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Dental Material Science Flashcards

1
Q

What is the composition of Nickel Chromium?

A

70-80% Nickel
10-25% Chromium

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2
Q

What are the features of nickel in nickel chromium?

A

High melting point
High casting shrinkage

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3
Q

What are the features of chromium in nickel chromium?

A

High Young’s modulus
Lowish bond strength

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4
Q

What are six features of cobalt chromium?

A

High melting point
2-3% casting shrinkage
High tensile strength (850MPa)
Lowish bond strength (50MPa)
High Young’s modulus (220 GPa)
High hardness (360-430VHN)

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5
Q

What is the tensile strength of cobalt chromium?

A

850MPa

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6
Q

What is the bond strength of cobalt chromium?

A

50MPa

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7
Q

What is the Young’s modulus of cobalt chromium?

A

220GPa

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8
Q

What is the hardness of cobalt chromium?

A

360-430VHN

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9
Q

What is the mechanical bonding mechanism for metal ceramic crowns?

A

Stressed skin effect

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10
Q

What is the stressed skin effect due to?

A

Slight difference in thermal contraction compared co-efficient that results in compressive forces that aid bonding

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11
Q

What is the chemical bonding mechanism in metal ceramic crowns?

A

Electron sharing in oxides

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12
Q

What is the ideal thermal expansion coefficient of the metal in a metal ceramic crown?

A

0.5ppm/C higher than porcelain (14ppm/C)

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13
Q

What is a wrought alloy?

A

An alloy which can be manipulated or shaped by cold working

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14
Q

What is the composition of steel?

A

> 98% Iron
<2% Carbon
0.5-1% Chromium

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15
Q

What % of chromium does cast iron have?

A

2%

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16
Q

What does allotropic mean?

A

Undergoes 2 phase changes with temperature

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17
Q

What phase is Iron in when temp >1400C

A

Body Centered Cubic (BCC)
Low carbon solubility
0.05

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18
Q

What phase is iron when temp 900-1400C?

A

Face centered cubic
Higher carbon solubility
0.2

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19
Q

What phase is iron when temp <900C?

A

Body centered cubic
Low carbon solubility
0.05

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20
Q

What is formed from the quenching of austinite?

A

Martensite

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21
Q

What is formed from the slow cool of martensite?

A

Pearlite
Ferrite
Cementite

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22
Q

What is austenite?

A

Interstitial solid solution of iron and carbon
Face centered cubic

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23
Q

What is ferrite?

A

Very dilute solid solution
Exists at low temperature

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24
Q

What is cementite?

A

Fe3C
Exists at low temperature

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25
Q

What is pearlite?

A

Eutecoid mixture of ferrite and cementite

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26
Q

What is a solid solution?

A

2 metals soluble in each other that form a common lattice structure

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27
Q

What are the two types of substitutional solid solution?

A

Random
Ordered

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28
Q

What does an interstitial solid solution contain?

A

Two atoms of different size

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29
Q

What are the features of martensite?

A

Distorted lattice
Hard
Brittle

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30
Q

What is tempering?

A

Heating to 450C followed by quenching

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31
Q

What does the addition of chromium in stainless steel provide?

A

Lowers the temperature of Austenite to martensite
Corrosion resistance from chromium oxide

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32
Q

What does the addition of nickel to stainless steel provide?

A

Increased strength
Increased corrosion resistance
Decreased austenite to martensite temperature

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33
Q

What is work hardening?

A

Work done on a metal at a low temperature that causes slip
The dislocations collect at grain boundaries and this results in a stronger metal

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34
Q

What is the difference between decorative and dental ceramics?

A

The kaolin is replaced with feldspar and silica
(Kaolin causes opacity)

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35
Q

What is feldspar?

A

Potassium or sodium alumina silicate

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36
Q

What does feldspar do?

A

Lowers the fusion and softening temperature of the glass

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37
Q

What colour does chromium oxide make?

