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BDS3 > DMS > Flashcards

DMS Flashcards

1
Q

Components of a preformed metal crown

A

Porcelain surface
Metal alloy substructure

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2
Q

Porcelain as a crown material

A

Good aesthetics but microcracks tend to for at the fitting surface, making it prone to mechanical failure

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3
Q

Metal alloys as crown material

A

Good mechanical properties
Poor aesthetics

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4
Q

Compressive strength

A

Stress required to cause fracture

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5
Q

Elastic modulus

A

Rigidity
Stress:strain ratio
Stress required to cause change in shape

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6
Q

Brittleness/ductility

A

Dimensional change experienced before fracture

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7
Q

Hardness

A

Resistance of surface to indentation or abrasion

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8
Q

Which mechanical properties does a stress strain curve give information on?

A

Rigidity/elastic modulus
Brittleness/ductility
Compressive/tensile strength

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9
Q

What does the difference between proportional limit and fracture stress on a stress-strain curve indicate?

A

The brittleness or ductility of a material
Big difference - ductile
Small difference - brittle

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10
Q

Describe hardness, strength, rigidity and ductility of metal alloy and porcelain

A

Porcelain - quite hard, strong and rigid, very brittle (not ductile at all)
Metal alloy - very hard, strong, rigid and quite ductile

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11
Q

Characteristics of porcelain

A

Rigid - large stress required to cause strain
Hard - surface withstands abrasion/indentation well
Strong - high compressive strength but low tensile strength
Tendency to form surface defects, leads to fracture at low stress
Brittle - low fracture toughness

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12
Q

What bonds porcelain to alloy in preformed metal crowns?

A

Metal oxide on the surface of the alloy

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13
Q

What is the role of an alloy in a preformed metal crown?

A

Support and limit the strain that porcelain experiences
With the two materials bonded together, the stress applied causes a small strain to be experienced, small enough for porcelain to withstand

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14
Q

Porcelain fused to metal alloys

A

High gold alloy
Low gold alloy
Silver palladium
Nickel chromium
Cobalt chromium

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15
Q

Required properties of alloy to be used in porcelain alloy crown

A

Form good bond to porcelain - i.e. good wetting, porcelain forms bond with metal oxides on the surface
Thermal expansion coefficient similar to porcelain, to avoid setting up stresses during fusing of porcelain on to alloy
Avoid discolouration of porcelain
Mechanical - bond strength, hardness and elastic modulus
Melting, recrystallisation temperature must be higher than fusion temperature of porcelain

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16
Q

Creep

A

Gradual increase in strain (permenant) experienced under prolonged application of stress (<EL)

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17
Q

Which alloys are more difficult to bond to porcelain?

A

Nickel chromium

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18
Q

Which alloys can cause discolouration of porcelain?

A

Silver in silver palladium

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19
Q

Which alloys have the highest elastic modulus?

A

Nickel chromium

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20
Q

Why is no copper usually in alloys to be bonded to porcelain for crowns?

A

Gives green hue to porcelain

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21
Q

High gold alloys constituents and their effects

A

80% gold
14% Pt/Pd - helps match thermal expansion to porcelain, increases melting point
Ag 1%
Small amount of indium or tin - form metal oxide layer

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22
Q

What is the issue with metal alloys having a lower melting/recrystallisation temp than fusion temperature of porcelain?

A

Can cause creep to occur

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23
Q

Problems with high gold alloys

A

Melting range may be too low
Young’s Modulus - too low

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24
Q

What alternative to high gold alloys can be used to improve its limitations?

A

Low gold alloys - Au 50%, Pd 30%, Ag 10%, Indium/Tin - 10%
Increased melting temperature
Slightly better mechanical properties

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25
Q

Constituents of silver palladium alloys and their characteristics

A

Pd 60%
Ag 30%
Indium/tin - 10%
High melting point
Care needed in casting

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26
Q

Constituents of Nickel chromium alloys and their characteristics

A

Ni 70-80%
Cr 10-25% (oxide bond)
High melting point
High young’s modulus
High casting shrinkage
Low-ish bond strength

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27
Q

Characteristics of cobalt chromium alloys

A

High melting point (1300-1400C)
Casting shrinkage 2.3%
Low ish bond strength 50MPa
High Young’s modulus 220GPa
High tensile strength 850MPa
High hardness 360-430 VHN

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28
Q

Bonding type between porcelain and metal in crowns

A

Mechanical - probably least important, due to irregularities on surfaces
Stressed skin effect - slight difference in thermal contraction coefficients - lead to compressive forces which aid bonding
Chemical - may be electron sharing in oxides, during firing porcelain flows and oxides in the metal-oxide coating migrate

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29
Q

Ideal thermal expansion coefficient of a metal alloy to be bonded to porcelain

A

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

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30
Q

Uses of stainless steel in dentistry

A

Denture bases
Orthodontic wire

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31
Q

Wrought alloy

A

An alloy which can be manipulated or shaped by cold working
e.g. drawn into a wire for use in ortho appliance or as denture clasps

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32
Q

Steel composition

A

Steel is an alloy
>98% iron
<2% carbon
0.5%-1% chromium to improve tarnish resistance
Manganese, molybdenum, nickel, cobalt, silicon
Any more than 2% carbon and it is considered as CAST IRON

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33
Q

Allotropic

A

Undergoes two solid state phase changes with temperature

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34
Q

Iron characteristics

A

Allotropic
Temp >1400C - Body Centred Cubic crystalline lattice structure, low carbon solubility (0.05%)
Between 900-1400C - face centred cubic lattice structure, higher carbon solubility (2%)
<900 - BCC structure, low carbon solubility again

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35
Q

Austenite

A

Interstitial solid solution of iron and carbon, FCC, exists at high temp >720C

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36
Q

Ferrite

A

Very dilute solid solution
Exists at low temp

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37
Q

Cementite

A

Fe3C exists at low temperature

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38
Q

Pearlite

A

Eutecoid mix of ferrite and cementite

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39
Q

Solid solution

A

Two metals that are soluble in one another form a common lattice structure

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40
Q

Substitutional solid solution

A

Two types
1. Random - both types of atom in the lattice structure are arranged at random
2. Ordered - types of atoms located in predictable layers

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41
Q

Interstitial solid solution

A

Two atoms of markedly different size
Larger atom forms lattice
Smaller fits into spaces in random fashion

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42
Q

Why is austenite more desirable than ferrite or cementite?

