DMS Flashcards
1
Q
what are types of impression material
A
- impression compound
- impression paste
- hydrocolloids
- alginate
- elastomers
- silicones
- polyethers
- polysulphides
2
Q
types of metals/alloys
A
- amalgam
- cobalt chromium
- titanium
- gold
- stainless steel
3
Q
types of restorative material
A
- amalgam
- composites
- glass ionomer cements
- compomers
- porcelain
4
Q
types of force
A
- compressive - squeezes material at top
- tensile - stretches material
- shear - force applied parallel to surface to which the object is attached
5
Q
what is stress
A
stress is = force/unit area
N/m2 - mega pascals
6
Q
what is strain
A
stress is applied object will change shape thus the material will undergo strain.
strain is = change in length/ original length
7
Q
what is stress-strain curve
A
this defines the elastic modulus (rigidity) of the material
8
Q
what is the mechanical properties of enamel
A
strong - fracture stress (262MPa)
hard
rigid
brittle
9
Q
what sort of forces do restorative materials need to withstand
A
- biting
-grinding/chewing - removal
10
Q
enamel and dentine characteristics
A
enamel
- high FS - 262 MPA
- high PL - 235MPA
- rigid/stiff
- EM (33.6 GPa)
dentine
- EM - 11.7 GPa
-less rigid
-lower PL (176MPA)
- Lower FS (234MPA)
11
Q
what design is needed for amalgam
A
- undercut
- mechanical retention
12
Q
what are some material failure mechanisms
A
also elasticity - ability of material to recover its dimensions following stress application
13
Q
is enamel bonding easy if so why ?
A
enamel bonding is easy due to the structure of enamel
- heterogeneous structure: densely packed prismatic
- highly mineralised: 95% inorganic
- ‘dry’
14
Q
how does acid etch work
A
- long enamel prisms packed with hydroxyapatite crystals
- acid roughens the surface of enamel
-allows micro mechanical interlocking of resin filling materials - this increases surface energy by removing surface contaminants leading to better wettability of enamel
- resin adapts better but enamel must be dry
- 35% phosphoric acid
15
Q
what is usually applied after etch for enamel
A
- low viscosity Bis-GMA resin
penetrates into rough surface and light cured
-this polymerises and allows strong bond of composite
16
Q
what is dentine composition
A
- 20% organic (collagen)
-70% inorganic (hydroxyapatite) - 10 % water
-full of permeable tubules - low surface energy
- hydrophilic
- aged dentine more mineralised
- smear layer
17
Q
requirements of dental bonding agents
A
- ability to flow
- potential for intimate contact with dentine surface
- low viscosity
- adhesion to substrate
- mechanical
- chemical
- Van der Waals
- combination of the above
- usually hydrophobic
18
Q
how is mechanical bonding in dentine achieved
A
- DBA and dentine surface meshing and interlocking with minimum gaps
19
Q
how is chemical adhesion achieved in dentine
A
mineralised dentine - ionic bond
organic dentine - covalent bond
20
Q
how is van Der Waals adhesion achieved
A
based onelectrostatic or dipole interaction between bonding agent and substrate
- strength depends on contact angle
<90 degrees = hydrophilic
21
Q
what is critical surface energy
A
the surface tension of a liquid that will just spread on the surface of a solid
liquid must have lower surface energy than surface its being placed on
22
Q
how does DBA work with critical surface energy
A
- dentine low surface energy
- for composite to stick dentine must have higher critical energy
-DBA increase this = surface wetting agent
23
Q
how does bonding through molecular entanglement work
A
-adhesive is absorbed onto the surface but can also penetrate into the surface of the dentine
- the absorbed component can form a long chain polymer
- this polymer meshes with the substrate - molecular engtanglement - leading to high bond strength
24
Q
what is smear layer
A
-layer of organic debris on dentine during prep
- 0.5-5 microns thickness
- got bacteria
need to remove by total etch or self etch
25
what was bonding mechanism in earlier dentine bonding agents
ionic bond to the calcium in the dentine by the chlorophosphate or hydroxyl group
26
how is smear layer removed/modified
- total etch
- self etch
27
what does total etch do
- remove smear layer
28
components of total etch
35% phosphoric acid
primer ; adhesive part which is hydrophobic/hydrophillic
adhesive ; resin penetrates into dentine surface attaching to primers hydrophobic surface
