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
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
3
Q
strain of file
A
amount of deformation a file undergoes
4
Q
elastic limit of file
A
- maximal strain aplied to file where returns to original deformation
- beyond this is fracture point
5
Q
define elastic deformation
A
reversible deformation that does not exceed elastic limit
6
Q
plastic deformation
A
permanent deformation occuring when elastic limit exceeded
7
Q
cyclic fatigue of endo materials
A
- generation of tension/compression cycles
- leads to failure
8
Q
what prevents rusting in endo file
A
- chromium in stainless steel
- passivation layer of chromium oxide
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
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
11
Q
describe shape memory alloys
A
- materials that can be deformed at one temperature
- when heated or cooled return to their original shape
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
13
Q
endodontic material
relief
A
reduction in surface or land (the surface which extends between flutes) on rotary instrument
14
Q
rotary instrument
helix angle
A
angle cutting axis forms with the long axis of file
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
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
17
Q
NaOCl what is responsible for antibacterial activty
A
hypochlorus acid
18
Q
factors important for NaOCl function
A
- concentration
- volume
- contact
- mechanical agitation
- exchange
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
20
Q
what can be used to remove smear layer in endo
A
- 17% EDTA
- 10% citric acid
- MTAD
- sonic and ultrasonic irrigation
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
22
Q
EDTA in endo conc, properties, use and amount per canal
A
- 17%
- smear layer removal
- penultimate rinse for 1 minute
- 3ml
23
Q
corsodyl in endo conc, properties, use
A
- 0.2 %
- used to check dam integrity
- also used to disinfect tooth surface
24
Q
irrigant interactions
A
- irrigants interact so need to be careful
- interaction with NaOCl forms para-chloroaniline
- cytotoxic and carcinogenic
25
properties of an ideal obturation material
* 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
26
gutta percher monomer
* isoprene
* trans isomer of polyisoprene
27
gutta percher crystalline forms
* 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
28
gutta percha constituents
* 20% gutta-percha
* 65% zinc oxide
* 10% radiopacifiers
* 5% plasticizers
29
endo sealer functions
* seales space between dentinal wall and core
* fills voids and irregularities in canal and lateral canals
* lubricates during obturation
30
properties of an ideal endo sealer
* tackiness to provide good adhesion
* provide seal
* radiopacity
* easily mixed
* no setting shrinkage
* non-staining
* bacteriostatic
* slow set
* insoluble in tissue fluids
31
zinc oxide and eugenol endo sealer
* 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
32
glass ionomer endo sealers
* dentine bonding properties
* removal upon retreatment is difficult
* minimal antimicrobial activity
* little clinical data to support use
33
resin endo sealers
AH plus
* AH plus
* epoxy resin
* paste-paste mixing
* slow setting
* good flow and sealing ability
* initial toxicity declining after 24 hours
34
resin endo sealers
epiphany
* dual cure dental resin composite sealer
* BisGMA
* UDMA
* fillers of calcium hyrdoxide, barium sulphate, barium glass and silica
* requires self-etch primer
35
resin endo sealers
endorez
* UDMA (urethane-dimethacrylate) sealer
* hydrophilic
* good penetration into tubules
* biocompatible
* radio-opaque
36
calcium silicate endo sealers
* high pH during first 24h of setting
* hydrophilic
* enhanced biocompatibility
* no setting shrinkage
* non-resorbale
* quick set
* easy to use
37
medicated sealers
* sealers containing paraformaldehyde not acceptable
* severe and permanent toxic effects on periradicular tissues
38
pulp cap/root end filling material uses
* direct pulp cap
* apexification
* epicoectomy
* root resorption repair
* furcation perforation repair
* pulpotomy
* lateral perforation repair
39
example of pulp cap/root end filling material
mineral trioxide aggregate (MTA)
40
mineral trioxide aggregate chemistry
* smaller particle size
* reduced discolouration
* tricalcium silicate
* dicalcium silicate
* calcium aluminate
* bismuth oxide
* calcium suphate dehydrated
41
MTA setting reaction
* 3 stages: mixing, dormancy, hardening
* when mixed with water chemical reaction occurs (hydration)
* requires water for setting
* extended setting times
42
bioceramic cements used in endo
* biodentine - similar material to MTA with modifications
43
tissue response to MTA
* induce osteogenesis - encourage bone formation
*could be due to change in pH
44
ideal properties of PMMA
* 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
45
mechanical properties of PMMA
* high youngs (elastic) modulus
* high proportional limit
* high transverse strength
* high fatigue strength
* high impact strength
* high hardness/abrasion resistance
46
PMMA transverse strength
* 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
47
summarise polymerisation of PMMA
* 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
48
acrylic polymerisation stages
* 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
49
heat cured acrylic powder components
* 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
50
heat cured acrylic liquid components
* 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
51
heat cure PMMA technique summary
* 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
52
thermal expansion PMMA
* 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
53
property disadvantages of PMMA
* 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
54
PMMA contraction/expansion
* contraction takes place during heat curing stage
* during usage PMMA absorbs water which expands about 0.4%
* makes up for the prior contraction
55
self cure acrylic main difference
* similar composition to heat cured version
* a tertiary amine in the liquid activates the initiator (benzyl peroxide) instead of heat activation
56
comparison of self cure and heat cure PMMA
* 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
57
why does self cure PMMA have poorer mechanical properties
* 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
58
acrylic resin PMMA alternatives
* improved form of acrylic
* high impact heat cure acrylic resin - Ultra-Hi
* pur n cure resins
* light activated denture resins
59
