DMT Stage 2 part 2 Flashcards
Which conditions require restoration ?
Fractures
Caries
Erosion and abrasion
What causes a tooth to fracture ?
trauma applies a high stress
What is enamel composed of ?
Biological apatite
A ceramic
What are the properties of enamel ?
hard but brittle
Relatively translucent
What is dentine composed of ?
A composite material- apatite and collagen
What are the properties of dentine ?
softer than enamel
tougher than enamel
opaque
Why is dentine tough ?
act as a shock absorber
What are the initial properties of a filling material ?
fluid and plastic so it can conform to the cavity
What must happen to properties of a filling material during setting ?
it needs to harden and strengthen - resist mechanical forces of mouth
In what way must filling materials be protective ?
protect the pulp from thermal and electrical changes
Which mechanical properties are important for filling materials ?
strength
stiffness
fatigue
toughness
Which 3 concepts are part of aesthetics ?
colour
shade - not possible for amalgam and gold
translucency - enamel is trnaslucnet but filling materials are opaque so not life like
Which factors can effect aesthetics in the long term ?
roughness- scratches can remove gloss and lead to colonisation
stains- food and drink can stain composites and amalgam tarnishes
Are enamel and dentine conductors or insualtors ?
insulators
low thermal condutivity and low thermal diffusivity
Is amalgam a conductor or insulator ?
a conductor - high thermal conductivity
How do we compensate for amalgams high thermal condutivity ?
Zinc oxide cmeent lining acts as an insulator
Do composites need insulator cements ?
not really
What are common allergies to materials ?
latex
nickel
Why is adhesion ideal ?
allows for conservative preparation
prevent microleakage
Which material expands on setting ?
amalgam
excessive expansion can lead to the filling protruding from the tooth and fracture of the tooth
What can absorption of water by fillings lead to ?
hydrolysis
softening - water acts as a plasticiser
leaching - positive and negative into oral cavity
staining
What are the factors in the oral environemnt the filling must endure ?
temperature variations- thermal cycling (5 to 60)
pH variations- alkaline conditions form medication and toothpaste
Solvent exposure- alcohol
corrosion- galvanation when dissimialr metals in contact
What is the pH of plaque ?
4
What can large dimensional changes lead to ?
stress at interface
percolation- pumping of food and bacteria round margins
What does high stress lead to ?
fracture
What does low stress over time ?
fatigue
What does rapidly applied stress lead to ?
impact
How does strength develop ?
setting time may be minutes or hours- long time for max strength to develop
How does the rate of development of strength impact on fillings ?
for the first 24h advise patients to not eat abrasively
What type of cavities can abrasion lead to ?
Class V cavity
V shaped
Can bacteria cause fillings to breakdown ?
bacteria cause resin breakdown
amalgam resist bacteria digestion
Should fillings be radiopaque or radiolucnet ?
radiopaque as caries is radiolucent
What determines the radioopactity of a material ?
its atomic number- the lower the atomic number the more radiolucent
composites are made from low atomic elements- need a radiopacifer adding
What does adding a radioopacifier do ?
lowers strength
compromise needed
What leads to a filling being technique sensitive ?
moisture control
numerous stages
Should working time be long or short ?
long to allow sufficient time for mixing and placing
Should setting time be long or short ?
short- for convenience
What is thermal diffusivity and what should it ideally be ?
thermal diffusivity is how a material reacts to transient thermal stimuli
it should be low
unless upper denture- should be high
What should the coefficient of thermal expansion be like for materials ?
similar to that of enamel and dentine
Why is a high modulus of elasticity beneficial for filling materials ?
allows a lot of elastic deformation before plastic occurs and permanently breaks marginal seal
What are the components of amalgam that ar mixed ?
liquid Hg
solid alloy powder
this is mixed
setting is via amalgamation to produce amaglam
Which metals are present in the alloy powder ?
