Dental Material Science Flashcards
What is porcelain prone to?
Mechanical failure due to microcracks formed at fitting surface
What is elastic modulus also called?
Young’s modulus
what is elastic modulus a measure of?
rigidity
what ratio is a measure of elastic modulus?
stress/strain ratio
stress required is to cause a change in what?
shape
what is brittleness/ductility?
dimensional change experienced before fracture
what is hardness?
resistance of surface to indentation or abrasion
What are the porcelain characteristics?
Rigid, Hard, Strong, tendency to form surface defects, brittle
what does tendency to form surface defects mean?
Leads to fracture at low stress
For porcelain-metal restorations bonding of what to porcelain? and what does it help eliminate?
bonding of metal oxide to porcelain helps eliminate defects/cracks on porcelain surface
what is function of alloy in a porcelain metal restoration?
Alloy acts as support and limits the strain that porcelain experiences
when forming a porcelain fused alloy restoration what does it have to be fired in?
furnace
after raising the temp of a porcelain fused alloy restoration what do you have to do?
cooling them without developing any thermal stresses that could cause either material or the metal oxide layer to develop defects or micro-cracks
what must porcelain and alloy have similar of?
thermal expansion coefficents
what is indium and tin role in high gold alloy’s
they enable a metal oxide layer to firm which enables bonding to porcelain
what has better mechanical properties high or low gold?
low gold
what are properties of silver palladium?
high mp, care needed in casting
what are properties of nickel chromium?
high melting point, high young modulus, high casting shrinkage, low-ish bond strength
what are advantage of cobalt chromium in porcelain fused metal restorations
high elastic modulus, hard, high tensile strength
what is the only alloy satisfactory in each criteria for porcelain fused to metal alloys?
low gold
what is the big disadvantage of high gold?
tendency to undergo creep
which is the only alloy to have a bicompatibility concern for porcelain fused to metal alloys?
nickel chromium due to allergic responses attributed to nickel
what are the required properties of a porcelain fused to metal alloy?
form good bond to porcelain, similar thermal expansion coefficient, avoid discolouration of porcelain, adequate material props (bond strength, hardness, elastic modulus), good melting recrystallisation temp of alloy
what is form good bond to porcelain also known as?
good wetting
why is similar thermal expansion coefficient imporantant?
To avoid setting up stresses during fusion of porcelain on to alloy
what is not used in high gold alloy as it can cause discolouration of porcelain?
copper
why is high elastic modulus important
to support porcelain and prevent fracture
what is the definition of creep?
gradual increase in strain experience under prolonged application of stress
what is the stressed skin effecgt?
slight differences in thermal contraction coefficients lead to compressive forces which aid bonding
what is the chemical mechanism in procelain metal bond?
The CHEMICAL mechanism is explained by oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself
what is the chemical mechanism also described as?
electron sharing mechanism
when does the stressed skin effect occur
during production process, after furnace stage when alloy contracts slightly more on cooling and generates compressive forces on porcelain
when does the chemical mechanism occur
during firing stage
what are the modes of failure in a porcelain fused restoration?
oxide layer itself fracturing, oxide layer delaminating from alloy, porcelain detaching from oxide layer
what are wrought alloys manipulated by?
cold working
what are uses of wrought alloys?
wires for ortho, partial denture clasps
composition for steel?
iron > 98 percent, carbon <2 percent
what are other constituents of steel?
chromium and maganese
what is the purpose of chromium?
improve tarnish resistance
what are the uses of steel?
cutting instruments and forceps
what is allotropic?
undergoes 2 solid state phase changes with temp
where does austenite exist?
exists at high temp
where does ferrite exist?
at low temp
where does cementite exist?
at low temp
what is pearlite?
eutectoid mixture of ferrite and cementite
what is an alloy?