A

They have large grains with poor mechanical properties

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43
Q

What does quenching of austenite produce?

A

Martensite

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44
Q

Martensite properties

A

Distorted lattice - no time for diffusion of carbon
Hard
Brittle

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45
Q

What does slow cooling of austenite give us?

A

Pearlite
Ferrite
Cementite

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46
Q

What is tempering of martensite and why is this done?

A

Heating 450C followed by quenching
Temp and duration affect conversion into ferrite (soft, ductile) and cementite (hard, brittle)
Non dental uses for these products

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47
Q

Stainless steel composition

A

Iron
Carbon
Chromium
Nickel
Only stainless steel if >12% Cr

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48
Q

What is the role of chromium in stainless steel?

A

To lower the temperature of austenite to martensite conversion and to lower the rate of this conversion

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49
Q

What gives stainless steel corrosion resistance?

A

Chromium oxide layer

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50
Q

What is the role of Nickel in stainless steel?

A

Improves strength and corrosion resistance
Lowers austenite to martensite transition temp

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51
Q

Type of stainless steel

A

Martensitic
Austenitic

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52
Q

Martensitic stainless steel properties

A

12-13% chromium, little carbon
Heat hardenable
Used to make dental instruments

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53
Q

Austenitic stainless steel properties

A

Contains sufficient chromium and nickel to suppress austenite to martensite transition
E.g. 18% Cr 8% Ni ratio
or
12% Cr 12% Ni ratio

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54
Q

Austenitic stainless steel uses

A

Dental equipment and instruments - can withstand WD temps
Wires - ortho, readily cold worked, resists corrosion
Sheet forms for denture bases - swaged

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55
Q

18-8 stainless steel composition

A

18% chromium
8% Nickel
0.1% carbon
74% iron

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56
Q

Properties of 18-8 stainless steel

A

Does not heat harden
Soft and malleable when cast but work hardens rapidly

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57
Q

Cold working

A

Work done on metal alloys at low temperature - below recrystallisation temp.
Bending, rolling or swaging, cause SLIP - dislocations collect at grain boundaries
Hence, stronger, harder material

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58
Q

18-8 stainless steel wire uses

A

Ortho springs, clasps
Partial denture clasp arms, wrought rests

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59
Q

Grades of 18-8 stainless steel wire

A

Soft
Half hard
Hard
Spring temper

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60
Q

Wire alloys

A

Stainless steel
Gold
Cobalt chromium
Titanium - various types

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61
Q

Cobalt chromium wire constituents

A

Co 40%
Cr 20%
Ni 15%
Fe 16%

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62
Q

Gold wire constituents

A

Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%

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63
Q

Nickel titanium wire constiuents

A

Ni 55%
Ti 45%
+ some cobalt

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64
Q

Springback ability

A

Ability of material to undergo large deflections (to form an arc) without permanent deformation
EL/YM

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65
Q

Requirements of wires

A

High springback ability
Stiffness (YM)
High ductility
Easily joined
Corrosion resistant

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66
Q

Stainless steel wire soldering

A

Can be done with gold or silver solder, care must be taken to avoid recrystallisation, which negatively impacts mechanical properties
Quench rapidly to maintain

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67
Q

Weld decay

A

Occurs when stainless steel is heated to between 500-900C
Chromium carbides precipitate at grain boundaries
Alloy becomes brittle
Less chromium in central region of solid solution - more susceptible to corrosion

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68
Q

How to minimise weld decay

A

Low carbon steels - expensive
Stabilised stainless steel - contain small quantities of titanium or niobium, forms carbides preferentially - not at grain boundaries

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69
Q

Stress relief anneal

A

Done to ensure the configuration of metal atoms into alloy grains settle into equilibrium
Stainless steel held at 450C for 1-2min

CAREFUL - Above 650C grain structure is affected and above 500C precipitation of carbides at grain boundaries occurs

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70
Q

Properties of stainless steel as a denture base

A

Thin 0.11mm (acrylic 1.52mm)
Light
Fracture resistant
Corrosion resistant
High polish obtainable
High thermal conductivity
High impact strength
Abrasion resistant

Disadvantages
Possible dimensional inaccuracy
Elastic recovery of steel - inaccuracy
Damage of die under hydraulic pressure during swaging
Loss of fine detail during the many stages
Difficult to ensure uniform thickness
Uneven pressure on die and counter die

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71
Q

Ideal properties of a denture

A

Replace function of natural teeth
Fit properly
Appropriate aesthetics

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72
Q

Ideal properties for denture base

A

Dimensionally accurate and stable in use
High softening temp
Unaffected by oral fluids
Thermal expansion
Low density
High thermal conductivity
Radiopaque
Non toxic
Colour
Easy and inexpensive to manufacture
Easy to repair

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73
Q

Mechanical properties required for a denture base material

A

High Young’s modulus
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistance

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74
Q

What type of polymerisation reaction does acrylic resin undergo?

A

Free radical addition polymerisation - chemical union of two molecules either the same or different to form a larger molecule, without the elimination of a smaller molecule
Involves molecules with C=C

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75
Q

Which three groups are joined to the central carbon atom in methacrylate monomer?

A

-CH3
=CH2
-COOCH3

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76
Q

Stages of polymerisation reaction

A

Activation - of initiator to provide free radicals
Initiation - free radicals break C=C bond in monomer and transfer free radical
Propagation - growing polymer chain
Termination - of polymerisation

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77
Q

What is the initiator in heat cured acrylic powder?

A

Benzoyl peroxide 0.2%-0.5%

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78
Q

Components of heat cured acrylic powder

A

Initiator - benzoyl peroxide
PMMA particles (pre polymerised beads)
Plasticiser - allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
Pigments to give natural colour
Co-polymers to improve mechanical properties e.g. ethylene glycol dimethacrylate

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79
Q

Components of heat cured acrylic liquid

A

Methacrylate monomer (dissolves PMMA particles, polymerises)
Inhibitor - Hydoquinone 0.006% prolongs shelf life - reacts with any free radicals produced by heat or UV light
Co-polymers to improve mechanical properties, particularly crosslinking of polymers

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80
Q

What is efficient polymerisation?