Ag Sn Zn Cu Hg sometimes
What is the role of Ag/Sn in amalgam ?
major component
combine to make the Ag3Sn gamma phase
What is the gamma phase ?
an intermatallic compound
Ag3Sn
What is the role of copper in the alloy powder ?
adds strength
What is the role of zinc in the alloy powder ?
acts as a scavenger
alloy is made in a high temperature where oxidation can occur
zinc reacts with the oxygen quickly removing it and saving the other metals from being oxidised
oxides severely affect the properties of the amalgam
What is a pre amalgamated alloy ?
alloy powder contains Hg already
these react more rapidly with Hg
most alloys arent pre amalgamated
Describe the setting reaction of dental amalgam ?
dissoloution of small alloy particles in liquid
Hg liquid dissolves into larger alloy particles
reaction to form gamma1 and gamma2
crystallisation of gamma1 and gamma2
Describe the structure of amalgam ?
core of gamma residual alloy (Ag3Sn)
matrix of reaction prodcuts- gamma1 and gamm2
Which has a higher tensile strength gamma or gamma1/2 ?
What is the consequence of this ?
gamma
need to control the amount of mercury- ensure that amalgamation happens but not too much that no gamma left to contribute to strength
Does amalgam have a higher compressive strength or tensile strength ?
What is the consequence of this ?
higher compressive strength of amalgam is higher
material is weak in thin sections as tensile strength is low
unsupported amalgam edges can fracture under heavy loads
What did convenional amalgams present with ?
ditching - marginal breakdown
What are the theories of ditching ?
creep model
corrosion model
Describe the creep model ?
- plastic deformation slowly and repeated over time
- leads to amalgam extending above the margins
- marginal fracture and ditch formation
What is thought to be responsible for creep ?
gamma 2 phase
What are the formuals of gamma 1 and gamma2 phases ?
gamma1- Ag2Hg3
gamma2- Sn7Hg
Describe the corrosion model of ditching ?
heterogenous multi-metal structure of amalgam makes it prone to corrosion
saliva is an electrolyte
gamma2 phase is more electrochemically reactive and forms anode
releases tin ions- weakens material and electrons which go to pulp
What does corrosion lead to ?
discolouration Hg release weakening metallic tastte ditching
What is thought happens to the corrosion products ?
they gather margins between restoration and tooth and seal margins
What is the solution to ditching ?
get rid of gamma2 phase
resulted in modern amalgams
What is present in modern amalgams ?
increased copper
What doe sincreased copper in modern amalgams do ?
makes the gamam2 phase unstable so it doesnt form
copper compounds form instead
What are the copper compounds formed in modern amalgams ?
exist as 2 forms
Cu6Sn5
Cu3Sn
What is the role of palladium ?
some amalgams have been made with palladium and have lower corrosion/creep rates but suggests that something else might be causing ditching
What are the 2 types of alloy powder size ?
lathe cut
atomisation
What are lathe cut particles ?
alloy cast and made into ingots
filings are made from this- usually irregular in size and shape
What is atomisation ?
molten alloy sprayed into inert atmosphere and then solidifies and drops to the ground in spheres
What are the dimensional changes that occur in amalgamation ?
slight contraction when Hg diffuses into alloy partivles
slight expansion when the gamma1 and gamma2 phases crystallisation
overall might be slight expansion/contraction
What are the ISO ranges for expansion/contraction ?
-0.1 to 0.2%
What does a large contraction do ?
marginal staining- fluid- secodnary caries
What does a large expansion do ?
material protrudes- tooth fractures
What is the phenomenon of delayed expansion with amalgam ?
with zinc containing amalgams delayed expansion can occur some time after setting
zinc is contaminated with moisture
leads to liberation of hydrogen - delayed expansion
the need for adequate moisture control
however can use zinc free amalgams- made by atomisation in inert atmosphere
How does the strength of amalgam develop ?
slowly- over 24 hours so there is a risk of early fracture
Compare the strengths of amalgam ?
Strong compression strength
weak tensile strength and weak flexural strength
How does alloy particle size effect setting rate ?
smaller particles have higher setting rate
How should the cavity design for amalgam be ?