A

Green

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38
Q

What colour does cobalt oxide make?

A

Blue

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39
Q

What colour does manganese oxide make?

A

Lavender

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40
Q

What colour does copper oxide make?

A

Green

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41
Q

What colour does iron oxide make?

A

Brown

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42
Q

What colour does nickel oxide make?

A

Brown

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43
Q

What size reduction is associated with sintering?

A

20%

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44
Q

What are the properties of dental ceramics?

A

Aesthetics
Chemical stability
Biocompatibility
Thermal
Dimensional stability
Mechanical
Stable colour
Smooth surface
Don’t stain
Optical properties

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45
Q

What does the Yttria do in Zirconia?

A

3-5%
Increases translucency
Decreases physical properties

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46
Q

What shape is a zirconia molecule?

A

Monocyclic

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47
Q

What shape is a yttria molecule?

A

Tetragonal

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48
Q

What type of crown is the strongest?

A

Milled crowns

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49
Q

What type of crown is stronger than lithium disilicate?

A

Zirconia

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50
Q

What advantage do crowns with layered porcelain have?

A

Better aesthetics but more likely to chip

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51
Q

What type of crown is preferred in posterior teeth?

A

Zirconia

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52
Q

What type of crown is preferred in anterior teeth?

A

Lithium disilicate

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53
Q

What are the two components of glass ionomer cement?

A

Silica
Polyacrylic acid/tartaric acid

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54
Q

What is the formula for the setting reaction of glass ionomer cement?

A

MO.SiO2 + H2A —> MA + SiO2 + H2O

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55
Q

What are the stages of glass ionomer cement reactions?

A

Dissolution
Gelation
Hardening

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56
Q

What happens during dissolution?

A

Mixing
H+ ions attack glass surface leaving silica gel around unreacted glass

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57
Q

What happens during gelation?

A

Forms a gel
Bivalent Ca+ ions crosslink with polyacrylic acid by chelation reaction

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58
Q

What happens during hardening?

A

Over 24 hours
Crystalisation
Trivalent Al+ ions crosslink increasing strength

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59
Q

How do glass ionomer cements bond?

A

Bonds the calcium ions in the enamel surface

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60
Q

What are the advantages of glass ionomer cements?

A

No setting contraction
Fluoride release
Bonds tooth
Good aesthetics
Minimal tooth prep

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61
Q

What are the contents of resin modified glass ionomer cement?

A

Fluoro-aluminio-silicate glass
Barium glass
HEMA
Polyacrylic/Tartaric acid

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62
Q

What are the pros of RMGIC?

A

Improved mechanical
Dual set
Longer working time

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63
Q

What are the cons of RMGIC?

A

Setting contraction
Unreacted monomer may be toxic

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64
Q

What type of cure can RMGIC have?

A

Dual or tri

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65
Q

What is dual cure?

A

Light
Mix

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66
Q

What is tri cure?

A

Light
Mix
Redox

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67
Q

What are some used of GIC?

A

Luting cement
Liner
Temporary restoration

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68
Q

What is the calculation for stress?

A

Force/Area

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69
Q

What are the four types of stress?

A

Tensile
Compressive
Shear
Torsional

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70
Q

What is a stress concentration point?

A

Abrupt changes in the shape of a file that results to a higher stress at that point

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71
Q

What is strain?

A

Response of a material to stress

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72
Q

What is elastic limit?

A

Maximal stress applied that allows material to return to original dimensions

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73
Q

What is elastic deformation?

A

Reversible deformation that doesn’t exceed elastic limit

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74
Q

What is plastic deformation?

A

Permanent bond displacement when elastic limit is exceeded

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75
Q

What happens when a material reaches its plastic limit?

A

Fracture

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76
Q

What is cyclic fatigue?

A

Generation of tension/compression angles

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77
Q

When does torsional fatigue occur?

