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DMS Flashcards

1
Q

endodontic metal files function

A
  • mechanical phase of chemomechanical disinfection
  • remove soft and hard tissues
  • remove micro-organisms
  • creates space for disinfectants/medicaments
  • creates appropriate shape for obturation
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2
Q

what leads to higher stress in k file

A

abrupt change in geometric shape of a file leads to higher stress at that point

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3
Q

strain of file

A

amount of deformation a file undergoes

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4
Q

elastic limit of file

A
  • maximal strain aplied to file where returns to original deformation
  • beyond this is fracture point
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5
Q

define elastic deformation

A

reversible deformation that does not exceed elastic limit

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6
Q

plastic deformation

A

permanent deformation occuring when elastic limit exceeded

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7
Q

cyclic fatigue of endo materials

A
  • generation of tension/compression cycles
  • leads to failure
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8
Q

what prevents rusting in endo file

A
  • chromium in stainless steel
  • passivation layer of chromium oxide
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9
Q

describe nitinol

A
  • equiatomic alloy of nickel and titanium
  • atypical metal
  • super-elasticity - application of stress does not result in usual proportional strain
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10
Q

NiTi crystal structure (nitinol)

A
  • martensite and austenite
  • crystal lattice structure altered by temp or stress
  • martensitic form is soft, ductile and easily deformed
  • austenitic form is quite strong and hard
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11
Q

describe shape memory alloys

A
  • materials that can be deformed at one temperature
  • when heated or cooled return to their original shape
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12
Q

components of endodontic rotary instrument

A
  • taper - diameter change along working surface
  • flute - groove which collects dentine and soft tissue
  • leading/cutting edge - forms and deflects dentine chips
  • land - surface extending between flutes
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13
Q

endodontic material
relief

A

reduction in surface or land (the surface which extends between flutes) on rotary instrument

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14
Q

rotary instrument
helix angle

A

angle cutting axis forms with the long axis of file

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15
Q

features of land on endodontic instrument

A
  • wide radial land - provides blade support and peripheral strength
  • radial land relief - reduces friction on canal wall
  • third radial land - stabilises instrument and keels it centred in canal
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16
Q

irrigant properties

A
  • facilitate removal of debris
  • lubrication
  • dissolution of organic and inorganic matter
  • penetration to canal periphery
  • kill bacteria/yeast/viruses
  • does not weaken tooth structure
  • biological compatibility
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17
Q

NaOCl what is responsible for antibacterial activty

A

hypochlorus acid

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18
Q

factors important for NaOCl function

A
  • concentration
  • volume
  • contact
  • mechanical agitation
  • exchange
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19
Q

describe smear layer in endo prep

A
  • organic pulpal material and inorganic dentinal debris
  • superficial 1-5 micrometres
  • some pack into tubules
  • prevents sealer penetration
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20
Q

what can be used to remove smear layer in endo

A
  • 17% EDTA
  • 10% citric acid
  • MTAD
  • sonic and ultrasonic irrigation
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21
Q

NAOCl in endo conc, properties, use and amount per canal

A
  • 3%
  • dissolve organic material
  • bactericidal
  • used for disinfection
  • 30ml continual irrigation time for at least 10 minutes after prep and before obturation
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22
Q

EDTA in endo conc, properties, use and amount per canal

A
  • 17%
  • smear layer removal
  • penultimate rinse for 1 minute
  • 3ml
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23
Q

corsodyl in endo conc, properties, use

A
  • 0.2 %
  • used to check dam integrity
  • also used to disinfect tooth surface
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24
Q

irrigant interactions

A
  • irrigants interact so need to be careful
  • interaction with NaOCl forms para-chloroaniline
  • cytotoxic and carcinogenic
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25
Q

properties of an ideal obturation material

A
  • easily manipulated and good working time
  • dimensionally stable
  • unaffected by tissue fluids
  • seals canal laterally and apically
  • non-irritant
  • inhibits bacterial growth
  • radiopaque
  • does not discolour tooth
  • sterile
  • easily removed if necessary
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26
Q

gutta percher monomer

A
  • isoprene
  • trans isomer of polyisoprene
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27
Q

gutta percher crystalline forms

A
  • exists in two crystalline forms alpha and beta
  • alpha is naturally occuring form and heated above 65 degrees melts into amorphous phase
  • cooled slowly and returns to alpha phase
  • cooled rapidly and recrystalises as beta phase
  • beta phase used in dentistry
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28
Q

gutta percha constituents

A
  • 20% gutta-percha
  • 65% zinc oxide
  • 10% radiopacifiers
  • 5% plasticizers
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29
Q

endo sealer functions

A
  • seales space between dentinal wall and core
  • fills voids and irregularities in canal and lateral canals
  • lubricates during obturation
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30
Q

properties of an ideal endo sealer

A
  • tackiness to provide good adhesion
  • provide seal
  • radiopacity
  • easily mixed
  • no setting shrinkage
  • non-staining
  • bacteriostatic
  • slow set
  • insoluble in tissue fluids
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31
Q

zinc oxide and eugenol endo sealer

A
  • mixing component mainly eugenol
  • zinc oxide enhances flow and atimicrobial
  • radiopacity below GP
  • cytotoxic but overall beneficial
  • free eugenol can be irritant
  • main negative is dissolution of material over time and apical seal can be diminished
  • setting results in zinc oxide in a matrix of zinc eugenolate
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32
Q

glass ionomer endo sealers

A
  • dentine bonding properties
  • removal upon retreatment is difficult
  • minimal antimicrobial activity
  • little clinical data to support use
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33
Q

resin endo sealers
AH plus

A
  • AH plus
  • epoxy resin
  • paste-paste mixing
  • slow setting
  • good flow and sealing ability
  • initial toxicity declining after 24 hours
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34
Q

resin endo sealers
epiphany

A
  • dual cure dental resin composite sealer
  • BisGMA
  • UDMA
  • fillers of calcium hyrdoxide, barium sulphate, barium glass and silica
  • requires self-etch primer
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35
Q

resin endo sealers
endorez

A
  • UDMA (urethane-dimethacrylate) sealer
  • hydrophilic
  • good penetration into tubules
  • biocompatible
  • radio-opaque
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36
Q

calcium silicate endo sealers

A
  • high pH during first 24h of setting
  • hydrophilic
  • enhanced biocompatibility
  • no setting shrinkage
  • non-resorbale
  • quick set
  • easy to use
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37
Q

medicated sealers

A
  • sealers containing paraformaldehyde not acceptable
  • severe and permanent toxic effects on periradicular tissues
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38
Q

pulp cap/root end filling material uses

A
  • direct pulp cap
  • apexification
  • epicoectomy
  • root resorption repair
  • furcation perforation repair
  • pulpotomy
  • lateral perforation repair
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39
Q

example of pulp cap/root end filling material

A

mineral trioxide aggregate (MTA)

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40
Q

mineral trioxide aggregate chemistry

A
  • smaller particle size
  • reduced discolouration
  • tricalcium silicate
  • dicalcium silicate
  • calcium aluminate
  • bismuth oxide
  • calcium suphate dehydrated
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41
Q

MTA setting reaction

A
  • 3 stages: mixing, dormancy, hardening
  • when mixed with water chemical reaction occurs (hydration)
  • requires water for setting
  • extended setting times
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42
Q

bioceramic cements used in endo

A
  • biodentine - similar material to MTA with modifications
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43
Q

tissue response to MTA

A
  • induce osteogenesis - encourage bone formation
    *could be due to change in pH
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44
Q

ideal properties of PMMA

A
  • dimensionally accurate - so dentures accurate
  • survive high temp without softening (Tg)
  • low thermal expansion
  • low density and high thermal conductivity
  • radiopaque
  • non toxic, non-irritant
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45
Q

mechanical properties of PMMA

A
  • high youngs (elastic) modulus
  • high proportional limit
  • high transverse strength
  • high fatigue strength
  • high impact strength
  • high hardness/abrasion resistance
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46
Q