Allows a sufficient bulk of amalgam undercuts provided no unsupported enamel cavo surface angle is 90 internal line angles are rounded protective pulp lining can use bonding agent to occlude dentine tubules
Why do amalgam fillings require a protective pulp lining ?
amalgam has a high thermal diffusivity and is a conductor
Compare the coefficient of thermal expansion for enamel/dentine and amalgam ?
amalgam has a much higher coefficient of thermal expansion
risk of microleakage, percolation and expansion
What does the final amalgam strength depend on ?
the concentration of gamma phase (Ag3Sn)
ideally less than 50%
How were traditional wet amalgam mixed made ?
by trituration
in an 8:5 ratio
How are amalgam mixes made today ?
amalgamator and encapsulated prodcuts
allows mechanical mixing in a 1:1 ratio
How is amalgam in capsules ?
impermeable membrane separates alloy powder from liquid mercury
membrane shatttered by the amalgamator
Why do we use amalgam capsules ?
prevent mercury contamination
How is amalgam placed in the cavity ?
by condensation
an amalgam condensor is used
packed in increments whilst still plastic
Why is amalgam packed ?
reduce porosity
How is a cavity filled with amalgam ?
cavity is overfilled
mercury rich layer is carved back to reduce Hg content
Describe how different condensation forces are needed for different particle types ?
lathe cut particles have significant friction between them so require higher condensation forces
spherical particles have less friction between them so require smaller condensation forces
How does the intensity of the force applied correlate with Hg content ?
the higher the condensation force the lower the Hg content
What is significant about a lower Hg content in amalgam ?
a lower Hg content means less amalgamation allowing more gamma phase to remain and contribute to compressive strength
Why is amalgam carved ?
to remove the Hg rich layer
to carve the anatomy for the opposing tooth - contact
Why is the timing of amalgam carving important ?
if too soon- significant amounts of material dragged out
if too late- too hard
How far should amalgam be carved back ?
cut back the amalgam to the margins
Which type of amalgams are easier to carve ?
spherical and fine grain
When are amalgams polished ?
on the next visit - 24 hours needed to achieve max strength
Why is amalgam polished ?
improve apprarance
precent accumulation of debris
imrove corrosion resistance
What is used to polish amalgam ?
abrasives suspended in fluid like zinc oxide and chalk
What does polishing amalgam produce ?
a beilby layer
this layer has good corrosion resistance
What is the problem with mercury ?
its a neurotoxin
When is a patient subjected to mercury ?
during amalgam placement and through corrosion products
Which alloys release less mercury ?
gamam2 phase free alloys- less corrosion
How do allergic reactions to mercury manifest ?
lichenoid reaction
allergic contact dermititis
due to previous mercury sensitisation
What are potentially bigger sources of mercury ?
environmental- rocks and fossil fuels
Is there a global drive to reduce amalgam use ?
no- mercury use is wanting to be reduced not amalgam specifically
How does dentistry take into account mercury concerns ?
no amalgam use in pregnant women
no amalgam use in under 16s
well ventilated surgeries etc
What were silicate cements used as ?
direct tooth coloured fillings
What were the constituents of silicate cements ?
powder- fluoroaluminosilicate glass
liquid- 50% phosphoric acid
What is the setting reaction for silicate cements ?
acid/base reaction
What is the structure of silicate cements ?
composite structure - glass filler and matrix of reaction products
How are silicate cements mixed ?
by hand
What were the advantages of silicate cements ?
shade simialr to enamel
coefficient of thermal expansion similar to enamel
low thermal diffusivity
fluoride releasing
What were the disadvantages of silicate cements ?
weak and brittle
moisture sensitive during setting
do not adhere to tooth
solubility was pH-dependent- below 7.4 (normal saliva) the solubility increased significantly
Why were silicate cements phased out ?
introduction of GIC (similar acid/base and F releasing)
What are acrylic resins ?
polymers based on acrylic monomers (monomers with acryalte bonds)
How are acrylic resins made ?
