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3rd Year - Module 5 > Dental materials > Flashcards

Dental materials Flashcards

1
Q

What is a thermoplastic material?

A

a material is solid at room temperature but at high temperatures melt with no decomposition

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2
Q

What kind of structure do waxes have?

A

crystalline structure

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3
Q

At softening temperature, what kind of transition do waxes undergo?

A

solid-solid transition

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4
Q

What kind of mineral hydrocarbon chain is paraffin wax?

A

straight chain

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5
Q

What kind of mineral hydrocarbon chain is microcrystalline wax?

A

branched chain

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6
Q

Name 6 uses of wax in dentistry

A

1) modelling
2) indirect - modelling wax
3) carding (teeth on card for lab)
4) boxing in
5) joining component (sticky wax)
6) modelling - direct - inlay wax

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7
Q

What kind of components can be incorporated into waxes? name 4

A

1) resins
2) oils
3) fats
4) pigments

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8
Q

At what temperature do paraffin waxes soften?

A

around 37-55 degrees

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9
Q

What is the melting point of paraffin wax?

A

around 48-70 degrees

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10
Q

What does the addition of microcrystalline wax do?

A

increase in melting point
decreases softening point

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11
Q

What does the addition of beeswax do?

A

decreases brittleness
decreases flow

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12
Q

What is the lost wax technique?

A

method of metal casting in which a molten metal is poured into a mould that has been created by means of a wax model. Once the mould is made, the wax model is melted and drained away

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13
Q

What is a refractory die?

A

any die material that is capable of enduring the high temperatures that occur during firing or casting procedures.

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14
Q

What is a refractory die formed of and what is done to them before use?

A

phosphate bonded investments
They are both fixed and glazed before use

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15
Q

What are the four types of investment materials?

A

1) dental plaster or stone
2) gypsum bonded
3) silica bonded
4) phosphate bonded

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16
Q

Is wax a thermoplastic material?

A

yes

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17
Q

What three components usually make up a wax?

A

animal, vegetable and mineral (hydrocarbon)

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18
Q

What is an example of a direct modelling wax?

A

inlay wax

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19
Q

What dictates the properties of a wax?

A

dependent on the blending of the different components incorporated into the wax

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20
Q

Why are paraffin waxes well suited to the oral environment?

A

softens between 37-55 degrees so will not melt in the mouth

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21
Q

What is modelling wax used for in the lost wax technique, explain the process

A

used to form restoration/prosthesis in wax which is then embedded in investment material (moulding material). It is then boiled out leaving a void in the shape of the restoration/prosthesis which material can be poured into to cast up the restoration

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22
Q

What type of wax is used for acrylic dentures?

A

modelling wax

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23
Q

Why is modelling wax well suited to the design of acrylic dentures?

A

withstands intraoral temperature
amenable to chairside adjustment
tough enough for undercut withdrawal
no tearing/chipping/flaking
no residue in investment mould upon boiling out

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24
Q

What type of wax is used for crowns/inlays/bridges?

A

inlay wax

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25
Q

What colour is inlay wax?

A

blue

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26
Q

What is a unique property of inlay wax?

A

it locks into undercuts and fractures - highlights the presence of undercut in inlay space which should ideally be parallel

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27
Q

What is a refractory die?

A

a dental cast which is heat resistant upon which we can form, in wax, complex structures so when we lose the wax via the lost wax technique, we are left with the shape of the complex structure in the heat resistant mould

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28
Q

Name two types of material in which a refractory die is used to form a framework

A

porcelain
cobalt chrome

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29
Q

What are the two main rigours of processing that refractory dies must be able to withstand?

A

heat and pressure

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30
Q

Of the four types of investment materials, which one is not heat resistant and therefore is not used for alloy casting?

A

Dental plaster and stone
instead it is used to form partial and fill acrylic dentures

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31
Q

What are dental plaster and stone investment materials used to form?

A

partial and full acrylic dentures

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32
Q

Which three investment materials are used for heat resistant investment?

A

Phosphate bonded
gypsum bonded
silica bonded

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33
Q

What are gypsum bonded investment materials mainly used to make moulds for?

A

gold casting alloys

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34
Q

What are silica bonded investment materials mainly used to make moulds for?