2 metal that form a common lattice structure - are soluble in one another and form a solid solution
what are substitional solid solution
Random and ordered
what does quenching of austenite lead to?
martensite not supersaturated austenite solution
why does martensite have a distorted lattice structure?
carbon being unable to diffuse normally within the array of iron atoms in each grain
slow cooling austenite leads to what?
pearlite
fast cooling of austenite leads to?
martensite
how do you get the conversion of martensite to pearlite
tempering
what is tempering?
heating followed by quenching
what does temp and during affect conversion to?
ferrite (soft, ductile) and cementite (hard, brittle)
what must stainless steel contain if it is to be considered stainless?
if contain >12 percent chromium
what are some properties of chromium?
lowers austentie to martensite temp and rate, decreases % of carbon at which eutectoid formed, corrosion resistant due to chromium oxide layer
what can attack chromium?
chlorides
what are some properties of nickel?
lowers austenite to martensite transition temp, improves fracture strength, improves corrosion resistance
what are the 2 types of stainless steel?
Martensitic and Austenitic
what are properties of martensitic steel?
contains 12-13 percent chromium and little carbon, can be tempered to produce hard materials
how do you get austenite instead of martensite
by having the right proportions of chromium and nickel specifically either 18:8 or 12:12 ratio
uses of austenitic steel?
dental equipment, corrosion resistant, wires, sheet forms for denture bases (swaged)
what are properties of 18-8 stainless steel?
does not heat harden, soft when cast but work hardens rapidly
what is cold working?
It’s work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature
what do cold working processes cause?
SLIP
what is slip?
dislocations at grain boundaries
what does cold working produce?
stronger harder material
what does the grade of stainless steel wire depend on?
degree of bending required
what are degrees of bending required?
soft - half hard, hard - spring temper
what is definition of springiness?
ability of a material to undergo large deflections (so as to form an arc) without permanent deformation – that is, the material will subsequently return to its original shape.
what are the requirements of wires?
high springiness, stiffness, high ductitlity, easily joined without impairing properties, corrosion resistant
what is ductility?
bending without fracture
what do you use to solder s/steel?
gold or silver
how do you stop s/steel grains from recrystallising?
quenching the alloy
when does weld decay occur?
between 500-900 celsius
what happens when weld decay occurs?
alloy becomes brittle, less chromium in central region of solid solution so more susceptible to corrosion
how to minimise weld decay?
low carbon steels - expensive, stabilised s/steel
what does stabilised s/steel contain?
titanium or niobium
what is stress relief annealing of s/steel which is needed?
process requires the temperature of s/steel needs to be held at around 450C for a minute or two
what are advantages of stainless steel denture base?
Thin 0.11m – acrylic 1.52mm
Light
Fracture resistant
Corrosion resistant
High polish obtainable
High thermal conductivity
High impact strength
High abrasion resistance
what are disadvantages of s/steel denture base?
Possible dimensional inaccuracy (contraction of die not marches by model expansion)
Elastic recovery of steel – inaccuracy
Damage of due under hydraulic pressure
Loss of dine detail during the many stages
Difficult to ensure uniform thickness
Uneven pressure on die and counter die wrinkling of steel
what are some ideal properties of denture base materials?
Replaces function of natural teeth
Dimensionally accurate and stable in use
High softening temp
Unaffected by oral fluids
Thermal expansion
Low density
High thermal conductivity
Radiopaque
Non toxic, non irritant
Colour/ translucency
Easy and inexpensive to manufacture
Easy to repair
what are the mechanical properties of a denture base material?
High young’s (elastic modulus)
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistance
what is transverse strength?
How well does upper denture cope with stresses that cause deflection?
what is free radical addition polymerisation?
Chemical union of 2 molecules either the same or diff to form. Larger molecule without elimination of smaller molecule. involves molecules with C=C bonds
what are components of acrylic polymerisation?
Activation – of initiator to provide free radicals
Initiation – free radicals break C=C bond in monomer and transfer free radical
Propagation – growing polymer chain
Termination – of polymerisation
what do you need for acrylic heat curing?
Need efficient polymerisation to give high molecular weight polymer. so high temp but gaseous porosity limits
what are the properties of acrylic?
non toxic, non-irritant if no monomer released, unaffected by oral fluid, poor mechanical properties but increase in bulk to compensate, fatigue strength - fairly resistant, high abrasion resistance, low thermal conductivity, low density, high softening temp, ok - linear contraction
why use self curing acrylic?
lower temp - less thermal contraction hence better dimensional accuracy
what is there more of in self curing acrylic that has a worse effect?
unreacted monomer, softening denture base, reducing transverse strength, potential tissue irritant which comprises its biocompatibility
what is dimensional accuracy of self curing acrylic?
