattice, causes CORING as composition varies throughout grain, which may reduce corrosion resistance but is otherwise undesirable

210
Q

How to reduce coring but maintain small grains

A

Homogenising anneal -reheat to allow atoms to diffuse and so cause grain composition to become homogenous, keep below recrystallisation temp or grains will be altered

211
Q

Eutectic alloys

A

Where liquidus and solidus coincide (crystallisation takes place at a single temperature)

212
Q

What is an alloy in solid solution?

A

Both metals co-existing in each grain

BDS3 (20 decks)

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