A

Low number of crosslinks in polymer, giving high molecular weight and good mechanical properties

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81
Q

Properties of acrylic resin

A

Non toxic/irritant
Unaffected by oral fluids
Mechanical properties - poor
Fairly resistant fatigue strength, but can fail
High hardness/abrasion resistance, some wear over time
Thermal expansion - ok if acrylic teeth used, far higher than porcelain teeth
Low thermal conductivity - bad
Low density - good
Ok softening temperature - must not clean in boiling water
Ok dimensional accuracy and stability in use

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82
Q

What is the difference in composition between self cured and heat cured acrylic?

A

Tertiary amine in liquid activates benzoyl peroxide initiator, instead of heat

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83
Q

Advantages of self cure acrylics

A

No heating stage - less thermal contraction and better fit

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84
Q

Disadvantages of self cure acrylic

A

Chemical activation leads to less efficient polymerisation - lower molecular weight and poorer mechanical properties and more unreacted monomer - acts as plasticiser softening the base and more likely to cause irritation
Poorer colour stability
Fits cast better BUT water absorption in mouth makes oversized

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85
Q

How much unreacted monomer can be expected in self cure vs heat cure acrylic?

A

Self - 3-5%
Heat - 0.2-0.5%

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86
Q

What can the issues with sizing of heat cured acrylic denture bases be?

A

Slightly undersized due to thermal contraction
BUT water absorption gives expansion, cancelling some of this out

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87
Q

Advantages of heat cured acrylic resin

A

Higher molecular weight - stronger
Better fit (likely slightly undersized rather than slightly oversized)
Less uncured monomer

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88
Q

Disadvantages of heat cured acrylic resin

A

Curing process may cause porosity

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89
Q

Improved versions of acrylic resin available

A

High impact resistant materials - incorporate rubber toughening agent to stop crack propagation - long term fatigue problems
and fibres (carbon, ultra high molecular weight polyethylene, glass) - difficult processing

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90
Q

Heat cure denture base product Ultra Hi

A

A high impact heat cure acrylic resin formulated with exceptional flexural strength and superior fracture toughness (ductility)
These two key features together gives ultra hi
A slight bending aspect which keeps material from being brittle to reduce cracking

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91
Q

Pour n Cure resins

A

Similar to self cure
Smaller powder particles creates fluid mix rather than dough like substance to be poured into mould
Good fitting but poor mechanical properties

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92
Q

Light Activated Denture Resins

A

Urethane dimethacrylate matrix plus acrylic copolymers, microfine silica fillers and photoinitiator system
Adapted to cast
Cured in light chamber - limits thickness
Used mostly as customised impression tray material and for repair of fractured dentures

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93
Q

Radiopaque polymers

A

Some additions are made to denture base materials to achieve the desired radiopaque quality
Metal inserts - weaken, poor aesthetics
Inorganic salts (high conc required for radiopaqueness) - weak base
Comonomers containing heavy metals - poor mechanical properties

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94
Q

What can be used for patients with allergies to acrylic resin, and what are their main drawbacks?

A

Nylons - water absorption, swelling
Vinyl polymers - low softening temp (60%)
Polycarbonates - require expensive injection moulding technique, become distorted

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95
Q

Most commonly used denture base material

A

Heat cured acrylic resin

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96
Q

What are investment materials used for?

A

To produce metal/alloy inlays, onlays, crowns and bridges by casting the molten alloy into a mould cavity of the required shape
The mould cavity is made of an investment material

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97
Q

Stages in casting an alloy by lost wax technique

A

Wax pattern of required prosthesis is made
Investment material poured around wax pattern and allowed to set to create a mould
Wax is then eliminated (with boiling water)
Molten alloy is forced into the cavity through sprues (hollow tubes prepared in the investment material)

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98
Q

Conditions for casting of molten alloy

A

1000C + and high pressure

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99
Q

Investment materials and their use

A

Dental stone or plaster - acrylic dentures
Gypsum bonded materials - gold casting alloys
Phosphate bonded materials - base metals/cast ceramics
Silica bonded materials - base metal alloys

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100
Q

Requirements of investment material of metal alloys

A

Must expand to compensate for cooling shrinkage of alloy
Porous to allow escape of trapped gases on casting to prevent back pressure effect
Strong - room temp ease of handing, casting temp withstand casting forces
Smooth surface - easy finishing
Chemically stable
Easy removal from cast
Handling not complicaed
Relatively inexpensive

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101
Q

Back pressure effect

A

Voids and defects in cast alloys due to gas trapped during the casting process

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102
Q

Typical contractions (by volume) from alloy melting point to room temp of alloys

A

Gold alloys - 1.4%
Ni/Cr alloys 2.0%
Co/Cr alloys 2.3%

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103
Q

Components of investment materials and their roles

A

Binder - to form coherent solid mass, gypsum, phosphate and silica
Refractory - withstand high temperature and undergoes expansion, silica (quarts or cristobalite)

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104
Q

Composition of gypsum bonded investment

A

Powder (mixed with water) Silica 60-65%, calcium sulphate hemihydrate 30-35%, reducing agent for oxides, chemicals to inhibit heating shrinkage and control setting time - boric acid, NaCl

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105
Q

Setting reaction of gypsum bonded investment material

A

(CaSO4)2.H2O +3H2O -> 2CaSO4.2H2O
Hemihydrate + water -> dihydrate

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106
Q

Dimensional changes of gypsum bonded investement

A

Silica undergoes thermal expansion and inversion expansion
Gypsum undergoes expansion during setting - hygroscopic expansion
and contraction above 320C

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107
Q

Hygroscopic expansion

A

Not fully understood
Varies, causing up to 5 fold change in volume within gypsum bonded investment
Thought to be water molecules attracted to gaps between crystals by capillary forces, forcing crystals apart

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108
Q

Factors increasing hygroscopic expansion

A

Lower powder/water ratio
Increased silica content
Higher water temp
Longer immersion time

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109
Q

What causes the contraction of gypsum above 320C?