PMMA transverse strength

A
  • flextrual strength - 3 point loading
  • how well does denture cope with stresses that cause deflection
  • if pivot point is baseplate and forces either side applied - wrst case fracture at pivot point
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47
Q

summarise polymerisation of PMMA

A
  • acrylic resin undergoes free radical addition polymerisation
  • chemical union of two molecules to form a larger molecule without the elimination of a smaller molecule
  • needs C to C double bonds
  • methacrylate monomer
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48
Q

acrylic polymerisation stages

A
  • activation - of initiator to provied free radicals (benzoyl peroxide)
  • initiation - free radicals break C=C in monomer and transfer free radical
  • propagation - growing polymer chain
  • termination - of polymerisation
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49
Q

heat cured acrylic powder components

A
  • initiator - benzoyl peroxide
  • PMMA particles pre polymerised beads to speed up reation
  • plasticiser - allows quicker dissolving in monomer liquid
  • pigments - to give natural colour
  • co-polymers - improve mechanical properties eg ethylene glycol dimethacrylate
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50
Q

heat cured acrylic liquid components

A
  • methacrylate monomer - dissolves PMMA particles and polymerises
  • inhibitor - hydroquinone prolongs shelf life of acrylic by reacting with any free radicals produced by heat, UV light etc
  • co-polymers to improve mechanical properties and enable cross-linking of polymers
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51
Q

heat cure PMMA technique summary

A
  • powder and liquid mixed to form dough like form
  • inserted into mould recess to shape of pts dentition
  • two halves clamped together and heat cycle started
  • polymerisation with lots of crosslinking of MMA monomers occurs - higher weight polymer produced so gd mechanical properties
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52
Q

thermal expansion PMMA

A
  • if acrylic theeth used expansion/contraction wont be issue
  • if porcelain teeth used will be mismatch as the teeth will expand less than acrylic resin denture base
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53
Q

property disadvantages of PMMA

A
  • low thermal conductivity
  • poor mechanical properties - need to make base thicker to overcome this
  • high softening temp but cant cope with boiling water so do not use this to clean
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54
Q

PMMA contraction/expansion

A
  • contraction takes place during heat curing stage
  • during usage PMMA absorbs water which expands about 0.4%
  • makes up for the prior contraction
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55
Q

self cure acrylic main difference

A
  • similar composition to heat cured version
  • a tertiary amine in the liquid activates the initiator (benzyl peroxide) instead of heat activation
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56
Q

comparison of self cure and heat cure PMMA

A
  • more unreacted monomer in sefl-cure PMMA - 3-5% VS 0.2-0.5% - self cure more at risk of being an irritant
  • self cure less polymerisation contraction so initially better fit - but as it expands due to water absorption ends up over-sized and will fall out
  • heat cured has polymerisation contraction coupled with expansion from water absorption so ends up slightly undersized which is tolerated better by pts
  • heat cure higher molecular weight so stronger
  • heat cure process may cause porosity
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57
Q

why does self cure PMMA have poorer mechanical properties

A
  • chemical activation is less efficient
  • so lower molecular weight polymer formed - hence poorer mechanical properties and Tg (softening temp)
  • more unreacted monomer which acts as a plasticiser - softening denture base and results in lower transverse strength
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58
Q

acrylic resin PMMA alternatives

A
  • improved form of acrylic
  • high impact heat cure acrylic resin - Ultra-Hi
  • pur n cure resins
  • light activated denture resins
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59
Q

acrylic resin PMMA alternatives
high impact heat cure acrylic resin

A
  • Ultra-Hi
  • new inredients give acrylic a greater degree of ductility - helps mitihate any micro-cracks present
  • gives greater flexural strength
  • keeps material from being brittle and prevents cracking/breaking
  • used in GDH production lab
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60
Q

acrylic resin PMMA alternatives
pour n cure resin

A
  • smaller powder particles so produces fluid mix - not dough form
  • fluid mix poured into mould
  • mechanical performance inadequate
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61
Q

acrylic resin PMMA alternatives
light activated denture resins

A
  • developed using UDMA ad additional acrylic copolymers
  • also has microfine sillica filers - controls flow of material
  • photoinitiator system
  • adapted to cast and cured in light chamber
  • mostly used for customised impression trays or for repairing fractured dentures
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62
Q

why do we want denture base to be radiopaque

A
  • if any fragments break off or risk of swallow can take radiograph to confirm this
  • some materials have been developed to try and achieve this but weaken material too much
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63
Q

other materials for denture if pt has allergy to acrylic resin
and their drawbacks

A
  • nylon - absorbs water causing it to swell
  • vinyl polymers - soften at temp of only 60degrees celcius
  • polycarbonate dentures - require injecting moulding process which is expensive and develop internal stresses during use causing distortion and poor fit
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64
Q
A
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65
Q
A
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66
Q

types of elastomers

A
  • polyethers
  • addition silicones
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67
Q

describe elastic behaviour

A
  • material can deform and then recover to original dimensions
  • assumings its perfectly elastic
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68
Q

summary of chemistry of elastomers

A
  • elastomers formed by polymerisation with cross-linking of polymer chains
  • cross linking generates elastic properties
  • causes fluid to solid transition
  • may produce byproducts (water, H2, alcohol) which affect dimensional stability and cast compatibility
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69
Q

elastomers often come in

A
  • large cartridges
  • base and catalyst paste
  • twin cartridge form
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70
Q

elastomer material properties
affecting accuracy of surface detail

A
  • surface detail reproduction
  • flow/viscosity
  • contact angle/wettability
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71
Q

elastomer material properties
affecting the accuracy of dimensions and shape of final impression

A
  • elastic recovery
  • stiffness (flexibility)
  • tear strength
  • setting shrinkage
  • dimensional stability
  • thermal expansion coefficient
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72
Q

elastomer material properties
ease of use, pt preferences

A
  • mixing time
  • working time
  • biocompatability
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73
Q

hardness test for impression material

A

shore A hardness

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74
Q

function of shark fin test

A
  • measures flow under pressure
  • relates to the ability of IM to deal with undercuts
  • cylindrical chamber with slot of specified depth
  • IM inserted in upper part of chamber and forced downwards
  • high flow = large fin length
  • low flow = short fin length
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75
Q

virtual type of material / comes in

A
  • addition silicone - addition polyvinylsiloxane
  • two forms
  • twin cartridge - base and catalyst pastes syringe gun pushes through mixing tip
  • putty form - spoonful of catalyst and base pastes mixed until colour uniform
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76
Q

elastomers ideal properties
viscosity

A
  • ability to flow
  • vital to reach all the dental tissues surface area
  • ranges low, med, high
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77
Q

elastomers ideal properties
wettability-contact angle

A
  • contact angle indicates how readily the IM wets the tooth surface
  • low contact angle means larger percentage of the volume will make contact with the target surface (IDEAL)
  • large contact angle results in spaces between globules of IM so some of tooth surface not replicated
  • small contact angle no spaces between globules of IM so all surface is replicated
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78
Q

elastomers ideal properties
surface reproducability

A
  • international standard measure for this is ISO 4823
  • test involves placing IM along a surface which has grooves of a specified width
  • 20, 50 and 75 micrometres
  • uniform pressure applied accross
  • ISO 50 considered norm
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79
Q

elastomers ideal properties
visco-elastic recovery

A
  • ideally 100% elastic recovery and no permanent strain
  • when load applied gradually reaches strain level and when load released strain level GRADUALLY drops
  • material does not return to original dimensions it experiences permanent deformation
  • if load time less - ie sharp short pull there is less overall permanent strain/less deformation
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80
Q