A

base metal casting alloys including cobalt chrome

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35
Q

Why are silica bonded investment materials rarely used?

A

Expensive

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36
Q

What are phosphate bonded investment materials mainly used to make moulds for?

A

base metal and gold casting alloys
cast ceramics and glasses
also used for refractory dies in ceramic build ups

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37
Q

For investment materials to be successful, what properties should they ideally have?

A

accurate - capture shape, size, detail
temperature shape stable (>1000 degrees)
high compressive strength
compensate for casting shrinkage
alloy compatible

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38
Q

What is an investment material?

A

materials are used to encase the wax designed for the lost wax casting technique

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39
Q

What are the two components of an investment material for alloy, porcelain and glass casting?

A

1) Binder - hold everything together
2) refractory material - usually silica, heat resistant material

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40
Q

Which materials shrink on casting as they cool?

A

alloys and ceramics

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41
Q

For an accurate fit of a restoration, what must an investment material compensate for?

A

Shrinkage of alloy or ceramic

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42
Q

Why should an investment material expand?

A

to compensate for alloy or ceramic shrinkage

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43
Q

Name 4 mechanisms of investment expansion

A

1) thermal
2) setting expansion of the binder
3) hygroscopic expansion
4) crystalline inversion of allotropic forms of silica

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44
Q

What is hygroscopic expansion?

A

When additional water is brought into contact with the setting material, an increased expansion is observed.

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45
Q

Explain how a thermal mechanism can cause investment expansion

A

heating up causes expansion of investment material

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46
Q

What are two examples of binders in investment materials which can expand upon setting?

A

Calcium sulfate dihydrate - gypsum bonded
magnesium ammonium phosphate hexahydrate - phosphate bonded

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47
Q

What are two ways of bringing about hygroscopic expansion?

A

1) placing mould into water at initial set
2) lining casting ring with damp asbestos equivalent

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48
Q

What two forms does silica exist in?

A

alpha form and beta form

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49
Q

Expansion can also be brought about by crystalline inversion of quartz silica, what does this mean and what do they change from and to?

A

sudden volume change in crystalline quartz particles as they pass 575 degrees. change from quartz alpha to quartz beta at 575 degrees

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50
Q

What is cristobalite?

A

crystalline form of silica

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51
Q

At what temperature does cristobalite low silica, an investment material, expand and turn into cristobalite high silica?

A

210 degrees

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52
Q

At what temperature do gypsum bonded investment materials decompose?

A

1200 degrees

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53
Q

At what temperature does gypsum bonded investment material decompose and how does this cause porosity?

A

Gypsum-bonded investments decompose above 1200°C by interaction of silica with calcium sulphate to liberate sulphur trioxide gas leading to porosity

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54
Q

Why are phosphate bonded investment materials the most popular?

A

higher strength

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55
Q

What materials can phosphate bonded investment materials be used for?

A

all dental alloys

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56
Q

What is a disadvantage of silica bonded investment material compared to gypsum bonded or phosphate bonded materials?

A

Silica bonded investment material lacks porosity which inhibits the escape of air, generating back pressure and possible incomplete casting

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57
Q

What is the importance of porosity in an investment material?

A

when we force alloy into the investment material, we want it to flow in. If air is unable to escape due to the lack of porosity back pressure is generated and casting will not fill, leading to incomplete cast

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58
Q

What does the sprue former look like?

A

A funnel into the sprue

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59
Q

What is the metal casting ring?

A

the cylinder the investment material is contained within

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60
Q

What is the reservoir regarding metal casting?

A

the dilated portion of the sprue

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61
Q

How long does it take to burn out wax prior to casting?

A

temperature held for 30 mins to 1 hour prior to casting

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62
Q

At what temperature is wax burnt out for a gold alloy cast?

A

700-750 degrees

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63
Q

At what temperature is wax burnt out for a palladium silver alloy cast?

A

730-815 degrees

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64
Q

At what temperature is wax burnt out for a base metal alloy cast?

A

815-900 degrees

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65
Q

What techniques can be used to melt an alloy and what are the two main techniques?

A

main 2 - furnace and electrical resistance
gas air torch
oxyacetylene torch
electrical induction

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66
Q

What techniques can be used to force the molten alloy into the mould and which are the main 2 techniques?