acrylic resin PMMA alternatives
high impact heat cure acrylic resin
* 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
60
acrylic resin PMMA alternatives
pour n cure resin
* smaller powder particles so produces fluid mix - not dough form
* fluid mix poured into mould
* mechanical performance inadequate
61
acrylic resin PMMA alternatives
light activated denture resins
* 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
62
why do we want denture base to be radiopaque
* 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
63
other materials for denture if pt has allergy to acrylic resin
and their drawbacks
* 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
64
65
66
types of elastomers
* polyethers
* addition silicones
67
describe elastic behaviour
* material can deform and then recover to original dimensions
* assumings its perfectly elastic
68
summary of chemistry of elastomers
* 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
69
elastomers often come in
* large cartridges
* base and catalyst paste
* twin cartridge form
70
elastomer material properties
affecting accuracy of surface detail
* surface detail reproduction
* flow/viscosity
* contact angle/wettability
71
elastomer material properties
affecting the accuracy of dimensions and shape of final impression
* elastic recovery
* stiffness (flexibility)
* tear strength
* setting shrinkage
* dimensional stability
* thermal expansion coefficient
72
elastomer material properties
ease of use, pt preferences
* mixing time
* working time
* biocompatability
73
hardness test for impression material
shore A hardness
74
function of shark fin test
* 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
75
virtual type of material / comes in
* 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
76
elastomers ideal properties
viscosity
* ability to flow
* vital to reach all the dental tissues surface area
* ranges low, med, high
77
elastomers ideal properties
wettability-contact angle
* 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
78
elastomers ideal properties
surface reproducability
* 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
79
elastomers ideal properties
visco-elastic recovery
* 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
80
elastomers ideal properties
visco-elastic recovery
* 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
81
elastomers ideal properties
development of elasticity
* 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
82
elastomers ideal properties
tear strength and rigidity
* 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
83
elastomers ideal properties
dimensional stability
* 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
84
product names for elastomers
* impregum (polyether)
* virtual (addition silicone)
* aquasil ultra (addition silicone)
* felxitime (addition silicone)
85
comparing polyethers and addition silicones
setting and working time
* polyether lower setting time
* polyther lower working time
* addition silicone greater setting and working time
86
comparing polyethers and addition silicones
best performing in terms of elastic recovery
* virtual (addition silicone) most elastic with 99.5% recovery
* flexitime a little behind
* impregum much poorer performer (polyether) 98%
87
comparing polyethers and addition silicones
best at recording deep undercuts
* based on results of shark fin test
* impregum (polyether) best at this
88
comparing polyethers and addition silicones
tear strength
* addition silicones better 9MPa
* virtual best
* impregum (polyether) least 1.9MPa
89
Ideal properties for elastomers
90
dental ceramic composition/%/brief function
* 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
91
conventional dental ceramic powder formation
* 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
92
chemistry of conventional dental ceramics
* 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
93
describe fabrication of a crown
* 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
94
describe sintering during crown fabrication
* 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
95
properties of conventional dental ceramics
aesthetics
* 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
96
properties of conventional dental ceramics
chemical and dimensional stability
* 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
97
properties of conventional dental ceramics
thermal properties
* 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
98
properties of conventional dental ceramics
mechanical properties
* 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
99
overcoming problems with conventional dental ceramics
* 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
100
overcoming problems with conventional dental ceramics
metal coping
* porcelain-fused alloys
* alumina core
* zirconia core
101
conventional dental ceramics
adding alumina core summary
* 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
101
conventional dental ceramics
adding zirconia core summary
* 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
102
zirconia core ceramic
describe yttria stabilisation of zirconia
* 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
103
describe fabrication of a zirconia core crown
* 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
104
milled core crowns and bridges
materials
* zironia
* lithium disilicate
* precious metal
* non-precious metal
* titanium
* composite
105
zirconia core ceramic
problems/poitives
* 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
describe cast and pressed ceramics
* 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
describe E max press ceramic fabrication
* 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
* 2. 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
advantages of different crown types
* 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
sintered vs milled crown
* 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
zirconia or Lithium Disilicate (E-max) what crown material to use where
* 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
luting zirconia and LiDiSi crowns
* 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
why do we have porcelain fused alloys
* 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
porcelain positive and negative characteristics
* 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
porcelain fused alloys
alloy support
* 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
porcelain fused alloys
layers
* 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