elastomers ideal properties
visco-elastic recovery

A
  • ideally 100% elastic recovery and no permanent strain
  • when load applied gradually reaches strain level and when load released strain level GRADUALLY drops
  • material does not return to original dimensions it experiences permanent deformation
  • if load time less - ie sharp short pull there is less overall permanent strain/less deformation
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81
Q

elastomers ideal properties
development of elasticity

A
  • elasticity only begins to develop when setting reaction has progressed to a certain extent
  • so if IM appears firm to touch it will still be developing elasticity
  • wait for an extra few minutes before removing it
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82
Q

elastomers ideal properties
tear strength and rigidity

A
  • tear strength - stress material will withstand before fracturing
  • rigidity - stress/strain ratio - ideally low value so material flexible for ease of IM removal especially from undercut regions
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83
Q

elastomers ideal properties
dimensional stability

A
  • little setting shrinkage so IM maintains shape of tooth or dentition
  • low thermal expansion coefficient - so IM doesnt change shape from drop of 37C in mouth to 22C at worktop
  • absorbing/release of moisture doesnt apply to polyethers or addition silicones unlike alginate
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84
Q

product names for elastomers

A
  • impregum (polyether)
  • virtual (addition silicone)
  • aquasil ultra (addition silicone)
  • felxitime (addition silicone)
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85
Q

comparing polyethers and addition silicones
setting and working time

A
  • polyether lower setting time
  • polyther lower working time
  • addition silicone greater setting and working time
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86
Q

comparing polyethers and addition silicones
best performing in terms of elastic recovery

A
  • virtual (addition silicone) most elastic with 99.5% recovery
  • flexitime a little behind
  • impregum much poorer performer (polyether) 98%
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87
Q

comparing polyethers and addition silicones
best at recording deep undercuts

A
  • based on results of shark fin test
  • impregum (polyether) best at this
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88
Q

comparing polyethers and addition silicones
tear strength

A
  • addition silicones better 9MPa
  • virtual best
  • impregum (polyether) least 1.9MPa
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89
Q

Ideal properties for elastomers

A
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90
Q

dental ceramic composition/%/brief function

A
  • kaolin 5%
  • quarz (silica) 12-25% - gives translucency
  • feldspar - acts as a flux, lowers the fusion and softening temp of the glass
  • borax
  • metallic oxides - convey colour to the ceramic
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91
Q

conventional dental ceramic powder formation

A
  • constituents heated to high temo >1000 degrees C
  • cooled rapidly (fritting) in water - creating cracks in ceramic mass
  • mill the frit into a fine powder
  • add binder - often starch
  • powder can then be mixed with distilled water and built up into the restoration
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92
Q

chemistry of conventional dental ceramics

A
  • due to feldspar in ceramic when heated to 1150-1500 degrees C - leucite is formed
  • leucite is potassium aluminium silicate
  • leucite forms around the glass phase of the ceramic
  • gives a powder of known physical and thermal properties
  • no further chemical reaction is required during fabrication of the restoration
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93
Q

describe fabrication of a crown

A
  • ceramic powder mixed with water and applied to die (mould) with a brush
  • crown built up using different porcelians for dentine and enamel - not tooth coloured
  • crown is heated in a furnace to merge the powder into the ceramic
  • heating leads to sintering
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94
Q

describe sintering during crown fabrication

A
  • crown heated in furnace which leads to sintering
  • occurs just above glass transition temp
  • ceramic particles begin to fuse into a single mass
  • glass phase softens and will merge
  • during sintering material contracts by around 20% - so skill required by technician to judge contraction in 3D
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95
Q

properties of conventional dental ceramics
aesthetics

A
  • ceramics have best aesthetic properties of any dental restorative material
  • colour stable
  • very smooth surface
  • less staining long-term
  • optical properties - reflectance, translucency, opacity, transparency
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96
Q

properties of conventional dental ceramics
chemical and dimensional stability

A
  • chemically very stable
  • generally unaffected by pH range in mouth
  • does not take up stain from food/drink
  • good biocompatibility - minimal adverse effects on biological tissues
  • once fully fired material very stable
  • during fabrication shrinkage problem and must be accomodated for my technician - 20% shrinkage normal for conventional crown
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97
Q

properties of conventional dental ceramics
thermal properties

A
  • similar to tooth substance
  • coefficient of thermal expansion similar to dentine - low stresses to the rest during use
  • thermal diffusivity low - protective or remaining tooth
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98
Q

properties of conventional dental ceramics
mechanical properties

A
  • high compressive strength
  • high hardness
  • very low tensile/flexural strength and fracture toughness - lead to failure during loading
  • static fatigue - decrease in strenght in absence of any applied load
  • surface micro-cracks can appear during manufacture or occlusal wear
  • means can only be used in low stress areas - anterior crowns only
  • not in all pts
  • too brittle for use elsewhere
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99
Q

overcoming problems with conventional dental ceramics

A
  • mechanical properties mean they can only be used as anterior crowns
  • produce a metal coping and cover in conventional porcelain
  • cast or press a block of harder ceramic
  • mill a labatory prepared block of ceramic
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100
Q

overcoming problems with conventional dental ceramics
metal coping

A
  • porcelain-fused alloys
  • alumina core
  • zirconia core
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101
Q

conventional dental ceramics
adding alumina core summary

A
  • doubled flexural strength
  • alumina particles stop cracks propagating through material - prevents #
  • not strong enough for posterior use
  • possibly more palatal reduction required than in metal ceramic crown but less labial reduction required
  • problem - still lack of flexural strength
  • not suitable for anything other than single crowns
  • more success anteriorly
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101
Q

conventional dental ceramics
adding zirconia core summary

A
  • zirconia - zirconium dioxide is naturally ocurring metal
  • use of zirconia not possible until CAD/CAM introduced
  • zirconia powder does not sinter unless heated to over 1600 degrees C
  • in dentistry Yttria-stabilised zirconia used - pure zirconia can crack on cooling
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102
Q

zirconia core ceramic
describe yttria stabilisation of zirconia

A
  • 3-5 % yttria present in material
  • more yttria = more translucency and reduces physical properties
  • adding yytria changes room temp structure from monoclinic crystal to tetragonal crystal
  • if a crack begins - when stresses at crack reaches critical level the crystal structure transforms to monoclinic structure - causes slight expansion of material and closes crack
  • makes material very hard, strong and tough
  • strong enough to use as bridge framework
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103
Q

describe fabrication of a zirconia core crown

A
  • impression taken of prep and sent to lab
  • model is cast and digitally scanned
  • software creates crown on virtual preparations
  • raw zirconia block milled - for three unit bridge takes around an hour
  • cut framework then heat treated around 850 degrees C - achieve final physical properties - 20% shrinkage but software deals with this during milling process
  • framework stained to appropriate colour
  • then veneered with conventional porcelain to produce fianl restoration
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104
Q

milled core crowns and bridges
materials

A
  • zironia
  • lithium disilicate
  • precious metal
  • non-precious metal
  • titanium
  • composite
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105
Q

zirconia core ceramic
problems/poitives

A
  • expensive equiptment required
  • potential for veneering porcelain to debond from core
  • zirconia core is opaque - although aesthetics already better than metal ceramic
  • inert fitting surface - cannot etch or bond
  • but once you have the equiptment cheaper to make - cost of metal is increasing
  • fit is generally excellent
106
Q

describe cast and pressed ceramics

A
  • restoration is waxed up and invested
  • cast from a heated ingtot of ceramic
  • no sintering occurs as the ceramic ingot is already fully condensed prior to firing
  • restoration then heated to improve crystal structure - produces crack inhibiting crystals (process called ceraming)
  • cast can then be stained
  • more often it is cut back labially and veneered wih feldspathic porcelains
107
Q