fits original cast better than heat cured but water absorption so expands hence self cured over size (heat cured under size - better tolerated)
what is stronger heat or self cure acyrlic
heat - stronger, will last longer
why is self cure more risk of being an irritant?
more monomer uncured
what are alternative polymers?
nylons and vinyl polymers and polycarbonates
what happens in nylon dentures?
water absorption leads to swelling and softening
what are advantages of polycarbonates over acrylic?
able to withstand large temps more than acrylic
what are weakness of polycarbonates over acrylic?
they develop internal stresses during use that causes distortion and so a poor fit
what is needed to produce, inlays, onlays, crown and bridges etc
casting process
what does casting process require?
an investment material of the required shape, to contain the molten alloy and withstand the high temps involved and ensure the alloy’s dimensions are sustained
outline stages involved with making inlays, outlays crowns etc
1) A wax pattern of the required prosthesis – crown, inlay – is created (ie a positive replica)
2) An InvMater is placed around this wax pattern and allowed to set. It forms a mould (a negative replica)
3) The wax is then removed – by burning or with boiling water. So now we have a cavity of the required shape surrounded by the InvMater
4) Next, the molten alloy is poured into the mould cavity – and this is done via the sprue – hollow tubes that allow the alloy to flow in.
known as lost wax technique
what are types of investment materials?
dental stone or plaster (acrylic dentures), gypsum bonded materials (gold casting alloys), phosphate bonded materials (base metals, cast ceramics), silica bonded materials (base metal alloys)
what are the requirements for an investment material?
expand, porous, strong, smooth surface, chemically stable, easy removal from cast, easy handling, inexpensive
why does an Investment material need to be able to expand?
to compensate for cooling shrinkage
why must an investment material be porous?
to allow escape of trapped gases on casting - back pressure effect
what are the 2 componenets of invmater and what are their purposes?
Binder - to form a coherent mass
refractory - usually type of silica, withstands high temps and undergoes expansion
what is meaning of hygroscopic?
absorb moisture from air
what factors increase hygroscopic expansion?
lower powder / water ratio
increased silica content
higher water temperature
longer immersion time
what happens when gypsum contraction above 320C
water loss and significant reduction by sodium chloride and boric acid
what are gypsum bonded properties?
smooth surface, easy manipulation and setting time controlled, good porous ability, adequate strength
what is needed to ensure adequate strength of gypsum bonded invmater
correct powder/liquid ratio and correct manipulation
what is heat soaking?
held at a high temperature for some time – and this enables the gases to gradually escape.
what is the composition of phosphate bonded investment?
powder - silica, mag oxide, ammonium phosphate
liquid - water or colloidal silica (increases strength)
what are properties of phosphate bonded investment?
High “green” strength:
Easy to use
High Strength
Porous
Chemically Stable
what are stages of silica investment?
Stage 1: prepare stock solution
Stage 2: add powder (quartz or cristobalite) – gelation
Stage 3: drying - tightly packed
silica particles
what are properties of silica investment?
Strength – sufficient
NOT porous - needs vents
complicated manipulation
what are 2 types of material for elastomers?
polyether and addition silicones
how are elastomers formed?
formed by polymerisation with cross-linking of polymer chains
what could byproducts of polymerisation affect?
dimensional stability and cast compatibility
state 3 types of elastomers?
polysulphides, silicones (addition curing and condensation curing) and polyethers
what are the ideal properties of impression materials?
quality of surface interaction between material and tooth/soft tissue surfaces, accuracy, dealing with removal and undercuts, dimensional stability
what are the components of quality of surface interaction between material and tooth/soft tissue surface?
viscosity - a measure of material’s ability to flow and determines a material potential for making close contact with hard/soft tissue surfaces so how well it records surface detail
surface wetting- must make intimate contact with teeth/mucosa
contact angle - determines how well material envelopes the hard/soft tissue surface to record fine detail
what is components for accuracy?
surface reproduction (ISO) and visco-elasticity/elastic recovery
what is components of dealing with removal and undercuts?
- flow under pressure (‘shark fin’ test)
- tear/tensile strength (removal)
- rigidity (removal)
what are the components of dimensional stability?