A

When a file binds dentinal walls and doesnt rotate

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78
Q

What are the pros PMMA?

A

High softening temp
Insoluble in oral fluid

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79
Q

What are the cons of PMMA?

A

Poor thermal conduction

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80
Q

What is the cause of gaseous porosity?

A

Monomer bubbling

81
Q

What is the cause of contraction porosity?

A

Poor pressure/too much monomer

82
Q

What is the cause of granular porosity?

A

Rough surface
Too much monomer

83
Q

What is the cause of residual monomer?

A

Insufficient terminal boil

84
Q

What is crazing?

A

Cracks at high stress areas (quenching and different thermal contractions)

85
Q

What are the properties of a luting cement?

A

Viscosity and film thickness
Ease of use
Radiopacity
Marginal seal
Aesthetics
Solubility
Cariostatic
Biocompatibility
Mechnanical properties

86
Q

What are the ideal properties of a luting cement in reference to viscosity and film thickness?

A

Must be low to allow seating without interference
Viscosity must increase as material seats
Ideal film thickness <25um

87
Q

What is viscosity and film thickness dependent on?

A

Size of powder/filler particles

88
Q

What are the ideal properties of a luting cement in reference to ease of use?

A

Easy to mix
Long working time, short setting time

89
Q

What are the ideal properties of a luting cement in reference to radiopacity?

A

Higher the better, easier to see marginal breakdown

90
Q

What are the ideal properties of a luting cement in reference to marginal seal?

A

Should be impenetrable to tooth and restoration

91
Q

What are the ideal properties of a luting cement in reference to aesthetics?

A

Tooth coloured
Non-staining

92
Q

What are the ideal properties of a luting cement in reference to solubility?

A

Low

93
Q

What are the ideal properties of a luting cement in reference to cariostaticity?

A

Fluoride-releasing
Anti-bacterial

94
Q

What are the ideal properties of a luting cement in reference to biocompatibility?

A

Non-toxic
No damage to pulp
Low thermal conductivity

95
Q

What are the ideal properties of a luting cement in reference to mechanical properties?

A

High compressive strength (similar to dentine 275MPa)
High tensile strength (similar to dentine 50MPa)
High hardness (dentine 70K, enamel 400K)
Young’s modulus (dentine 15GPa)

96
Q

What are examples of luting materials?

A

Dental cements- zinc phosphate, zinc polycarboxylate
GIC: conventional, resin modified
Composite resin

97
Q

How are composite resin luting agents applied?

A

Total etch for use with DBA
Self etch (still requires etch but has own bonding agent incorporated)

98
Q

What are the features of zinc phosphate luting agent?

A

Acid-base reaction
Powder and liquid
Excellent clinical service
Easy to use
Cheap

99
Q

What does the powder component of zinc phosphate luting agent contain?

A

Zinc oxide (90%)
Magnesium Dioxide (<10%)
Other oxides- alumina and silica

100
Q

What does the presence of magnesium dioxide in zinc phosphate luting cement provide?

A

White colour
Increases compressive strength

101
Q

What does the presence of other oxides in zinc phosphate luting cement provide?

A

Improve physical properties
Alters shade

102
Q

What does the liquid component of zinc phosphate cement contain?

A

Aqueous solution of phosphoric acid (50%)
Oxides; aluminium and zinc

103
Q

What does the presence of aluminium oxide in zinc phosphate luting cement provide?

A

Even consistency of set

104
Q

What does the presence of zinc oxide in zinc phosphate luting cement provide?

A

Slows reaction for better working time

105
Q

What is the acid base reaction for zinc phosphate luting cement?

A

ZnO + 2H3PO4 –> Zn(H2PO4)2 + H2O

106
Q

What is the hydration reaction for zinc phosphate luting cement?

A

ZnO + Zn(H2PO4) + 2H2O –> Zn3(H2PO4)2 .H2O

107
Q

What is the chemical formula of hopiete?