A

Main 2 - vacuum and centrifugal force
air pressure
gravity
steam pressure

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67
Q

Name 4 casting faults which can occur

A

1) finning and bubbling
2) incomplete casting
3) porosity in casting
4) over/under sized casting

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68
Q

What is finning?

A

a fault where sharp metal fins appear perpendicular to the surface of the casting

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69
Q

What can finning and bubbling form as a consequence of?

A

due to investment mould cracking, for example, under the force applied when forcing the alloy into the investment mould

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70
Q

What can incomplete casting occur as a result of?

A

poor space design or failure - eg. sprue not facilitating flow of molten alloy
alloy not molten
lack of force
back pressure
cooling shrinkage

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71
Q

What can porosity of a casting occur as a result of? 2 reasons

A

1) embedding of fractured investment material in the casting
2) gaseous porosity due to alloy entrapment of oxygen usually taken up by Cu, Au, Ag, Pt or Pd in alloys when molten - but could be insufficient amount of elements if alloy reused

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72
Q

What elements do base metal casting alloys definitely NOT contain?

A

Au, Ag, Pt or Pd

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73
Q

What are the two types of base metal casting alloys?

A

CoCr
NiCr

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74
Q

What is CoCr mainly used to construct?

A

partial denture frameworks and complete denture bases

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75
Q

What is NiCr mainly used for?

A

applications in crown and bridge

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76
Q

What are the constituents of CoCr?

A

cobalt
chromium - no less than 25%
molybdenum - no less than 4%

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77
Q

Proportions of what three elements added together should make up no less than 85% of CoCr?

A

Cobalt + nickel + chromium

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78
Q

Traces of what three elements are found in CoCr to improve its casting and handling properties?

A

Be, Si, C

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79
Q

What is the role of Co and Ni in CoCr?

A

confer hardness and strength

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80
Q

What is the role of Cr in CoCr?

A

confers both hardness and corrosion resistance via the passivating effect

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81
Q

Why are the trace elements Be, Si and C included in the composition of CoCr?

A

improve casting and handling properties

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82
Q

What is the passivating effect?

A

spontaneous oxidation in the air improves corrosion resistance of alloys

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83
Q

What are the constituents of NiCr?

A

Nickel - main constituent
Chromium - no less than 20%
Molybdenum - no less than 4%
Beryllium - no less than 2%

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84
Q

Proportions of what three elements must make up no less than 85% of NiCr?

A

Nickel + chromium + cobalt

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85
Q

What is the fusion temperature of both CoCr and NiCr?

A

1200-1500 degrees C

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86
Q

How is the fusion temperature of CoCr and NiCr achieved?

A

oxyacetylene torch or electrical induction furnace

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87
Q

The fusion temperature of NiCr and CoCr is higher than that of what alloy?

A

higher than gold alloys

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88
Q

The density of NiCr and CoCr is lower than what alloy?

A

gold alloys

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89
Q

To avoid miscasting CoCr and NiCr, what is required?

A

additional casting thrust
porous investment material required to allow egress of air

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90
Q

What is steel an alloy of?

A

alloy of iron and less than 2% carbon

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91
Q

Why should the percentage of carbon present in steel not be increased past 2%?

A

it will become brittle and unsuitable for cold working

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92
Q

Steel can be shaped at temperatures well below the metals recrystallisation temperature, what can this cause and what are the pros/cons?

A

may lead to work hardening
pro - can toughen up the alloy
con - predisposed to fracture if overdone

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93
Q

What structure does steel exist in when heated above 723 degrees C?

A

Austenite

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94
Q

What is austenite (structure)?

A

an interstitial solid solution of carbon in a face centred cubic iron matrix

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95
Q

At what temperature does steel exist in its austenite phase?

A

above 723 degrees

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96
Q

Below 723 degrees, which two phases of steel exist?

A

Ferrite
Cementite

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97
Q

What is a mixture of ferrite and cementite termed?

A

Pearlite

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98
Q

What is ferrite?

A

phase of steel
dilute solution of C (up to 0.02%) in Fe
exists at temperatures below 723 degrees

99
Q

What is cementite?