describe E max press ceramic fabrication

A
  • ceramics used in these processes are called glass ceramics - lithium disilicate glass / leucite reinforced glass
  • ceraming takes place - 2 stage process
    1. maximum number of crystal nuclei formed during crystal formation
    1. crystal growth to maximise physical properties
  • strong materials have small crystal size and high volume fraction of crystals - makes crack propagation through crystals difficult
  • good flexural strength 350 MPa
108
Q

advantages of different crown types

A
  • monolithic block crowns strongest - milled from a single block of material
  • zirconia based crowns stronger than LiDiSi
  • LiDiSi have better translucency hence better aesthetics
  • crowns with layered porcelain have better aesthetics than stained monolithic block
  • layered crowns more likely to chip
109
Q

sintered vs milled crown

A
  • for same material milled crown stronger than a build up/layered crown
  • block used for milling will have been subjected to ideal heat treatments to maximise its properties and all blocks will be consistent
  • aesthetics of sintered/layered crowns better
  • as aesthetics of blocks of ceramic improve these will beome the most commonly used crown
110
Q

zirconia or Lithium Disilicate (E-max) what crown material to use where

A
  • posterior teeth - monolithic zirconia - for single crowns and shorter span bridges
  • anterior teeth with important aesthetics - LiDiSi
  • anterior bridgework - LiDiSi if short span with no parafunction
  • longer span or heavier occlusion - zirconia core with zirconia where occlusal contacts will meet
111
Q

luting zirconia and LiDiSi crowns

A
  • can be cemented with conventional or resin cements
  • both crown types do not rely on being bonded to tooth to prevent fracture - as they have intrinsic strength
  • LiDiSi crowns can be etched with hydrofluoric acid to produce a retentive surface - etched surface can be bonded to tooth using bonding agent and resin cement ( as they contain silica)
  • zirconia crowns do not contain silica so not affected by etch but can be air abraided to create retentive surface although not necessary
112
Q

why do we have porcelain fused alloys

A
  • porcelain is ceramic material with excellent aesthetics however microcracks inevitable on surface - prone to mechanical failure
  • bonded to an supported to by metal structure (alloy) which has good mechanical properties
113
Q

porcelain positive and negative characteristics

A
  • rigid - large stress required to cause strain
  • hard - surface withstands abrasion/indentation well
  • strong - high compressive strength
  • LOW tensile strength - tendency to form surface defects which leads to fracture at low stress
  • brittle - low fracture toughness
114
Q

porcelain fused alloys
alloy support

A
  • alloy acts as a support and limits strain porcelain experiences
  • alloy with its own oxide layer is more rigid
  • when subjected to large stress - will change shape very little and return to its original dimensions
115
Q

porcelain fused alloys
layers

A
  • metal oxide layer in between porcelian and alloy - bonding to each material
  • metal oxide bonding to porcelain helps eliminate defects/cracks on porcelain surface
  • alloy acts as support and limits strain that porcelain experiences - improves rigidity
116
Q

porcelain fused alloys
alloy technician steps

A
  • alloy has been cast to the desired shape, before bonded to porcelain, by technician
  • porcelain-alloy rest put in furnice at very high temp - produces the oxide layer and to avoid thermal stress which could cause all 3 layers to develop defects or micro-cracks
  • porcelain and alloy should have similar thermal expansion coefficients - expand at same rate during heating and contract at same rate on cooling
117
Q

porcelain fused alloys
alloy options

A
  • high gold
  • low gold
  • silver palladium
  • nickel chromium
  • cobalt chromium - different type to CoCr used i RPD
118
Q

porcelain fused alloys
alloy required properties

A
  1. good bond to porcelain - good wetting/surface contact - bond is through metal oxide layer on alloy surface
  2. similar thermal expansion coeffecient - ideally alloy 0.5ppm per degrees C greater so that during cooling alloy slightly compresses porcelain
  3. avoid discolouration of porcelain
  4. mechanical properties - good bond strenght, hardness and high elastic modulus
  5. melting/recrystalisation temp higher than porcelain otherwise creep may occur
119
Q

porcelain fused alloys
high gold summary

A
  • 80% gold
  • 14% platinum or palladium - helps match thermal expansion of alloy with porcelain and increases melting point temp
  • 1% silver
  • small amount of indium and/or tin which enables metal oxide layer to form - bonding to porcelain
  • disadvantages - MP too low, not sufficiently rigid
120
Q

porcelain fused alloys
low gold summary

A
  • 50% gold
  • 30% palladium
  • 10% silver
  • 10% indium and tin
  • higher melting temp to high gold
  • better mechanical properties than high gold
  • satisfactory at all porcelain fused alloys properties criteria
121
Q

porcelain fused alloys
silver palladium summary

A
  • 30% silver
  • 60% palladium
  • 10% indium and tin
  • high melting point
  • casting this alloy is challenge for technicians
122
Q

porcelain fused alloys
nickel chromium summary

A
  • 70-80% nickel
  • 10-25% chromium
  • high melting point
  • rigid
  • high casting shrinkage - challenging to use
  • quite low bond strength to porcelain
  • only alloy with biocompatibility concerns - allergic response to Ni
123
Q

porcelain fused alloys
cobalt chromium summary

A
  • high elastic modulus
  • hard material
  • high tensile strength
  • high melting point
  • significant casting shrinkage
  • low-ish bond strength
  • casting difficult
124
Q

porcelain fused alloys
porcelain to metal bond types

A
  • mechanical - due to irregularities on alloys metal oxide layer and porcelain which allows materials to interlock
  • stressed skin effect - due to slight differences in thermal contraction - alloy contracts slightly more which compresses porcelain and grips it
  • chemical - oxides in metal oxide migrate with oxides in porcelain - occurs during firing stage (electron sharing)
125
Q

porcelain fused alloys
modes of failure

A
  • metal oxide later itself fracturing
  • oxide layer delaminating from the alloy
  • porcelain detaching from the oxide layer
  • porcelain fracture - ideally this type of failure
126
Q

endodontic material categories

A
  • instruments
  • irrigants
  • intra-canal medicaments
  • obturation materials
  • sealers
  • pulp capping materials
  • root-end filling materials
127
Q

endodontic instruments function

A
  • mechanical phase of chemomechanical disinfection
  • metal files used to remove soft and hard tissues
  • removes micro-organisms
  • creates space for disinfectants/medicaments
  • creates appropriate shape for obturation
128
Q

stress

A
  • force measured across a given area
  • tensile/compressive/shear/torsional
  • stress = F/A
129
Q

elastic limit

A

a set value representing the maximal strain that when applied to a material, allows the material to return return to original dimensions

130
Q

cyclic fatigue

A
  • generation of tension/compression cycles
  • repeted forces eventually resulting in fracture
  • failure
131
Q

torsional fatigue of endodontic instruments

A
  • rotating instrument between 0 and 400 degrees dimensional changes to instrument are reversible
  • 400 degrees to 1100 degrees is plastic phase and instrument will undergo irreversible changes
  • repeted rotational movement will cause torsional fatigue and eventually lead to fracture
  • best to set reciprocated rotational movements lower than elastic limit
  • the lower the angles of rotation the safer the procedure
132
Q

classification of endodontic intruments

A
  • manually operated
  • low-speed instruments
  • engine-driven nickel-titianium rotary instruments
  • engine-driven instruments that adapt to canal shape
  • enging-driven reciprocating instruments
  • ultrasonic instruments
133
Q

stainless steel endodontic instrument summary

A
  • alloy of iron, carbon and chromium
  • nickel may also be present
  • 13-26% chromium prevents rusting
  • passivation layer of chromium oxide
134
Q

summary of stainless steel endodontic instrument manufacture

A
  • machined stainless steel wire into desired shape
  • work-hardening occurs
135
Q

summarise work hardening

A
  • strengthening of a metal by plastic deformation
  • crystal structure dislocation
  • dislocations interact and create obstructions in crystal lattice
  • resistance to dislocation formation develops
136
Q