29
what is dentine conditioner
- phosphoric acid
-removes smear layer
- opens dentinal tubules
-decalcifies uppermost layer of dentine
30
what is a primer
-adhesive component
- hydrophilic end to bond to hydrophilic dentine
- hydrophobic end bonds to resin
- spacer group = flexible
- molecule is dissolved in suitable solvent
31
what is adhesive
BIS-GMA and HEMA
hydrophobic
filler particles make stronger
contains camphorguinone to allow light cure
penetrates primed dentine which is now hydrophobic
micro mechanical bond - molecular entanglement
forms the hybrid layer of collagen and resin
32
total etch problems
-over etch - collapse of collagen fibres , too deep etch
moisture dependent - moist dentine gives good bond
33
what is self etching primer
- infiltrate and incorporate into smear layer
- not as technique sensitive but not as strong bond
34
how does self etch primer work
- contain acidic methacrylate monomer
- bifunctional monomers which infiltrate the dentine with hydrophilic end while polymerising at hydrophobic end
- create amorphous calcium chelate on surface
-smear layer disollved but then incorported into hybrid layer
35
what is AD concept
-adhesion decalcification concept
Iinitially all acid monomers bond to the calcium in HA ionically
- whether they stay bonded depends on the stability of the HA-monomer bond
- Monomers with lower pK (not necessarily pH) do not form a stable bond as they continue to dissolve HA
- this leads to a hybrid layer with unstable calcium phosphates incorporated
36
smear layer problems
- smear layer is thick then mild self etch may not penetrate
37
what is mild self etch
- mild self etch only partially demineralises the dentine
- HA crystals remain around the collagen
- protective against hydrolytic breakdown
- remaining Ca ions allow ionic bonding
38
benefits of self etch
- less technique sensitive
- no rinsing
- no excessive drying
- no dentine collapse leading to low bond strength
- simulatenous demineralisation and resin infiltration
- less chance of post-op sensitivity
39
disadvantages of self etching bonding agents
- there is great variability between products with regard to initial pH of the solution
- the difference in pH results in different etch and different penetration of resin
- those materials with a lower initial pH may not etch enamel efficiently
- there is little evidence of stronger bond to dentine than with total etch
40
direct filling materials ideal properties
- mechanical - strength, rigidity, hardness
- bonding to tooth/ compatible with bonding systems
- thermal properties
- aesthetics
- handling/viscosity
- smooth surface finish/polishable
- low setting shrinkage
- radiopaque
- anticariogenic
- biocompatible
41
what is in composite resin
- filler particles
- resin
-camphorquinone - photo-initiator
- low weight dimethacrylates
-silane coupling agents
42
what are the filler particles in composite
- glass (types)
- microfine silica
- quartz
- borosilicate glass
- lithium aluminium silicate
- barium aluminium silicate
- others!
43
what makes up resin in composite
- Bis-GMA - reaction product of bisphenol-A and glycidyl methacrylate
- urethane dimethacrylates
- undergoes free radical polymerisation
44
what is camphorquinone
-- activated by blue light
- produces radical molecules
- these initiate free radical addition - polymerisation of Bis-GMA
- leading to change in resin properties (ie increased molecular weight, so increased viscosity, and strength)
45
what are low weight dimethacrylates
- TEGDMA
- added to adjust viscosity and reactivity rate
- give some control over products characteristics
46
what is silane coupling agent
- good bond between filler particle and resin is essential
- normally water will adhere to glass filler particles, preventing resin from bonding to the glass surface
- a coupling agent is used to preferentially bond to glass and also bond to resin
47
what happens when smaller filler particles are in composite
-adding smaller particles achieves a greater volume uptake
48
effect of adding smaller filler particles
- improved mechanical properties
strength,rigidity,hardness,abrasion resistance
- low thermal expansion (still not perfect)
- lower polymerisation shrinkage (still a problem)
- less heat of polymerisation (but not negligible)
- improves aesthetics
- some radiopaque
49
composite activation ?