A

Zn3(H2PO4)2.H2O
(hydrated zinc phosphate)

108
Q

Discuss the reaction in zinc phosphate luting cement:

A

The aluminium oxide prevents crystallisation leading to an amorphous glossy matrix of the acid salt surrounding unreacted ZnO
This matrix is almost insoluble, but it is porous and contains free water from the setting material
The cement subsequently matures binding this water, leading to a stronger less porous cement

109
Q

What are the problems with zinc phosphate as a luting cement?

A

Initial low pH (pulpal irritation)
Exothermic setting reaction
Non-adhesive to tooth surface or restoration
Non-cariostauc
Final set takes 24 hours
Brittle
Opaque

110
Q

What is the difference between zinc phosphate and zinc polycarboxlate cement?

A

Zinc polycarboxylate uses polyacrylic acid instead of phosphoric acid

111
Q

What are the features of zinc polycarboxylate cement?

A

Bonds to tooth
Less exothermic
Low initial pH, returns to neutral more quickly
Cheap

112
Q

What are the benefits of glass ionomer cement as a luting cement?

A

Fairly strong
Durable
Low shrinkage
Long term stability
Relatively insoluble
Better aesthetics than zinc phosphates
Self adhesive to tooth
Fluoride release
Cheap

113
Q

What are the downsides of glass ionomer cement as a luting cement?

A

No chemicals bond to restoration surface
Surface must be sandblasted to allow mechanical adhesion
Easy to use clinically

114
Q

What is the hydrophilic monomer in resin modified glass ionomer cement?

A

HEMA (hydroxethyl methacylate)

115
Q

What does a secondary cure via redox allow?

A

Shorter setting tine
Longer working time
Increased compressive and tensile strength
Higher bond to tooth
Decreased solubility

116
Q

What are the problems with RMGIC as a luting cement?

A

HEMA monomer is cytotoxic and swells

117
Q

What can RMGIC not be used as a luting cement with?

A

Porcelain crowns
Posts

118
Q

What are the benefits of composite luting agents?

A

Better physical properties
Lower solubility
Better aesthetics

119
Q

What are the downsides of composite luting agents?

A

Need dentine bonding agent
Technique sensitive

120
Q

Discuss the bonding between an indirect composite restoration and a composite luting agent?

A

Composite bonds composite
Bond strength is lower to inlay fitting surface than new composite
Bond is micromechanical and chemical (C=C) on the fitting inlay surface
Dual cure required as light penetration through inlay will be poor

121
Q

Discuss the bonding to porcelain crowns?

A

Porcelain is brittle and requires to be bonded to tooth to prevent fracture
Porcelain must be etched with HF to etch the surface and create a rough retentive surface
Surface is hydrophobic, surface wetting agent required

122
Q

What is an example of a silane coupling agent?

A

Gamma methacryoxypropyltrimothoxysilane

123
Q

How does a silane coupling agent work?

A

Applied to etched porcelain: strong bond between oxide group on porcelain surface and silane
Other end of the silane has a C=C which bonds the composite luting agent

124
Q

What must be done with bonding to a metal crown?

A

Etching; electrolytic etching
Sand blasting: needs a bonding agent

125
Q

Why must dual cure be used for metal crowns?

A

Light cannot penetrate metal

126
Q

What must be done when bonding precious metals?

A

Change composition to alloy alloy formation
Increase copper content and heat 400C

127
Q

How does a self adhesive composite resin work?

A

Metal coupling agent incorporated into composite resin
Simplifies bonding process

128
Q

What is an example of a self adhesive composite resin?

A

Panavia

129
Q

What are examples of self etching composite resin cements?

A

RelyX
Unicem

130
Q

What are self etching composite resin cements?

A

Combination of composite resin cement and self etching DBAs

131
Q

What are the benefits of a self etching composite resin cement?