nitinol endodontic instrument summary

A
  • equiatomic alloy of nickel and titanium
  • super-elasticity - application of stress does not result in usual proportional strain
137
Q

nickel titianium crystal structure

A
  • temperature dependent structures: martensite and austenite
  • crystal lattice structure altered by temperature or stress
  • martensite form - soft and ductile and easily deformed
  • austenite form - quite strong and hard
138
Q

describe shape memory alloys

A
  • shape memory alloys are materials that can be deformed at one temp
  • but when heated or cooled return to their original shape
139
Q

components of endodontic rotary instruments

A
  • taper - diameter change along working surface
  • flute - groove to collect dentine and soft tissue
  • cutting edge - forms and deflects dentine chips
  • land - surface extending between flutes
  • relief - reduction in surface of land
  • helix angle - angle cutting edge forms with long axis of file
140
Q

irrigant properties/function

A
  • facilitate removal of debris
  • lubrication
  • dissolution of organic or inorganic matter
  • penetration to canal periphery
  • kill bacteria/yeasts/viruses
  • biofilm disruption
  • biological compatibility
  • does not weaken tooth structure
141
Q

irrigants
sodium hypochlorite summary

A
  • ionises in water into Na+ and OCl-
  • establishes equilibrium with hypochlorus acid (HOCl)
  • acid/neutral - HOCl predominates
  • > pH9 OCl predominates
  • HOCl responsible for antibacterial activity
142
Q

irrigants
NaOCl effect

A
  • effect on organic material
  • inability to remove smear layer by itself
  • possible effect on dentine properties
143
Q

factors important for NaOCl function

A
  • concentration
  • volume
  • contact
  • mechanical agitation
  • exchange
144
Q

preparation of the canal for obturation

A
  • smear layer formed during preparation - organic pulpal material and inorganic dentinal debris
  • superficial 1-5um with packing into tubules
  • bacterial contamination, substrate and interferes with disinfection
  • prevents sealer penetration
145
Q

removal of smear layer during endo
options

A
  • 17% EDTA
  • 10% citric acid
  • MTAD
  • sonic and ultrasonic irrigation
  • watch apical control
146
Q

irrigant interactions

A
  • interaction with NaOCl forms para-chloroanaline
  • cytotoxic and carcinogenic
  • uncertain bioavbailability
147
Q

properties of an ideal obturation material

A
  • easily manipulated with ample working time
  • dimensionally stable
  • seals canal laterally and apically
  • non-irritant
  • unaffected by tissue fluids
  • inhibits bacterial growth
  • radiopaque
  • does not discolour tooth
  • sterile
  • easily removed if necessary
148
Q

gutta percha summary

A
  • polymer of isoprene
  • trans isomer of polyisoprene
  • exists in two crystalline forms: alpha and beta
  • beta phase used in dental gutta-percha
  • alpha phase heated above 65 melts and if cooled clowly returns to alpha phase BUT if cooled RAPIDLY recrystalises as beta phase
149
Q

gutta percha cone composition

A
  • 20% gutta percha
  • 65% zinc oxide
  • 10% radiopacifiers
  • 5% plasticisers
150
Q

endo sealer functions

A
  • seals space between dentinal wall and core
  • fills voids and irregularities in canal, lateral canals and between GP points used in lateral condensation
  • lubricates during obturation
151
Q

properties of an ideal endo sealer

A
  • tackiness to provide good adhesion
  • radiopacity
  • easily mixed
  • no setting on shrinkage
  • non-staining
  • bacteriostatic
  • slow set
  • insoluble in tissue fluids
152
Q

endo sealer
zinc oxide and eugenol summary

A
  • less radiopaque than GP
  • zinc oxide effective antimicrobial
  • resin acids in rosin component affect lipids in cell membrane - antimicrobial/cytotoxic
  • although toxic may overall be beneficial
  • during setting zinc oxide embedded in zinc eugenolate matrix
  • free eugenol which remains can act as an irritant
  • loses volume during time due to dissolution - resins can modify this
153
Q

endo sealer
glass ionomer summary

A
  • dentine bonding properties
  • removal upon retreatment is difficult
  • minimal antimicrobial activity
  • little clinical data to support use
154
Q

endo sealer
resin sealers summary

A
  • epoxy resin
  • paste-paste mixing
  • slow setting- 8 hours
  • good flow and sealing ability
  • initial toxicity declining after 24 hours
  • examples:
  • epiphany- dual cure resin composite sealer which requires self etch primer
  • endorez is a UDMA resin-based sealer
155
Q

endo sealer
calcium silicate sealers summary

A
  • high pH (12.8) during initial 24hours of setting
  • hydrophilic
  • enhanced biocompatibility
  • does not shrink on setting
  • non-resorbable
  • escellent seal
  • quick set - 3-4 hours
  • easy to use
156
Q

endo sealer
medicated sealers

A
  • sealers containing paraformaldehyde NOT acceptable
  • severe and permanent toxic effects on periradicular tissues
  • sargenti paste, endomethasone, SPAD etc
157
Q

mineral trioxide aggregate summary

A
  • earlier forms were grey - better setting characteristics but tooth discolouration
  • white formulation - smaller particle size and reduced discolouration
  • tricalcium silicate
  • dicalcium silicate
  • bismuth oxide
158
Q

MTA setting reaction

A
  • composed of several phases
  • when mixed with water a chemical reaction occurs between these phases and water (hydration)
  • white and grey MTA undergo different setting reactions
159
Q

properties of a luting agent

A
  • viscosity and film thickness
  • ease of use
  • radiopaque
  • marginal seal
  • aesthetics
  • solubility - low
  • cariostatic
  • biocompatible
  • mechanical properties
160
Q

properties of a luting agent
viscosity and film thickness

A
  • dependant on size of powder or filler particles in the material
  • must be low to allow seating of restoration without interference
  • viscosity increases as material sets - must seat restoration quickly and maintain pressure
  • film thickness should be as thin as possible ideally 25um or less
161
Q

properties of a luting agent
marginal seal

A
  • ideally luting agent should bond chemically to the tooth and indirect restoration
  • with a permanent and impenetrable bond
  • some of the newer materials approach this
162
Q

properties of a luting agent
aesthetics and biocompatability

A
  • tooth coloured and non staining - variation in shade and translucency
  • biocompatible - not toxic, not damaging to the pulp, low thermal conductivity
163
Q

properties of a luting agent
radiopaque and cariostatic

A
  • some ceramic crowns are radiolucent - makes it easier to see marginal breakdown
  • cariostatic - fluoride releasing, antibacterial
  • important in preventing secondary caries around crown margins
164
Q

luting angents
types of materials

A
  • dental cement: zinc phosphate, zinc polycarboxylate
  • glass ionocer cement: conventional, resin modified
  • composite resin luting agents: total etch for use with DBA, self etch
165
Q

properties of a luting agent
mechanical properties

A
  • high compressive strength
  • high tensile strength
  • high hardness value
  • youngs modulus similar to tooth
166
Q

luting cements
zinc phosphate overview

A
  • in use for 100+ years
  • acid base reaction
  • powder and liquid
  • easy to use
  • cheap
167
Q

luting cements
zinc phosphate powder

A
  • zinc oxixe > 90% main reactive ingredient
  • magnesium dioxide - gives white colour and increases compressive strength
  • other oxides alumina and silica - improve physcial properties and alter shade of set material
168
Q

luting cements
zinc phosphate liquid

A
  • phosphoric acid (aq) 50% approx
  • oxides which buffer the solution
  • aluminium oxide - ensures even consistency of set material, prevents crystalisation leading to amorphous acid/salt matrix surrounding unreacted ZnO powder
  • zinc oxide - slows the reaction giving better working time
169
Q