camphorguinone + blue light (430-490nm)
50
advantages of light curing
- extended working time
- less finishing
- immediate finishing
- less waste
- higher filler levels (not mixing two pastes)
- less porosity
51
how does the light cure work curing composite
the light source spectral range matches the absorption spectrum of the photo initiator
52
what is done to assure composite is cured correctly
- depth of cure
- 2mm
- depth of cure is depth at which hardness is 80%
53
what happens if composite isn't placed in increments
- soggy bottom
-poor bonding to tooth
54
what is a bulk fill composite
- lucerin initiator as well as camphorquinone
- has different optical absorption specturm hence UV and blue light needed to polymerise and cure material fully
55
problems with light curing
- overexposure
- premature polymerisations
-depth of cure
- setting time too short
-polymerisation shrinkage
56
properties of composite
- strong - 350 MPs
- rigid - 15GPa
-PL = 300MPa
- El= 320MPA
57
properties of conventional composite
strong but problems within finishing and staining due to soft resins and hard particles
58
properties of conventional composite
- smaller particles - smoother surface better aesthetics for longer period
- but inferior mechanical properties
59
properties of hybrid composite
- originally compromise between conventional and microfine
- most modern composite are hybrids
60
typical bond strength go composite to dentine and enamel
40 MPa
61
composite mechanical properties numbers
62
composite thermal properties
- low thermal conductivity
-low thermal diffusivity
- high thermal expansion coefficient
63
thermal expansion coefficient of materials
- affects micro leakage
64
purpose of lining material
- prevents gaps
- protective barrier
(<0.5mm over dentine )
65
what could a cavity base be used for
- thick
- block out undercuts
66
how is liner protective
- pulpal protection - bacteria, thermal stimuli, chemical stimuli
- can calm down inflammation and promote pulpal healing
- can reduct symptoms prior to definitive TX
67
desirable properties of lining materials
- ease of use
- thermal properties
- mechanical properties
- radiopaque
- marginal seal
- solubility
- cariostatic
- biocompatible
- compatible with restorative materials
68
types of liners
setting calcium hydroxide
glass ionomer and resin modified glass ionomer cements
69
what are good base liners
zinc oxide based cements
palliative cements
70
what is in setting calcium hydroxide
- base - calcium hydroxide, zinc oxide, plasticiser
-catalyst - butylene glycol disalicylate (reactive element) , filler and radiopaqur
71
what is the setting reaction of CaOH
- the setting reaction is a chelation reaction between the ZnO and the butylene glycol disalicylate
- this results in a cement with an initial pH of around 12
72
what is mode of action of CaOH
- bactericidal
-irritation causing tertiary dentine formation
73
properties of CaOH
- quick setting time
- radiopaque
- easy to use
- low compressive strength
- poor solubility
- unstable and soluble
- if the cavity leaks then the lining will disappear
- it may even disappear just because it is in contact with moist dentine
74
what are zinc oxide based cements
- zinc phosphate
- zinc polycarboxylate
- zinc oxide eugenol (ZOE)
- resin modified ZOE
- ethoxybenzoic acid (EBA) ZOE
75
what is ZOE used for
- linings/base in deep cavities
- under amalgam restorations
- temporary restorations
- resin modified or EBA ZOE
- root canal sealer
- slow setting 24hrs
- periodontal dressings
- fast setting, 5 minutes
76
what is the ZOE reaction
base - ZnO
acid - eugenol
chelation reaction
77
ZOE properties
- adequate working time
- relatively rapid setting time
- can be modified by addition of accelerators
- sets quicker in the mouth due to moisture and heat
- low thermal conductivity
- low strength around 20MPa
- weak hydrogen bonds between the eugenolate molecules
- not strong enough to use as a base beneath an amalgam filling. the packing pressure would damage it
- radiopaque
- high solubility
- eugenol is constantly released
- this is good and bad
- eugenol is replaced by water which leads to disintegration of the material BUT
- eugenol when liberated has an obtundant effect on the pulp and can reduce pain
- the released eugenol inhibits the set of resin based filling materials. it softens them and can cause discolouration
- ZOE materials should not be used under composite resin materials
78
resin role in resin modified ZOE
- these do not take part in the reaction but give a stronger backbone to the set material
- this increases the compressive strength to >40MPa making it suitable as a cavity lining
-makes crack propagation harder
79
glass ionomer lining properties
- thermal conductivity and diffusivity are lower than dentine for both GIC and RMGIC
- thermal expansion is similar to dentine for GIC
- compressive strength is >170MPa, higher than any of the ZnO based materials
- most materials are radiopaque, the radiopacity varies between materials
- marginal seal is better than any of the other materials as there is a chemical bond to enamel and dentine
- they are the only material to predictably seal dentinal tubules. this decreases micro-leakage and helps prevent post treatment sensitivity
- solubility is greater for GIC than RMGIC and is greatest initially
- however, GIC materials are less soluble than any of the other liners apart from Zinc phosphate cement
- RMGIC is less soluble than any other cement
- can bond to amalgam
80
uses of GI and RMGI
restorative
- filling material
- Eg. Riva, vitremer
core build up
- prior to restoration with crowwn
- Eg. vitremer crown core
lining
- underneath permenant fillings
luting
- cementing inderect restorations
81
components of GI
acid - liquid
base - glass powder
82
what makes up the powder in GI
Silica, SiO2 (Silicone dioxide) 30% - 40%
Alumina, Al2O3 (Aluminium dioxide) 15% - 30%
Calcium Fluoride, CaF2 15% - 35%
Aluminium Fluoride 2% - 10%
Aluminium phosphate 4% - 20%
Sodium fluoride 4% - 10%
Adding Strontium and lithium salts can increase the radiopacity but these play no part in the reaction chemistry.
The ratio of alumina/silica alters the translucency.
More silica more translucent.
83
what is the acid in GI
- poly acrylic
- tateric acid
84
what size of particles in GI needed for luting cements
<20um required for luting cement to give a low film thickness. The smaller the particle size the quicker the setting reaction and the more opaque the set cement.