A

phase of steel
specific compound Fe3C
exists below 723 degrees

100
Q

What does Tc stand for in a phase diagram?

A

Critical temperature

101
Q

What does the X highlight in a phase diagram?

A

eutectic alloy

102
Q

What percentage of carbon in steel gives the eutectic alloy?

A

0.8%

103
Q

What is a eutectic alloy?

A

A mixture of metals having a melting point lower than that of any of its components

104
Q

Below Tc, what phases of steel exist?

A

Ferrite, cementite and pearlite

105
Q

Above Tc, what phases of steel exist?

A

austenite
austenite and ferrite
austenite and cementite

106
Q

Different structures exist above Tc in a phase diagram depending on what?

A

% of carbon

107
Q

Out of austenite and ferrite or austenite and cementite, which has the lower percentage of carbon?

A

austenite and ferrite

108
Q

What is the very highest line on a phase diagram called?

A

liquidus

109
Q

What is the line at the top of the austenite phase but below the liquidus called?

A

solidus

110
Q

What line indicates where austenite, cementite, ferrite and pearlite all exist in solid form?

A

solidus

111
Q

What is the solidus on a phase diagram?

A

line below which only solid is present. where melting begins

112
Q

What is the liquidus on a phase diagram?

A

line above which only liquid is present.
temperature at which an alloy is completely melted

113
Q

What exists between the solidus and liquidus lines?

A

temperatures between the solidus and liquidus the alloy is part solid, part liquid

114
Q

What can be indicated via use of the phase diagram?

A

indicated how we can use the ability to customise hardness by heat treatments

115
Q

How can we change martensite into ferrite and cementite?

A

tempering in the range of 200-400 degrees for a defined time

116
Q

How can we change ferrite and cementite into austenite?

A

heating above Tc below the solidus

117
Q

What side of a phase diagram is hyper-eutectoid?

A

RHS - cementite

118
Q

What side of a phase diagram is hypo-eutectoid?

A

LHS - ferrite

119
Q

A material is either hypo or hyper eutectoid depending on what?

A

% of carbon

120
Q

Does a hypereutectoid have a higher or lower % of carbon than a hypoeutectoid?

A

Hypereutectoid = higher % C
hypoeutectoid = lower % C

121
Q

Is cementite a hyper or hypo-eutectoid?

A

hypereutectoid

122
Q

Is ferrite a hyper of hypo-eutectoid?

A

hypo-eutectoid

123
Q

What are the characteristics of a hyper-eutectoid and what are they used for?

A

hard and brittle
cutting instruments eg. burs

124
Q

What are the characteristics of a hypo-eutectoid and what are they used for?

A

softer and more ductile
non-cutting instruments such as forceps

125
Q

Name a hypo-eutectoid

A

Ferrite

126
Q

Name a hyper-eutectoid

A

cementite

127
Q

What does stainless steel have added to it?

A

chromium is added

128
Q

Other than chromium, what else may be added to stainless steel and why?

A

Ni - increases corrosion resistance and strength

129
Q

The addition of nickel and chromium reduces what?

A

reduces critical temperature

130
Q

Stainless steel has an improved corrosion resistance due to what?

A

by the passivating effect

131
Q

What is the composition of 18/8 stainless steel?

A

18% Cr
8% Ni

132
Q

Why is 18/8 stainless steel not amenable to heat treatment?

A

because Tc is below where atomic movements are possible

133
Q

What type of working is used to form wrought structures from 18/8 stainless steel?

A

cold working

134
Q

What wrought structures can be formed from cold working 18/8 stainless steel?

A

non-cutting instruments, wires, denture bases

135
Q

18/8 stainless steel exists in 3 grades depending on the carbon content, what are these 3 grades?

A

soft
half hard
hard

136
Q

Soft, half hard and hard 18/8 stainless steel all have a carbon composition of less than what?

A

less than 0.15%

137
Q

18/8 Stainless steel may be joined by what two techniques?

A

soldering
welding

138
Q

18/8 SS can be joined through soldering, what metal element is solder formed from?

A

Ag - silver

139
Q

What does the welding technique do?

A

press two pieces together and heat the join

140
Q

temperature change is proportional to IsquaredRt where I, R and t represent what?