luting cements
zinc phosphate setting reaction

A
  • initial reaction is acid base
  • ZnO + phosphoric acid = zinc phosphate + water
  • followed by hydration reaction resulting in formation of crystalised phophate matrix
  • ZnO + zinc phosphate + water = hopiete
170
Q

luting cements
zinc phosphate material problems

A
  • low initial pH approx 2 - can cause pulpal irritation as pH can take 24h to return to normal
  • exothermic setting reaction
  • not adhesive to tooth or restoration - retention may be slightly mechanical due to surface irregularities on prep and restoration
  • not cariostatic
  • final set takes 24hours
  • brittle
  • opaque
171
Q

luting cements
zinc polycarboxylate overview/comparison to zinc phosphate

A
  • similar material to zinc phosphate but phosphoric acid replaced by polyacrylic acid
  • has advantage of bonding to tooth surface
  • less heat reaction
  • pH low to begin with but returns to neutral more quickly
  • difficult to mix and manipulate
  • soluble in oral environments at lower pH
  • opaque
  • lower modulus and compressive strength than zinc phosphate
172
Q

luting cements
glass ionomer cements overview and chemistry

A
  • chemistry sam as filling material
  • particle size less than 20um to allo for suitable film thickness
  • acid base reaction between glass and acid
  • glass is SiO, AlO, CaF
  • polyacid mixture of acrylic, maleic and itaconic acid and thier co-polymers
  • reaction goes through dissolution, gelation and hardening stages
  • cement bonds to tooth surface though ion exchange with calcium in enamel and dentine
  • hydrogen bonding with collagen in dentine
173
Q

luting cements
glass ionomer cement properties

A
  • fairly strong and durable bond to tooth
  • no chemical bond to restoration surface - surface of restoration should be sandblasted to allow mechanical adhesion
  • low shrinkahe
  • long term stability
  • relatively insoluble once fully set
  • aesthetically better than ZnPhos
  • self adhesive to tooth
  • fluoride release
  • cheap
174
Q

luting cements
resin modified glass ionomer overview and chemistry

A
  • same as RMGI filling material
  • glass particle size smaller to allow acceptable film thickness
  • liquid contains a hydophilic monomer - as GIC water based material so monomer needs to be hydrophilic
  • HEMA is monomer ^
  • same acid base reaction occurs
  • light activation causes polymerisation of the HEMA and any copolymers - rapid initial set
  • some materials have secondary cure via REDOX reaction - dark curing where material not exposed to light
175
Q

luting cements
resin modified glass ionomer properties

A
  • shorter setting time
  • longer working time
  • higher compressive and tensile strengths
  • higher bond strength to tooth
  • decreased solubility
176
Q

luting cements
resin modified glass ionomer potential problems

A
  • HEMA is cytotoxic - important no monomer remains as it can damage pulp
  • HEMA expands in wet environment - cannot be used to cement conventional porcelain crowns as they may crack
  • shouldnt be used to cement posts as it may split the root
  • no bond to indirect restoration
177
Q

luting cements
composite luting agents overview

A
  • variants on composite filling materials with suitable viscosity and filler particle size
  • must be used in conjunction with a suitable DBA
  • can be light cured or dual cured
  • better physical properties, lower solubility and better aesthetics BUT
  • technique sensitive
  • physical properties reduced by 25% if not lgith cured - despite dual cure
178
Q

luting cements
composite luting agents - bonding to indirect composite

A
  • composite bonds to composite
  • bond strength 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 bonds on fitting surface of inlay
  • use a dual cure cement as light penetration through the inlay will be poor
179
Q

luting cements
composite luting agents - bonding to porcelain

A
  • porcelain is brittle and requires to be bonded to tooth to prevent fracture
  • porcelain treated with HF to etch the surface - produces rough retentive surface but still not hydrophobic and compatible with composite resin luting agents
  • surface wetting agent is required - silane coupling agent applied to etched porcelain surface
  • very strong bond between oxide groups on porcelain surface and silane
  • other end of silane has C=C bond which reacts with composite resin luting agent
  • works in same manner as DBA does with tooth
  • produces strong durable bond
  • use light cure composite if porcelain restoration is thin and if thick use dual cure
180
Q

luting cements
composite luting agents - bonding to metal overview

A
  • metal surface needs to be roughened - can be done by etching or sandblasting
  • sandblasting does not give the undercut surface of etcing - chemical bonding is required to strengthen the bond
  • must use a dual cure material as light will not penetrate metal
  • can be used to cement most crowns, bridges and posts
  • materials will not bond to precious metal
  • techniqe sensitive and will not work unless moisture control is adequate
181
Q

luting cements
composite luting agents - metal
etching metal

A
  • electrolytic etching - removes different phases of the alloy at different rates
  • gives very retentive surface
  • BUT
  • technique sensitive
  • beryllium containing metals work best
  • cannot etch preciou metals at all
182
Q

luting cements
composite luting agents bonding to non-precious metak

A
  • materials with carboxylic and phosphoric acid derived resin monomers (metal bond agent)
  • MDP and 4-META
  • have an acidic end and a C=C end ^
  • acidic end of molecule reacts with metal oxide and renders the surface hydrophobic
  • same as DBA and silane
183
Q

luting cements
composite luting agents - bonding to precious metal

A
  • cannot bond to precious metal
  • change precious alloy composition to allow oxide formation - increase copper content and heat 400degrees C in air
  • tin plate
  • sulphur based chemistry of bonding agent
  • all complicated and technique sensitive
184
Q

luting cements
composite luting agents - self adhesive to metal composite resin

A
  • metal coupling agent is incorporated into the composite resin
  • simplifies the bonding process
  • is an anaerobic self cured material
  • good film thickness
  • opaque
  • moisture sensitive
  • expensive
  • called panvia Ex
185
Q

luting cements
composite luting agents - self etching comp resin cements overview

A
  • combination of comp resin cement and a self etching dentine bonding agent
  • on paper good idea
  • hower requires good moisture control
  • there is doubt about the bond strength to enamel due to inadequate etching
  • pH of carboxylic monomer doesnt stay low for long enough to give a good etch
186
Q

luting cements
self etching composite luting agents bonding

A
  • acidic groups bind with calcium in hydroxyapatite
  • ions from dissolution of filler neutralise the remaining acidic groups - frming chelate reinforced methacrylate network
  • limited removal of smear layer or significant infiltration into tooth surface
  • good bond strength to dentine
  • bonding to enamel lower - enamel should be etched prior to application
  • bonding to ceramics - brand specific
  • bonding to metal - better to non-precious
187
Q

luting cements
self etching composite luting agents
mechanical properties

A
  • compressive strength, tensile, hardness all slightly lower than conventional resin luting agents
  • very few clinical studies
  • do not get round problem of moisure control
188
Q

luting cements
temporary cements overview

A
  • made to cement temp restorations in place
  • need to be soft for easy removal - some do not set at all
  • supplied as two paste systems and catalyst or accelerator
  • base contains ZnO, starch and mineral oil
  • accelerator contains resins, eugenol or ortho-EBA and carnauba wax
  • wax weakens the structure of set cement making it easier to remove
  • two main types: with and without eugenol
  • eugenol type should NOT be used if permanent rest cemented with resin cement - residual eugenol may interfere with setting
195
Q

types of temporary materials overview

A
  • PMMA - used indirectly (not at chairside) - short and long crown/bridge
  • PEMA - direct/chariside for single crowns
  • Bis-acrylate composite - direct/chairside for short span bridges
196
Q

temporary materials
brand names

A
  • polymethylmethacrylate PMMA: jet
  • polyethylmethacrylate PEMA: trim II, snap
  • Bis-acryl composite: protemp 4, quicktemp
197
Q

temporary materials
methacrylate monomer

A
  • mono-functional monomer: one C=C double bond
  • double bond enables polymer development via cross-linking and free radical polymerisation
  • PPMA: forms linear, long chain polymer, greater width, rigidity and strength
198
Q