85
setting reaction in GI
- dissolution
-gelation
-hardening
86
dissolution phase in GI reaction
87
gelation phase in GI reaction
- initial set is due to calcium ion crosslinking with the polyacid by chelation with the carboxyl groups
- calcium ions are bivalent so they can react with two molecules joining them
- crosslinking is not ideal as the Ca can chelate with two carboxyl groups on the same molecule
- this intial set is caused by formation of calcium polyacrylate
- following this reaction the material will appear hard in the mouth
88
hardening reaction in GI
- trivalent aluminium ioins ensure good crosslinking with an increase in strength
- aluminium polyacrylate formation takes a long time
- this process does not start for at least 30 minutes and can take a week or longer to be complete
- the aluminium reaction ensures a much higher degree of crosslinking
- this process greatly improves the mechanical properties of the material
89
gic protection
- varnishes
-resins
-vaseline
90
bonding mechanism for GI
- chelation between carboxyl groups in the cement and Ca on the tooth surface
- re-precipitation of complex mixture of calcium phosphate (from apatite) and calcium salts from the polyacid onto and into the tooth surface
- hydrogen bonding or metallic ion bridging to collagen
- can bond without intermediate material
- conditioned not etched
91
thermal properties of liners
- conductivity *
How well heat energy is transferred through a material
Cavity lining should have as low thermal conductivity as possible!!
Thermal expansion coefficient
change in length per unit length for a rise of 1*C (in ppm*C-1)
Liner should match thermal coefficient of tooth!
GIC has better TEC than RMGI
Thermal Diffusivity
similar to conductivity
Liners have similar or lower thermal diffusivity than enamel
Amalgam much higher than tooth tissue- hence use of liner
92
mechanical properties of liner
high compressive strengthh
similar modulus to dentine
93
properties of GIC
Can bond to enamel and Dentine w/o use of intermediate material
Bond strength pretty poor (5-20 MPa)
Poor Tensile strength
Lower compressive strength than composite (less than 50%)
Higher solubility than composite due to unprotected material during gelation phase
Usually seals well
Fluoride release (for short time)
colour is ok
poorer mechanical properties then composite
94
RMGI components
- Powder
* Fluro-Alumino-Silicate glass
* Barium glass (provides radiopacity)
* Vacuum dried polyacrylic acid
* Pottasium persulphate (redox catalyst - cures resin in the dark) * Ascorbic acid
* Pigments (vary in shade for aesthetics)
- Liquid
* HEMA (water miscible resin)
* Polyacrylic acid with pendant methyacrylate (undergo acid/base reactions and polymerisation) * Tartaric Acid (speeds up setting reaction)
* Water (allows reaction between polyacid and glass)
* Photoinitiators - ALLOW LIGHT CURE!
95
advantages of GIC
- stable chemical bond to enamel and dentine
- low microleakage
- fluoride release
- good thermal properties
- no contraction on setting
96
disadvantages of GIC
- brittle
- poor wear resistance
- moisture susceptible when first placed
- poor aesthetics
- poor handling characteristics
- suceptible to acid attack and drying out over time
- possible problems bonding to composite
97
dual curing RMGI setting reaction
* On mixing proceeds like the normal GIC (dissolution)
* Light activation causes free radical methycrylate reaction to occur = resin matrix formed * Acid Base reaction occurs for several hours afer
98
tri curing RMGI setting reaction
* On mixing proceeds like the normal GIC (dissolution) * Redox reaction begins
* Light activation- resin matrix formed
* Redox reaction continues for 5 mins after initial mix
* Acid base reaction occurs for several hours
* Final hardening may take days
99
RMGI over GI
- Better physical properties
- Lower solubility
- Fluoride release
- Better translucency/aesthetics - Better handling
100
components of amalgam
- mercury (powder)
- silver and tin and copper (liquid)
101
what particle types of amalgam exist
- lathe cut - filing ingots
-spherical/spheroidal - spraying molten metal into atmosphere
102
amalgam setting reaction
--Silver/Tin Powder reacts with Liquid Mercury
- Some unreacted Silver/Tin Powder remains
- Silver Mercury (y1) and Tin Mercury (y2) form amalgam matrix
103
some properties of Gamma phases in amalgam
small contraction <0.2
104
zincs effect in amalgam
- interacts with saliva
- forms H2
Hydrogen gas causes pressure expansion of amalgam
downward pressure = pulpal pain
upward prressure= sitting proud of surface - chipped off
∴ Zinc free materials!