A

Very good moisture control
Good bond to dentine
Good compressive strength
Good tensile strength
Good hardness
Good wear resistance

132
Q

What are the downsides of a self etching composite resin cement?

A

Limited smear layer removal

133
Q

How do self etching composite resins work?

A

Acidic groups bind with calcium in hydroxyapatite forming stabilising attachment between tooth and resin
Ions from dissolution of filler neutralise the remaining acidic groups forming chelate reinforced methacrylate network

134
Q

What is the 2 paste system of temporary cements?

A

Base: zinc oxide, starch and oil
Accelerator: resins, Eugenol, ortho-EBA, carnauba wax

135
Q

What is the role of carnauba wax in temporary cements?

A

Prevents full set

136
Q

When should a Eugenol containing temporary cement not be used?

A

If the permanent luting cement is composite based

137
Q

What is the composition of stainless steel?

A

Iron 72%
Chromium 18%
Nickel 8%
Titanium 1.7%
Carbon 0.3%

138
Q

What makes hard stainless steel?

A

Work hardening

139
Q

What are the reasons for stainless steel fracture?

A

Overworked
Mechanical abrasion
Fatigue
Weld decay

140
Q

What are the features of austenitc steel?

A

Crystal lattice structure
Soft unless cold hardened

141
Q

What does chromium provide to stainless steel?

A

Lowers temperature for martensite formation
Corrosion resistance

142
Q

What are the features of martensite?

A

Formed quenching from high temperatures
Inter grown plate-like crystals with a distorted cubic structure arising from the presence of carbon atoms in the iron structure

143
Q

What does nickel provide to stainless steel?

A

Help achieve austenitic structure at room temperature by lowering critical temperature for austenic structure breakdown on cooling improves strength and corrosion resistance

144
Q

What does titanium provide to stainless steel?

A

Prevents precipitation of chromium at grain boundaries when the alloy is heated during welding or soldering
Carbon combines with titanium in preference to chromium

145
Q

What are the differences between austenite and martensite?

A

The average unit of austenite is a perfect cube
Martensite is distorted by interstitial carbon atoms that do not have time to diffuse during quenching
Material is harder as particle layers cannot slide past one another

146
Q

What are the ideal features of an investment material?

A

Porous
Expands
Easily removed from cast

147
Q

What are the two components of an investment material?

A

Binder and refractory

148
Q

What does a binder do?

A

Forms coherent mass

149
Q

What does a refractory do?

A

Allows expansion
Withstands high temperatures

150
Q

What is hydroscopic expansion?

A

Water molecules attracted between crystals forcing crystals apart

151
Q

What factors lead to hydroscopic expansion?

A

High water concentration
High water temperature
Long immersion time
Higher silica content

152
Q

What are the types of dental stone/plaster?

A

Gypsum bonded
Silica bonded

153
Q

What are the ideal properties of dental cement?

A

Easy to use
Radiopaque
Aesthetic
Cariostatic

154
Q

What are the types of dental cements?

A

Zinc phosphate
Zinc poly carboxylate
GIC
RMGIC

155
Q

What is weld decay?

A

Carbon chromides forming at grade boundaries
Increases brittleness and corrosion

156
Q

What are the key features of an impression material?

A

Low viscosity
Low viscoelasticity
High tear strength
High elastic recovery
Good wettability
Good surface detail

157
Q

What is static fatigue?

A

Time dependent reduction in function regardless of applied strength

158
Q

What are the pros of alumina cores?

A

Good aesthetics
High flexural strength
Cheap

159
Q

What are the pros of zirconia cores?

A

Hard
Strong
Excellent fit

160
Q

What colour is copper oxide?

A

Green

161
Q

What colour is chromium oxide?

A

Green

162
Q

What colour is cobalt oxide?

A

Blue

163
Q

What colour is manganese oxide?

A

Lavender

164
Q

What colour is iron oxide?

A

Brown

165
Q

What colour is iron oxide?