A

Water loss
Presence of NaCl and boric acid

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110
Q

Properties of gypsum bonded investment material

A

Expansion 1.4% - sufficient for gold alloys
Smooth surface due to fine particles
Manipulation is easy and setting time is controlled
Porous
Strength is adequate if correct powder/liquid ratio and correct manipulation

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111
Q

What is heat soaking

A

About 700C, if there is any wax residue remaining, a reaction between CaSO4 and C occurs, releasing carbon monoxide
CaSO4 +4C -> CaS + +4CO
Then the CaS reacts with CaSO4 to give SO2 gas
3CaSO4 +CaS -> 4CaO +4SO2
Crucial these gases escape, to ensure this happens, heat soaking is done
Held at this temperature for some time to allow gases to escape

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112
Q

When is gypsum bonded investment suitable?

A

If the metal to be cast melts below 1200C
Above 1200, the CaSO4 in gypsum reacts with SiO2 to create SO3 which will cause voids in the cast

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113
Q

Composition of phosphate bonded investment

A

Powder - silica, magnesium oxide, ammonium phosphate
Liquid - water or colloidal silica

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114
Q

What is the purpose of mixing phosphate bonded investment with colloidal silica?

A

Increases strength
Gives hygroscopic expansion (2%) to compensate for alloy shrinkage on cooling to room temp

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115
Q

Setting reaction for phosphate bonded investment material

A

NH4H2PO4 +MgO +5H2O –> MgNH4PO4+6H2O

Ammonium phosphate reacts with magnesium oxide and water to give
MAGNESIUM AMMONIUM PHOSPHATE and water

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116
Q

What are the effects of heating phosphate bonded investment to around 1000-1100C?

A

At 330C water and ammonia are liberated
At higher temps complex reactions with silico-phosphates take place, leading to increased strength of the material

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117
Q

Properties of phosphate bonded investment

A

High strength
Sufficiently porous
Chemically stable
Easy to use
High green strength

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118
Q

What is green strength?

A

Strength for handling at room temperature

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119
Q

Properties of silica bonded investment

A

Sufficient strength
Not porous - would create weak alloy therefore needs vents
Complicated manipulation

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120
Q

Types of elastomer used

A

Polyether
Addition silicone

121
Q

Factors in selecting an impression material

A

Material characteristics
Clinical performance - ease of use and patient acceptance

122
Q

How are elastomers formed?

A

Polymerisation and crosslinking of polymer chains
Crosslinking generates elastic properties, causing fluid -> solid transition
Polymerisation also may produce byproducts such as water, hydrogen and alcohol which affect dimensional stability and cast compatibility

123
Q

Types of silicone elastomers

A

Addition curing
Condensation curing

124
Q

How are addition cured silicones and polyethers usually sold?

A

Large twin cartridges of base paste and catalyst paste

125
Q

Important material properties of elastomers

A

Surface detail
Flow/viscosity
Contact angle/wettability
(first 3 determine quality of impression small details recorded)
Elastic recovery
Stiffness
Tear strength
(Determine accuracy of dimensions and shape of impression)
Mixing time
Working time
(Practical considerations)
Shore A hardness
Shark fin (flow under pressure, essential for undercuts)
Setting shrinkage
Dimensional stability
Thermal expansion coefficient
Biocompatibility

126
Q

What type of material is Virtual?

A

Addition silicone elastomer
Polyvinylsiloxane

127
Q

Factors affecting the quality of surface interaction between material and tooth/soft tissue surfaces

A

Viscosity - must be able to flow readily
Surface wetting - must make intimate contact with teeth/mucosa
Contact angle - determines how well material envelopes the hard/soft tissue surface

128
Q

Factors affecting the accuracy of an impression material

A

Surface reproduction
Visco-elasticity/elastic recovery

129
Q

Factors affecting an impression materials ability to cope with removal stage and undercuts

A

Flow under pressure (Shark fin test)
Tear/tensile strength
Rigidity

130
Q

Factors affecting impression materials dimensional stability

A

Setting shrinkage
Thermal expansion/contraction
Storage

131
Q

Viscosity of impression materials

A

A measure of materials ability to flow
Must flow readily in order to make close contact with hard/soft tissue surfaces
This therefore affects how well it records surface detail

132
Q

Wettability - contact angle of impression material

A

How readily the material wets the surface of tooth and how closely it envelopes the tooth surface
Small contact angle = large % of volume will make contact with target surface - ideal

133
Q

What is the effect of a high contact angle of impression material on the final impression?

A

Results in spaces between globules of material when applied to teeth, so some of the tooth surface will not be replicated

134
Q

Hydrophilic silicones benefits

A

Incorporate a non-ionic surfactant to wet tooth surface
Makes better contacts with tooth surface

135
Q

ISO 4823:2000

A

International standard for measuring how well surface is represented by impression material

Measured by spreading material over surface with grooves of A 20micrometres B 50 and C 75

136
Q

Ideal elastic behaviour of impression material

A

100% elastic recovery following removal of impression tray from mouth - no permanent strain

137
Q

How can clinical technique influence elastic recovery of an impression?

A

If load time is less (tray is removed quickly) there is less overall permanent strain, better elastic recovery

138
Q

Why should impression trays be removed from the mouth quickly?

A

To reduce the permanent strain on the impression and achieve better elastic recovery, by reducing the load time while the impression is stretched over the bulbous part of the tooth

139
Q

Viscoelastic behaviour

A

Occurs when after being stretched or compressed, a material fails to return to its original dimensions/shape and there is permanent deformation

140
Q

What happens to elastomers ability to flow following mixing?

A

Gradually decreases while the material sets

141
Q

What happens to an elastomers elasticity following mixing of the material?

A

Starts to increase gradually

142
Q

How is the shark fin test carried out?

A

Impression material is applied to a cylindrical chamber with slot of specified depth
Slot is shark fin shape
Pressure is applied from above
Longer fin length = high flow, better for recording undercuts

143
Q

Tear strength

A

Stress material will withstand before fracturing

144
Q

Rigidity

A

Stress/strain rate
Low rigidity - flexible - ideal for impression materials as removal can cause strain and tears, especially from undercut regions

145
Q

Dimensional stability of impression materials

A

Setting shrinkage should be low to maintain shape of the dentition
Thermal expansion coefficient should be low to reduce the change in shape when the impression material is taken through the large temperature shift from the mouth to room temp
Storage - some materials absorb/release moisture, causing a change in dimensions

146
Q

Which type of elastomers tend to have higher setting time and working time?