PMMA jet overview

A
  • temporary material
  • indirect fabrication
  • powder/liquid formulation
  • self-curing
  • good marginal fit, transverse strength and polishable
  • negatives: poor abrasion resistence, high shrinkage, high thermal release, free monomer may be toxic
199
Q

temporary materials
PMMA temperature

A
  • temperature generated by polymerisation reaction
  • exothermic reaction
  • safety issue?
  • two studies looking at effect of temp rises on dental pulp: zach and cohen + baldissaria
200
Q

results of zach and cohen study

A
  • study looking at effect of temperature rises on the dental pulp
  • findings:
  • 2 degrees C increase: no effect on pulp histology
  • 5.5 increase: significant tissue changes over first few days and after 56 days MOST pulps had overcome thermal trauma BUT some of smaller teeth were necrotic
  • 11 increase: 2/3 of sample suffer irreparable necrosis
  • study also did not factor duration of temp rise experienced which also affects pulpal damage
201
Q

baldassaria study results

A
  • in study thermal stimulus applied equivalent to heat released from a temporary material
  • no histological changes in dental pulp were observed - so no trauma
  • strongly suggests an 11 degreeC rise would not affect the dental pulp
  • duration also a factor - results show 3 min exposure still produced no detrimental effect on the pulp
202
Q

temporary materials
colour stability

A
  • protemp showed least amount of colour change
  • then trim
  • then jet
  • both trim and jet changed in appearance a significant amount more than protemp
203
Q

temporary materials
temporature rise

A
  • thermal stimulus generated by polymerisation of temp materials varies from 3.5 degrees C to 10.5
  • the temp changes are ALL less than 11 degrees C - so they should all be safe
  • be aware of exposure time as also affects potential for thermal trauma to pulp
  • refer back to baldassaria study
204
Q

temporary materials
polymerisation shrinkage

A
  • important for assessing a temporary materials accuracy of fit
  • the lower the shrinkage the better
  • protemp and integrity shows shrinkage of 2.5% and 3%
  • trim and jet is 4% and 5%
205
Q

temporary materials
compressive strength

A
  • protemp 3 has greatest compressive strength
  • protemp G similar to trim etcbut significantly weaker than protemp 3
206
Q

temporary materials
abrasion resistence

A
  • compared to amalgam TMs are much more likely to be abraided
  • protemp G abrades 10 x more readily than amalgam
  • trim 17 x more readily
207
Q

temporary materials
surface roughness

A
  • appearance of material influenced by surface roughness
  • protemp 3 G has 0.3 units of roughness
  • integrity has 0.6 so twice the roughness
  • jet is about 6 times rougher than protemp
208
Q

what are wrought alloys and their uses

A
  • alloy which can be manipulated/shaped by cold working
  • eg drawn into wire
  • uses: wires for orthodontics and partial denture clasps
209
Q

steel constituents

A
  • 98% iron
  • <2% carbon = steel - if over 2% not regarded as steel but as cast/pig iron
  • chromium 0.5-1% to improve tarnish resistance
  • magnese: sulphur scavenger
  • many others: nickel, cobalt, silicon etc
210
Q

steel uses and carbon %

A
  • cutting instruments >0.8% carbon
  • forceps <0.8% carbon
211
Q

iron structure

A
  • allotropic: meaning in a solid state it can exist in 2 crystalline forms (two phases) depending on temperature
  • <900C or >1400C it has body centred cubic (BCC) lattic structure
  • in between 900 and 1400C it forms a face centred cubic (FCC) lattice structure
212
Q

Fe-C phase diagram

A
  • austenite: interstitial solid solution, face centred cubic, exists at high temp ie >720
  • ferrite: very dilute solid solution, exists at low temp
  • cementite: axists at low temp (Fe3C)
  • pearlite: eutectoid mixture of ferrite and cementite
213
Q

what is solid solution and types of solid solution

A
  • two metals that form a common lattic structure and are soluble in one another
  • substitutional solid solution - random or ordered
  • interstitial solid solution - two atoms are markedly different in size
214
Q

Fe-C phase diagram
on cooling rapidly grain structure is

A
  • austenite
  • quenching should therefore give us austenite - but in practice we get martensite
  • martensite behaves quire differently
215
Q

martensite behaviour

A
  • has a distorted lattice structure
  • a result of carbon unable to diffuse within iron atoms of each grain
  • forms a hard and brittle material
  • formed by fast cooling of austenite (Fe-C)
216
Q

describe tempering martensite

A
  • heating (450C) followed by quenching
  • will determine the proportion of ferrite and cementite produced
  • ferrite is soft and ductile
  • cementite is hard and brittle
  • not used in dentistry
217
Q

stainless steel four main components and features

A
  • iron, carbon, chromiun, nickel
  • steel on regarded stainless in >12% chromum
  • chromium lowers austenite to martensite temp and converstion rate and decreases % carbon at which eutectoid formed
  • good corrosion resistence due to chromium oxide layer - forms on surface but can be attacked by chlorides
  • nickel lowers austenite to martensite transition temp and also improves fracture strength and corrosion reistence
218
Q

stainless steel types

A
  • martensitic
  • austenitic
219
Q

martensitic stainless steel and use

A
  • has round 12-13% chromium and little carbon
  • can be heat hardened by tempering process
  • used to make dental instruments
220
Q

austenitic stainless steel and use

A
  • either 18:8 or 12:12 ratio of cromium to nickel
  • uses:
    1. dental equiptment and instruments to be sterilised - corrosion resistence and withstands autoclave
    2. wires eg orthodontics - readily cold worked and corrosion resistant
    3. sheet form for denture bases
221
Q

18:8 stainless steel overview

A
  • 18% chromium, 8% nickel, 0.1% carbon and 74% iron
  • does not heat harden
  • soft/malleable when cast
  • work hardens rapidly so cannot be repetedly manipulated to form desired shape
222
Q

describe cold working

A
  • work done on metal/allow at low temperature (below recrystallisation temp)
  • ie bending, rolling, swaging
  • causes slip - dislocations collect at grain boundaries
  • hence stronger, harder material
  • also called work or strain hardening
223
Q

18:8 stainless steel uses

A
  • orthodontic appliances: springs and clasps
  • partial dentures: clasp arms, wrought rests
  • range of grades from soft to hard refered to as “spring temper”
  • soft, half hard, hard spring temper
224
Q

alloys used as wires

A
  • stainless steel (austenitic) 18:8
  • cobalt chromium (dif composition to RPD) : 40% Co, 20% Cr, 15% Ni and Fe
  • gold (similar to type IV) : 60% Au, 15% Ag and Cu
  • various types of nickel-titanium
225
Q

define springiness

A
  • ratio EL/YM
  • anility of a material to undergo large deflections (such as to form an arc) without permanent deformation
  • it returns to original shape after force fremoved
    *
226
Q

wire property requirements

A
  • high springiness (EL/YM)
  • stiffness (YM) depends on required force for tooth movement
  • high ductility - bending without fractures - allows wire to be manipulated to desired configuration
  • easily joined without impairing properties - soldering or welding
  • corrosion resistant
227
Q

describe stainless steel soldering

A
  • can be soldered using gold or silver
  • care to be taken as temp rise created is close to SS melting point - risk ss grains may recrystalise - decreasing mechanical properties
  • quenching the alloy would avoid this
228
Q

describe weld wecay SS

A
  • a risk when welding stainless steel
  • occurs when SS temp is raised to between 500-900C
  • can push the Cr and C atoms to the grain boundaries which would allows CrC to precipitate there
  • if CrC forms at grain boundaries then SS becomes brittle - less chromium at central region of solid solution, more susceptible to corossion
229
Q