- setting shrinkage - should be low
- thermal expansion/contraction (ppm/C- should be low)
- storage - some materials absorb/release moisture causing a change in its dimensions
what are the components of dimensional stability?
- setting shrinkage - should be low
- thermal expansion/contraction (ppm/C- should be low)
- storage - some materials absorb/release moisture causing a change in its dimensions
what does contact angle indicate?
how readily the investment materials wets the tooth surface
high contact angle - more gaps
low contact angle - less gaps
what do hydrophilic silicones incorporate?
non ionic surfactant which wets tooth surface and more easily wetted by water containing die materials
what happens if impression is removed with sharp pull?
less overall permanent strain
when does viscoelastic behaviour occur?
when after being stretched material fails to return to its original dimensions so a permanent strain?
what is a shark fin test?
To record an undercut, the IM must first reach the extremities of what is a narrow zone, with a complex shape. Between the gingiva and the tooth surface
what is tear strength?
stress material will withstand before fracturing
should an IM have a low or high rigidity value and why?
low so it can be easily removed
what is tear resistance?
ability to withstand large stresses
properties of PMMA (jet) has a temporary material?
- Powder/liquid formulation
- self-curing
- Good marginal fit
- Good transverse strength
- Polishable
disadvantage of jet as a temporary material?
poor abrasion resistance
high shrinkage
high thermal release
free monomer may be toxic
types of temporary materials?
- PMMA eg Jet
- PEMA eg Trim II, Snap
- Bis-acryl(ate) composite e g Protemp4, Quicktemp
- Urethane dimethacrylate eg Provipont DC
key properties to assess of a temp material you are going to use?
- temperature and it’s ability to harm pulp
- colour stability for aesthetics
which temp material has best colour stability?
protemp
what is polymerisation shrinkage important in assessing for temp materials?
important for assessing a temporary material’s accuracy of fit
low value gives good clinical fit
what are other factors that are important in choosing which temp materials to use?
ease of use
working and setting time
key properties required for temp materials?
- temperature reached during setting (thermal release);
- compressive strength
- colour stability
- abrasion resistance
- polymerisation shrinkage
other term for luting agent?
dental cement
what are the properties of a luting agent? and what should they ideally be?
- Viscosity and film thickness - low
- ease of use - long working time
- radiopaque - easier to see marginal breakdown
- marginal seal - agent should bond chemically to tooth
- aesthetics - non staining & tooth coloured
- solubility - low
- cariostatic - fluroide releasing & antibacterial
- biocompatible - not toxic & not damaging pulp & low thermal conductivity
what is the purpose of a cariostatic luting agent?
important in preventing secondary caries around crown margins
what are the ideal mechanical properties of a luting agent?
- high compressive strength
- high tensile strength
- high hardness value
- young’s modulus similar to tooth
*no luting agent gets close to tooth values for more than one or 2 physical props
types of luting agent materials?
- dental cement
- glass ionomer cement
- composite resin luting agents
- self adhesive comp resin
types of dental cement?
- zinc phosphate
- zinc polycarboxylate
types of glass ionomer cement?
- conventional
- resin modified
what kind of reaction is zinc phosphate?
acid base
main reactive ingredient in powder of zinc phosphate?
zinc oxide
what is purpose of magnesium oxide in zinc phosphate?
- gives white colour
- increases compressive strength
what is purpose of aluminium oxide in liquid of zinc phosphate?
prevents crystalisation
what is the purpose of zinc oxide in the liquid of zinc phosphate?
slows the reaction giving better working time
what does the matrix of zinc phosphate contain?
free water so is almost insoluble
for zinc phosphate what does the cement maturing and binding this water lead to?
stronger less porous material
what are some problems of zinc phsopate?
- low initial pH - 2 so could cause pulpal irritation
- exothermic setting reaction
- not adhesive to tooth or restoration
not cariostatic - final set takes 24hrs
- brittle
- opaque
how long does it take for pH in zinc phosphate to return to normal?
24 hrs
what are some properties of zinc polycarboxylate?
- bonds to tooth surfaces
- less heat of reaction
- pH returns to neutral more quickly
- difficult to mix
- difficult to manipulate
- soluble in oral environment at lower pH
- opqaue
- lower modulus and compressive strength than zinc phosphate
how does conventional glass ionomer cement bond to tooth surface?