A

I - current
R - electrical resistance
t - time current passes

141
Q

What is temperature change proportional to?

A

IsquaredRt

142
Q

What is the composition of martensitic SS , what treatment can it undergo and what can be made from it?

A

No Ni present, 12% Cr
Heat treatments possible
Probes and cutting instruments

143
Q

Orthodontic wire has a springback ability, what does this mean?

A

ability to undergo large deflections without permanent deformation

144
Q

What is the equation for springback potential?

A

springback potential = yield stress/modulus of elasticity

145
Q

The thickness of orthodontic wire impacts its stiffness, with an increase in thickness, what happens to stiffness?

A

thicker = stiffer

146
Q

What are the two presentations of impression compound?

A

impression compound in 5mm sheet
green stick

147
Q

What is a thermoplastic material?

A

polymers that can be softened through heating before being processed and then left to cool and harden.

148
Q

Are impression compounds thermoplastic?

A

Yes

149
Q

What are sheets of impression compound used for?

A

in stock trays for impressions of edentulous ridges
in stock trays for impressions of free end saddles for partial dentures prior to alginate imp

150
Q

What are the characteristics of sheets of impression compound?

A

mucocompressive, non-elastic, displace tissue, low surface detail obtained

151
Q

What is green stick used for?

A

for border extensions of impression trays

152
Q

What is the composition of an impression compound?

A

thermoplastic material - 47%
filler - 50%
lubricant - 3%

153
Q

What thermoplastic materials are used in impression compounds?

A

natural/synthetic resins or waxes

154
Q

What is the function of the thermoplastic material in an impression compound?

A

characterise softening temperature

155
Q

What filler material is used in impression compounds?

A

Talc

156
Q

What is the function of talc, the filler material, in impression compounds?

A

gives body to material
decreases thermal contraction

157
Q

What lubricant is used in impression compounds?

A

stearic acid

158
Q

What is the function of stearic acid in impression compounds?

A

lubricant, improves flow

159
Q

What can over conditioning of an impression compound cause?

A

leaching out of stearic acid

160
Q

How is an impression compound conditioned?

A

placed in a 55-65 degree napkin/cloth lined bath to soften it

161
Q

Once an impression compound has been softened, what must you do to improve flow characteristics?

A

incorporate water to act as an additional plasticiser by kneading the material

162
Q

Why must you avoid prolonged immersion during conditioning of an impression compound?

A

can cause leaching of stearic acid which can reduce the flow of the material

163
Q

How do you test the temperature of impression compound before use?

A

test on the back of a gloved hand

164
Q

Is impression compound elastic?

A

No - but it may remove from small undercuts if still warm but this can cause distortion

165
Q

What type of denture is zinc oxide eugenol impression material used for?

A

complete dentures

166
Q

What is zinc oxide eugenol impression material mixed from?

A

two paste presentation

167
Q

What are the two pastes composed of in zinc oxide eugenol impression material?

A

1) ZnO, olive oil and zinc acetate +/- water
2) Eugenol and Kaolin/talc

168
Q

What are the functions of the three components of paste 1 in zinc oxide eugenol impression material?

A

ZnO - reactive ingredient
olive oil - inert, forms paste
zinc acetate +/- water - accelerants

169
Q

What are the functions of the two components of paste 2 in zinc oxide eugenol impression material?

A

Eugenol - reactive ingredient
Kaolin/talc - inert filler

170
Q

What kind of setting reaction is there in the making of zinc oxide eugenol impression material?

A

acid base reaction with ZnO forming salt of zinc eugenolate plus water

Phenolic -OH of eugenol acts as a weak base

171
Q

What kind of tray is used with zinc oxide eugenol impression material?

A

close fitting special tray

172
Q

What depth of spacer is there in a close fitting special tray?

A

1mm spacer

173
Q

What allows zinc oxide eugenol impression material to capture fine detail in an impression?

A

low initial viscosity

174
Q

Name two mucostatic impression materials

A

alginate
dental impression plaster

175
Q

The expansion of dental impression plaster is reduced by the incorporation of what?

A

potassium sulphate and borax

176
Q

Why may patients complain of a dry mouth following impression using dental impression plaster?

A

it absorbs moisture

177
Q

What is an ideal glass transition temperature (Tg) of an impression material?