powder and liquid are mixed
powder- PMMA, initiator and pigment
liquid- MMA and activator
Why is pre- polymerised PMMA used in the polymerisation of acrylic resins ?
pre polymerised PMMA acts as a filler to reduce shrinkage during contraction
How do acrylic resins set ?
free radical polymerisation reaction
What type of reaction is free radical polymerisation ?
exothermic
What were the advantages of acrylic resins ?
less acidic than silicates
less oluble than siliciates
low thermal diffusivity- insulators
tooth coloured
What were the disadvantages of acrylic resins ?
setting is exothermic- can damage pulp
resisdual monomer can be left- tissue irritant
setting contraction
coefficient of thermal expansion very high- percolation and microleakage
flexible- marginal breakdown and destroyed by masticatory forces
no adhesion to enamel/dentine
What is the problem with residual monomer ?
tissue irritant
acts as a plasticiser- makes acrylic resin more flexible
What type of monomer is MMA ?
monofunctional
one c-c double bond only
small- low viscosity but high shrinkage
What is meant by a composite structure ?
2 distinct phases formed by blending components
properties would not be achieved by not mixing the components
2 phases are the matrix and the filler
matrix- reaction products
filler- glass, pre PMMA etc
Describe the composite structure of composie filling materials ?
matrix is resin
filler is glas
What are the different components of composite fillings ?
resin- matrix glass-filler coupling agent to bond matrix and filler stabiliser- extend shelf life radio-opacifier pigment initiator/activators
Describe the problems of convnetional composites ?
they used simple quartz as a filler
and had to add radio-opacfier
this weakened the materials strength
What was the solution to convnetional composites ?
increase glass filler contnet
What happens as the glass filler content increases ?
hardness
rigidity
strength
increase
What decreases as the glass filler content in composites increases ?
polymerisation shrinkage
coefficient of thermal expansion
exothermic temperature rise
How can composites be classified based on filler size ?
conventional
microfilled
hybrid
What are conventional composites ?
filler particle range from 1-50 um
filler volume is 50%
What are microfilled composites ?
particle size from 0.01 to 0.1 um
filler volume is 30-60%
possible to achieve high filler content with high pressure and high temperature
How are filler particles for microfilled composites made ?
combine glass particles with monomer and polymerise to make resin
resin shattered into small pieces
mixed with monomer and polymerised
pre polymerised glass filler surrounded by matrix
What are the disadvantages of microfilled composites ?
the filler and matrix bind weakly
limits use to anterior cavities
What are the advantages of microfilled composites ?
the filler particles are small- smooth surface and better gloss
What are hybrid composites ?
particle size ranges from 0.01 to 20 um
60-80%
What is advantageous about a range of particle sizes in hybrid composite s?
allows more efficient filler packing as small particles can fit between large particles
improved properties- allows them to be used posteriorly
What are nanocomposites ?
produced with particles less than 1 um
build up method
small particles can lead to clumping
can also be nanohybrids
Are there any advantages of nanohybrids ?
evidence in the lab only to suggest better wear resistance than microfilmed and hybrid
Compare microfilled and hybrid composites ?
flexural and tensile strength greater for hybrid
What is the problem with conventional composites and unfilled acrylic resins ?
still have high coefficient of thermal expansion - percoaltion
What is the thermal diffusivity of composites like ?
considered insualtors
What are alternative fillers to glass ?
amorphous silica
silica based glasses
colloidal silica
ceramics
What is BisGMA ?
alternative to MMA for the matrix phase
difucntional- cross linked polymers are made
larger monomer- lower shrinkage but higher viscosity so blending filler is difficult
steric hindrance
How does BisGMA display steric hindrance ?
aromatic groups make it harder for chains to move over each other- stiffer polymers
What is TEGMA ?
a diluent monomer- used to dilute BisGMA and reduce its viscosity
less viscous but higher shrinkage as smaller monomer than BisGMA
difucntional
Why are polymers made of BisGMA and TEGMA considered copolymers ?
made from more than one type of monomer- they are comonomers
What are the problems posed by contraction setting ?
microleakage marginal staining secodnary caries enamel microcracks post operative sensitivity- leave to pulp deonding
Describe different monomers in order of contraction setting ?