- ion exchange with calcium in enamel and dentine
- hydrogen bonding with collage in dentine
what does surface filling sandblaster in conventional glass ionomer cement lead to?
mechanical adhesion
good properties of conventional glass ionomer cement?
- easy to use
- durable
o Low shrinkage
o Long term stability.
o Relatively insoluble once fully set.
o Aesthetically better than ZnPhos.
o Self adhesive to tooth substance.
o Fluoride release.
o Cheap.
for resin modified glass ionomer cement it contain conventional GIC powder and liquid but what does the liquid also contain?
hydrophilic monomer
how does rapid initial set occur in resin modified glass ionomer cement?
Light activation causes polymerisation of the HEMA and any copolymers in the material
incorporation of resin in glass ionomer cement improves which material properties?
o Shorter setting time
o Longer working time
o Higher compressive and tensile strengths
o Higher bond strength to tooth
o Decreased solubility
what are some potential problems of resin modified glass ionomer cement?
- HEMA is cytotoxic - very important no monomer remains as it can damage pulp
- HEMA swells it expands in a wet environment
- can’t bond to indirect restoration
what can resin modified glass ionomer cement not be used for?
- conventional porcelain crowns
- can’t cement posts as it may split root
what are pros of composite resin luting agents?
- better physical properties
- lower solubility
- better aesthetics
what is biggest con for composite resin luting agents?
technique sensitive
what is required for bonding to porcelain?
surface wetting agent
what is applied to etched porcelain surface?
silane coupling agent
what much you use if porcelain restoration is thin?
light cured comp luting agent and increase curing time
what doesn’t bonding directly to metal?
porcelain comp
what needs to be done to the metal surface? and how is it done?
- roughened
- done by etching or sandblasting
what does sandblasting not give you for bonding to metal?
does not give the undercut surface
when bonding to precious metal what must you change ? and how is this done?
change precious alloy composition to allow oxide formation
- done by increasing copper content and heat 400C
what are some properties of self-adhesive comp resin?
- anaerobic self cured material
- good film thickness
- opqaue
- moisture sensitive
- expensive
what are mechanical properties of self etching comp resin?
- compressive strength
- tensile strength
- hardness
- wear resistant
how does self etching comp resin cements bond to enamel, dentine, ceramics and metal
enamel - lower than to dentine
dentine - better than to enamel
ceramics - brand specific
metal - better than to non precious
what must base of a temp cement contain?
zinc oxide
what are the 2 main types of temp cement?
with eugenol and without eugenol
when not to use with eugenol type of temp cement?
Not be used to cement provisional restoration where permanent will be cemented with a resin
what types of dental ceramics are there?
- feldspar
- borax
- silica
- metallic oxides
what are the 2 types of feldspar?
- potash feldspar
- soda feldspar
metallic oxides convey colour to the ceramix so what are they?
chromium - green
cobalt - blue
copper - green
iron - brown
manganese - lavender
nickel - brown
what are conventional dental ceramics supplied as?
powder
how are conventional dental ceramics made?
- heating constiuents to over 100C
- cool rapidly
- mill the fritt to powder
- add binder (starch)
- powder mixed with water to go in restoration
cooling rapidly of a conventional dental ceramic is also known as?
fritting
when you heat a feldspathic to 1150-1500C you get what?
leucite
what is leucite of conventional dental ceramic?
potassium aluminium silicate?
what is the process of a fabrication of crown?
- powder mixed with water
- applied to die with brush
- crown built using diff procelains for dentine and enamel
- crown heated in furnace to coalesce the powder into ceramic
what does heating lead to in fabrication of crown in conventional dental ceramics?
sintering
what is sintering?
when ceramic particles begin to fuse to single mass
what are properties of conventional dental ceramics?
- aesthetics - best of any rest material
- chemical stability - very stable - unaffected by wide range of pH
- biocompatibility - good
- thermal properties - similar to tooth - thermal diffusivity is low
- dimensional stability - very stable when fully fired
in terms of aesthetics for conventional dental ceramics what are the optical properties?
o Reflectance
o Translucency
o Opacity
o Transparency
o Opalescence
what are the mechanical properties of conventional dental ceramics?