105
advantages of spherical amalgam
less Hg required
higher tensile strength
higher early compressive strength
less sensitive to condensation
easier to carve
106
amalgam properties
- suitable for posterior
- strength ok
- subject to creep
-corncern about toxicity
conductivity - high - may need liner
diffisuvity - high
thermal expansion - x3 of tooth
- needs mechanical retention
- looks poor
- not anti cariogenic
107
factors decreasing strength
-undermixing
-too high Hg content after condensation
- too low condensation pressure
- slow rate of packing
- increments do not bond
- corrosion
108
what is creep
When a material experiences low-level stress levels (ie below elastic limit stress) which are applied repeatedly over a prolonged time period, it may FLOW, resulting in PERMANENT DEFORMATION
109
corrosion in amalgam
gamma 2 most electronegative
weakens material particularly at margins
reduce by
- copper enriched, polishing margins
avoiding galvanic cells
110
what is copper enriched amalgam
- non gamma 2
- >6% copper
111
what are types of copper enriched amalgam
- dispersion modified
- single composition types
112
dispersion modified amalgam setting reaction
113
single composition amalgam reaction
114
benefits of copper enriched amalgam
Higher early strength
Less creep
Higher corrosion resistance
Increased durability of margins
115
mechanical properties of amalgam compared to enamel and dentine
116
what is the function of impression materials
produce an accurate replica of the surface and shape of hard and soft oral tissues
117
what is positive and negative replica in impressions
- impression = negative replica
- dental stone = positive replica
118
what is a mucostatic impression
- ZOE, low viscosity alginates
- fluid that displaces soft tissues slightly
- gives impression of undisplaced mucosa
119
what is mucocompressive material
- impression compound, high viscosity alginates, elastomers
- impression of mucosa under load
120
what are elastic materials in impressions
-hydrocolloids - alginate
-elastomers - silicone, polyether
121
ideal properties of impressions
- no dimensional change
- complete elastic recovery
-good surface detail
- good storage over time
- low thermal expansion coefficient
-non toxic
122
what are some non-elastic impression materials
- impression compound /paste
123
what is a hydrocolloid
- 2 phase system of fine particles
- one phase dispersed in another
- water is medium
124
composition of alginate
125
alginate setting reaction
Cross linking with Calcium allows the alginate to set
intermediate reaction between sodium to calcium stage allows a
delay in setting
Use perforated tray with adhesive for alginate!
remove tray with a sharp pull
large bulk reduces strain on material
126
reproduction of surface detail
ISO4823
127
ideal properties of PMMA
- Replaces function of natural teeth
- Goes into patient mouth
- Has to be cost effective
- Dimensionally accurate and stable in use - fit and be retained
- High softening temp (Tg) - must not distort when eating or cleaning
- Unaffected by oral fluids over time
- Non-toxic/Non-Irritant
- Easy to repair
- Radiopaque - helps with detection of inhaled or ingested fragments if broken and swallowed
128
ideal mechanical properties for PMMA
High YM
High Proprotional limit - only large stresses will cause permanent deformation
High Transverse strength - upper denture has 3 pt loading (2 lateral - 1 middle donward force)
High fatigue strength - can withstand low stresses over a long time (design dependent)
Thermal
High Impact strength - with stand large stresses applied rapidly e.g dropping onto hard surface - may form hairline fractures
129
setting reaction of PMMA
- free radical addition polymerisation - adding 2 molecules of same of diff to form a large molecule without elimination of smaller molecule
- methacrylate monomer
activation - to provide free radicals
initiation - free radicals break down c=c and transfer free radical
propagation - growing polymer chain
termination - of polymerisation
130
composition of heat cured acrylic
POWDER
- benzoyl peroxide
-PMMA particles
- plasticiser
- pigments
-copolymers
LIQUID
- methacrylate monomer
- inhibitors
copolymers
131
problems with acrylic curing
undercured
- free monomer (irritant)
-poor molecular weight
- gaseous porosity
- polymerisation shrinkage
132
what are metals and alloys
metal
- aggregate of atoms in crystalline structs
alloy
- combination of 2 or more metals in crystalline structure
133
what are some crystal or lactice structures
- cubic
- face centred cubic
-body centred cubic
134
cooling curve in metal
- cooling from molten state
-atoms flow readily and arranged randomly
some change from liquid to solid and crystallise
135
what is crystal growth in metals
- atoms at these sites act as nuclei of crystallisation
- crystals form dendrites
- grow until they impinge on other crystals
-regions where they make contact is grain boundaries
136
types of grain structure
- equi-axed - is equal dimension
-radial - molten metal cooled quickly in cylindrical mould
-fibrous - wire pulled through die (cold worked)
137
what is fast cooling effect in metals
(quenching)
- more nuclei
-small fine grains
138
what are nucleating agents
- controlling grain size
- impurities or additives act as foci for crystal growth
139
what is a grain
- single crystal (lattice) with atoms orientated in given directions (dendrites)
140
what is the best type of grain and why
- small fine grains
-high elastic limit, FS and UTS and hardness
141
what is quenching