A

32-70 degrees C

178
Q

Acrylic denture base materials come in what form prior to mixing?

A

Powder liquid form

179
Q

What is the powder mixed to form acrylic denture base material composed of?

A

PMMA beads
initiator - benzoyl peroxide (0.5%)
pigments - colouring

180
Q

What is the liquid mixed to form acrylic denture base material composed of?

A

monomer - methyl methacrylate (MMA)
cross linking agent -ethylene glycol dimethacrylate
inhibitor - hydroquinone

181
Q

What is only included in the liquid aspect of acrylic denture base materials when a self-cure material is desired?

A

activator - N,N’-dimethyl-p-toluidine

182
Q

Why is it so important to use the correct P:L ratio when mixing acrylic denture base material?

A

reduces shrinkage by large degree

183
Q

What is the correct P:L ratio for mixing acrylic denture base material and what does this reduce shrinkage to?

A

2.5:1
reduces shrinkage to 5-6%

184
Q

What are the five stages of mixing acrylic denture base material?

A

sandy
stringy
doughy - ideal
rubbery
hard

185
Q

What is the setting reaction in the production of acrylic denture base material and is this exo or endothermic?

A

free radical addition polymerisation
exothermic

186
Q

Once the acrylic has been packed into the flask, what cycle is it put into in the water bath or electrical oven?

A

7hrs at 70 degrees C followed by 100 degrees C for 3 hours

187
Q

Where is a particular weak point on lower dentures?

A

along the midline

188
Q

Why should frenal relief always be cut in a rounded shape, not notched in acrylic?

A

poor impact strength of material - notch focuses pressure on one area

189
Q

Does PMMA absorb water?

A

Yes, candidal growth risk

190
Q

Why does MMA pose a risk to technicians?

A

it is a powerful dermatological irritant

191
Q

What are the preferred investment materials for CoCr denture base construction?

A

Si or phosphate bonded
NOT gypsum bonded

192
Q

What is a tissue conditioner?

A

material temporarily added to the fit surface of a denture to permit a more equal load distribution and assumption of more normal mucosal position

193
Q

In what situation may a tissue conditioner be required?

A

for an immediate denture a few weeks after extractions
following infection of denture bearing area to adapt fit to healing tissues

194
Q

What is the function of a functional impression material?

A

applied to fit surface to allow a more definitive impression to be taken under functional conditions

195
Q

What is the function of a permanent resilient lining (soft lining)?

A

applied as “shock absorber” to avoid pain from movement of denture upon nerves (particularly mental nerve with atrophic ridge)

196
Q

What is the ideal glass transition temperature of soft lining materials, tissue conditioners and functional impression materials?

A

37 degrees or less

197
Q

What three types of materials (polymers) can be used to make soft lining materials, tissue conditioners and functional impression materials?

A

1) acrylic polymers with solvent
2) acrylic polymers or co-polymers with an inert ester to act as a plasticiser
3) silicones

198
Q

What kind of hardness would we want in a tissue conditioner?

A

soft but not capable of undergoing substantial permanent deformation - so is elastic

199
Q

What kind of hardness would we want in a functional impression material?

A

soft but deforms plastically to record impression of denture bearing area or neutral zone. plastically - meaning does not change shape again

200
Q

What does visco-elastic mean?

A

a material that flows under pressure but is resilient under dynamic forces such as chewing

201
Q

What are the two different presentations of tissue conditioners and functional impression materials?

A

1) powder/liquid
2) preformed sheets of acrylic gel

202
Q

What are the names of the powders and liquids mixed to form a tissue conditioner or functional impression material?

A

powder - poly ethyl methacrylate or co-polymer
liquid - ethyl alcohol plus aromatic ester such as butyl phthaly glycolate

203
Q

What kind of reaction occurs between the powder and liquid in the formation of tissue conditioners/functional impression material?

A

physical formation of a gel, NOT chemical

204
Q

What are the four materials that soft linings/permanent resilient linings can be made of?

A

Heat cured acrylic
Self cured acrylic
Heat cured silicone
Self cured silicone

205
Q

What is a heat cured acrylic composed of?