- High compressive strength
- High hardness - abrasion with opposing teeth if not glazed
- Tensile strength – very low
- Flexural strength – very low
- Fracture toughness – very low
o All lead to failure during loading - Static fatigue
o Time dependant decrease in strength even in absence of any applied load. (due to hydrolysis in material in water environment) - Surface micro-cracks
o Occur during manufacture finishing or due to occlusal wear. Areas where fractures can initiate - Slow crack growth
o Cyclic fatigue under occlusal forces in a wet environment over time
what do the mechanical properties of conventional feldspathic ceramics lead you to believe?
can only be used in low stress areas like anterior crown and not in all patients and too brittle for use elsewhere
how to overcome problems of conventional ceramics?
cover in conventional porcelain
what are the cores of strong coping?
- aluminia core
- zirconia core
what are properties of aluminia core?
- reinforce feldspathic core ceramics
- core material in PJC
- alumina particle are crack stoppers
- not strong enough for posterior use
what does increased aliumina content in alumina core lead to?
increased alumina content increases strength and leads to new techniques INCERAM AND PROCERA
what are problems with alumina core?
- Lack flexural strength
- Only single crowns
- More successful anteriorly
what kind of zirconia core is used in dentistry?
yttria stabilised zirconia
what does more yttria in zirconia lead to?
- more translucency
- reduced physical properties
properties of yttria stabilised zirconia
- hard
- strong
- tough
how is the fabrication of zirconia core done?
-* Impression is taken of the prep and sent to the lab
* A model is cast and then scanned digitally
* Raw Zirconia block is selected for milling
* A presintered block is much easier to mill
* Milling for a three unit bridge will take around an hour
* The cut framework is then heat treated at around 850oC to achieve its final physical properties
* This causes a 20% shrinkage but the computer softwear deals with this during the milling process.
* The framework is also stained to an appropriate colour
o Some modern zirconias are available in different shades
* The Zirconia core is then veneered with feldspathic porcelain to produce the final restoration
what are problems with yttria zirconia?
expensive
potentil for veneering porcelain to debond from core
opaque
what are the milled core crowned and bridges?
Zirconia
Lithium Disilicate (E-Max)
Precious metal
Non-precious metal
Titanium
Composite
how is the fabrication of milled crown done?
Cast goes in scanner
Same method irrespective of material being used
* Zirconia
* LiDiSi
* Metal
Ceramic filled composite resin
Scanned image of cast
Lower cast scanned and articulated
Select crown margin
Adjust crown margin
Select crown type and place on ‘model’
Adjust shape and size of selected crown
what is the process of cast and pressed ceramics called?
ceraming
how is ceraming done?
Diff technique more like casting a metal restoration
Restoration is waxed up
Invested
Cast from heat ingot of ceramic (1100 Celsius)
No sintering occurs - ceramic ingot already condensed prior firing
Once devested and clean
* Rest is heated to improve crystal structure so doesn’t crack
Process is called CERAMING
what is usually done with cast crowns?
- veneered with appropriate feldspathic porcelains
what are the ceramics used in the cast and pressed ceramics process?
- Lithium Disilicate Glass
- Leucite Reinforced Glass
what are the advantages of diff crown types?
- Monolithic block crowns, milled from a single block of material are strongest
- zirconia stronger than LiDiSi
- LiDiSi better translucency so better aesthetics
what is stronger sintered or milled?
milled
how to decide what to use when it comes to crowns?
posterior teeth
- monolithic zirconia - used for single crowns
anterior teeth
- LiDiSi - used as far back as first premolar
anterior birdgework
- LiDiSi
longer span or heavier occlusion
- zirconia cored
what are zirconia and ZiDiSi cemented with?
conventional or resin cements
what are silica containing ceramic etched with?
hydrofluoric acid to produce a retentive surface (LiDiSi)
what do you do to zirconia crowns to create a retentive surface?
Zirconia cored crowns do not contain silica and are not affected by acid but can be air abraded to create retentive surface
what are metal files used for?
to remove hard and soft tissues
what do endo instruments create?
space for disinfectants/medicaments and creates appropriate shape for obturation
how is stress measured?
deforming force over given area
what do abrupt changes in geometric shape of a file lead to?
higher stress at that point
what is strain?
amount of deformation a file undergoes
what is elastic limit?