MMA most shrinkage
TEGMA
BIsGMA
How can we classifiy composites on handling characteristics ?
univerable
flowable
packable
What is universal composite ?
useful for general work
What is flowable composite ?
lower viscosity due to lower filler or more diluent monomer
weaker and less wear resistant
useful for lining or repair of non carious surface loss
What is packable composite ?
higher viscosity ad more filler
needs to be packed- potential for porosity
useful for posterior restorations
What are the alternative monomers to TEGMA and BisGMA ?
BisEMA
UDMA
Silorane
Why do we need alternative monomers ?
BPA used in making BisGMA is related t health issues
(BPA free composites)
to get Better properties- handling and shrinkage
What is UDMA ?
difunctional mathacrylate- makes a cross linked polymer
Low viscosity
High molecular weight
No BPA used in production
What is BisEMA ?
ethoxylated BisGMA
low viscosity
little BPA
What is silorane ?
different in that it undergoes ring opening not free radical polymerization - has an oxirane ring
undergoes slight expansion- might eliminate any shrinkage but not shown clinically
Why is silane coupling needed ?
resins dont bond to silicates (glass)
applying heat and stress leads to pores between filler and matrix
What happens in silane coupling ?
filler particles are pre treated- with a silane to allowmbetter bonding between matrix and filler
What agent is used for silane coupling ?
silane agent
difucntional molecule- one end is like MMA and can bind to the resin and the other end is like a silane and can bind to the glass filler
bond forms between resin and filler
What is problematc about silanes ?
they hydrolyse and breakdown
What is advantageous about silanated composites ?
higher fatigue life than unsilanated composites
How are chemically activated composite pastes made ?
liquid and pastes have to be mixed by hand
technique sensitive and can allow porosity to develop
When does setting commence for chemcially activated composites ?
setting starts when mixign starts- working time is very small
What are the characteristics of chemically activated polymerisation ?
uniform rate benzyl peroxide inittiaotr DMPT activaor residual monomer possible increase in viscosity
What is light activated polymerisation ?
light is delivered at the correct wavelength to activate polymerisation
command setting- unlimited working time
What are the characteristics of light activated polymerization ?
high degree of polymerisation- not much residual non uniform rate however need an LCU limited depth of cure - use increments marginal stress develops
What are the different types of light activation ?
visible light
UV light
How does the depth of cure decrease ?
as the distance between composite and LCU increases
Why is there a greater temeprature rise with light-activated composites ?
the heat is released over a short period of time when the LCU is placed
Why is the light from the LCU blue ?
filters are used to remove the hotter parts of the spectrum
Desrcribe UV light activation ?
hand unit- but not visible 2mm depth of cure benzoinmethyl ether initiator health risks- melanoma and cornel burns not used now
What is visible light activation ?
vast majority use 470 nm wavelength
camphoroquinone photoinitiator - yellow
tertiary amine still used as an activator
Compare UV and visible light activation ?
improved degree of polymerisation with visible light
no health risk with visible
improved depth of cure with visible light
What are the different designs for LCUs ?
fibre optic cable- glass fibres used to break and reduce transmission
gun design- has a cooling element and bulk and is handheld as well as a timing system
What is the conventional source of light in a LCU ?
QTH light bulb
need exposure for 20 secs
Can plasma and LED sources be used ?
they are availabel in different wavelengths and need shorter exposure
filter out the unwanted wavelengths
How do LEDs work as a light source ?
emit light over a narrow spectrum needed for CQ
environmentally friendly
no heat and only specific wavelengths
What is the formula for radiant energy ?
li x exposure time
What is the optimum light intensity for an LCU ?
16 J/ cm^2
What is the optimuum exposure time for an LCU ?