minimise risks of weld decay by

A
  • using low-carbon (stainless?) steel - its expensive
  • using stabilised SS - has small amounts ofTi and niobium and forms carbides preferentially which decreases CrC forming at grain boundaries
230
Q

describe stainless steel stress relief annealing

A
  • needed for SS wires as undergone various processes
  • ensures the configuration of metal ions in each of the alloy grains settle to an equilibrium
  • temp of SS held at around 450C for 1/2mins
  • crucial temp does not exceed this value - grain structure affected >650C and metal carbides forming at grain boundaries >500C
231
Q

how is stainless steel denture base made

A
  • stainless steel sheet positioned between die and counter die
  • these are then pressed together
  • SWAGING occurs - sheet of alloy is swaged into denture base shape
232
Q

advantages of stainless steel denture base

A
  • thin - less than 1/10 thickness of acrylic resin
  • light
  • fracture resistant
  • corrosion resistant
  • high polish ability
  • conducts heat readily
  • high impact strength
  • high abrasion resistence
233
Q

disadvantages of stainless steel denture base

A
  • possible dimensional inaccurace - contraction of die not matched by model expansion
  • elastic recovery of steel - inaccuracy
  • damage of die under hydraulic pressure
  • loss of fine detail during many stages
  • difficult to ensure uniform thickness
  • uneven pressure on die and counter die = wrinkling of steel
234
Q

investment material uses

A
  • for production of inlays, onlays, crowns and bridges
  • ^^ that are made of an alloy
  • materials are used by lab technicians
235
Q

investment materials
technique used involves/which requires

A
  • involves casting molten alloy under pressure by centrifugal forces
  • requires investment material of required shape (to contain molten alloy) that can withstand high temp and ensure alloys dimensions are sustained
  • melting point of an alloy is determening factor as to which investment material most suitable
236
Q

investment materials stages overview

A
  1. wax pattern of required prosthesis made
  2. investment material placed around wax pattern an allowed to set - mould is e negative replica
  3. wax then removed - burning or boiling water
  4. molten alloy poured into mould cavity - done via sprue (hollow tubes) that allow the alloy to flow in

aka lost wax technique

237
Q

when alloy is cast
conditions

A
  • pressure must be applied - to ensure no gaps or voids form within it
  • investment material has to be strong enough to withstand forces generated
  • gases are produced which is inevitable
  • important gases allowed to escape - or alloy will have voids/be porous - gases captured by inv M
  • also on cooling alloy CONTRACTS - wont be same shape as mould cavity
238
Q

investment materials
types

A
  • dental stone or plaster - acrylic dentures
  • gypsum bonded materials - gold casting alloys
  • phosphate bonded materials - base metals/cast ceramics including CoCr
  • silica bonded materials - base metal alloys
239
Q

investment materials
ideal properties

A
  • expands - compensate for cooling shrinkage of alloys
  • porous - allows escape of trapped gases on casting
  • strong - withstand forces created during casting process
  • room temp strength - when handled - often referred to as GREEN STRENGTH
  • smooth surface - so alloys surface wont need much finishing work
  • chemically stable
  • easy removal from cast and easy to handle
  • relatively inexpensive - as destroyed after use
240
Q

typical contractions from alloy melting point to room temp

A
  • gold alloys: 1.4%
  • Ni/Cr alloys: 2%
  • Co/Cr alloys: 2.3%
241
Q

investment materials components

A
  • they all have two components: binder and refractory
  • binder determines what kind of IM it is: gypsum; phosphate or silica
  • refractory component withstands high temp and also gives expansion: type of silica (quartz or cristobalite)
  • binder forms coherent mass - to provide substance
  • each binder has different setting reaction and material characteristics
242
Q

investment materials
different refractory component expansions

A
  • quartz and cristobalite expand as their temp is increased
  • quartz expands by 0.8%
  • cristabolite exoasnds by 1.3%
  • bear in mind alloys shrink by different amounts
243
Q

investment materials
desribe refractory component quartz expansion

A
  • linear thermal expansion gradually rising until 570C
  • then climbs more rapidly
  • reaches 0.8% expansion
  • below 570C exists in alpha quartz form - squashed crystalline lattice structure
  • beyond 570C changes to beta quartz form - it explodes to maximum volume (not squashed)
244
Q

gypsum-bonded investment material
composition

A
  • powder (mixed with water)
  • silica 60-65% - refractory component
  • calcium sulphate hemihydrate 30-35%
  • reducing agent for oxides
  • boric acid and sodium chloride - inhibit heat shrinkage and control setting time
245
Q

gypsum-bonded investment material
setting reaction

A
  • gypsum products undergo reaction
  • calcium sulphate hemihydrate + water = calcium sulphate di-hydrate
246
Q

gypsum-bonded investment material
dimensional changes

A
  • silica undergoes thermal expansion and inversion expansion
  • gypsum undergoes expansion during setting - hydroscopic expansion as well as contraction above 320C
247
Q

what is gypsum hydroscopic expansion

A
  • setting expansion up to 5 X
  • mechanism not fully understood
  • considered to be due to water molecules being pulled into gaps between crystals (of hemi-hydrate) forcing them apart
  • due to capillary forces
248
Q

factors increasing hydroscopic expansion

A
  • lower powder/water ratio
  • increased silica content
  • higher water temperature
  • longer immersion time
249
Q

investment materials
gypsum-bonded IM contraction

A
  • occurs above 320C
  • due to:
  • water loss
  • presence of sodium chloride and boric acid
250
Q

gypsum bonded investment materials properties

A
  • 1.4% total expansion - sufficient for gold alloys
  • smooth surface - due to fine particles
  • easily manipulated
  • setting time controlled
  • sufficiently porous to uptake gases released by casting alloys
  • adequate strength
251
Q

gypsum-bonded investment material
unwanted reaction

A
  • when casting there is an unwanted reaction at about 700C
  • if any wax residue or graphite in invest M - reaction between calcium sulphate and carbon
  • releases carbon monoxide and releases calsium sulphide - calcium sulphide may react with calcium sulphate to produce sulphur dioxide gas
  • ensure IM undergoes heat soaking - held at high temp for some time - to allow gases to gradually escape
252
Q

gypsum-bonded investment material
chemical stability

A
  • below 1200C: satisfies requirements
  • above 1200C: calcium sulphate reacts with silica to produce SULPHUR TRIOXIDE
  • sulphur trioxide will produce voids in cast alloy and contributes to corrosion
  • for alloys that melt above 1200C need different investment materials
253
Q

phosphate-bonded investment material
composition

A
  • powder consists of: silica; magnesium oxide; ammonium phosphate
  • liquid: water or colloidal silica
  • colloidal sillica increases strength and gives 2% expansion (hydroscopic expansion) - will compensate for alloy shrinkage
254
Q

phosphate-bonded investment material
setting

A
  • ammonium phosphate + magnesium oxide + water
  • = magnesium ammonium phosphate
255
Q

heating phosphate-bonded investment material

A
  • to around 1000 results in:
  • at 330C water and ammonia released
  • at higher temp complex reactions with silico-phosphates - increases strength
256
Q

phosphate bonded investment material
properties

A
  • high strength “green strength”
  • sufficiently porous
  • chemically stable
  • easy to use
  • porous
257
Q

silica investment materials stages

A
  1. prepare stock solution
  2. add powder (quartz or cristabolites) - gelation
  3. drying - tightly packed silica particles

not used in GDH so dont need to know in detail

258
Q

silica investment material dimensional changes

A
  • contraction at early stages of heating - water and alcohol loss from gel
  • substantial thermal and inversion expansion - lots of silica present
259
Q

silica investment material properties

A
  • sufficiently strong
  • NOT porous - needs vents
  • complicated manipulation
  • not used in GDH

BDS3 - CDS (13 decks)

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