porcelain fused alloys
alloy technician steps
* 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
porcelain fused alloys
alloy options
* high gold
* low gold
* silver palladium
* nickel chromium
* cobalt chromium - different type to CoCr used i RPD
118
porcelain fused alloys
alloy required properties
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
porcelain fused alloys
high gold summary
* 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
porcelain fused alloys
low gold summary
* 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
porcelain fused alloys
silver palladium summary
* 30% silver
* 60% palladium
* 10% indium and tin
* high melting point
* casting this alloy is challenge for technicians
122
porcelain fused alloys
nickel chromium summary
* 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
porcelain fused alloys
cobalt chromium summary
* high elastic modulus
* hard material
* high tensile strength
* high melting point
* significant casting shrinkage
* low-ish bond strength
* casting difficult
124
porcelain fused alloys
porcelain to metal bond types
* 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
porcelain fused alloys
modes of failure
* 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
endodontic material categories
* instruments
* irrigants
* intra-canal medicaments
* obturation materials
* sealers
* pulp capping materials
* root-end filling materials
127
endodontic instruments function
* 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
stress
* force measured across a given area
* tensile/compressive/shear/torsional
* stress = F/A
129
elastic limit
a set value representing the maximal strain that when applied to a material, allows the material to return return to original dimensions
130
cyclic fatigue
* generation of tension/compression cycles
* repeted forces eventually resulting in fracture
* failure
131
torsional fatigue of endodontic instruments
* 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
classification of endodontic intruments
* 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
stainless steel endodontic instrument summary
* alloy of iron, carbon and chromium
* nickel may also be present
* 13-26% chromium prevents rusting
* passivation layer of chromium oxide
134
summary of stainless steel endodontic instrument manufacture
* machined stainless steel wire into desired shape
* work-hardening occurs
135
summarise work hardening
* strengthening of a metal by plastic deformation
* crystal structure dislocation
* dislocations interact and create obstructions in crystal lattice
* resistance to dislocation formation develops
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nitinol endodontic instrument summary
* equiatomic alloy of nickel and titanium
* super-elasticity - application of stress does not result in usual proportional strain
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nickel titianium crystal structure
* 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
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describe shape memory alloys
* shape memory alloys are materials that can be deformed at one temp
* but when heated or cooled return to their original shape
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components of endodontic rotary instruments
* 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
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irrigant properties/function
* 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
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irrigants
sodium hypochlorite summary
* 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
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irrigants
NaOCl effect
* effect on organic material
* inability to remove smear layer by itself
* possible effect on dentine properties
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factors important for NaOCl function
* concentration
* volume
* contact
* mechanical agitation
* exchange
144
preparation of the canal for obturation
* 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
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removal of smear layer during endo
options
* 17% EDTA
* 10% citric acid
* MTAD
* sonic and ultrasonic irrigation
* watch apical control
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irrigant interactions
* interaction with NaOCl forms para-chloroanaline
* cytotoxic and carcinogenic
* uncertain bioavbailability
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properties of an ideal obturation material
* 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
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gutta percha summary
* 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
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gutta percha cone composition
* 20% gutta percha
* 65% zinc oxide
* 10% radiopacifiers
* 5% plasticisers
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endo sealer functions
* 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
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properties of an ideal endo sealer
* tackiness to provide good adhesion
* radiopacity
* easily mixed
* no setting on shrinkage
* non-staining
* bacteriostatic
* slow set
* insoluble in tissue fluids
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endo sealer
zinc oxide and eugenol summary
* 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
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endo sealer
glass ionomer summary
* dentine bonding properties
* removal upon retreatment is difficult
* minimal antimicrobial activity
* little clinical data to support use
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endo sealer
resin sealers summary
* 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
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endo sealer
calcium silicate sealers summary
* 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
endo sealer
medicated sealers
* sealers containing paraformaldehyde NOT acceptable
* severe and permanent toxic effects on periradicular tissues
* sargenti paste, endomethasone, SPAD etc
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mineral trioxide aggregate summary
* earlier forms were grey - better setting characteristics but tooth discolouration
* white formulation - smaller particle size and reduced discolouration
* tricalcium silicate
* dicalcium silicate
* bismuth oxide
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MTA setting reaction
* 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
