dm (all topics) Flashcards
NiTi in endo ratio
56% Ni
44% Ti
1:1 atomic ratio
properties of NiTi that makes it suitable for endo
- martensite / austenite phase
- phase tf under stress
- twinning + superelasticity
what is austenite phase
- parent phase
- simple cubic structure
what is martensite phase
- mono clinic crystal struucture allows NiTi to undergo limited deformation wo breaking atomic bonds
reasons for NiTi fracture in endo
- torsional load (plastic deformation -> fracture)
- cyclic fatigue (instrument is work hardened at same location until it becomes brittle and fractures)
common solutions used in endo chemical disinfectants
- sodium hypochlorite (>0.5%)
- chlorhexidine (2%)
- EDTA
- MTAD
- chloroform
what is ceramic
rigid material of metallic & non metalllic elements inlcuding oxides, nitrides, carbides, sillicates that are organised in a crystalline / vitreous mixture
properties of ceramics
- translucency
- chromatic stability
- biocompatible (insoluble)
- chemically inert
- low thermal conductivity
- high compressive strength
- decent tensile strength, fracture toughness
how does porcelain solidify
Vitrification - porcelain solidify w a liquid structure instead of crystalline structure -> this creates a more stable structure
what is the mechanism of failure for ceramics
brittle fracture
- single crack extends around a pre existing flaw
how does ceramic break down
ceramics completely non ductile & is v brittle -> doesn’t allow any distortion
fracture occurs suddenly wo prior plastic deformation
so the entire structure just break
what is fatigue
fatigue = damage caused by alternating application of load. repeated cyclical damage OVER TIME causes crack growth over microstructural elements
define fracture toughness
fracture toughness = ability of a material to withstand crack propagation
what is usually the cause of failure:
a) load
b) compression
c) tension / tensile stresses
tension & tensile stresses
what is the purpose of fillers in ceramics
fillers are added to resist & inhibit crack propagation
- the more dispersed the fillers the better
- the smaller the filler size the better
ideal properties of fillers for ceramics
- bond to matrix (glass)
- liner coefficient of expansion same as matrix so no stress during cooling
- small and well dispersed fillers
how does brittle fracture occur in ceramics
conc stress from surface defects more than strength of ceramic body hence resulting in brittle fracture
fatigue crack growth caused by repeated cyclical damage to microstructural elements
consequences of surface defects on ceramics
- lower colour stability
- higher chance of fracture
purpose of glazes on ceramics
glaze = a ceramic veneer added to porcelain resto after it has been fired
- seals pores on surface so that surface becomes poreless
- produce glossy surface
- induce compression to increase strength
what is the strongest dental material
zirconia
what is used to colour ceramics
metal oxides
define fluorescence
material absorb light at high energy, short wavelength, and emits light at lower energy, typically visible wavelength
what is the function of leucite in ceramics
leucite is a reinforcement particle to inhibit crack propagation
what does a solid solution metal mean
metals are soluble in each other in the solid state
meaning atoms interspersed randomly in a common space lattice, single phase
*size diff of atoms within 15%
substitutional solid solutions
solute atom occupy space normally occupied by solvent atoms
- atoms have similar diameters
interstitial solid solutions
solute atoms positioned in interstitial spaces of solvent atoms
- solute much smaller than solvent
effects of alloying
- increase strength, proportional limit, hardness
- decrease ductility
in a binary phase diagram, what does the area between liquidus & solidus represent
represents solid solution and liquid solution present at thermal eqm
liquidus and solidus represents the limits of solubility at each composition
what are eutectic alloys
alloys that show complete liquid solubility but limited solid solubility
in a eutectic alloy phase diagram graph, what does the liquidus and solidus line represent
when liquidus and solidus meet = eutectic composition
this is the lowest temp in which any alloy composition of 2 metals can be entirely liquid.