10-20 secs
What is light intensity related to ?
LED power output
distance of light form surface
correct alignment and full coverage
What is the wavelenght needed or CQ ?
470 nm
What are the alternative photoinitiators to CQ ?
PPD, TPO and BAPO
have their own specific absorption wavelengths so a specialist LCU is needed
How should composite be dispensed ?
from a syringe into a pad
and taken from the pad with a spatula
What are the stages of free radical polymerisation ?
activation
initiation
propagation
termination
What happens in activation ?
activator allows the initiator to form radcials
What happens in initiation ?
the radicals react with the monomer producing a molecule capable of further reaction
What happens in propagation ?
molecule reacts with further monmers
this stage determines the molecular weight and the chain length
What happens in termination ?
the viscosity has reached the gel point - the monomers can no longer come into contact with radicals
radicals cancel each other out
impurities can prematurely terminate the reaction
How can oxygen prematurely terminate polymerisation ?
oxygen reacts a radical
this nullifies the radical and creates an oxygen radical
oxygen radical has a low reactivity- reaction terminated
What does oxygen inhibition lead to ?
prematuretly terminated layer on top that is soft, weak and can be easily damaged
What is the purpose of matrix strips ?
to prevent air inhibition
to guide the morphology of the restoration
to produce a glossy surface which reflects the matrix
Why are composites polished ?
to remove any excess at margins and produce a smooth finish whilst maintaining the shape of the restoration
What is the basic principle of polishing ?
start with the coarsest abrasive firdt and then gradually reduce the coarsness
Is the polished surface glossier than the original matrix surface ?
no - the matrix surface will always be the smoothest possible
Why do composite surfaces get rougher over time ?
the original matrix strip surface is rich in resin and soft
over time this is roughned away and the filler particles start to extrude from the surface
abrasion removes rich resin and gloss
Why do microfilled composites retain a smooth surface better ?
the filler particles are smaller
What does polishing do to surface roughness ?
polishing increasess surface roughness
Why do composites stain ?
solvents soften resin
food and drink
acids get rid of filler and silane bond
CQ
What does polymerisation shrinkage do ?
ruptures the adhesive seal at tooth interface
microleakage and secondary caries
enamel microcracks
fracture possibly
What does the magnitude of shrinkage depend on ?
the size of the monomer- smaller means more shrinkage
degree of polymerisation- higher degree more shrinkage as polymer Is smaller than monomer
filler content
strength of adhesion
configuration factor
amount of diluent monomer TEGMA- more shrinage as smaller monomer
What techniques have been devised to overcome shrinkage ?
incremental placement of composite
What is the configuration factor ?
ratio of bonded surfaces : unbonded surfaces
What does a higher C factor indicate ?
more stress will develop at tooth interface
more potential for shrinkage to cause a problem
Where do the greatest C factors apply ?
with coating agents- a high proorption of the volume of the agent is made of bonded surfaces
What is needed for successful light activated polymerisation for composite s?
good illumianation
incremental build up
single paste used- no porosity risk
What needs to be consdiered during light activation ?
polymerisation is exo
greater temp rise in greater cavities
heat from LCU as well
greater cavities have less tertiary dentine- less pulp protection
What is the biocompatabiltiy of composite ?
BPA used in production of BisGMA
residual monomer is cytotoxic and tissue irritant
tertiary amines and monomers are cytotoxic
polishing composites can lead to silica aerosls- penumosilicosis
What is the biocompatability of amalgam ?
concerns over mercury use
What has to be consdiered for cavity design for amaglam ?
is mechanically retained- need undercuts and grooves
needs a bulk
can destroy sound tooth tissue
What has to be considered for cavity design with composite ?
might be more conservative as adhesive can be used
How can composite margin seals be overcome ?
shrinkage
poor adhesion bond
How can amalgam seal it margins ?
products of corrosion
What is the hardness, toughness and strength like for amalgam and composite?
similar
Is amalgam or composite more durable ?
amalgam- provided it is in bulk
Do enamel and dentine adhere to composites ?
no
they require an adhesive
What are some of the problems with trying to establish a bond ?