A

powder of higher methacrylates (such as polyethyl methacrylate) activated by benzoyl peroxide in liquid containing monomer and plasticiser

206
Q

What are heat cured silicones processed against?

A

processed against acrylic

207
Q

What are issues with soft linings over time?

A

food can become embedded in them

208
Q

What is an issue over time with acrylic soft lining materials?

A

plasticisers are lost with time leading to hardening and dimensional changes

209
Q

Are silicones susceptible to candida albicans growth?

A

Yes

210
Q

What two materials can be difficult to achieve a bond between relating to soft linings?

A

silicone and PMMA

211
Q

How does the thickness of soft lining effect its function?

A

Thicker the soft lining, the greater the cushioning effect

212
Q

What is the ideal thickness of a soft lining material?

A

2-3mm

213
Q

How are acrylic artificial teeth produced?

A

produced in reusable moulds by dough moulding techniques or injection moulding

214
Q

How are acrylic teeth retained to the denture base?

A

chemical interaction

215
Q

What kind of moulds are porcelain artificial teeth produced in?

A

produced in moulds that are 30% larger than required as they shrink when they are fired

216
Q

How are porcelain teeth retained to the denture base?

A

mechanical retention via holes or pins

217
Q

What happens to porcelain teeth upon chewing and how do they transmit the load compared to acrylic teeth?

A

click on chewing
transmit greater load to denture bearing area

218
Q

What is absolutely necessary to optimise attachment to denture base?

A

effective boiling out

219
Q

Are acrylic or porcelain teeth more likely to suffer abrasion?

A

acrylic teeth

220
Q

Are acrylic or porcelain teeth more likely to chip and fracture?

A

Porcelain teeth

221
Q

Is occlusal adjustment easier with acrylic or porcelain teeth?

A

acrylic teeth

222
Q

What two materials can endodontic files be made of?

A

Stainless steel
Nickel titanium

223
Q

Can files be sterilised and re-used?

A

No they are single use

224
Q

With an increase in number of radial lands on a file, what also increases?

A

friction

225
Q

What do radial lands do on a file?

A

they cut

226
Q

How does the angle/rake affect effectiveness of a file?

A

the angle/rake directs debris and affects cutting ability

227
Q

what is quenching?

A

rapid cooling of a metal

228
Q

What do heat treatments affect in metals?

A

grain/crystalline structure

229
Q

How are stainless steel files manufactured? 3 techniques

A

1) machined into blank (square/triangle shaped)
2) twisted into spiral - brings about work hardening
3) machine from a stainless steel rod

230
Q

What are two drawbacks of SS files?

A

1) lack of resilience
2) no shape memory

231
Q

What is the composition of NiTi?

A

Ni - 55%
Ti - 45%
sometimes Ni substituted with <2% Co

232
Q

What is NiTi with 55% Ni also known as?

A

55NiTiNOL

233
Q

60NiTiNOL has been manufactured and is harder than 55NiTiNOL but what is its disadvantage?

A

less shape memory effect

234
Q

What are the advantages of NiTi over SS?

A

Softer
greater strength
tougher
greater resilience
shape memory
superelasticity
therefore, less likely to fracture

235
Q

What does superelasticity mean?

A

resists stress without permanent deformation

236
Q

The stress of RCT causes shape, volume and density changes, causing the change of NiTi between who two forms?

A

austenitic to martensitic

237
Q

What form of NiTi exists at high temperature and what are the properties of it?

A

Austenitic
hexagonal lattice, less brittle, higher modulus of elasticity

238
Q

What form of NiTi exists at low temperature and what are the properties of it?

A

Martensitic
body centred cubic lattice, more brittle, lower modulus of elasticity

239
Q

What are the Vickers hardness numbers of SS, NiTi and dentine?

A

SS - 530
NiTi - 300-350
Dentine - 70

240
Q

Why can’t NiTi wire not be twisted like SS to form a file?

A

due to its superelasticity

241
Q

When NiTi corrodes what is released?

A

Ti

242
Q

What biological impact has Ni been found to have?

A

reduces mitosis of human fibroblasts

243
Q

Does NiTi have a biological impact?

A

no such effect as Ni alone has been found, presumably due to being bound tightly in NiTi alloy

3rd Year - Module 5 (12 decks)

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