A

Addition silicones

147
Q

How to choose the appropriate material checklist

A

Know KEY material properties
Review product specification data
Know typical values expected for specific properties
Identify properties NOT mentioned
REJECT claims not supported with evidence

148
Q

Temporary materials

A

PMMA
PEMA
Bis-acryl composite
UDMA

149
Q

Methacrylate monomer

A

CH3
I
C = CH2
I
COOCH3

150
Q

Polymethylmethacrylate - JET temporary material

A

Powder/liquid formation
Self curing
Good marginal fit
Good transverse fit
Polishable
BUT - poor abrasion resistant, high shrinkage, high thermal release and FREE MONOMER TOXIC

151
Q

What type of monomer does PMMA have?

A

Mono-functional - has only one C=C bond, forms linear chain like polymer

152
Q

Brand names for PEMA temporary materials

A

Trim II
Snap

153
Q

Bis GMA monomer type

A

Bifunction - 2 C=C bonds - forms 3D polymer network

154
Q

Brand names of Bisacrylate composite temporary materials

A

Protemp4
Quicktemp

155
Q

Brand name of urethane dimethacrylate temporary material

A

Provipont DC

156
Q

How are temporary restoration materials sold?

A

Powder and liquid
OR
Two pastes in cartridge system

157
Q

Why is product literature not always the best source of information when choosing a material?

A

Can be biased
Not always complete or accurate data

158
Q

What is a reliable source of information when choosing a dental material?

A

Journals

159
Q

Examples of product literature

A

Brochures
Web sites
Product profile

160
Q

Findings of Zach and Cohen study of temperature increase impact on the pulp in 1965

A

2C increase - no effect on pulp histology
5.5C increase - significant tissue changes over first few days, two months for recovery of MOST pulps, some smaller teeth were necrotic
11C increase - most suffer irreparable necrosis

161
Q

Two factors of heat stimulus effects on dental pulp

A

Duration of stimulus
Temp increase

162
Q

What did the Baldissara study from 1997 find, in comparison to the earlier Zach and Cohen study?

A

It contradicts what Zach and Cohen found that temp increase of 11.5C will cause pulp tissue damage

163
Q

Baldissara experiment

A

Pair of teeth from same individual
One tooth exposed to thermal stimulus believed to be harmful, monitored for symptoms for 2-3months then extracted and examined histologically for damage to pulp
Other tooth extracted and exposed to same stimulus in order to assess the temp increase induced in tooth
No damage <11C increase

164
Q

Temperature increase shown by a range of temporary materials

A

3.5-10.5C, 1.5-2 mins taken
All safe

165
Q

What was the worst case involved in the Baldissara experiment?

A

Change of more than 7C, held for 170 sec, with a peak of over 14C - pulp was unaffected

166
Q

Three important characteristics of temporary materials

A

Temperature - exothermic reaction, safety of the pulp important
Colour stability - aesthetics
Polymerisation shrinkage - clinical fit

167
Q

Biting forces can reach what force?

A

700N

168
Q

Luting agents include

A

Dental cements
Composite resins
Self adhesive composite resins
Surface modifying chemicals

169
Q

Properties of a luting agent

A

Viscosity and film thickness
Ease of use
Radiopaque
Marginal seal
Aesthetics
Solubility
Cariostatic
Biocompatible
Suitable mechanical properties

170
Q

Viscosity and film thickness of luting agents

A

Dependent on the size of powder or filler particles in the material
Must be low to allow seating of the restoration without interference
Viscosity increases as material sets - must seat rest. quickly and maintain pressure
Film thickness should be as thin as possible ideally 25 micrometers or less

171
Q

Ease of use of luting agents

A

Easy to mix - many products encapsulated, clicker system
Working time should be long to allow for seating of restoration
Setting time to be short

172
Q

Radiopaque - luting agents

A

Some ceramic crowns are radiolucent - makes it easier to see marginal breakdown

173
Q

Marginal seal of luting agents

A

Ideally should bond chemically to the tooth and the indirect restoration with a permanent impenetrable bond

174
Q

Aesthetics of luting agents

A

Ideally tooth coloured - variation in shade and translucency
Non staining

175
Q

Solubility of luting agents

A

LOW

176
Q

Cariostatic - luting agents

A

Fluoride releasing
Antibacterial
Important in prevent secondary caries around crown margins

177
Q

Biocompatibility of luting agents

A

Non toxic
Not damaging to pulp - pH or heat on setting
Low thermal conductivity

178
Q

Ideal mechanical properties of luting agents

A

High compressive strength (dentine around 275 MPa)
High tensile strength (Dentine around 50MPa)
High hardness (dentine around 70K enamel around 400K)
Young’s modulus similar to tooth (dentine around 15GPa)
Currently no luting agent gets close to tooth values for more than one or two properties

179
Q

Types of material used as luting agents

A

Dental cements - zinc phosphate, zinc polycarboxylate
GIC - conventional and RM
Composite resin - total etch for use with DBA, self etch, requires etch but has own bonding agent incorporated

180
Q

Zinc phosphate as luting agent

A

Acid base reaction
Powder (ZO90% MgO2 10% to change colour) and liquid (phosphoric acid 50%, oxides to buffer and slow down setting)
Excellent clinical service
Easy to use
Cheap
ZnO + 2H3PO4 -> Zn(H2PO4)2 +H2O followed by hydration reaction resulting in formation of crystalised phosphate matrix Zn3(H2PO4)2.4H2O

181
Q

Problems with zinc phosphate cement

A

Initial pH 2
Exothermic setting reaction
Not adhesive to tooth or restoration - just fills gap
Not cariostatic
Final set takes up to 24h
Brittle
Opaque

182
Q

Zinc polycarboxylate cement as luting agent

A

Similar but polyacrylic acid instead of phosphoric acid
This material has advantage of bonding to tooth
Less heat of reaction
pH low to begin with but returns to neutral more quickly AND longer chain acids which do not penetrate dentine as easily
Cheap

183
Q

Disadvantages of zinc polycarboxylate luting agent

A

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

184
Q

In what circumstances are dental cements used today?

A

Rarely
If trying to cement a temporary indirect such as onlay, without many retentive surfaces, use zinc phosphate as it will stay much better than with temporary cement, and it does not stick to tooth like other permanent cements

185
Q

What type of luting agents are used most?

A

Glass ionomer cement

186
Q

What is the main difference between GI for restorations and GI as a luting agent?