- small bulk of metal
- heat just above tM
-store in mould
-allowing for high thermal conduction
-quench
142
what is dislocation in metal/alloy
- defect - discontinuity in lattice
143
what happens when force is placed onto a dislocation in metal/alloy
- defect moves along lattice plane
- crystal changes shape
- no defects as its propagated to grain boundary - SLIP
144
effect of dislocation
increases
- elastic limit
- fracture stress/UTS
- hardness
decreases
- ductility
- impact resistance
145
what factors imped dislocation movement
- grain boundaries (hence fine grains)
- alloys : different atom sizes - has an inherent resistance to the movement of dislocations
- cold working - dislocations stopped at grain boundaries
146
what is cold working
- work done on material
- low temp
-causes slip = stronger or harder material
- improves some mechanics properties
-but causes residual stress
147
how is residual stress relieved
- annealing
-this is heating metal so that greater thermal vibrations allow migration of atoms
148
what is recrystallisation in meta/alloy
- metal heated after cold work fails
- spoils benefit of cold work but allows further cold work
- all redone until correct shape obtained
149
what is phase and solution in alloy
Phase
- physically distinct homogenous structure
solution
- homogenous mixture at an atomic scale
- grains of only one metal = single phase
- individual metals of metal A and B in lattice network = two phases
-metals A and B in homogenous mixture - one phase
150
on crystallisation 2 metals may be
A ) be insoluable - no common lattice - exist as two phases
- B) form a intermetallic compound with a specific chemical formulation (ag3Sn)
- C) be soluable and form a solid solution - common lattice
151
what are the 3 types of solid solution
1. substitutional - atoms of one metal replace the other metal in crystal lattice
A) random substitution
B) ordered substitution
2. interstitial - atoms diff size, smaller atoms in spaces in lattice/grain
152
metal/alloy cooling curve temp
- metal - crystallises at one temp
- alloy - crysyallises over temp range
153
what state may metal be in
soluble:- solid solution formed (homogeneous mixture of metals in each grain)
insoluble:- grains of individual metals formed
154
what are terms in metal phase diagram
LIQUIDUS
line representing the temperatures which different alloy compositions begin to crystallise
SOLIDUS
line representing the temperatures which different alloy compositions have completely crystallised
155
how is cooling of alloy taken place
- slow cooling
- allows metal atoms to diffuse through lattice
- ensures homogenous
156
what is coring
prevents atoms diffusing through lattice
causes CORING
causes concentration gradient
as composition varies throughout grain.
157
how is coring resolved
Homogenising Anneal- anneal to gently heat and vibrate atoms, help atoms to diffuse (below recrystallisation temperature) to reduce
the coring but not altering the grain structure allowing a more homogenous proportion of metal right the wat through its depth
158
dislocation in alloys
- in metals the defect rolls smoothly over the lattice along the slip plane
- in alloys the defect has to ‘fall’ into the space between large and small atoms and climbs over the atoms along to the grain boundary,
but requires a great degree of effort to do so.
*instead of flying over a crowd of people you have to climb over them because you cant get through them
- Requires more stress to move dislocations in a solid solution = inherently better mechanical properties
159
what are eutectic alloys
Eutectic- when an alloy melts at a temperature higher than that of its individual metals (together- stronger)
- physically distinct grains
- soluble in liquid state
- insoluble in solid state
- *UNUSUAL IN THAT IT IS AN ALLOY WHICH COOLS AT A SINGLE TEMPERATURE NOT OVER A RANGE!
160
mechanical properties of porcelain fused alloys
porcelain
- hard
-strong
-rigid
-brittle
alloy
- hard
-strong
- rigid
-ductile
161
explain porcelain metal restorations
metal oxide between porcelain and alloy
metal oxide helps eliminate defects/cracks on porcelain
alloy supports and limits porcelain strain
162
required properties for porcelain fused alloy
- similar thermal expansion coefficient - expand and contract at same rate to prevent thermal stress
-good bond to porcelain - for longevity
- avoid discolouration of porcelain- Ag and copper
-need good bond strength - not Ni-Cr
- good hardness
-need high elastic modulus
- recrystallisation temp of alloy must be higher than fusion temp of porcelain to avoid creep
163
high gold alloy properties
- match thermal expansion
-increased melting point
- form oxide (good bonding)
-biocompatable
- YM too low
-melting range too low
- Cu can cause green discolouration
164
low gold alloy properties
- increased melting temp
-better mechanical properties
- good biocompatible
165
silver palladium alloys properties
- high MP
-care needed in casting
166
Ni-Cr alloy properties
- high MP
- High YM
-high casting shrinkage
- low bond strength
167
cobalt chromium properties
- high MP
high YM
high tensile strength and hardness
minimal casting shrinkage
168
porcelain - metal bond mechanism
- mechanical
-stressed -skin
- chemical
169
what is stressed skin effect
- slight diff in thermal contraction coefficient
-lead to compressive forces which aid bonding
170
chemical mechanism in porcelain metal bonding
- The CHEMICAL mechanism is explained by oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself.