at this point, the alloy solidifies at a constant temp
properties of eutectic alloys
- brittle
- increase strength and hardness
- poor resistance to corrosion
why is CoCr corrosion resistant
Cr on surface forms CrO, an inert layer that is tarnish resistance and has stainless properties
what does adding Be do to CoCr
smoother casting
MOE of CoCr
v high MOE = req lotsa force to deform -> less thickness of material required to achieve the same strength -> denture base can be thinner and lighter
CoCr vs gold shrinkage
CoCr has higher shrinkage
ductility of CoCr
low ductility -> fracture easily
impt things abt CoCr
CoCr cannot use hypochlorite cleansers / chlorine stuff bc it will corrode
oxygenating denture cleansers also tend to stain CoCr
constituents of gold alloys & its purpose
gold - tarnish and corrosion resistant, ductile, yellow
platinum - hardens alloy, white
palladium - tarnish and corrosion resistant, hardener, white
copper - increase strength and hardness, increase ductility (decreases tarnish resistance)
zinc - oxide scavenger, improve castability
indium - oxide scavenger, casting fluidity
what are the 4 types of gold alloys
type 1 - soft
type 2 - medium
type 3 - hard
type 4 - extra hard
summary of type 1-4 gold
type 1 -4
gold content high - low
MOE, strength, hardness, low - high
ductility, melting range high - low
how to compensate for alloy shrinkage
controlled expansion of investment so final cast before cooling will be slightly bigger, after cooling will be just right
tarnish vs corrosion
tarnish = staining corrosion = when metals degrade into oxides/hydroxides through chem rxn
what are wrought metal alloys
wm = cold worked metals that are plasticly deformed to change its structure & mechanical properties
a pure metal/alloy that is permanently deformed = wm
define
a) malleability
b) ductility
c) joinability
a) malleability - ability to form thin sheets by hammering or rolling
b) ductility - ability to be stretched into a wire
c) joinability - ability to be joined via soldering/welding
outcome of work hardening
metal become stronger harder more ductile less ductile, lower corrosion resistance
NiTi composition
54% Ni
44% Ti
definition of twinning
twinning = an atomic arrangement within a crystal where a region of crystal takes on a different crystallographic axis orientation from the rest of the crystal without breaking atomic bonds
define shape memory
shape memory = the ability to return to previously manufactured shape when it is thermally induced
important property of NiTi
v high fatigue strength - can take on low loads but at very high frequency
composition of ss implants
ss (316L)
Fe - 60-65%
Cr - 17-19%
Ni - 12-14%
18/8 SS
18% Cr 8% Ni 0.2% C
composition of Ti
Ti-6Al-4V
6% Al
4% V
although Ti fracture is uncommon, what is the cause
metal fatigue from high cyclic ocl loading
Ti casting
must be done in vacuum / inert atmp bc Ti itself is v reactive
passivation definition
passivation = enhancement of oxide layer to minimise release of metallic ions
- done by immersing in 40% nitric acid / anodisation
INCREASES CORROSION RESISTANCE
self healing properties of YTZP
when crack is present, localised stress at the crack tip causes crack propagation
when stress at crack tip reaches a certain level, metastable tetragonal crystal structure transform into a stable monoclinic form
this stress induced change in structure leads to large increase in volume = induce surface compression strength and close up crack, prevent propagation
phase transformation of YTZP
tetragonal (more stable) -> monoclinic
leads to substantial increase in volume that induces surface compressive strength
fun facts abt zirconia
- strongest dental material
- zirconia is 2x stronger than alumina based ceramics
- 3% of yttria is added to pure zirconia to form YTZP
gypsum bonded investment used for what alloys
only gold
type 1 vs type 2 gold alloy shrinkage compensation during casting?