A

Much smaller glass particle size in the luting agent which is less than 20 micrometres to allow for suitable film thickness

187
Q

Reaction for glass ionomer luting agent

A

Acid base between glass (SiO2,Al2O3, CaF2) and acid (poly acid mix of acrylic, maleic and itaconic acid)
Reaction goes through dissolution, gelation and hardening stages

188
Q

Glass ionomer cement bonds

A

Ion exchange with calcium
Hydrogen bonding with collagen
Strong durable and possibly dynamic bond to tooth
No chemical bond to restoration surface - surface of the restoration should be sandblasted to allow mechanical adhesion

189
Q

Benefits of GIC as luting agents

A

Low shrinkage
Easy to use
Long term stability
Relatively insoluble once fully set
Self adhesive to tooth surface
Fluoride release
Cheap

190
Q

RMGI cements

A

Chemistry same as RMGI filling material however glass particles smaller to allow acceptable film thickness
In addition to conventional GIC powder and liquid, the liquid contains hydrophilic monomer - HEMA
Same acid base reaction occurs
Light activation causes polymerisation of HEMA - rapid initial set, dark cure by REDOX reaction
Acid base reaction continues for some time

191
Q

Benefits of resin modified GIC luting agents

A

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

192
Q

Problems with RMGIC that do not affect conventional GIC luting agents

A

HEMA - cytotoxic
HEMA swells/expands in wet environment
No bond to indirect restoration

193
Q

Where can resin modified glass ionomer cements not be used, and why?

A

Cement posts in as may split the root
To cement porcelain crowns as they may crack
both due to swelling when HEMA expands in wet environment

194
Q

Composite luting agents

A

Variants on composite filling materials with suitable viscosity and filler particle size
Must be used in conjunction with dental bonding agents
Can be light cured or dual cured
Have better physical properties, lower solubility and better aesthetics
Technique sensitive
Moisture control important
Physical properties reduced 25% if not light cured

195
Q

Bonding to indirect composite

A

Composite bonds to composite
Bond strength is lower to inlay fitting surface than to new composite
Bond is micromechanical to rough internal surface of inlay
Bond is also chemical to remaining C=C bond on the fitting surface of the inlay
Use a dual curing cement as light penetration through the inlay will be poor

196
Q

What is the role of DBA between tooth and composite luting resin?

A

Hydrophilic end bonds to tooth and hydrophobic end bonds to composite luting resin

197
Q

Bonding to porcelain

A

Porcelain is brittle and requires to be bonded to tooth to prevent fracture
Untreated porcelain is smooth and non retentive
It can be treated with HF to etch surface but toxic
Produces rough retentive surface but it is still not hydrophobic and compatible with composite resin luting agents there for a surface wetting agent is required

198
Q

Silane coupling agent

A

Hydrophobic and hydrophilic ends
Applied to etched porcelain - very strong bond between oxide groups
Other end has C=C bonds which react with composite resin luting agent
Works in the same manner as dentine bonding agent
Strong durable bond

199
Q

Why do composite resin luting agents not bond directly to tooth?

A

Tooth - wet/hydrophilic
Composite - oily/hydrophobic

200
Q

Bonding to metal

A

Like porcelain, composite materials do not bond directly to metal
Metal surface needs roughened - done by etching or more often sandblasting
Chemical bonding required to strengthen the bond
MDP and 4META used, acidic end bonds to metal oxides on surface, C=C bond to composite

201
Q

Why must metal bonding agents be dual cure?

A

Light will not penetrate metal

202
Q

Limitations of metal bonding agents

A

Technique sensitive
Moisture control essential
Will not bond to precious metals

203
Q

Bonding to precious metal

A

Very complex
Must change precious alloy composition to allow oxide formation

204
Q

Self adhesive composite resin

A

Simplifies bonding process
Contains MDP for bonding to metal surfaces
Anaerobic self cured material
Consistent results over many years
Good film thickness
Opaque
Moisture sensitive
Expensive
Mostly used to retain bridgework

205
Q

Self etching composite resin cements

A

New material
Composite cement with self etching dentine bonding agent built in
Requires good moisture control
Acidic groups bind with calcium in hydroxyapatite - stabilising attachment
Ions from dissolution of filler neutralise the remaining acidic groups - chelate reinforced methacrylate network
Limited removal of smear layer
Good bond strength to dentine
Good mechanical properties

206
Q

Self etching composite resin cements bonds to different materials

A

Enamel - lower than to dentine, should be acid etched first
Dentine - should not be etched first
Ceramics - brand specific
Metal - better to non precious, not good enough to cement ortho brackets

207
Q

Uses for GIC cement

A

MCC
Metal post
Zirconia crown
Gold restoration

(NOT fibre post, veneer, adhesive bridge, composite inlay, porcelain inlay)

208
Q

Uses for RMGIC luting agent

A

MCC
Zirconia crown
Gold rest

209
Q

Uses for Light cure composite + DBA as luting agent

A

Veneer

210
Q

Uses for dual cure composite + DBA luting agent

A

Fibre post
Zirconia crown
Composite inlay
Porcelain inlay

211
Q

Uses for anaerobic cure composite luting agent

A

Adhesive bridge
Zirconia crown

212
Q

Uses for self adhesive composite luting agent

A

Fibre post
Zirconia crown
Composite inlay
Porcelain inlay

213
Q

Suitable luting agents for MCC

A

GIC
RMGIC

214
Q

Suitable luting agents for metal post

A

GIC

215
Q

Suitable luting agents for fibre post

A

Dual cure comp + DBA
Self adhesive comp

216
Q

Suitable luting agents for veneers

A

Light cure comp + DBA

217
Q

Suitable luting agents for adhesive bridge

A

Anaerobic cure comp

218
Q

Suitable luting agents for zirconia crown

A

GIC
RMGIC
Dual cure comp +DBA
Anaerobic cure comp
Self adhesive comp

219
Q

Suitable luting agents for composite or porcelain inlay

A

Dual cure comp + DBA
Self adhesive comp

220
Q

Suitable luting agents for gold restoration

A

GIC
RMGIC

221
Q

Temporary cements

A

Made to cement temporary rests in place while permanent is fabricated
Soft for easy removal - some don’t set at all
Prep must be physically retentive or they don’t work
Can be used for trial lute of permanent rest to allow assessment by pt or clinician
Two pastes base and catalyst/accelerator
Base - ZnO, starch and mineral oil
Accelerator - resins, eugenol or ortho EBA and carnauba wax
Wax weakens structure of set cement making it easier to remove
Incorporate petroleum jelly to weaken further

222
Q

When can eugenol containing temporary cements not be used and why?