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properties of a luting agent
* viscosity and film thickness
* ease of use
* radiopaque
* marginal seal
* aesthetics
* solubility - low
* cariostatic
* biocompatible
* mechanical properties
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properties of a luting agent
viscosity and film thickness
* 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
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properties of a luting agent
marginal seal
* 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
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properties of a luting agent
aesthetics and biocompatability
* tooth coloured and non staining - variation in shade and translucency
* biocompatible - not toxic, not damaging to the pulp, low thermal conductivity
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properties of a luting agent
radiopaque and cariostatic
* some ceramic crowns are radiolucent - makes it easier to see marginal breakdown
* cariostatic - fluoride releasing, antibacterial
* important in preventing secondary caries around crown margins
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luting angents
types of materials
* dental cement: zinc phosphate, zinc polycarboxylate
* glass ionocer cement: conventional, resin modified
* composite resin luting agents: total etch for use with DBA, self etch
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properties of a luting agent
mechanical properties
* high compressive strength
* high tensile strength
* high hardness value
* youngs modulus similar to tooth
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luting cements
zinc phosphate overview
* in use for 100+ years
* acid base reaction
* powder and liquid
* easy to use
* cheap
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luting cements
zinc phosphate powder
* 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
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luting cements
zinc phosphate liquid
* 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
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luting cements
zinc phosphate setting reaction
* 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
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luting cements
zinc phosphate material problems
* 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
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luting cements
zinc polycarboxylate overview/comparison to zinc phosphate
* 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
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luting cements
glass ionomer cements overview and chemistry
* 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
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luting cements
glass ionomer cement properties
* 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
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luting cements
resin modified glass ionomer overview and chemistry
* 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
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luting cements
resin modified glass ionomer properties
* shorter setting time
* longer working time
* higher compressive and tensile strengths
* higher bond strength to tooth
* decreased solubility
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luting cements
resin modified glass ionomer potential problems
* 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
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luting cements
composite luting agents overview
* 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
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luting cements
composite luting agents - bonding to indirect composite
* 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
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luting cements
composite luting agents - bonding to porcelain
* 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
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luting cements
composite luting agents - bonding to metal overview
* 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
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luting cements
composite luting agents - metal
etching metal
* 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
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luting cements
composite luting agents bonding to non-precious metak
* 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
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luting cements
composite luting agents - bonding to precious metal
* 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
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luting cements
composite luting agents - self adhesive to metal composite resin
* 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
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luting cements
composite luting agents - self etching comp resin cements overview
* 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
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luting cements
self etching composite luting agents bonding
* 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
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luting cements
self etching composite luting agents
mechanical properties
* compressive strength, tensile, hardness all slightly lower than conventional resin luting agents
* very few clinical studies
* do not get round problem of moisure control
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luting cements
temporary cements overview
* 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
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191
192
193
194
195
types of temporary materials overview
* 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
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temporary materials
brand names
* polymethylmethacrylate PMMA: jet
* polyethylmethacrylate PEMA: trim II, snap
* Bis-acryl composite: protemp 4, quicktemp
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temporary materials
methacrylate monomer
* 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
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PMMA jet overview
* 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
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temporary materials
PMMA temperature
* temperature generated by polymerisation reaction
* exothermic reaction
* safety issue?