type 1 - shrinkage compensated by thermal expansion
type 2 - shrinkage compensated by hygroscopic expansion
purpose of silica in gypsum bonded investment
- refractory material
- regulates thermal expansion
cristobalite expands more than quartz but at a lower temp
what is thermal expansion affected by
lower w/p ratio, increase thermal expansion
when does thermal expansion occur
occurs during setting
when does hygroscopic expansion occur
occurs when setting in contact w water
what is hygroscopic setting expansion
it is a continuation of ordinary setting expansion when investment is allowed to set in contact w water -> surface tension of water is no longer a limiting factor
when is hygroscopic setting used
used to provide extra expansion esp for casting alloys of high COTE or higher contraction upon solidifcation
factors influencing hygroscopic expansion
- more silica = more expansion
- more w/p ratio = less expansion, less strength
- more spatulation more expansion
- smaller particle size, more hygroscopic expansion
where should sprue be attached
part where there is greatest bulk and least anatomy
what is purposed of wax reservoir in casting
wax reservoir = small amt of additional wax attached to sprue near the junction of wax pattern to prevent localised shrinkage porosity
reasons for these casting failure:
a) poor margins
b) investment crack
a) poor margins - too little casting pressure
b) investment crack - too much casting pressure
purpose of flux for casting
flux = a material added to molten alloy before alloy is spun into the mould
o Prevents O2 from causing hot metal to oxidise
o Flux will dissolve oxides that may form when metal is heated
o increase fluidity of metal
o decrease porosity
define strain
strain = deformation from tensile force, elongation is in the same axis that force is applied in
define modulus of elasticity
MOE = stress / strain, the amount of stress required for the corresponding strain
high MOE = large amount of force applied but relatively low amount of deformation (material is very rigid)
define plastic deformation
deformation until material cannot go back to its original shape
define toughness
toughness = ttl energy required to fracture a material
aka AUC of a stress/strain
define hardness
hardness = resistance of a material to penetration
define COTE
COTE = change in length per increase in 1 degree
define creep
creep = permanent deformation after being exposed to stress constantly even if stress is within elastic limit
what is the composition of amalgam
dental amalgam = AgHg matrix containing filler particles of AgSn
[matrix] = gamma 1 (AgHg) and gamma 2 (SnHg)
[filler] = gamma (AgSn)
amalgam LCP vs SP
LCP better corrosion resistance
SP harder to pack tightly, tend to overhang
SP smaller SA:Vol ratio, req less Hg, better properties and surface finish
functions of diff components of amalgam
Ag - increase strength, expansion, reactivity, decrease flow, creep
Sn - decrease strength, hardness, expansion, increase flow, setting time, corrosion, contraction
Cu - increase strength hardness, expansion, flow, decrease creep, corrosion
Zn - oxide scavenger, cause delayed expansion, better clinical performance
3 principles of adhesion
- adherents need to be close together
- surface of adherent needs to be clean
- adhesive must have good wetting with adherent, contact angle less than 90degree
what does etchant do to enamel
etchant = 37% phosphoric acid
- removes smear layer
- produce pores on surface for mechanical interlocking of adhesives, improved retention of resto
- increased SA and roughness of enamel for resto to bond to
what does conditioner do to dentin
!cannot etch dentin!
conditioner = 10-25% polyacrylic acid
- alter surface characteristic of dentin
- removes smear layer wo demineralising tooth surface
- produce pores in dentin surface
- opens dentinal tubules and collagen network for primer and adhesive to flow in
function of primer
primer = coupling agent for dentin and resin
primer consist of bifunctional monomer dissolved in suitable solvent to bond hydrophobic resin to hydrophillic dentin
polar component of primer binds to col fibres + apolar component of adhesive bind to resto = binding a hydrophobic surface to a hydrophillic surface
consequence of insufficient polymerisation
#1 poor colour stability inferior physical and mechanical properties higher solubility
why cr discolour over time
water hydrolyses silane filler bonds, forces polymer chains apart, weaken CR structure, allows contaminants to enter and cause discolouration
how to activate polymerisation in CR
- chemical (free radicals)
- UV light / visible light (diketone)
- blue light activated (cpq)
purpose of fillers in CR
- improve physical and mechanical properties
- decrease polymerisation shrinkage
- more viscous, less liquidy
COTE of CR
relatively close to enamel and dentin (compared to amalgam)
COTE of amalgam
much higher than CR/enamel/dentin
how does CR bond to enamel
Relies on micromechanical interlocking with unfilled resin and the acid-etched enamel
how does CR shrink upon polymerisation
CR shrinks away from less retentive surface (NOT towards light source)
*place CR in increments reduce shrinkage stress but not volumetric shrinkage constant
light curing for CR
cure in increments of <2mm depth to decrease shrinkage stress
- light tip must place <6mm from resin
- LED blue light only works for CPQ initiated polymerisation
what rxn is GIC
acid base rxn btwn polyalkenoic acid and calcium fluoro alumino silicate glass base
function of tartaric acid in GIC
increase working time decrease setting time
how does GIC bond to tooth