A

When the definitive is to be cemented with a resin cement
Any residual eugenol may interfere with setting

223
Q

Endo material categories

A

Instruments
Irrigants
Intra-canal medicaments
Obturation materials
Sealers
Pulp capping materials
Root end filling materials

224
Q

Purpose of endodontic instruments

A

Used in mechanical phase of chemomechanical disinfection
Help create space for introduction of irrigants, medicaments and for obturation
Metal files are used to remove soft and hard tissues, as well as micro-organisms
Provides appropriate shape for obturation

225
Q

Strain formula

A

Change in length/length

226
Q

Stress

A

Deforming force measured across a given area F/A

227
Q

Stress concentration point - endo files

A

Abrupt changes in the geometric shape of a fil that leads to a higher stress at that point

228
Q

Strain

A

Response of a material to stress, amount of deformation undergone

229
Q

Elastic limit - endo files

A

Set value representing the max strain that when applied to a file, allows the file to return to original dimensions

230
Q

Elastic deformation

A

Reversible deformation that does not exceed the elastic limit

231
Q

Plastic deformation

A

Permanent bond displacement when elastic limit exceeded

232
Q

Prevention of endodontic instrument failure

A

Manual glide path
Straight line access
NiTi files should be used in constant motion with gentle pressure
Avoid use of rotary files in abruptly curved or dilacerated canals

233
Q

Ideal root filling (materials)

A

Various endo materials used
Generally, core material and a sealer
Always use sealer to create a fluid-tight seal
Biologically acceptable chemical and mechanical treatment of the root canal to promote healing and repair of periradicular tissues

234
Q

Modern alternative to bioceramic cement

A

Biodentine - similar material with modifications allowing much more rapid setting, good for pulp capping
Reduced discolouration

235
Q

Classifications of endo instruments

A

Manually operated
Low speed
Engine driven NiTi rotary instruments
Engine driven instruments that adapt to canal shape
Engine driven reciprocating instruments
Ultrasonic instruments

236
Q

Stainless steel hand instruments - endo

A

Alloy or iron, carbon, chromium and nickel
13-26% chromium - anti rust
Machined stainless steel wire cut into square or triangular cross sections, twisted together creating work hardening, modifying properties
Fluted pattern

237
Q

Work hardening

A

Strengthening of a material by plastic deformation
Crystal structure dislocated, becomes less regular, leading to less easily dislocated structure as no easy sliding of lattice over itself

238
Q

Shape memory

A

Alloys can be deformed at one temp but when heated or cooled, return to their original shape

239
Q

Nitinol

A

Equiatomic alloy of nickel and titanium
Does not display normal metal characteristics
Super elasticity
Crystal structure in either austenite or martensite structure depending on temp
Proportions determine the mechanical properties

240
Q

Austenite vs martensite

A

Martensite - soft, ductile, easily deformed
Austenite - strong, hard

241
Q

Components of endo rotary instrument

A

Taper - diameter change along working surface
Flute - groove to collect dentine and soft tissue
Leading/cutting edge - forms and deflects dentine chips
Land - surface between flutes
Relief - reduction in surface of land
Helix angle - angle cutting axis forms with long axis of file

242
Q

Purpose of lands and reliefs on endo rotary instrument

A

Modify the friction created between instrument and dentine, also affecting the force applied to instruments

243
Q

What are irrigants used for in endo treatment?

A

To help remove debris, lubricate and act as anti-microbials by dissolving organic tissue in the root canal
Also penetrate canal periphery and kill bacteria/yeast/viruses
Cause biofilm disruption creating biological compatibility in the canal, without weakening tooth

244
Q

Primary irrigant used

A

NaOCl sodium hypochlorite

245
Q

NaOCl as an irrigant

A

Ionises in water into Na+ and the hypochlorite ion OCl- Establishes an equilibrium with hypochlorous acid (HOCl)
In acid or neutral HOCl predominates
pH 9 + OCl- predominates
HOCl responsible for antibacterial activity
NaOCl effects organic material but is not able to remove smear layer by itself
Potentially has effects on the dentine by reacting with organic component

246
Q

Concentration of NaOCl as irrigant

A

3%

247
Q

What must be removed from root canals before obturation?

A

Smear layer - would cause bacterial contamination and interfere with disinfection, as well as preventing penetration with sealer

248
Q

Smear layer

A

Organic pulpal material and inorganic dentinal debris layer of 1-5 micrometers, with packing into tubules

249
Q

What is used to remove the smear layer from root canals?

A

17% EDTA (or 10% citric acid) with sonic or ultrasonic irrigation - careful with apical control
Do not mix irrigants, make sure NaOCl is fully removed with sterile water rinse before

250
Q

Obturation materials ideal properties

A

Easily manipulated with ample working time
Dimensionally by tissue fluids stable
Seals canals laterally and apically
Non irritant
Impervious to moisture
Unaffected by tissue fluids
Inhibits bacterial growth
Radiopaque
Does not discolour tooth
Sterile
Easily removed if necessary

251
Q

Most commmon obturation material

A

Gutta percha

252
Q

Monomer in GP

A

Isoprene
CH3
I
C = CH2
I
C = CH2
I
H

253
Q

Two crystalline forms of GP

A

Alpha - naturally occurring, heated above 65C it melts into amorphous phase and cooled slowly returns to alpha, cooled rapidly it recrystallises as beta phase
Beta phase is used in commercially produced GP

254
Q

GP cones constituents

A

20% GP
65% ZO
10% radiopacifiers
5% plasticisers

255
Q

Sealers - endo

A

Fill the void between dentinal wall and core material
Also fills irregularities in the canal, lateral canals and between GP points used in lateral condensation
Lubricates during obturation

256
Q

Ideal sealer properties - endo

A

Tackiness for good adhesion
Hermetic seal
Radiopacity
Easily mixed
No shrinkage on setting
Non staining
Bacteriostatic or doesn’t encourage growth
Slow set
Insoluble in tissue fluids
Tissue tolerant
Soluble on retreatment

257
Q

Most common endo sealer

A

Resin sealers

258
Q

Radiopacity of ZOE compared to GP

A

ZOE less radiopaque

259
Q

What is added to ZOE to improve dentine adhesion?