* two studies looking at effect of temp rises on dental pulp: zach and cohen + baldissaria
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results of zach and cohen study
* 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
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baldassaria study results
* 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
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temporary materials
colour stability
* protemp showed least amount of colour change
* then trim
* then jet
* both trim and jet changed in appearance a significant amount more than protemp
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temporary materials
temporature rise
* 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
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temporary materials
polymerisation shrinkage
* 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%
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temporary materials
compressive strength
* protemp 3 has greatest compressive strength
* protemp G similar to trim etcbut significantly weaker than protemp 3
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temporary materials
abrasion resistence
* 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
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temporary materials
surface roughness
* 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
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what are wrought alloys and their uses
* alloy which can be manipulated/shaped by cold working
* eg drawn into wire
* uses: wires for orthodontics and partial denture clasps
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steel constituents
* 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
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steel uses and carbon %
* cutting instruments >0.8% carbon
* forceps <0.8% carbon
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iron structure
* 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
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Fe-C phase diagram
* 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
what is solid solution and types of solid solution
* 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
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Fe-C phase diagram
on cooling rapidly grain structure is
* austenite
* quenching should therefore give us austenite - but in practice we get martensite
* martensite behaves quire differently
215
martensite behaviour
* 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)
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describe tempering martensite
* 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
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stainless steel four main components and features
* 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
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stainless steel types
* martensitic
* austenitic
219
martensitic stainless steel and use
* has round 12-13% chromium and little carbon
* can be heat hardened by tempering process
* used to make dental instruments
220
austenitic stainless steel and use
* 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
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18:8 stainless steel overview
* 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
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describe cold working
* 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
18:8 stainless steel uses
* 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
alloys used as wires
* 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
define springiness
* 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
wire property requirements
* 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
describe stainless steel soldering
* 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
describe weld wecay SS
* 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
minimise risks of weld decay by
* 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
describe stainless steel stress relief annealing
* 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
how is stainless steel denture base made
* 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
advantages of stainless steel denture base
* 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
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disadvantages of stainless steel denture base
* 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
investment material uses
* for production of inlays, onlays, crowns and bridges
* ^^ that are made of an alloy
* materials are used by lab technicians
235
investment materials
technique used involves/which requires
* 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
investment materials stages overview
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
when alloy is cast
conditions
* 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
investment materials
types
* 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
investment materials
ideal properties
* 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
typical contractions from alloy melting point to room temp
* gold alloys: 1.4%
* Ni/Cr alloys: 2%
* Co/Cr alloys: 2.3%
241
investment materials components
* 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
investment materials
different refractory component expansions
* 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
investment materials
desribe refractory component quartz expansion
* 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
gypsum-bonded investment material
composition
* 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
gypsum-bonded investment material
setting reaction
* gypsum products undergo reaction
* calcium sulphate hemihydrate + water = calcium sulphate di-hydrate
246
gypsum-bonded investment material
dimensional changes
* silica undergoes thermal expansion and inversion expansion
* gypsum undergoes expansion during setting - hydroscopic expansion as well as contraction above 320C
247
what is gypsum hydroscopic expansion
* 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
factors increasing hydroscopic expansion
* lower powder/water ratio
* increased silica content
* higher water temperature
* longer immersion time
249
investment materials
gypsum-bonded IM contraction
* occurs above 320C
* due to:
* water loss
* presence of sodium chloride and boric acid
250
gypsum bonded investment materials properties
* 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
gypsum-bonded investment material
unwanted reaction
* 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
gypsum-bonded investment material
chemical stability
* 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
phosphate-bonded investment material
composition
* 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
phosphate-bonded investment material
setting
* ammonium phosphate + magnesium oxide + water
* = magnesium ammonium phosphate
255
heating phosphate-bonded investment material
* to around 1000 results in:
* at 330C water and ammonia released
* at higher temp complex reactions with silico-phosphates - increases strength
256
phosphate bonded investment material
properties
* high strength "green strength"
* sufficiently porous
* chemically stable
* easy to use
* porous
257
silica investment materials stages
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
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silica investment material dimensional changes
* contraction at early stages of heating - water and alcohol loss from gel
* substantial thermal and inversion expansion - lots of silica present
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silica investment material properties
* sufficiently strong
* NOT porous - needs vents
* complicated manipulation
* not used in GDH
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