A set value representing the maximal strain that when applied to a file, allows the file to return to original dimensions
what is elastic deformation?
Reversible deformation that does not exceed elastic limit
what is shape memory?
Shape memory alloys are materials that can be deformed at one temperature but when heated or cooled, return to their original shape.
what is plastic deformation?
Permanent bond displacement occurring when elastic limit exceeded
what is plastic limit?
the point at which a plastic deformed file breaks
what are different types of engine driven instruments?
Engine-driven nickel-titanium rotary instruments
Engine-driven instruments that adapt to canal shape
Engine-driven reciprocating instruments
what does chromium in stainless steel prevent?
rusting
how is metal of endo material strengthened?
plastic deformation
what are the components of a endo roary instrument?
taper, flute, cutting edge, land, relief, helix angle
what does the flute of rotary instrument do?
groove to collect dentine and soft tissue
what does cutting edge of rotary instrument do?
forms and deflects dentine chips
what are features of endo instrument?
- positive rake angle
- wide radial land
- third radial land
- radial land relief
what does positive rake angle provide?
provides the active cutting action of the K3
what does the wide radial land provide?
provides blade support while adding peripheral strength to resist torsional and rotary stresses
what does third radial land stabilise?
stabilises and keeps the instrument centred in the canal and minimises over engagement
what does radial land relief reduce?
reduces friction on canal wall
what are the central role of irrigants in endo treatment?
o Facilitate removal of debris
o Lubrication
o Dissolution of organic and inorganic matter
o Penetration to canal periphery
o Kill bacteria/yeasts/viruses
o Biofilm disruption
o Biological compatibility
o Does not weaken tooth structure
what does sodium hypochlorite establish and equilibrium with?
hypochlorous acid
what is hypochlorus acid responsible for?
antibacterial acitivity
what are factors important for sodium hypochlorite (NaOCl) function?
Concentration
Volume
Contact
Mechanical agitation
Exchange
what are some diff types of irrigant?
NaOCl, EDTA, chlorohexidine digluconate, sterile saline
what are irrigant interactions?
cytoxic and carcinogenic
what are properties of an ideal obturation material?
- Easily manipulated with ample working time
- Dimensionally by tissue fluids stable
- Seals the canal laterally and apically
- Non-irritatant
- Impervious to moisture
- Unaffected by tissue fluids
- Inhibits bacterial growth
- Radiopaque
- Does not discolour tooth
- Sterile
- Easily removed if necessary
what are the 2 forms gutta percha exist as?
alpha and beta
what is alpha GP heated above?
65 degrees celsius into amorphous phase, cooled slowly returns to alpha, cooled rapidly recrystallises as beta
what does sealer seal between in endo?
- Seals space between dentinal wall and core
what are properties of an ideal sealer?
- Exhibits tackiness to provide good adhesion
- Establishes a hermetic seal
- Radiopacity
- Easily mixed
- No shrinkage on setting
- Non-staining
- Bacteriostatic or does not encourage growth
- Slow set
- Insoluble in tissue fluids
- Tissue tolerant
- Soluble on retreatment
what is zinc oxide in endo effective as?
- Zinc Oxide effective antimicrobial and may afford cytoprotection
what does free eugenol which remain in endo act as?
irritant
what do you lose with time to dissolution and what can modify this?
lose volume and resin can modify
resin sealers take how long to set?
8 hours -slow , initial toxics declining after 24 hours
what are features of calcium silicate sealer?
- High pH(12) during initial setting 24 hrs
- Hydrophilic
- Enhanced biocompatibility
- Does not shrink when setting
- Non-resorbable
- Excellent sealing ability
- Quick set 3-4 hrs
- Easy to use
how should NiTi files be used?
NiTi files should be used in constant motion using gentle pressure
what are some features of nitinol?
o Alloy of nickel and titanium
o Exotic metal – does not conform to typical rules of mettalurgy
o Super-elasticity – application of stress does not result in usual proportional strain
what is structure of Nitinol?
Temperature-dependent structures martensite and austenite
Lattice structure altered by temp and stress
Martensite form – soft and ductile easily deformed
Austenitic form – strong hard
what does the AAE state NSRCT?
involves the use of biologically acceptable chemical and mechanical treatment of the root canal system to promote healing and repair of the periradicular tissues