chemical adhesion
- ion exchange w HAP to form intermediate layer of polyacrylate, phosphate, ca2+ or bond directly to ca2+
- hydrogen bonding w dentin col network, ionic bond to HAP
bc enamel has more HAP, bonding to enamel > dentin
factors that allow GIC to set faster
- higher temp
- tartaric acid
- smaller particle size (larger SA)
- more alumina / silica ratio
- higher powder : liquid ratio
advantages of GIC
- v close COTE to enamel
- cariostatic fx that is rechargeable
- weak acid + big chonky molecule = gentle to pulp
- self adhesive = conservative prep
- lower acid erosion values than other cement
GIC & CR
- no need to etch GIC before putting on CR on top of GIC
- GIC only mechanically bond to CR
what is RMGIC
water in GIC replaced to water-HEMA, PAA replaced to PAA w side chains that polymerises upon light curing
amt of resin = 4-6%
RMGIC & CR
chemical bond bc of HEMA
synersis
loss of water hence causing shrinkage
imbition
gain of water hence causing expansion
how to minimise deformation when using alginates
snap removal
pour within 15mins
keep in humid environment
polysulfide polymerisation reaction
condensation polymerisation
by pdt = water
polyethers polymerisation reaction
additional polymerisation
compare setting time of elastomers
PS
CPS
APS
PE (shortest)
compare hardness of elastomers
PE (hard af)
APS
CPS
PS (softest)
compare dimensional stability (opposite of shrinkage) of elastomers
APS
PE
PS
CPS
compare deformation (opposite of elastic recovery) of elastomers
PS
PE
CPS
APS
compare tear resistance of elastomers
PS (v tear resistant)
APS, CPS
PE
disadvantage of PE
taste like shit
imbition weakness
disadvantage of APS
latex cannot touch
expensive
hydrophobic
properties of compound
- thermoplastic
- soften when heated, hard and brittle when cooled
- poor conductors of heat (inside often not softened enough)
- no undercuts
- not dimensionally stable, deforms upon storage, pour impression within 1h
plaster vs stone
plaster: large irregular porous particles that dont pack well -> need lots water
stone: smaller regular no porous particles that pack well -> req less water
factors that decreases setting time of stone
- smaller particle size
- more impurities
- less water
- more mixing
- accelerators / retarders
factors to increase strength of stone
- less water
- less porosity
- less spatulation time
- less acc/ret
stages of heat cure acrylic resin
[74deg] sandy stage stringy stage doughy stage rubber stage - can't be moulded, cannot make any more adjustments stiff stage
how is cold cure acrylic resin activated
tertiary amine
cement reaction
mixing basic ceramic powder w acidic liquid
ZnOE advantages and disadvantages
+ bacteriostatic + soothing fx
- high solutbility, doesnt adhere to dentin and enamel
ZnPO
- mechanical interlocking
- brittle af
working time = 3-6mins
setting time = 5-12 mins - not anticariogenic
- v acid
Zn polycarboxylate
- chemical bonding
- gd mechanical properties
- not as brittle as ZnPO
- antibacterial
consequence of over trituration of amalgam
increase Hg consumption
net contraction
consequence of overdrying etched dentine
col fibres may collapse
can ZnOE be used for undercut
NO!!!!!!
in visible lc resto material, what light absorber is used
CPQ
in elastomers, what conveys stiffness
a) chain lengthening
b) cross linking
chain lengthening
in elastomers, what conveys elasticity
a) chain lengthening
b) cross linking
cross linking
martensite -> austenite transformation by what means
heating
austenite -> martensite transformation by what means
stress / forces
how does sodium hypochlorite work as an endo chemical disinfectant
- saponification to form organic products
- interfere cell metab and inhib bacterial enzymes
- inactivate enzymatic sites by hydroxyl ions and chloramination rxn
mechanism of CaOH as a medicament
high pH level help to hydrolyse proteins, fats, bacterial enzymes
dental wax melting temp exists as a _______
range (not as a point)
properties of wax
low MOA
low proportional limit (deforms easily)
brittle and fragile at low temp
dental wax have better flow when
- higher temp
- more force
- long time that force is applied
dental wax ductility increases w _________
temp
how to clean dentures
use chemical cleansers, brush gently w soap and water
don’t leave denture out to dry
which type denture cleanser least discolouration
acid type denture cleanser
which type denture cleanser most discolouration
alkaline peroxide
amalgam reaction
AgSn + Hg -> AgSn (gamma) + AgHg (gamma 1) + SnHg (gamma2)
high Cu amalgam
AgCu + SnHg (gamma 2) -> AgHg (gamma 1) + CuSn
less gamma 2, more creep and corrosion resistant, stronger, harder
undertrituration
grainy crumply amalgam, too much Hg
spherical amalgam
less Hg
less expansion
tendency for overhang but better properties
consequences of poor amalgam condensation
- porosity, inferior properties
- poor marginal seal
- poor mechanical retention
- excess gamma 2, excess Hg not removed
- harder to carve and polish
what is smear layer
inorganic + organic components of tooth structure, pieces of burr, proteins, pulp tissue, saliva, blood, bacteria
amorphous irregular granular appearance that interferes w adhesion
purpose of adhesive
adhesive contains hydrophillic resins that polym w primer to form mechanical interlocking w dentin + form chemical bond w CR
what is hybrid layer
hybrid layer = interpenetrating layer of dentin and resin, consists of dentin primer and adhesive
can bis GMA be activated by blue light
Not activated by blue light
bisgma uses TPO (a diff light absorber)
if blue light = cpq as light absorber, bis gma = TPO, which will not be blue light!