- may be electron sharing in oxides
- during firing porcelain flows and oxides in the metal-oxide coating migrate
171
components of SS
- 72% iron
- 18% chromium
-8% Nickel
-1.7% titanium
-0.3% carbon
172
iron in SS
allotropic - 2 solid state phase changes with temp
(1) Temp. > 1400 C
BCC lattice structure; low Carbon solubility (0.05%)
(2) 900 < Temp. < 1400 C
FCC lattice; higher Carbon solubility (2%)
(3) Temp < 900 C: as (1)
173
solid solutions formed from iron
AUSTENITE:
- interstitial solid solution, FCC;
- exists at high temp (ie >720 **0**C)
FERRITE
-very dilute solid solution;
- exists at low temp
CEMENTITE
-Fe3C ; exists at low temp
PEARLITE
- Eutectoid mixture of Ferrite and Cementite
174
what happens when austenite is quenched
-forms martensite
- distorted lattice
-hard
-brittle
175
what is tempering
altering its temp and duration its kept at specific temp then quenching
- heating 450 followed by quenching
- can give ferrite and cementite
- more control of mechanical properties
176
what is chromium in SS
- STAINLESS if > 12% Cr
- lowers Austenite to Martensite temperature
- lowers Austenite to Martensite rate
- decreases % carbon at which Eutectoid formed
- helps with corrosion resistance
177
nickel in SS
- lowers Austenite to Martensite transition temperature
- improves fracture strength
- improves corrosion resistance
178
types of SS
- martensitic
-austenitic
179
martensitic SS
- 12 - 13% chromium + little carbon
- heat hardenable (tempering process)
- dental instruments
180
austenitic SS
contains sufficient Chromium and Nickel to suppress austenite to martensite transition i.e more than normal amounts of these metals -
Used in sterilisable instruments which don’t have a cutting edge
Ortho wire, due to them being readily cold worked and their corrosion resistance
Sheet form for denture bases
Corrosion resistance is more important than strength and hardness
181
what is a wrought alloy
- alloy that can be manipulated/shaped by cold working
182
SS wire
- does not heat harden
- soft (malleable when cast)
- work hardens fast - cant be repeatedly manipulated
- can be used as ortho appliances or partial denture claps
183
characteristic needed for alloy wires
-high springiness
- stiffness
-high ductility
-easily joined without impairing properties
-corrosion resistant
184
what is weld decay
- risk when welding
- Occurs when s/steel temp between 500 - 900 **0**C
- pushes Chromium carbides to precipitate at grain boundaries
- alloy becomes brittle
- less manipulation of wire to match desired configuration
- less chromium in central region of solid solution
- more susceptible to corrosion
185
solution to weld decay
- low carbon content steel
-stabilised SS - small amounts of titanium forms carbides
186
advantages of SS denture base
1. Thin 0.11mm - acrylic 1.52mm
2. Light
3. Fracture resistant
4. Corrosion resistant
5. High polish obtainable
6. High thermal conductivity
7. High impact strength
8. High abrasion resistance
187
disadvantages of SS denture base
1. Possible dimensional inaccuracy (contraction of die not matched by model expansion)
2. Elastic recovery of steel – inaccuracy
3. Damage of die under hydraulic pressure
4. Loss of fine detail during the many stages
5. Difficult to ensure uniform thickness
6. Uneven pressure on die and counter die~~~> wrinkling of steel
188
difference between decorative and dental ceramic
189
composition of dental ceramic
- feldspar
- silica
-borax
-koalin
- metal oxides
190
setting reaction of dental ceramics
vitreous pgase
- feldspar breaks tetrahedral silica
- forms 2d structure
- feldspar lowers fusion and softening temp of the glass
-forms solid mass around other components
fusion
-feldspar reacts with kaolin and forms a leucite when headed at 1150-1500
-molten mass is quenched and ground to a powder (grit)
sintering
- heating leads to sintering
-occurs just above glass transition temp
- glass phase softens and coalesce
-contraction of about 20%
191
properties of ceramics
- aesthetics - good
-chemically stable
-good biocompatibility
-thermally similar to tooth
-dimensionally stable
-hard
192
problems with ceramics
- surface microcracks
-static fatigue
193
what metal copings can be used for ceramic
- alumina core
-zirconia core
194
alumina core in porcelain
Core material in PJCs
Better flexure strength than feldspathic porcelain
alumina particles stop cracks propagating
however its opaque so cant be used as a restorative material
195
zirconia core in porcelain
- diff forms at diff temps
- zirconia doesn't sinter unless heated to over 1600
- yttria-stabilised zirconia
- yttrium is tetragonal and zirconia is monoclinic
-prevents chrack
196
what is cast/pressed ceramics
- wax up
-invested
-cast from heated ingot of ceramic
- heated to improve crystal structure - ceraming
either
- lithium dislocate glass
-leucite reinforced glass
197
what is ceraming
- Stage 1 crystal formation maximum number of crystal nuclei are formed
- Stage 2 crystal growth to maximise the physical properties
198
what ceramics are best to use in diff areas of mouth