Time- adhesives take a long time to develop max strength
cavity size effect- larger cavities more reason for the bond to fail
lack of tooth affinity- resins are hydrophobic but tooth is hydrophilic
mouth is moist/warm- leads to thermal expansion/contraction
What is the prerequisite for all adhesion ?
intimate contact
What can get in the way of adhesion ?
surface roughness
What is an adherned ?
a surface that is joined to another by an adhesive
/Why are adhesives liquid usually ?
to allow intimate contact
What is wetting ?
the ability of a liquid to flow over a surface and form a consistent film
What is wetting dependent on ?
surface tension of the liquid
surface energy of the substrate
What is surface tension of a liquid ?
in its lowest energy form tension can hold liquid into bubbles
What is the surface energy of a substrate ?
the number of availbale bonds the surface can make compared to the bulk
What does wetting require ?
the surface energy of the substrate is greater of the surface tension of the liquid
What is the contact angle ?
the angle between bubble and the surface
What can we do to the contact angle ?
lower the contact angle
this will lead to flattening of the droplet
increases the surface energy and lowers the surface tension
What is a primer ?
alters the surface of the adhered so the adhesive can interact better with it
alters the surface energy
What are the 3 bonding mechanisms ?
mechanical
chemical
complex
What is a mechanical bonding mechanism ?
macromechanical retention- amaglam uses undercuts in cavities
micromechanical retention- liquids flow into microundercuts and form tags
What is a chemical bonding mechanism ?
reversible or irreversible covalent and ionic bonds
subject to hydrolysis, temperature and pH changes
What is a complex bonding mechanism ?
mixture of chemical and mechanical bonding mechanisms
What are the obstales to bonding to enamel ?
the enamel pellicle - weakled edhered to enamel must be removed
the amorphous HaP layer- has a non hydroxyapatite structure and is formed during remineralisation so must be removed
Describe the stages of the acid/etch technique ?
wash
etch
wash
dry
Why is enamel etched ?
leads to etched structure
etched prisms act as microscale undercuts
resin can flow into the undercuts
Why is enamel washed initially ?
to remove the pellicle
How do you etch enamel ?
apply etchant which is 37% phosphoric acid
for 10-60 secs
can be liquid/gel and coloured for control
if left on too long- precipitation of phosphates
Why do we wash the etchant off enamel ?
rinse enamel with oil free water
remove debris and etchant
Why do we need to dry etched enamel ?
reveal frosty appearance
bonding resins are hydrophobic and the enamel is hydrophillic- must dry as much as can
What must you avoid during etching ?
salivary/moisture contamination
must re etch if contaminated
What is bonding via etching sufficient for ?
resisting composite shrinkage
Why cant you apply a composite directly to etched enamel ?
low viscosity - BisGMA is a large molecule
filler particles are large and can occlude the undercuts
What do you apply before the composite to enamel ?
Bonding resin
What are the properties of enamel bonding resins ?
similar monomer composition to the composite
fluid
originally unfilled
How do bonding resins set ?
light set- viscosity is consistent and can enter the microundercut
chemically set- mix 2 pastes, viscosity will increase and might compromise ability to enter micorundercut
What is the action of bonding resins ?
enter microscale undercuts and form resin tags
leads to micromechanical retention
mechanical bonding only
What are applications of the acid/etch technique ?
class IV cavities- proximal incisal edges
fissure sealnts
orthodontic brackets
resin bonded bridges
What is the structure of enamel ?
highly clacfied prisms
96% apatite
3% water
1% organic
What is the structure of dentine ?
tubular structure
70% hydroxyapatite
20% collagen
10% water
both hydrophilic and hydrophobic components
impossible to obtain dry field- tubules are always pumping fluid
What are the concerns with applying the acid/etch technique to engine ?
acid applied to vital dentine- damage the pulp ?