A

Rosin or Canada balsam

260
Q

Setting process of ZOE sealer

A

Chemical process combined with physical embedding of ZO in eugenolate matrix
Eugenolate formation constitutes hardening
CaOH accelerates this so must be fully removed

261
Q

Advantage of GIC sealers in endo treatment

A

Bond to dentine

262
Q

Disadvantages of GIC sealers in endo treatment

A

Removal upon retreatment difficult, minimal antimicrobial activity

263
Q

Main advantage of ZO sealers

A

Effective antimicrobial

264
Q

How are resin sealers sold?

A

As paste-paste mix

265
Q

Advantages of resin sealers

A

Good seal
Good flow
Slow set

266
Q

Disadvantages of resin sealers

A

Initial toxicity - declining after 24 hours

267
Q

Why is it good to have a hydrophilic material as endo sealer?

A

Good penetration into dentinal tubules

268
Q

Calcium silicate sealers

A

High 12.8 pH during initial 24 hours
Hydophilic, enhanced biocompatible, no shrink on setting, non resorbable, easy to use, excellent seal, quick set

269
Q

Options for endo sealers

A

Resin sealers - mainly used
ZO sealers - long lasting antimicrobial, but toxic
GI sealers - dentine bonding good but other undesirable traits
Medicated sealers - not favoured
Calcium silicate sealers - many advantages

270
Q

Uses for pulp capping materials

A

Pulp exposure due to trauma or removal of deep caries
Pulpotomy and pulp regeneration
Root resorption repair
Apexification
Lateral perforation repair
Apicoectomy

271
Q

Characteristics of white vs grey MTA

A

Grey - Better setting characteristics but cause tooth discolouration
White - smaller particle size and reduced colouration

272
Q

Constituents of grey MTAs

A

Tricalcium silicate (both)
Dicalcium silicate (both)
Tricalcium aluminate
Tetracalcium aluminoferrite
Bismuth oxide (both)

273
Q

Constituents of white MTAs

A

Tricalcium silicate (both)
Dicalcium silicate (both)
Calcium aluminate
Bismuth oxide (both)
Calcium sulphate dehydrated

274
Q

Setting reaction of MTAs

A

Hydraulic cements composed of several phases
When mixed with water, chemical hydration reaction takes place
Different setting reactions for white and grey MTA

275
Q

Porcelain vs ceramic

A

Porcelain is a type of ceramic
All porcelain is ceramic but all ceramic is not porcelain

276
Q

What must be removed from ceramics for dental uses?

A

Kaolin - it is opaque and dental ceramics must be translucent

277
Q

What component in dental ceramics is not in decorative ceramics?

A

Glass (up to 15%)

278
Q

Constituents of dental ceramics

A

Kaolin <5%
Quartz (silica) 12-25%
Feldspar 70-80%
Metal oxides 1%
Glass up to 15%

279
Q

Feldspar

A

Potash feldspar - potassium alumina silicate
Soda feldspar - sodium alumina silicate
Acts as a flux - lowers the fusion and softening temp of the glass
It is the lowest fusing component and flows during firing, forming a solid mass around the other components

280
Q

Metal oxide colours conveyed to ceramic

A

Chromium - green
Cobalt - blue
Copper - green
Iron - brown
Manganese - lavender
Nickel - brown

281
Q

How are conventional dental ceramics supplied?

A

As a powder
Powder is made by heating the constituents to >1000C, then cooling rapidly (fritting) in water creating cracks
Mill the frit to fine powder
Add binder - often starch
Powder is mixed with distilled water and built up into restoration

282
Q

What do feldspathic ceramics for when heated to 1150-1500 in crown fabrication?

A

Leucite - potassium aluminium silicate around the glass phase of the ceramic
Gives a powder
Powder melts together to form the crown

283
Q

Properties of dental ceramics

A

Good aesthetics
Chemical stability
Biocompatibility
Thermal properties similar to tooth substance and low thermal diffusivity
Dimensional stability - shrinkage is an issue during firing but once fired very stable
Mechanical properties - high compressive strength and hardness but low tensile, flexural and fracture strength leading to fracture during loading

284
Q

Which dental material has best aesthetics?

A

Ceramics - stable colour and very smooth surface, less staining in the long term

285
Q

Rule of thumb for choosing a luting agent

A

If it won’t break when thrown at a wall use GIC

286
Q

What is the effect of mechanical properties on use of ceramics?

A

Can only be used in low stress areas

287
Q

Alumina core

A

Used in ceramic crowns to improve mechanical properties - not strong enough for ruse in posterior crowns - lack of flexural strength

288
Q

Most popular material for a ceramic crown core

A

Zirconia

289
Q

What type of zirconia is used in dentistry?

A

Ytria stabilised (3-5% Ytria)

290
Q

Problems with zirconia cored crowns

A

Expensive equipment required
Potential for veneered porcelain to debond
Zirconia core is opaque - doesn’t improve aesthetics much from MCC
Inert fitting surface - cannot etch or bond

291
Q

Fabrication of Milled Crown

A

CAD/CAM technology used
Scanned image of cast
Lower cast scanned and articulated
Select and adjust crown margin
Select crown type and place on virtual model
Adjust shape and size
Save file and send to milling machine 30-40 to produce crown
Still requires final finishing

292
Q

Cast and pressed ceramics

A

Restoration is waxed up
Invested
Cast from a heated block of ceramic 1100C
Devested
Heated to form crack inhibiting crystals - ceraming

293
Q

Is the sintered or milled version of the same material stronger?

A

Milled

294
Q

What crown should be used for posterior teeth?

A

Monolithic zirconia

295
Q

Which crown for anterior use?

A

LiDiSi (Lithium disilicate)

296
Q

What should be used for longer bridgework or heavily occluding crowns?

A

Zirconia cored with zirconia where occlusal contacts meet

297
Q

What should be used to lute crowns?

A

Zirconia or LiDiSi - conventional or resin cements

298
Q

Sintering

A

Occurs on heating of ceramic crowns - ceramic particles begin to fuse into a single mass, glass phase softens and comes together
Over time there is controlled diffusion and a solid ceramic mass is formed
During sintering material contracts about 20%

BDS3 (20 decks)

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