A

Brown

166
Q

What colour is nickel oxide?

A

Brown

167
Q

What are the stages of cast and pressed ceramics?

A

Wax up
Investment
Cast from heated ingot
Ceraming
Staining

168
Q

What are the ceramic stages?

A

Crystal formation
Crystal growth

169
Q

What is a study cast?

A

Positive replica of the dentition produced from an impression

170
Q

What are the uses of study casts?

A

To record positions, shape and dimensions of teeth
Aids visualisation/assessment of dentition
Enables manufacture of dental prostheses

171
Q

What is the reaction for the manufacture of gypsum?

A

Calcium sulfate dihydrate + heat –> Calcium sulphate hemihydrate

(CaSO4.2H2O) + heat —> (CaSO4)2. H2O + H2O

172
Q

What are the types of gypsum?

A

Plaster (beta hemihydrate)
Stone (alpha- hemihydrate
Improved stone (densite)

173
Q

What are the features of plaster (b hemihydrate)?

A

Heated in open vessel
Large, porous
Irregular crystals

174
Q

What are the features of stone (a hemihydrate)?

A

Heated in autoclave
Non- porous
Irregular crystals
Requires less water

175
Q

What are the features of improved stone (densite)

A

Heated in presence of Ca and Mg chloride
Compact, smooth particles

176
Q

What is the reaction for setting gypsum?

A

Calcium sulfate hemihydrate + water –> calcium sulfate dihydrate

(CaSO4)2.H2O + 3H2O –> 2CaSO4.2H2O

177
Q

What happens during the initial set of gypsum?

A

Dihydrate crystals come in contact
Expansion begins

178
Q

What happens during the final set of gypsum?

A

Hardens
Excess water evaporates

179
Q

What is the mixing ratio for dental plaster?

A

50/60ml : 100g

180
Q

What is the mixing ratio for dental stone?

A

20-35ml : 100g

181
Q

What is ditching?

A

Ditching is a failure of amalgam which is characterised as marginal breakdown where the restoration breaks away from the cavity margins

182
Q

What are the causes of ditching?

A

Inappropriate cavity preparation
Creep

183
Q

What is wettability?

A

A measure of how well a material spreads/adheres to the surface

184
Q

When does wettability increase?

A

Wettability increases when the critical surface energy of the surface is higher than the liquid

185
Q

How does light curing work?

A

Blue light activates camphorquinone which releases free radicals
The free radicals released polymerase the monomers

186
Q

What is the depth of cure?

A

2mm

187
Q

What are the properties of gypsum?

A

Reproduction of surface detail
Strength
Surface hardness (low)
Setting time
Exansion

188
Q

What is the compressive strength of gypsum?

A

28-38MPa

189
Q

What is the expansion of gypsum?

A

0.05-0.3

190
Q

What is the effect of increasing the powder in gypsum?

A

Decreases setting time
Increases expansion

191
Q

What is the effect of increasing spatulation in gypsum?

A

Decreases setting time
Increases expansion

192
Q

What is the effect of increasing impurities in gypsum?

A

Decreases setting time
Increases expansion

193
Q

What is there effect of increasing temperature on gypsum?

A

Increases the rate of diffusion of ions but decreases hemihydrate solubility

194
Q

What are examples of additives to gypsum?

A

Potassium sulfate
Borax

195
Q

What is the role of potassium sulfate as an additive in gypsum?

A

Produces synergite (K2(CaSO4)2.H2O)
Rapid crystallisation (increased growth)
Decreased setting time

196
Q

What is the role of borax as an additive in gypsum?

A

Forms calcium borate (deposits on dihydrate)
Delays setting process

197
Q

What are the disadvantages of gypsum?

A

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

198
Q

What are the advantages of gypsum?

A

Dimensionally accurate
Stable
Low expansion (<0.1)
Good colour contrast

199
Q
A

Finals (20 decks)

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