now thought ineffective marginal selaing- acid irritation
would need to dry dentine before bonding resin is applied which is hydrophobic and the dentine is hydrophilic
In what circumstances does dnetinal tubular fluid decrease ?
in anaesthetised teeth
adrenaline is vasconstrictor to pulp blood vessels
less tubular fluid which is a direct exudate of the pulp blood vessels
What is the smear layer ?
gelatinous layer that is weakly chemically bonded to dentien and prevents the adhesive bonding to bulk dentine
formed from cavity prep- contains debris- dentien chips, collagen, microbes, salivary protein and collagen
covers the entire surface of prepared dnetine
Why do we need to deal with the smear layer ?
to form an effective bond between the adhesive and the engine
How can we deal with the smear layer ?
remove it in the total acid etch approach- you will end up bonding to bulk dentine
modfiy it with weak acid or self etching primer and you will bond to a mix of modified smear layer and engine
What is the stage called where we deal with the smear layer ?
conditioning- equiavalent of acid etch
Why is the priming stage carried out after conditioning ?
designed to change the chemical nature of the dentine surface to overcome the repulsion between the hydrophilic dentine and the hydrophobic monomers in the resin
What is the most commonly used priming agent ?
HEMA
Which molecules are used for priming agents ?
difunctional molecules
one end is a methacryalte group- can bind to the bonding resin
the other end is a reactive group that has affinity for moist dentine
What is the general structure of a priming agent ?
M-S-R
M is the methacrylate groups which binds to the bonding resin and can polymerise
S is a spacer that allows flexibility and viscosity
R is the reactive group and can have an affinity for moist dentine -
What does HEMA stand for ?
Hydroxyethyl methacrylate
What type of molecule is HEMA ?
amphiphillic- hydrophobic and hydrophillic componenets
hydrophobic- methacryalte
hydrophilic- OH group
Does the primer carry out mechanical or chemical bonding ?
mechanical bonding to the dentine in the hybrid layer only has affinity for dentine via OH group
not chemically bonding
What is 10 MDP ?
another primer molecule
might have possible chemical bonding on the dentine
contains phosphate group which can bind to calcium in the dentine
What happens after priming ?
the dentine is now more hydrophobic and able to bind to the resin
What does the conditioning do ?
smear layer removed or modified
demineralises dentine and leads to 3D collagen network
What happens to the collagen network ?
the primer and the resin can infiltrate the 3D collagen network forming the hybrid layer
How does infiltration affect bond strength ?
poor infiltration of the primer and resin in the hybrid layer leads to weak bond strength
Good infiltration leads to strong bond strength
What type of structure does the hybrid layer have ?
composite structure
2 phases- resin and collagen phases
What would happen if we dried dentine after conditionng ?
this would lead to pulp damage
leads to collapse of the 3D collagen network
What is the dilemma with drying conditioned dentine ?
we need to dry dentine to make hydrophobic for the monomers but drying leads to collagen network collapse so we dont get a hybrid layer
How do we get over the dentine drying dilemma /?
we use displacing primers
the primer contains water/solvents like ethanol that will displace the water around the collagen fibrils
What is the total etch approach for dentine ?
use 37% phosphoric acid
this will remove the smear layer and demineralise the dentine leading to the collagen network
you have to rinse the acid
can bond to wet dentine
What is the self etch approach for dentine ?
use a self etching primer or a weak acid
HEMA in maleic acid
you dont need to rinse the acid off
the smear layer is not fully removed so the dentinal tubules are not exposed directly- less post op sensittivity ?
What are dentine bonding resins like ?
difucntional methacrylate
bond to the primer methacrylate
bond to the methacryalte groups in the composite
traditionally unfilled
What are the stages of dentine bonding ?
conditioning - remove smear layer and get collagen network
priming - make dentine hydrophobic
bonding- bonding with resin - infiltrate the tubules and make hybrid layer with primer
How long does it take for bond strength to develop ?
24 hours
takes 20secodns for the composite to cure so there is a disparity between bond development and curing - can lead to marginal failure