posterior - zirconia
anterior - LiDiSi
anterior bridgework - LiDiSI
long span bridge - zirconia
199
what are luting crowns
200
ideal properties of luting agent
- viscosity/film thickness - low to allow restoration to sit , 25 microns
- easy to mix
-short setting time
- impenetrable bond
- low solubility
- high compressive, tensile, harndess
- YM similar to tooth
201
types of cements
dental cement
- zinc phosphate
-zince polycarboxate
GIC
- conventional
- RMGIC
- composite retin luting agents
202
zinc phosphate composition
powder
- zinc oxide
-magnesium dioxide
- other oxides
liquid
- phosphoric acid
-aluminum oxide
-zinc oxide
203
setting reaction of zinc phosphate
- acid base
- hydration reaction - crystallised phosphate matrix
204
properties of zinc phosphate
- low initial ph - pulpal irritation
-exothermic setting reaction
- not adhesive to tooth
-not cariostatic
-final set takes 24 hrs
- brittle
- opaque
205
zinc polycarbonate cement properties
- has polyarylic acid
- bonding to tooth similar to GIC
-less exothermic
- ph is low but increases
-cheap
BUT
- difficult to mix and manipulate
-opaque
206
GIC properties (CEMENT)
- particle size of glass smaller
bonds to tooth through ion exchange and hydrogen bonding
-ease of use
-strong
-flouride release
-insoluble once set
- stable
207
RMGIC (CEMENT)
same as GIC but resin properties too
- Shorter setting time
- Longer working time
- Higher compressive and tensile strengths
- Higher bond strength to tooth
- Decreased solubility
however
- HEMA cytotoxic
-no bond to indirect restoration
208
composite luting agents
- variants on composite filling materials with suitable viscosity and filler particle size
- need to be used with DBA
- can be dual cured or light cured
209
composite luting agents bonding to indirect composite
- composite bonds to composite
- micromechanical bonding occurs on rough internal surface of composite inlay
- bond is also chemical to remaining unbroken c=c bonds on inlay surface
210
composite luting agents bonding to porcelain
- porcelain is brittle needs to be bonded to tooth to prevent fracture
-untreated porcelain smooth and non-retentive
- HF to etch surface
-rough retentive surface but still not hydrophobic and compatible with resin luting agents
use SILANE coupling agent
- allows strong bond between silicon group in porcelain and base carbon
if thin - light cure composite
if thick - dual cure composite
211
bonding to metal (composite luting agent)
- need to be roughened -
- etching/sandblasting
- electrolytic etching
212
bonding to non-precious metal (luting agent)
- MDP and 4 meta
- acidic end and c=c end
213
bonding to precious metal (luting cement)
- change precious alloy composition to allow oxide formation
-increase copper content
-sulpher based bonding agent
214
self adhesive composite resin
- metal coupling agent incorporated into resin
-simplifies bonding process
- Acidic groups bind with calcium in hydroxyapatite forming a stabilising attachment between the tooth and the resin
- Ions from dissolution of filler neutralise the remaining acidic groups forming a chelate reinforced methacrylate network
- Limited removal of smear layer or significant infiltration into the tooth surface. (only a couple of microns)
- Good bond strength to dentine
215
self adhesive composite resin bonding to materials
- Bonding to Enamel
- Lower than to dentine: should be etched with acid prior to application
- Bonding to Dentine
- Better than to enamel: should not be etched with acid prior to application
- Bonding to Ceramics
- Brand specific. RelyX unicem seem to bond quite well to sandblasted Zirconia
- Bonding to Metal
- Better to non-precious
- Not good enough to cement ortho brackets
216
what are temporary cements
- two paste systems
-base - ZnO , starch
-accelatator - resin, eugenol , wax
217
chemistry of elastomers
- formed by polymerisation with cross-linking of polymer chains
-crosslinking - elastic properties, causes fluid to solid transition
218
polysiloxanes formula
Base paste
- polydimethylsiloxane - some methyl (CH3) groups replaced by hydrogen - filler - variations change viscosity
Catalyst paste
- polydimethylsiloxane - some methyl groups replaced by vinyl (CH2 =CH) - filler - variations change viscosity
- platinum catalyst eg chloroplatinic acid
Base Paste ((PDS (Hydrogen)) + Catalyst ((PDS) Vinyl)) + Chlorplatinic acid ——> Cross linked Polymer formed (NO BYPRODUCTS)
219
condensation cured silicones
220
polyether components
221
ideal qualities of impression materials
- viscosity
- suface wetting /contact angle - want small contact angle so more makes contact with tooth
- needs accuracy of surface detail - at least 50um
- needs elastic recovery
- needs high large fin length - to flow into undercuts
- low rigidity
222
what is cyclic fatigue
- Freely rotating in a curvature
- Generation of tension/compression cycles
- Cyclic fatigue
- Failure
223
what is torsional fatigue
When the load is suddenly reversed i.e turning the file in the opposite
direction to which has been