what are coupling agents
coupling agents = intermediary substance that help to bond one material to another. in the absence of a coupling agent these 2 materials would not be able to bond to each other effectively
e.g. bonding hydrophillic to hydrophobic
what is the purpose of silanes in CR
silanes = coupling agent = a molecule w central Si and 4 bonds
- bonds resin matrix and fillers tgt
- efficient stress tf from resin to glass, better properties
- promotes wettability and dispersion
- decrease viscosity
light cure depth problem
light activated CR conversion from paste to solid relies on ability of light to access and initiate curing in all parts of resto
when light hits composite, photons are reflected, scattered, absorbed. this limits the penetration that light can achieve and limits the depth of material that can be cured
inverse square law: light cure efficiency drops significantly further away from tip
cr failure
- contamination of water, bacteria, plaque
- discolouration
- fracture from wear and heavy ocl contact
- poor marginal seal
- weak foundation
- residual monomer causing post op sensi
GIC setting rxn
- dissolution: acid attack glass, release Ca Al F and leave behind silica gel. H+ diffuse into glass and replace lost ions
- gelation: cross linking of carboxyl and Ca/Al forms polysalt and physical properties
- hardening: glass particles sheathed by siliceous hydrogel bonded by a matrix of hydrated fluoridate Ca Al polysalt
GIC components
PAA + Ca F Al silicate glass base
most impt factor for gd detail capturing for impressions
low viscosity
dihydrate vs hemihydrate
dihydrate: CaSO4.2H2O
hemihydrate: (CaSO4)2.H2O
plaster vs stone:
plaster greater degree of hemi -> di
benefit of cold cure acrylic resin
less shrinkage
ZnOE
- rigid once set, no undercuts
- sets in 4-10 mins to form Zn eugenolate crystals
- bacteriostatic
- gd surface reproduction, low viscosity
- high solubility
- gd for temp resto
- acid base rxn
why ZnPO need to mix in increments
exothermic rxn, heat increases setting rate
mix in increments to delay rxn, prolong working time
spread over large area to dissipate heat decrease setting rate
able to mix in more powder, high powder/liquid ration stronger material
can mix into ideal consistency
define eutectic composition
eutectic composition = the lowest point in which any alloy composition of 2 metals can be entirely liquid. at this point allow solidifies at a constant temp
CoCr composition
varies but majority Co, followed by Cr
carbon content <0.4%
CoCr vs gold in terms of shrinkage
CoCr has higher shrinkage lvl
reasons for back pressure porosity
- low w/p ratio, insufficient porosity
- short sprue
- insufficient casting pressure
heat treatment for softening alloys
heat alloy to 700deg for 10mins then quench in water
heat treatment for hardening alloys
heat alloy for 424deg for 2 mins, 250deg for 30 mins, quench
composition of 18/8 ss
Fe 60-65%
Cr 17-19%
Ni 12-14%
which has greatest amt of expansion: quartz/cristobalite/tridymite
cristobalite
purpose of boric acid/NaCl in gypsum investment
regulate setting expansion and setting time, prevents most of gypsum shrinkage
phosphate bonded investment for what
for metal ceramic alloys
austenite -> twinned martensite
cooling
gutta percha is susceptible to degradation, how should it be stored?
5 to -10deg
e.g. of ceramic fillers
leucite
lithium disilicate
